Central Nervous System 1
ananthatiger
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May 04, 2010
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NERVOUS NERVOUS
SYSTEMSYSTEM
SYSTEMSYSTEM
NERVOUS FUNCTIONSNERVOUS FUNCTIONS
Body’s master controlling and communicating Body’s master controlling and communicating
systemsystem
Three functionsThree functions
•Sensory inputSensory input
Gathers information Gathers information
from sensory receptorsfrom sensory receptors
•IntegrationIntegration
Processes and interprets Processes and interprets
sensory inputsensory input
•Motor outputMotor output
Activates effector organs to cause a responseActivates effector organs to cause a response
Body’s master controlling and communicating Body’s master controlling and communicating
systemsystem
Three functionsThree functions
•Sensory inputSensory input
Gathers information Gathers information
from sensory receptorsfrom sensory receptors
•IntegrationIntegration
Processes and interprets Processes and interprets
sensory inputsensory input
•Motor outputMotor output
Activates effector organs to cause a responseActivates effector organs to cause a response
Nervous System Nervous System
OrganizationOrganization
OrganizationOrganization
ORGANIZATIONORGANIZATION
Two Principal Parts of the SystemTwo Principal Parts of the System
Central nervous system (CNS)Central nervous system (CNS)
•Brain and spinal cordBrain and spinal cord
•Integrating and command centerIntegrating and command center
Interprets sensory inputInterprets sensory input
Dictates motor responsesDictates motor responses
Peripheral nervous system (PNS)Peripheral nervous system (PNS)
•Nerves extending from brain and spinal cordNerves extending from brain and spinal cord
•Carry impulses to and from the CNSCarry impulses to and from the CNS
Two Principal Parts of the SystemTwo Principal Parts of the System
Central nervous system (CNS)Central nervous system (CNS)
•Brain and spinal cordBrain and spinal cord
•Integrating and command centerIntegrating and command center
Interprets sensory inputInterprets sensory input
Dictates motor responsesDictates motor responses
Peripheral nervous system (PNS)Peripheral nervous system (PNS)
•Nerves extending from brain and spinal cordNerves extending from brain and spinal cord
•Carry impulses to and from the CNSCarry impulses to and from the CNS
PERIPHERAL DIVISIONSPERIPHERAL DIVISIONS
Two Functional Subdivisions of the PNSTwo Functional Subdivisions of the PNS
Sensory divisionSensory division
• “ “afferent division”afferent division”
•Nerve fibers conveying impulses to the CNSNerve fibers conveying impulses to the CNS
Somatic afferent fibersSomatic afferent fibers convey impulses from the skin, convey impulses from the skin,
muscles, and jointsmuscles, and joints
Visceral afferent fibersVisceral afferent fibers convey impulses from visceral convey impulses from visceral
organsorgans
Motor divisionMotor division
•, “efferent division”, “efferent division”
•Nerve fibers conveying impulses from the CNSNerve fibers conveying impulses from the CNS
Two Functional Subdivisions of the PNSTwo Functional Subdivisions of the PNS
Sensory divisionSensory division
• “ “afferent division”afferent division”
•Nerve fibers conveying impulses to the CNSNerve fibers conveying impulses to the CNS
Somatic afferent fibersSomatic afferent fibers convey impulses from the skin, convey impulses from the skin,
muscles, and jointsmuscles, and joints
Visceral afferent fibersVisceral afferent fibers convey impulses from visceral convey impulses from visceral
organsorgans
Motor divisionMotor division
•, “efferent division”, “efferent division”
•Nerve fibers conveying impulses from the CNSNerve fibers conveying impulses from the CNS
ORGANIZATIONORGANIZATION
HISTOLOGYHISTOLOGY
Nervous system consists mainly of nervous Nervous system consists mainly of nervous
tissuetissue
Highly cellularHighly cellular
•e.g., <20% extracellular space in CNSe.g., <20% extracellular space in CNS
Two principal cell typesTwo principal cell types
•NeuronsNeurons
Excitable nerve cells that transmit electrical signalsExcitable nerve cells that transmit electrical signals
•Supporting cellsSupporting cells
Smaller cells surrounding and wrapping neuronsSmaller cells surrounding and wrapping neurons
““Neuroglia”Neuroglia”
Nervous system consists mainly of nervous Nervous system consists mainly of nervous
tissuetissue
Highly cellularHighly cellular
•e.g., <20% extracellular space in CNSe.g., <20% extracellular space in CNS
Two principal cell typesTwo principal cell types
•NeuronsNeurons
Excitable nerve cells that transmit electrical signalsExcitable nerve cells that transmit electrical signals
•Supporting cellsSupporting cells
Smaller cells surrounding and wrapping neuronsSmaller cells surrounding and wrapping neurons
““Neuroglia”Neuroglia”
NEUROGLIANEUROGLIA
““Nerve glue”Nerve glue”
Six types of small cells associated with neuronsSix types of small cells associated with neurons
•4 in CNS4 in CNS
•2 in PNS2 in PNS
Most have central cell body and branching Most have central cell body and branching
processesprocesses
Several functionsSeveral functions
•e.g., Supportive scaffolding for neuronse.g., Supportive scaffolding for neurons
•e.g., Electrical isolation of neuronse.g., Electrical isolation of neurons
•e.g., Neuron health and growthe.g., Neuron health and growth
““Nerve glue”Nerve glue”
Six types of small cells associated with neuronsSix types of small cells associated with neurons
•4 in CNS4 in CNS
•2 in PNS2 in PNS
Most have central cell body and branching Most have central cell body and branching
processesprocesses
Several functionsSeveral functions
•e.g., Supportive scaffolding for neuronse.g., Supportive scaffolding for neurons
•e.g., Electrical isolation of neuronse.g., Electrical isolation of neurons
•e.g., Neuron health and growthe.g., Neuron health and growth
CNS NEUROGLIACNS NEUROGLIA
AstrocytesAstrocytes
MicrogliaMicroglia
Ependymal cellsEpendymal cells
OligodendrocytesOligodendrocytes
AstrocytesAstrocytes
MicrogliaMicroglia
Ependymal cellsEpendymal cells
OligodendrocytesOligodendrocytes
CNS NEUROGLIACNS NEUROGLIA
AstrocytesAstrocytes
Most abundant and versatile glial cellsMost abundant and versatile glial cells
Numerous processes support branching neuronsNumerous processes support branching neurons
•Anchor neurons to capillary blood supplyAnchor neurons to capillary blood supply
Guide migration of young neuronsGuide migration of young neurons
Facilitate nutrient delivery to neuronsFacilitate nutrient delivery to neurons
•(blood (blood glial cell glial cell neuron) neuron)
Control chemical environment Control chemical environment
around neuronsaround neurons
•Uptake of KUptake of K
++
, neurotransmitters, neurotransmitters
Communicate with astrocytes Communicate with astrocytes
& neurons& neurons
•Gap junctions Gap junctions
AstrocytesAstrocytes
Most abundant and versatile glial cellsMost abundant and versatile glial cells
Numerous processes support branching neuronsNumerous processes support branching neurons
•Anchor neurons to capillary blood supplyAnchor neurons to capillary blood supply
Guide migration of young neuronsGuide migration of young neurons
Facilitate nutrient delivery to neuronsFacilitate nutrient delivery to neurons
•(blood (blood glial cell glial cell neuron) neuron)
Control chemical environment Control chemical environment
around neuronsaround neurons
•Uptake of KUptake of K
++
, neurotransmitters, neurotransmitters
Communicate with astrocytes Communicate with astrocytes
& neurons& neurons
•Gap junctions Gap junctions
CNS NEUROGLIACNS NEUROGLIA
MicrogliaMicroglia
Small ovoid cellsSmall ovoid cells
Relatively long “thorny” Relatively long “thorny”
processesprocesses
•Processes touch nearby neuronsProcesses touch nearby neurons
Migrate toward injured neuronsMigrate toward injured neurons
Transform into macrophageTransform into macrophage
•Phagocytize microorganisms, debrisPhagocytize microorganisms, debris
•(Cells of immune system cannot enter the CNS)(Cells of immune system cannot enter the CNS)
MicrogliaMicroglia
Small ovoid cellsSmall ovoid cells
Relatively long “thorny” Relatively long “thorny”
processesprocesses
•Processes touch nearby neuronsProcesses touch nearby neurons
Migrate toward injured neuronsMigrate toward injured neurons
Transform into macrophageTransform into macrophage
•Phagocytize microorganisms, debrisPhagocytize microorganisms, debris
•(Cells of immune system cannot enter the CNS)(Cells of immune system cannot enter the CNS)
CNS NEUROGLIACNS NEUROGLIA
Ependymal CellsEpendymal Cells
Line central cavities of brain and spinal cordLine central cavities of brain and spinal cord
•Form permeable barrier between cerebrospinal fluid inside Form permeable barrier between cerebrospinal fluid inside
these cavities and tissue fluid of CNS tissuethese cavities and tissue fluid of CNS tissue
Shapes range from squamous to columnarShapes range from squamous to columnar
Many are ciliatedMany are ciliated
•Beating helps circulate cerebrospinal fluid cushioning brain Beating helps circulate cerebrospinal fluid cushioning brain
and spinal cordand spinal cord
Ependymal CellsEpendymal Cells
Line central cavities of brain and spinal cordLine central cavities of brain and spinal cord
•Form permeable barrier between cerebrospinal fluid inside Form permeable barrier between cerebrospinal fluid inside
these cavities and tissue fluid of CNS tissuethese cavities and tissue fluid of CNS tissue
Shapes range from squamous to columnarShapes range from squamous to columnar
Many are ciliatedMany are ciliated
•Beating helps circulate cerebrospinal fluid cushioning brain Beating helps circulate cerebrospinal fluid cushioning brain
and spinal cordand spinal cord
CNS NEUROGLIACNS NEUROGLIA
OligodendrocytesOligodendrocytes
Fewer processes than astrocytesFewer processes than astrocytes
Wrap processes tightly around thicker neuron Wrap processes tightly around thicker neuron
fibers in CNSfibers in CNS
•““Myelin sheath”Myelin sheath”
•Insulating coveringInsulating covering
OligodendrocytesOligodendrocytes
Fewer processes than astrocytesFewer processes than astrocytes
Wrap processes tightly around thicker neuron Wrap processes tightly around thicker neuron
fibers in CNSfibers in CNS
•““Myelin sheath”Myelin sheath”
•Insulating coveringInsulating covering
PNS NEUROGLIAPNS NEUROGLIA
Satellite cellsSatellite cells
Schwann cellsSchwann cells
Satellite cellsSatellite cells
Schwann cellsSchwann cells
PNS NEUROGLIAPNS NEUROGLIA
Satellite cellsSatellite cells
Surround neuron cell bodies within gangliaSurround neuron cell bodies within ganglia
•(A ganglion is a collection of nerve cell bodies (A ganglion is a collection of nerve cell bodies
outside of the CNS)outside of the CNS)
Function poorly understoodFunction poorly understood
Satellite cellsSatellite cells
Surround neuron cell bodies within gangliaSurround neuron cell bodies within ganglia
•(A ganglion is a collection of nerve cell bodies (A ganglion is a collection of nerve cell bodies
outside of the CNS)outside of the CNS)
Function poorly understoodFunction poorly understood
PNS NEUROGLIAPNS NEUROGLIA
Schwann cellsSchwann cells
“ “Neurolemmocytes”Neurolemmocytes”
Surround and form myelin sheaths around larger nerve Surround and form myelin sheaths around larger nerve
fibers of PNSfibers of PNS
•Functionally similar to oligodendrocytesFunctionally similar to oligodendrocytes
Vital to regeneration of peripheral nerve fibersVital to regeneration of peripheral nerve fibers
Schwann cellsSchwann cells
“ “Neurolemmocytes”Neurolemmocytes”
Surround and form myelin sheaths around larger nerve Surround and form myelin sheaths around larger nerve
fibers of PNSfibers of PNS
•Functionally similar to oligodendrocytesFunctionally similar to oligodendrocytes
Vital to regeneration of peripheral nerve fibersVital to regeneration of peripheral nerve fibers
NEURONSNEURONS
Nerve cellsNerve cells
Structural units of nervous systemStructural units of nervous system
•Billions are present in nervous systemBillions are present in nervous system
Conduct messages throughout bodyConduct messages throughout body
•Nerve impulsesNerve impulses
Extreme longevityExtreme longevity
•Can function optimally for entire lifetimeCan function optimally for entire lifetime
AmitoticAmitotic
•Ability to divide is lost in mature cellsAbility to divide is lost in mature cells
•Cannot be replaced if destroyedCannot be replaced if destroyed
Some (very few) exceptionsSome (very few) exceptions
e.g., stem cells present in olfactory epithelium can produce new neuronse.g., stem cells present in olfactory epithelium can produce new neurons
•Stem cell research shows great promise in repairing damaged neuronsStem cell research shows great promise in repairing damaged neurons
High metabolic rateHigh metabolic rate
•Require large amounts of oxygen and glucoseRequire large amounts of oxygen and glucose
Nerve cellsNerve cells
Structural units of nervous systemStructural units of nervous system
•Billions are present in nervous systemBillions are present in nervous system
Conduct messages throughout bodyConduct messages throughout body
•Nerve impulsesNerve impulses
Extreme longevityExtreme longevity
•Can function optimally for entire lifetimeCan function optimally for entire lifetime
AmitoticAmitotic
•Ability to divide is lost in mature cellsAbility to divide is lost in mature cells
•Cannot be replaced if destroyedCannot be replaced if destroyed
Some (very few) exceptionsSome (very few) exceptions
e.g., stem cells present in olfactory epithelium can produce new neuronse.g., stem cells present in olfactory epithelium can produce new neurons
•Stem cell research shows great promise in repairing damaged neuronsStem cell research shows great promise in repairing damaged neurons
High metabolic rateHigh metabolic rate
•Require large amounts of oxygen and glucoseRequire large amounts of oxygen and glucose
NeurotrophinsNeurotrophins
Promote neuron growth.Promote neuron growth.
Nerve growth factors include:Nerve growth factors include:
•Nerve growth factor (NGF), brain-derived neurotrophic factor Nerve growth factor (NGF), brain-derived neurotrophic factor
(BDNF), glial-derived neurotrophic factor (GDNF), (BDNF), glial-derived neurotrophic factor (GDNF),
neurotrophin-3, and neurotrophin-4/5.neurotrophin-3, and neurotrophin-4/5.
Fetus:Fetus:
•Embryonic development of sensory neurons and sympathetic Embryonic development of sensory neurons and sympathetic
ganglia (NGF and neurotrophin-3).ganglia (NGF and neurotrophin-3).
Adult:Adult:
•Maintenance of sympathetic ganglia (NGF).Maintenance of sympathetic ganglia (NGF).
•Mature sensory neurons need for regeneration.Mature sensory neurons need for regeneration.
•Required to maintain spinal neurons (GDNF).Required to maintain spinal neurons (GDNF).
•Sustain neurons that use dopamine (GDNF).Sustain neurons that use dopamine (GDNF).
Myelin-associated inhibitory proteins:Myelin-associated inhibitory proteins:
•Inhibit axon regeneration.Inhibit axon regeneration.
Promote neuron growth.Promote neuron growth.
Nerve growth factors include:Nerve growth factors include:
•Nerve growth factor (NGF), brain-derived neurotrophic factor Nerve growth factor (NGF), brain-derived neurotrophic factor
(BDNF), glial-derived neurotrophic factor (GDNF), (BDNF), glial-derived neurotrophic factor (GDNF),
neurotrophin-3, and neurotrophin-4/5.neurotrophin-3, and neurotrophin-4/5.
Fetus:Fetus:
•Embryonic development of sensory neurons and sympathetic Embryonic development of sensory neurons and sympathetic
ganglia (NGF and neurotrophin-3).ganglia (NGF and neurotrophin-3).
Adult:Adult:
•Maintenance of sympathetic ganglia (NGF).Maintenance of sympathetic ganglia (NGF).
•Mature sensory neurons need for regeneration.Mature sensory neurons need for regeneration.
•Required to maintain spinal neurons (GDNF).Required to maintain spinal neurons (GDNF).
•Sustain neurons that use dopamine (GDNF).Sustain neurons that use dopamine (GDNF).
Myelin-associated inhibitory proteins:Myelin-associated inhibitory proteins:
•Inhibit axon regeneration.Inhibit axon regeneration.
NeuronsNeurons
Axon of another
neuron
Cell BodyDendrites
Axon
Myelin
Sheath
Dendrites of
another neuron
Axon of another
neuron
Cell BodyDendrites
Axon
Myelin
Sheath
Dendrites of
another neuron
NEURONSNEURONS
Generally large, complex cellsGenerally large, complex cells
Structures vary, but all neurons have the same basic Structures vary, but all neurons have the same basic
structurestructure
•Cell bodyCell body
•Slender processes Slender processes
extending from cell extending from cell
bodybody
•Plasma membrane Plasma membrane
is site of signalingis site of signaling
Generally large, complex cellsGenerally large, complex cells
Structures vary, but all neurons have the same basic Structures vary, but all neurons have the same basic
structurestructure
•Cell bodyCell body
•Slender processes Slender processes
extending from cell extending from cell
bodybody
•Plasma membrane Plasma membrane
is site of signalingis site of signaling
NEURON CELL NEURON CELL
BODYBODY
Most neuron cell bodies are located in the CNSMost neuron cell bodies are located in the CNS
•Protected by bones of skull or vertebral columnProtected by bones of skull or vertebral column
Clusters of cell bodies in the CNS are termed Clusters of cell bodies in the CNS are termed
“nuclei”“nuclei”
Clusters of cell Clusters of cell
bodies in the bodies in the
PNS are PNS are
termed termed
“ganglia”“ganglia”
BODYBODY
Most neuron cell bodies are located in the CNSMost neuron cell bodies are located in the CNS
•Protected by bones of skull or vertebral columnProtected by bones of skull or vertebral column
Clusters of cell bodies in the CNS are termed Clusters of cell bodies in the CNS are termed
“nuclei”“nuclei”
Clusters of cell Clusters of cell
bodies in the bodies in the
PNS are PNS are
termed termed
“ganglia”“ganglia”
NEURON CELL NEURON CELL
BODYBODY
a.k.a., “perikaryon” or “soma”a.k.a., “perikaryon” or “soma”
5 – 140 5 – 140 mmm in diameterm in diameter
Transparent spherical nucleusTransparent spherical nucleus
•Contains Contains
conspicuous conspicuous
nucleolusnucleolus
BODYBODY
a.k.a., “perikaryon” or “soma”a.k.a., “perikaryon” or “soma”
5 – 140 5 – 140 mmm in diameterm in diameter
Transparent spherical nucleusTransparent spherical nucleus
•Contains Contains
conspicuous conspicuous
nucleolusnucleolus
NEURON CELL NEURON CELL
BODYBODY
Major biosynthetic center of neuronMajor biosynthetic center of neuron
Other usual organelles presentOther usual organelles present
•ER & ribosomes most active and best developed in bodyER & ribosomes most active and best developed in body
What do they do?What do they do?
•Centrioles absentCentrioles absent
What do centrioles What do centrioles
do?do?
•Sometimes contains Sometimes contains
pigment inclusionspigment inclusions
BODYBODY
Major biosynthetic center of neuronMajor biosynthetic center of neuron
Other usual organelles presentOther usual organelles present
•ER & ribosomes most active and best developed in bodyER & ribosomes most active and best developed in body
What do they do?What do they do?
•Centrioles absentCentrioles absent
What do centrioles What do centrioles
do?do?
•Sometimes contains Sometimes contains
pigment inclusionspigment inclusions
NEURON CELL NEURON CELL
BODYBODY
Focal point for the outgrowth of neuron processes Focal point for the outgrowth of neuron processes
during embryonic developmentduring embryonic development
•Some processes receive signalsSome processes receive signals
•Plasma membrane Plasma membrane
generally also acts generally also acts
as part of the as part of the
receptive surfacereceptive surface
BODYBODY
Focal point for the outgrowth of neuron processes Focal point for the outgrowth of neuron processes
during embryonic developmentduring embryonic development
•Some processes receive signalsSome processes receive signals
•Plasma membrane Plasma membrane
generally also acts generally also acts
as part of the as part of the
receptive surfacereceptive surface
NEURON NEURON
PROCESSESPROCESSES
Extend from the neuron’s cell bodyExtend from the neuron’s cell body
CNS contains both neuron cell bodies and their CNS contains both neuron cell bodies and their
processesprocesses
•Bundles of CNS processes are termed “tracts”Bundles of CNS processes are termed “tracts”
PNS consists mainly of neuronal processesPNS consists mainly of neuronal processes
•Bundles of PNS processes are termed “nerves”Bundles of PNS processes are termed “nerves”
Two types of neuron processesTwo types of neuron processes
•DendritesDendrites
•AxonsAxons
PROCESSESPROCESSES
Extend from the neuron’s cell bodyExtend from the neuron’s cell body
CNS contains both neuron cell bodies and their CNS contains both neuron cell bodies and their
processesprocesses
•Bundles of CNS processes are termed “tracts”Bundles of CNS processes are termed “tracts”
PNS consists mainly of neuronal processesPNS consists mainly of neuronal processes
•Bundles of PNS processes are termed “nerves”Bundles of PNS processes are termed “nerves”
Two types of neuron processesTwo types of neuron processes
•DendritesDendrites
•AxonsAxons
NEURON NEURON
PROCESSESPROCESSES
Typical DendriteTypical Dendrite
Short, tapering, diffusely branching extensionsShort, tapering, diffusely branching extensions
•Generally hundreds clustering close to cell bodyGenerally hundreds clustering close to cell body
•Most cell body organelles also present in dendritesMost cell body organelles also present in dendrites
Main receptive / input regionsMain receptive / input regions
•Large surface area for Large surface area for
receiving signals from receiving signals from
other neuronsother neurons
•Convey incoming messages Convey incoming messages
toward cell bodytoward cell body
•Short-distance signals are Short-distance signals are
“graded potentials”“graded potentials”
Not action potentialsNot action potentials
PROCESSESPROCESSES
Typical DendriteTypical Dendrite
Short, tapering, diffusely branching extensionsShort, tapering, diffusely branching extensions
•Generally hundreds clustering close to cell bodyGenerally hundreds clustering close to cell body
•Most cell body organelles also present in dendritesMost cell body organelles also present in dendrites
Main receptive / input regionsMain receptive / input regions
•Large surface area for Large surface area for
receiving signals from receiving signals from
other neuronsother neurons
•Convey incoming messages Convey incoming messages
toward cell bodytoward cell body
•Short-distance signals are Short-distance signals are
“graded potentials”“graded potentials”
Not action potentialsNot action potentials
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Single axon per neuronSingle axon per neuron
““Axon hillock” of cell body narrows to form a slender Axon hillock” of cell body narrows to form a slender
process of uniform diameterprocess of uniform diameter
Sometimes very shortSometimes very short
Sometimes very longSometimes very long
•e.g., axons controlling e.g., axons controlling
big toe are 3 – 4 feet big toe are 3 – 4 feet
longlong
PROCESSESPROCESSES
Typical AxonTypical Axon
Single axon per neuronSingle axon per neuron
““Axon hillock” of cell body narrows to form a slender Axon hillock” of cell body narrows to form a slender
process of uniform diameterprocess of uniform diameter
Sometimes very shortSometimes very short
Sometimes very longSometimes very long
•e.g., axons controlling e.g., axons controlling
big toe are 3 – 4 feet big toe are 3 – 4 feet
longlong
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Single axon may branch along lengthSingle axon may branch along length
““Axon collaterals” extend from neurons at ~ 90Axon collaterals” extend from neurons at ~ 90
oo
angles angles
Usually branches Usually branches
profusely at endprofusely at end
•10,000 or more 10,000 or more
terminal branches terminal branches
is commonis common
•Distal endings Distal endings
termed “axonal termed “axonal
terminals”terminals”
PROCESSESPROCESSES
Typical AxonTypical Axon
Single axon may branch along lengthSingle axon may branch along length
““Axon collaterals” extend from neurons at ~ 90Axon collaterals” extend from neurons at ~ 90
oo
angles angles
Usually branches Usually branches
profusely at endprofusely at end
•10,000 or more 10,000 or more
terminal branches terminal branches
is commonis common
•Distal endings Distal endings
termed “axonal termed “axonal
terminals”terminals”
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Conducting component of neuronConducting component of neuron
Generates nerve impulsesGenerates nerve impulses
•Generated at axon hillock / axon junction in motor neuronsGenerated at axon hillock / axon junction in motor neurons
•““Trigger zone”Trigger zone”
Transmits nerve Transmits nerve
impulses away from impulses away from
cell bodycell body
•To axonal terminalsTo axonal terminals
PROCESSESPROCESSES
Typical AxonTypical Axon
Conducting component of neuronConducting component of neuron
Generates nerve impulsesGenerates nerve impulses
•Generated at axon hillock / axon junction in motor neuronsGenerated at axon hillock / axon junction in motor neurons
•““Trigger zone”Trigger zone”
Transmits nerve Transmits nerve
impulses away from impulses away from
cell bodycell body
•To axonal terminalsTo axonal terminals
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Axonal terminals are Axonal terminals are secretory componentsecretory component of neuron of neuron
Sequence of eventsSequence of events
•Signal reaches terminalsSignal reaches terminals
•Membranes of vesicles fuse with Membranes of vesicles fuse with
plasma membraneplasma membrane
““Axolemma”Axolemma”
•Neurotransmitters releasedNeurotransmitters released
•Neurotransmitters interact Neurotransmitters interact
with either other neurons with either other neurons
or effector cellsor effector cells
Excite or inhibitExcite or inhibit
PROCESSESPROCESSES
Typical AxonTypical Axon
Axonal terminals are Axonal terminals are secretory componentsecretory component of neuron of neuron
Sequence of eventsSequence of events
•Signal reaches terminalsSignal reaches terminals
•Membranes of vesicles fuse with Membranes of vesicles fuse with
plasma membraneplasma membrane
““Axolemma”Axolemma”
•Neurotransmitters releasedNeurotransmitters released
•Neurotransmitters interact Neurotransmitters interact
with either other neurons with either other neurons
or effector cellsor effector cells
Excite or inhibitExcite or inhibit
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Contains most of the same organelles found in dendrites Contains most of the same organelles found in dendrites
and cell bodyand cell body
•Lacks ER and Golgi apparatusLacks ER and Golgi apparatus
PROCESSESPROCESSES
Typical AxonTypical Axon
Contains most of the same organelles found in dendrites Contains most of the same organelles found in dendrites
and cell bodyand cell body
•Lacks ER and Golgi apparatusLacks ER and Golgi apparatus
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Rely on cell body for some moleculesRely on cell body for some molecules
Rely on efficient transport mechanisms for deliveryRely on efficient transport mechanisms for delivery
•AnterogradeAnterograde movement toward axonal terminals movement toward axonal terminals
e.g., Mitochondria, e.g., Mitochondria,
membrane components, membrane components,
neurotransmitters or neurotransmitters or
enzymes required for enzymes required for
neurotransmitter neurotransmitter
synthesis, etc.synthesis, etc.
•RetrogradeRetrograde movement movement
toward cell bodytoward cell body
e.g., Organelles being e.g., Organelles being
returned for recyclingreturned for recycling
PROCESSESPROCESSES
Typical AxonTypical Axon
Rely on cell body for some moleculesRely on cell body for some molecules
Rely on efficient transport mechanisms for deliveryRely on efficient transport mechanisms for delivery
•AnterogradeAnterograde movement toward axonal terminals movement toward axonal terminals
e.g., Mitochondria, e.g., Mitochondria,
membrane components, membrane components,
neurotransmitters or neurotransmitters or
enzymes required for enzymes required for
neurotransmitter neurotransmitter
synthesis, etc.synthesis, etc.
•RetrogradeRetrograde movement movement
toward cell bodytoward cell body
e.g., Organelles being e.g., Organelles being
returned for recyclingreturned for recycling
NEURON NEURON
PROCESSESPROCESSES
Typical AxonTypical Axon
Some viruses and bacterial toxins use retrograde Some viruses and bacterial toxins use retrograde
transport to reach the cell bodytransport to reach the cell body
•e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus
toxin, etc.toxin, etc.
Such viruses can be Such viruses can be
used as vehicles for the used as vehicles for the
therapeutic delivery of therapeutic delivery of
engineered DNAengineered DNA
•““Gene therapy”Gene therapy”
PROCESSESPROCESSES
Typical AxonTypical Axon
Some viruses and bacterial toxins use retrograde Some viruses and bacterial toxins use retrograde
transport to reach the cell bodytransport to reach the cell body
•e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus e.g., poliovirus, rabies virus, herpes simplex viruses, tetanus
toxin, etc.toxin, etc.
Such viruses can be Such viruses can be
used as vehicles for the used as vehicles for the
therapeutic delivery of therapeutic delivery of
engineered DNAengineered DNA
•““Gene therapy”Gene therapy”
MYELIN SHEATHMYELIN SHEATH
Whitish, fatty covering of many nerve fibers Whitish, fatty covering of many nerve fibers
•Particularly those long are large in diameterParticularly those long are large in diameter
Protects and electrically insulates fibersProtects and electrically insulates fibers
Increases speed of nerve impulse transmissionIncreases speed of nerve impulse transmission
•Some axons and all dendrites are unmyelinatedSome axons and all dendrites are unmyelinated
Whitish, fatty covering of many nerve fibers Whitish, fatty covering of many nerve fibers
•Particularly those long are large in diameterParticularly those long are large in diameter
Protects and electrically insulates fibersProtects and electrically insulates fibers
Increases speed of nerve impulse transmissionIncreases speed of nerve impulse transmission
•Some axons and all dendrites are unmyelinatedSome axons and all dendrites are unmyelinated
MYELIN SHEATHMYELIN SHEATH
In PNS, myelin sheaths formed by Schwann cellsIn PNS, myelin sheaths formed by Schwann cells
•Continually wrap around nerveContinually wrap around nerve
•Cytoplasm gradually squeezed from intracellular spaceCytoplasm gradually squeezed from intracellular space
•Result is many concentric layers of plasma membrane Result is many concentric layers of plasma membrane
surrounding the axonsurrounding the axon
These plasma membranes contain little proteinThese plasma membranes contain little protein
•Some proteins present interlock adjacent membranesSome proteins present interlock adjacent membranes
•Thickness depends on number of wrappingsThickness depends on number of wrappings
Nucleus and most of Nucleus and most of
cytoplasm exist as a bulge cytoplasm exist as a bulge
external to the myelin sheathexternal to the myelin sheath
•““Neurilemma”Neurilemma”
In PNS, myelin sheaths formed by Schwann cellsIn PNS, myelin sheaths formed by Schwann cells
•Continually wrap around nerveContinually wrap around nerve
•Cytoplasm gradually squeezed from intracellular spaceCytoplasm gradually squeezed from intracellular space
•Result is many concentric layers of plasma membrane Result is many concentric layers of plasma membrane
surrounding the axonsurrounding the axon
These plasma membranes contain little proteinThese plasma membranes contain little protein
•Some proteins present interlock adjacent membranesSome proteins present interlock adjacent membranes
•Thickness depends on number of wrappingsThickness depends on number of wrappings
Nucleus and most of Nucleus and most of
cytoplasm exist as a bulge cytoplasm exist as a bulge
external to the myelin sheathexternal to the myelin sheath
•““Neurilemma”Neurilemma”
MYELIN SHEATHMYELIN SHEATH
Adjacent Schwann cells on axon do not touch Adjacent Schwann cells on axon do not touch
each othereach other
•Gaps in sheath occur at regular intervals Gaps in sheath occur at regular intervals
““Nodes of Ranvier”Nodes of Ranvier”
a.k.a., “Neurofibril a.k.a., “Neurofibril
nodes”nodes”
•Axon collaterals Axon collaterals
can emerge at can emerge at
these nodesthese nodes
Adjacent Schwann cells on axon do not touch Adjacent Schwann cells on axon do not touch
each othereach other
•Gaps in sheath occur at regular intervals Gaps in sheath occur at regular intervals
““Nodes of Ranvier”Nodes of Ranvier”
a.k.a., “Neurofibril a.k.a., “Neurofibril
nodes”nodes”
•Axon collaterals Axon collaterals
can emerge at can emerge at
these nodesthese nodes
MYELIN SHEATHMYELIN SHEATH
CNS contains both myelinated and CNS contains both myelinated and
unmyelinated axonsunmyelinated axons
•Those long are large in diameter are typically Those long are large in diameter are typically
myelinatedmyelinated
Oligodendrocytes, not Schwann cells, form CNS Oligodendrocytes, not Schwann cells, form CNS
myelin sheathsmyelin sheaths
•Oligodendrocytes possess numerous processes that Oligodendrocytes possess numerous processes that
can coil around numerous (up to 60) axons at once can coil around numerous (up to 60) axons at once
CNS myelin sheaths lack a neurilemmaCNS myelin sheaths lack a neurilemma
CNS contains both myelinated and CNS contains both myelinated and
unmyelinated axonsunmyelinated axons
•Those long are large in diameter are typically Those long are large in diameter are typically
myelinatedmyelinated
Oligodendrocytes, not Schwann cells, form CNS Oligodendrocytes, not Schwann cells, form CNS
myelin sheathsmyelin sheaths
•Oligodendrocytes possess numerous processes that Oligodendrocytes possess numerous processes that
can coil around numerous (up to 60) axons at once can coil around numerous (up to 60) axons at once
CNS myelin sheaths lack a neurilemmaCNS myelin sheaths lack a neurilemma
MYELIN SHEATHMYELIN SHEATH
White matterWhite matter
•Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing
dense collections of myelinated fibersdense collections of myelinated fibers
Gray matterGray matter
•Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing
mostly nerve cell bodies and unmyelinated fibersmostly nerve cell bodies and unmyelinated fibers
White matterWhite matter
•Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing
dense collections of myelinated fibersdense collections of myelinated fibers
Gray matterGray matter
•Regions of the brain and spinal cord containing Regions of the brain and spinal cord containing
mostly nerve cell bodies and unmyelinated fibersmostly nerve cell bodies and unmyelinated fibers
NEURON NEURON
CLASSIFICATIONCLASSIFICATION
Structural classification based upon number of Structural classification based upon number of
processesprocesses
•Multipolar neuronsMultipolar neurons
•Bipolar neuronsBipolar neurons
•Unipolar neuronsUnipolar neurons
Functional classification based upon direction Functional classification based upon direction
nerve impulse travelsnerve impulse travels
•Sensory (afferent) neuronsSensory (afferent) neurons
•Motor (efferent) neuronsMotor (efferent) neurons
•Interneurons (association neurons)Interneurons (association neurons)
CLASSIFICATIONCLASSIFICATION
Structural classification based upon number of Structural classification based upon number of
processesprocesses
•Multipolar neuronsMultipolar neurons
•Bipolar neuronsBipolar neurons
•Unipolar neuronsUnipolar neurons
Functional classification based upon direction Functional classification based upon direction
nerve impulse travelsnerve impulse travels
•Sensory (afferent) neuronsSensory (afferent) neurons
•Motor (efferent) neuronsMotor (efferent) neurons
•Interneurons (association neurons)Interneurons (association neurons)
NEURON NEURON
CLASSIFICATIONCLASSIFICATION
Structural ClassificationStructural Classification
Multipolar neuronsMultipolar neurons
•Three or more processesThree or more processes
•Most common neuron Most common neuron
type in humanstype in humans
(> 99% of neurons)(> 99% of neurons)
Bipolar neuronsBipolar neurons
•Two processes – axon and Two processes – axon and
dendritedendrite
•Found only in some Found only in some
special sense organs special sense organs
e.g., retina of eyee.g., retina of eye
•Act as receptor cellsAct as receptor cells
Unipolar neuronsUnipolar neurons
•Single short processSingle short process
•““Pseudounipolar neurons”Pseudounipolar neurons”
Originate as bipolar neuronsOriginate as bipolar neurons
Two processes converge and Two processes converge and
fuse fuse
•Process divides into proximal Process divides into proximal
and distal branchesand distal branches
Distal process often associated Distal process often associated
with a sensory receptorwith a sensory receptor
•““Peripheral process”Peripheral process”
Central process enters CNSCentral process enters CNS
•Most are sensory neurons in Most are sensory neurons in
PNSPNS
CLASSIFICATIONCLASSIFICATION
Structural ClassificationStructural Classification
Multipolar neuronsMultipolar neurons
•Three or more processesThree or more processes
•Most common neuron Most common neuron
type in humanstype in humans
(> 99% of neurons)(> 99% of neurons)
Bipolar neuronsBipolar neurons
•Two processes – axon and Two processes – axon and
dendritedendrite
•Found only in some Found only in some
special sense organs special sense organs
e.g., retina of eyee.g., retina of eye
•Act as receptor cellsAct as receptor cells
Unipolar neuronsUnipolar neurons
•Single short processSingle short process
•““Pseudounipolar neurons”Pseudounipolar neurons”
Originate as bipolar neuronsOriginate as bipolar neurons
Two processes converge and Two processes converge and
fuse fuse
•Process divides into proximal Process divides into proximal
and distal branchesand distal branches
Distal process often associated Distal process often associated
with a sensory receptorwith a sensory receptor
•““Peripheral process”Peripheral process”
Central process enters CNSCentral process enters CNS
•Most are sensory neurons in Most are sensory neurons in
PNSPNS
Classification of Classification of
neurons by shapeneurons by shape
neurons by shapeneurons by shape
NEURON NEURON
CLASSIFICATIONCLASSIFICATION
Functional ClassificationFunctional Classification
Sensory (afferent) neuronsSensory (afferent) neurons
•Transmit impulses Transmit impulses towardtoward CNS CNS
From sensory receptors or internal From sensory receptors or internal
organsorgans
•Most are unipolarMost are unipolar
•Cell bodies are located outside Cell bodies are located outside
CNSCNS
Motor (efferent) neuronsMotor (efferent) neurons
•Carry impulses Carry impulses away fromaway from CNS CNS
Toward effector organsToward effector organs
•MultipolarMultipolar
•Cell bodies generally located in Cell bodies generally located in
the CNSthe CNS
Interneurons Interneurons
•a.k.a., association a.k.a., association
neuronsneurons
•Lie between motor and Lie between motor and
sensory neurons in sensory neurons in
neural pathwaysneural pathways
•Shuttle signals through Shuttle signals through
CNS pathways where CNS pathways where
integration occursintegration occurs
•> 99% of neurons in > 99% of neurons in
body body
•Most are multipolarMost are multipolar
•Most are confined Most are confined
within the CNSwithin the CNS
CLASSIFICATIONCLASSIFICATION
Functional ClassificationFunctional Classification
Sensory (afferent) neuronsSensory (afferent) neurons
•Transmit impulses Transmit impulses towardtoward CNS CNS
From sensory receptors or internal From sensory receptors or internal
organsorgans
•Most are unipolarMost are unipolar
•Cell bodies are located outside Cell bodies are located outside
CNSCNS
Motor (efferent) neuronsMotor (efferent) neurons
•Carry impulses Carry impulses away fromaway from CNS CNS
Toward effector organsToward effector organs
•MultipolarMultipolar
•Cell bodies generally located in Cell bodies generally located in
the CNSthe CNS
Interneurons Interneurons
•a.k.a., association a.k.a., association
neuronsneurons
•Lie between motor and Lie between motor and
sensory neurons in sensory neurons in
neural pathwaysneural pathways
•Shuttle signals through Shuttle signals through
CNS pathways where CNS pathways where
integration occursintegration occurs
•> 99% of neurons in > 99% of neurons in
body body
•Most are multipolarMost are multipolar
•Most are confined Most are confined
within the CNSwithin the CNS
NEUROPHYSIOLOGNEUROPHYSIOLOG
YY
Neurons are highly irritableNeurons are highly irritable
•Responsive to stimuliResponsive to stimuli
Response to stimulus is action potentialResponse to stimulus is action potential
•Electrical impulse carried along length of axonElectrical impulse carried along length of axon
•Always the same regardless of stimulusAlways the same regardless of stimulus
•The underlying functional feature of the nervous The underlying functional feature of the nervous
systemsystem
YY
Neurons are highly irritableNeurons are highly irritable
•Responsive to stimuliResponsive to stimuli
Response to stimulus is action potentialResponse to stimulus is action potential
•Electrical impulse carried along length of axonElectrical impulse carried along length of axon
•Always the same regardless of stimulusAlways the same regardless of stimulus
•The underlying functional feature of the nervous The underlying functional feature of the nervous
systemsystem
ION CHANNELSION CHANNELS
Plasma membranes contain various ion channelsPlasma membranes contain various ion channels
Passive channels (leakage channels)Passive channels (leakage channels)
•Always openAlways open
Active channels (gated channels)Active channels (gated channels)
•Ligand-gated channelsLigand-gated channels
Open when specific chemical bindsOpen when specific chemical binds
•Voltage-gated channelsVoltage-gated channels
Open and close in response to membrane potentialOpen and close in response to membrane potential
•Mechanically-gated channelsMechanically-gated channels
Open in response to physical deformation of receptorOpen in response to physical deformation of receptor
•e.g., touch and pressure receptorse.g., touch and pressure receptors
Plasma membranes contain various ion channelsPlasma membranes contain various ion channels
Passive channels (leakage channels)Passive channels (leakage channels)
•Always openAlways open
Active channels (gated channels)Active channels (gated channels)
•Ligand-gated channelsLigand-gated channels
Open when specific chemical bindsOpen when specific chemical binds
•Voltage-gated channelsVoltage-gated channels
Open and close in response to membrane potentialOpen and close in response to membrane potential
•Mechanically-gated channelsMechanically-gated channels
Open in response to physical deformation of receptorOpen in response to physical deformation of receptor
•e.g., touch and pressure receptorse.g., touch and pressure receptors
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
A voltage exists across the plasma membraneA voltage exists across the plasma membrane
•Due to separation of oppositely charged ionsDue to separation of oppositely charged ions
Potential difference in a resting membrane is Potential difference in a resting membrane is
termed its termed its “resting membrane potential”“resting membrane potential”
•~ -70 mV in a resting ~ -70 mV in a resting
neuronneuron
•Membrane is Membrane is
“polarized”“polarized”
POTENTIALSPOTENTIALS
A voltage exists across the plasma membraneA voltage exists across the plasma membrane
•Due to separation of oppositely charged ionsDue to separation of oppositely charged ions
Potential difference in a resting membrane is Potential difference in a resting membrane is
termed its termed its “resting membrane potential”“resting membrane potential”
•~ -70 mV in a resting ~ -70 mV in a resting
neuronneuron
•Membrane is Membrane is
“polarized”“polarized”
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Neurons use changes in membrane potentials as Neurons use changes in membrane potentials as
signalssignals
•Used to receive, integrate, and send signalsUsed to receive, integrate, and send signals
Changes in membrane potentials produced byChanges in membrane potentials produced by
•Anything changing membrane permeability to ionsAnything changing membrane permeability to ions
•Anything altering ion concentrationsAnything altering ion concentrations
Two types of signalsTwo types of signals
•Graded potentialsGraded potentials
Short-distance signalsShort-distance signals
•Action potentialsAction potentials
Long-distance signalsLong-distance signals
POTENTIALSPOTENTIALS
Neurons use changes in membrane potentials as Neurons use changes in membrane potentials as
signalssignals
•Used to receive, integrate, and send signalsUsed to receive, integrate, and send signals
Changes in membrane potentials produced byChanges in membrane potentials produced by
•Anything changing membrane permeability to ionsAnything changing membrane permeability to ions
•Anything altering ion concentrationsAnything altering ion concentrations
Two types of signalsTwo types of signals
•Graded potentialsGraded potentials
Short-distance signalsShort-distance signals
•Action potentialsAction potentials
Long-distance signalsLong-distance signals
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Short-lived local changes in membrane potentialShort-lived local changes in membrane potential
•Either depolarizations or hyperpolarizationsEither depolarizations or hyperpolarizations
Cause current flows that decrease in magnitude Cause current flows that decrease in magnitude
with distancewith distance
Magnitude of potential dependent upon stimulus Magnitude of potential dependent upon stimulus
strengthstrength
•Stronger stimulus Stronger stimulus larger voltage change larger voltage change
•Larger voltage change Larger voltage change farther current flows farther current flows
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Short-lived local changes in membrane potentialShort-lived local changes in membrane potential
•Either depolarizations or hyperpolarizationsEither depolarizations or hyperpolarizations
Cause current flows that decrease in magnitude Cause current flows that decrease in magnitude
with distancewith distance
Magnitude of potential dependent upon stimulus Magnitude of potential dependent upon stimulus
strengthstrength
•Stronger stimulus Stronger stimulus larger voltage change larger voltage change
•Larger voltage change Larger voltage change farther current flows farther current flows
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Triggered by change in neuron’s environmentTriggered by change in neuron’s environment
•Change causes gated ion channels to openChange causes gated ion channels to open
Small area of neuron’s plasma membrane becomes Small area of neuron’s plasma membrane becomes
depolarized (by this stimulus)depolarized (by this stimulus)
Current flows on both sides of the membraneCurrent flows on both sides of the membrane
•+ moves toward – and + moves toward – and vise versavise versa
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Triggered by change in neuron’s environmentTriggered by change in neuron’s environment
•Change causes gated ion channels to openChange causes gated ion channels to open
Small area of neuron’s plasma membrane becomes Small area of neuron’s plasma membrane becomes
depolarized (by this stimulus)depolarized (by this stimulus)
Current flows on both sides of the membraneCurrent flows on both sides of the membrane
•+ moves toward – and + moves toward – and vise versavise versa
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Inside cell: + ions move away from depolarized areaInside cell: + ions move away from depolarized area
Outside cell: + ions move toward depolarized areaOutside cell: + ions move toward depolarized area
•(+ and – ions switch places)(+ and – ions switch places)
Membrane is leakyMembrane is leaky
•Most of the charge is quickly lost through membraneMost of the charge is quickly lost through membrane
•Current dies out after traveling a short distanceCurrent dies out after traveling a short distance
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Inside cell: + ions move away from depolarized areaInside cell: + ions move away from depolarized area
Outside cell: + ions move toward depolarized areaOutside cell: + ions move toward depolarized area
•(+ and – ions switch places)(+ and – ions switch places)
Membrane is leakyMembrane is leaky
•Most of the charge is quickly lost through membraneMost of the charge is quickly lost through membrane
•Current dies out after traveling a short distanceCurrent dies out after traveling a short distance
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Act as signals over very short distancesAct as signals over very short distances
Important in initiating action potentialsImportant in initiating action potentials
POTENTIALSPOTENTIALS
Graded PotentialsGraded Potentials
Act as signals over very short distancesAct as signals over very short distances
Important in initiating action potentialsImportant in initiating action potentials
MEMBRANE MEMBRANE
POTENTIALSPOTENTIALS
Action PotentialsAction Potentials
Principal means by which neurons communicatePrincipal means by which neurons communicate
•Brief reversal of membrane potentialBrief reversal of membrane potential
Total amplitude of ~ 100 mV (-70 Total amplitude of ~ 100 mV (-70 +30) +30)
•Depolarization followed by repolarization, then brief Depolarization followed by repolarization, then brief
period of hyperpolarizationperiod of hyperpolarization
•Time for entire event is only a few millisecondsTime for entire event is only a few milliseconds
Events in generation and transmission of an Events in generation and transmission of an
action potential identical between neurons and action potential identical between neurons and
skeletal muscle cellsskeletal muscle cells
POTENTIALSPOTENTIALS
Action PotentialsAction Potentials
Principal means by which neurons communicatePrincipal means by which neurons communicate
•Brief reversal of membrane potentialBrief reversal of membrane potential
Total amplitude of ~ 100 mV (-70 Total amplitude of ~ 100 mV (-70 +30) +30)
•Depolarization followed by repolarization, then brief Depolarization followed by repolarization, then brief
period of hyperpolarizationperiod of hyperpolarization
•Time for entire event is only a few millisecondsTime for entire event is only a few milliseconds
Events in generation and transmission of an Events in generation and transmission of an
action potential identical between neurons and action potential identical between neurons and
skeletal muscle cellsskeletal muscle cells
ACTION ACTION
POTENTIALSPOTENTIALS
POTENTIALSPOTENTIALS
ACTION ACTION
POTENTIALSPOTENTIALS
Not all local depolarizations produce action Not all local depolarizations produce action
potentialspotentials
Depolarization must reach threshold valuesDepolarization must reach threshold values
•Brief, weak stimuli produce subthreshold Brief, weak stimuli produce subthreshold
depolarizations that are not translated into nerve depolarizations that are not translated into nerve
impulsesimpulses
•Stronger threshold stimuli produce depolarizing Stronger threshold stimuli produce depolarizing
eventsevents
POTENTIALSPOTENTIALS
Not all local depolarizations produce action Not all local depolarizations produce action
potentialspotentials
Depolarization must reach threshold valuesDepolarization must reach threshold values
•Brief, weak stimuli produce subthreshold Brief, weak stimuli produce subthreshold
depolarizations that are not translated into nerve depolarizations that are not translated into nerve
impulsesimpulses
•Stronger threshold stimuli produce depolarizing Stronger threshold stimuli produce depolarizing
eventsevents
ACTION ACTION
POTENTIALSPOTENTIALS
Action potential is all-or-nothing phenomenonAction potential is all-or-nothing phenomenon
•Happens completely or doesn’t happenHappens completely or doesn’t happen
Independent of stimulus strength once generatedIndependent of stimulus strength once generated
•Strong stimuli generate Strong stimuli generate moremore impulses of the impulses of the samesame
strength per unit timestrength per unit time
•Intensity is determined by number of impulses per Intensity is determined by number of impulses per
unit timeunit time
POTENTIALSPOTENTIALS
Action potential is all-or-nothing phenomenonAction potential is all-or-nothing phenomenon
•Happens completely or doesn’t happenHappens completely or doesn’t happen
Independent of stimulus strength once generatedIndependent of stimulus strength once generated
•Strong stimuli generate Strong stimuli generate moremore impulses of the impulses of the samesame
strength per unit timestrength per unit time
•Intensity is determined by number of impulses per Intensity is determined by number of impulses per
unit timeunit time
ACTION ACTION
POTENTIALSPOTENTIALS
Refractory PeriodsRefractory Periods
Neuron cannot respond to a second stimulus Neuron cannot respond to a second stimulus
while the Nawhile the Na
++
channels are still open from channels are still open from
previous stimulusprevious stimulus
•This period of time is termed the This period of time is termed the “absolute “absolute
refractory period”refractory period”
““Relative refractory period”Relative refractory period” follows the follows the
absolute refractory periodabsolute refractory period
•Repolarization is occurringRepolarization is occurring
•Threshold for impulse generation is elevatedThreshold for impulse generation is elevated
Only strong stimuli can generate impulsesOnly strong stimuli can generate impulses
POTENTIALSPOTENTIALS
Refractory PeriodsRefractory Periods
Neuron cannot respond to a second stimulus Neuron cannot respond to a second stimulus
while the Nawhile the Na
++
channels are still open from channels are still open from
previous stimulusprevious stimulus
•This period of time is termed the This period of time is termed the “absolute “absolute
refractory period”refractory period”
““Relative refractory period”Relative refractory period” follows the follows the
absolute refractory periodabsolute refractory period
•Repolarization is occurringRepolarization is occurring
•Threshold for impulse generation is elevatedThreshold for impulse generation is elevated
Only strong stimuli can generate impulsesOnly strong stimuli can generate impulses
ACTION ACTION
POTENTIALSPOTENTIALS
Conduction VelocitiesConduction Velocities
Conduction velocities of neurons vary widelyConduction velocities of neurons vary widely
Rate of impulse propagation dependent uponRate of impulse propagation dependent upon
•Axon diameterAxon diameter
Larger axons conduct impulses fasterLarger axons conduct impulses faster
•Degree of myelinationDegree of myelination
Myelin sheath dramatically increases rate of propagationMyelin sheath dramatically increases rate of propagation
•Myelin acts as an insulator to prevent almost all leakage from Myelin acts as an insulator to prevent almost all leakage from
axonaxon
POTENTIALSPOTENTIALS
Conduction VelocitiesConduction Velocities
Conduction velocities of neurons vary widelyConduction velocities of neurons vary widely
Rate of impulse propagation dependent uponRate of impulse propagation dependent upon
•Axon diameterAxon diameter
Larger axons conduct impulses fasterLarger axons conduct impulses faster
•Degree of myelinationDegree of myelination
Myelin sheath dramatically increases rate of propagationMyelin sheath dramatically increases rate of propagation
•Myelin acts as an insulator to prevent almost all leakage from Myelin acts as an insulator to prevent almost all leakage from
axonaxon
ACTION ACTION
POTENTIALSPOTENTIALS
Multiple Sclerosis (MS)Multiple Sclerosis (MS)
Autoimmune disease mainly affecting young Autoimmune disease mainly affecting young
adultsadults
Myelin sheaths in CNS are gradually destroyedMyelin sheaths in CNS are gradually destroyed
Interferes with impulse conductionInterferes with impulse conduction
•Visual disturbances, muscle control problems, Visual disturbances, muscle control problems,
speech disturbances, etc.speech disturbances, etc.
Some modern treatments showing some promise Some modern treatments showing some promise
in delaying problemsin delaying problems
POTENTIALSPOTENTIALS
Multiple Sclerosis (MS)Multiple Sclerosis (MS)
Autoimmune disease mainly affecting young Autoimmune disease mainly affecting young
adultsadults
Myelin sheaths in CNS are gradually destroyedMyelin sheaths in CNS are gradually destroyed
Interferes with impulse conductionInterferes with impulse conduction
•Visual disturbances, muscle control problems, Visual disturbances, muscle control problems,
speech disturbances, etc.speech disturbances, etc.
Some modern treatments showing some promise Some modern treatments showing some promise
in delaying problemsin delaying problems
NERVE FIBERSNERVE FIBERS
Classified based onClassified based on
•DiameterDiameter
•Degree of myelinationDegree of myelination
•Conduction speedConduction speed
Classified based onClassified based on
•DiameterDiameter
•Degree of myelinationDegree of myelination
•Conduction speedConduction speed
NERVE FIBER NERVE FIBER
CLASSIFICATIONCLASSIFICATION
Group A fibersGroup A fibers
•Largest diameterLargest diameter
•Thick myelin sheathsThick myelin sheaths
•Conduct impulses at high speeds (> 300 mph)Conduct impulses at high speeds (> 300 mph)
•Mostly somatic sensory ad motor fibers serving skin, skeletal Mostly somatic sensory ad motor fibers serving skin, skeletal
muscles, and jointsmuscles, and joints
Group B fibersGroup B fibers
•Intermediate diameterIntermediate diameter
•Lightly myelinatedLightly myelinated
•Transmit impulses at moderate speeds (40 mph)Transmit impulses at moderate speeds (40 mph)
Group C fibersGroup C fibers
•Smallest diameterSmallest diameter
•UnmyelinatedUnmyelinated
•Transmit impulses comparatively slowly (2 mph or less)Transmit impulses comparatively slowly (2 mph or less)
CLASSIFICATIONCLASSIFICATION
Group A fibersGroup A fibers
•Largest diameterLargest diameter
•Thick myelin sheathsThick myelin sheaths
•Conduct impulses at high speeds (> 300 mph)Conduct impulses at high speeds (> 300 mph)
•Mostly somatic sensory ad motor fibers serving skin, skeletal Mostly somatic sensory ad motor fibers serving skin, skeletal
muscles, and jointsmuscles, and joints
Group B fibersGroup B fibers
•Intermediate diameterIntermediate diameter
•Lightly myelinatedLightly myelinated
•Transmit impulses at moderate speeds (40 mph)Transmit impulses at moderate speeds (40 mph)
Group C fibersGroup C fibers
•Smallest diameterSmallest diameter
•UnmyelinatedUnmyelinated
•Transmit impulses comparatively slowly (2 mph or less)Transmit impulses comparatively slowly (2 mph or less)
Nerve Fiber Nerve Fiber
ClassificationClassification
General classification scheme (Erlanger-Gasser):General classification scheme (Erlanger-Gasser):
•A fibers: MyelinatedA fibers: Myelinated
Subtypes: Subtypes: a, b, g, d, a, b, g, d, some overlap in rangessome overlap in ranges
Fastest conducting and largest diameter – Fastest conducting and largest diameter – a (120 a (120 m/sec, m/sec, 20 m)20 m)
““A” often dropped: alpha motor neuronA” often dropped: alpha motor neuron
•B fibers: Slower myelinated (seldom used)B fibers: Slower myelinated (seldom used)
•C fibers: UnmyelinatedC fibers: Unmyelinated
Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 mm))
ClassificationClassification
General classification scheme (Erlanger-Gasser):General classification scheme (Erlanger-Gasser):
•A fibers: MyelinatedA fibers: Myelinated
Subtypes: Subtypes: a, b, g, d, a, b, g, d, some overlap in rangessome overlap in ranges
Fastest conducting and largest diameter – Fastest conducting and largest diameter – a (120 a (120 m/sec, m/sec, 20 m)20 m)
““A” often dropped: alpha motor neuronA” often dropped: alpha motor neuron
•B fibers: Slower myelinated (seldom used)B fibers: Slower myelinated (seldom used)
•C fibers: UnmyelinatedC fibers: Unmyelinated
Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 Slower conducting than As and smallest diameter (0.5 m/sec, 0.5 mm))
Nerve Fiber Nerve Fiber
ClassificationClassification
Sensory nerve classification (Lloyd-Hunt):Sensory nerve classification (Lloyd-Hunt):
•I, II, III fibers: MyelinatedI, II, III fibers: Myelinated
Subtypes: Ia, IbSubtypes: Ia, Ib
Fastest conducting and largest diameter – IaFastest conducting and largest diameter – Ia
•IV fibers: UnmyelinatedIV fibers: Unmyelinated
Slower conducting than IIIs and smallest diameterSlower conducting than IIIs and smallest diameter
ClassificationClassification
Sensory nerve classification (Lloyd-Hunt):Sensory nerve classification (Lloyd-Hunt):
•I, II, III fibers: MyelinatedI, II, III fibers: Myelinated
Subtypes: Ia, IbSubtypes: Ia, Ib
Fastest conducting and largest diameter – IaFastest conducting and largest diameter – Ia
•IV fibers: UnmyelinatedIV fibers: Unmyelinated
Slower conducting than IIIs and smallest diameterSlower conducting than IIIs and smallest diameter
SYNAPSESYNAPSE
Junction mediating information transfer from one Junction mediating information transfer from one
neuron to another neuron or an effector cellneuron to another neuron or an effector cell
Axodendritic synapsesAxodendritic synapses
•Axonal endings Axonal endings dendrites of second neuron dendrites of second neuron
Axosomatic synapsesAxosomatic synapses
•Axonal endings Axonal endings cell body of neuron cell body of neuron
Presynaptic neuronPresynaptic neuron
•Conducts impulses toward the synapseConducts impulses toward the synapse
Postsynaptic neuronPostsynaptic neuron
•Transmits impulse away from the synapseTransmits impulse away from the synapse
Junction mediating information transfer from one Junction mediating information transfer from one
neuron to another neuron or an effector cellneuron to another neuron or an effector cell
Axodendritic synapsesAxodendritic synapses
•Axonal endings Axonal endings dendrites of second neuron dendrites of second neuron
Axosomatic synapsesAxosomatic synapses
•Axonal endings Axonal endings cell body of neuron cell body of neuron
Presynaptic neuronPresynaptic neuron
•Conducts impulses toward the synapseConducts impulses toward the synapse
Postsynaptic neuronPostsynaptic neuron
•Transmits impulse away from the synapseTransmits impulse away from the synapse
SYNAPSE TYPESSYNAPSE TYPES
Electrical SynapsesElectrical Synapses
Less common than chemical synapsesLess common than chemical synapses
Correspond to gap junctions found elsewhereCorrespond to gap junctions found elsewhere
•Cytoplasm of adjacent neurons connected through Cytoplasm of adjacent neurons connected through
protein channelsprotein channels
•Ions flow directly between neuronsIons flow directly between neurons
•Neurons are Neurons are “electrically coupled”“electrically coupled”
Transmission across synapse is very rapidTransmission across synapse is very rapid
Electrical SynapsesElectrical Synapses
Less common than chemical synapsesLess common than chemical synapses
Correspond to gap junctions found elsewhereCorrespond to gap junctions found elsewhere
•Cytoplasm of adjacent neurons connected through Cytoplasm of adjacent neurons connected through
protein channelsprotein channels
•Ions flow directly between neuronsIons flow directly between neurons
•Neurons are Neurons are “electrically coupled”“electrically coupled”
Transmission across synapse is very rapidTransmission across synapse is very rapid
SYNAPSE TYPESSYNAPSE TYPES
Chemical SynapsesChemical Synapses
Specialized for release & reception of Specialized for release & reception of
neurotransmittersneurotransmitters
Two partsTwo parts
•Axonal terminal of presynaptic neuronAxonal terminal of presynaptic neuron
Contains numerous synaptic vesicles filled with Contains numerous synaptic vesicles filled with
neurotransmitter moleculesneurotransmitter molecules
•Neurotransmitter receptor regionNeurotransmitter receptor region
Present on dendrite or cell body of postsynaptic neuronPresent on dendrite or cell body of postsynaptic neuron
Separated by synaptic cleftSeparated by synaptic cleft
•Remember this stuff in muscles?Remember this stuff in muscles?
Chemical SynapsesChemical Synapses
Specialized for release & reception of Specialized for release & reception of
neurotransmittersneurotransmitters
Two partsTwo parts
•Axonal terminal of presynaptic neuronAxonal terminal of presynaptic neuron
Contains numerous synaptic vesicles filled with Contains numerous synaptic vesicles filled with
neurotransmitter moleculesneurotransmitter molecules
•Neurotransmitter receptor regionNeurotransmitter receptor region
Present on dendrite or cell body of postsynaptic neuronPresent on dendrite or cell body of postsynaptic neuron
Separated by synaptic cleftSeparated by synaptic cleft
•Remember this stuff in muscles?Remember this stuff in muscles?
SYNAPSESYNAPSE
Nerve impulse reaches axonal terminalNerve impulse reaches axonal terminal
Voltage-gated CaVoltage-gated Ca
2+2+
channels open in axon channels open in axon
•CaCa
2+2+
enters presynaptic neuron enters presynaptic neuron
Neurotransmitter is released via exocytosisNeurotransmitter is released via exocytosis
•Vesicles fuse with axonal membraneVesicles fuse with axonal membrane
Neurotransmitter binds to postsynaptic receptorsNeurotransmitter binds to postsynaptic receptors
Ion channels open in Ion channels open in
postsynaptic membranepostsynaptic membrane
•Result is excitation or Result is excitation or
inhibitioninhibition
Nerve impulse reaches axonal terminalNerve impulse reaches axonal terminal
Voltage-gated CaVoltage-gated Ca
2+2+
channels open in axon channels open in axon
•CaCa
2+2+
enters presynaptic neuron enters presynaptic neuron
Neurotransmitter is released via exocytosisNeurotransmitter is released via exocytosis
•Vesicles fuse with axonal membraneVesicles fuse with axonal membrane
Neurotransmitter binds to postsynaptic receptorsNeurotransmitter binds to postsynaptic receptors
Ion channels open in Ion channels open in
postsynaptic membranepostsynaptic membrane
•Result is excitation or Result is excitation or
inhibitioninhibition
SYNAPSESYNAPSE
Binding of neurotransmitter to its receptor is Binding of neurotransmitter to its receptor is
reversiblereversible
Permeability affected as long as neurotransmitter Permeability affected as long as neurotransmitter
is bound to its receptoris bound to its receptor
Neurotransmitters do not persist in the synaptic Neurotransmitters do not persist in the synaptic
cleftcleft
•Degraded by enzymes associated with postsynaptic Degraded by enzymes associated with postsynaptic
membranemembrane
•Reuptake by astrocytes or presynaptic terminalReuptake by astrocytes or presynaptic terminal
•Diffusion of neurotransmitters away from synapseDiffusion of neurotransmitters away from synapse
Binding of neurotransmitter to its receptor is Binding of neurotransmitter to its receptor is
reversiblereversible
Permeability affected as long as neurotransmitter Permeability affected as long as neurotransmitter
is bound to its receptoris bound to its receptor
Neurotransmitters do not persist in the synaptic Neurotransmitters do not persist in the synaptic
cleftcleft
•Degraded by enzymes associated with postsynaptic Degraded by enzymes associated with postsynaptic
membranemembrane
•Reuptake by astrocytes or presynaptic terminalReuptake by astrocytes or presynaptic terminal
•Diffusion of neurotransmitters away from synapseDiffusion of neurotransmitters away from synapse
SYNAPSESYNAPSE
Transmission of impulses along axon can be Transmission of impulses along axon can be
very fastvery fast
•Up to 300 mph (150 m/s)Up to 300 mph (150 m/s)
Transmission of a signal across a synapse is Transmission of a signal across a synapse is
slow in comparisonslow in comparison
•Leads to Leads to “synaptic delay”“synaptic delay”
•~0.3 0 5.0 milliseconds~0.3 0 5.0 milliseconds
•Rate-limiting step of neural transmissionRate-limiting step of neural transmission
•Transmission along multisynaptic pathways is Transmission along multisynaptic pathways is
slower than along pathways with fewer synapsesslower than along pathways with fewer synapses
Transmission of impulses along axon can be Transmission of impulses along axon can be
very fastvery fast
•Up to 300 mph (150 m/s)Up to 300 mph (150 m/s)
Transmission of a signal across a synapse is Transmission of a signal across a synapse is
slow in comparisonslow in comparison
•Leads to Leads to “synaptic delay”“synaptic delay”
•~0.3 0 5.0 milliseconds~0.3 0 5.0 milliseconds
•Rate-limiting step of neural transmissionRate-limiting step of neural transmission
•Transmission along multisynaptic pathways is Transmission along multisynaptic pathways is
slower than along pathways with fewer synapsesslower than along pathways with fewer synapses
SYNAPSESYNAPSE
Postsynaptic PotentialsPostsynaptic Potentials
Many receptors present on postsynaptic Many receptors present on postsynaptic
membranes open ion channelsmembranes open ion channels
•Ligand-gated channelsLigand-gated channels
•Electrical signal converted to chemical signal Electrical signal converted to chemical signal
converted to electrical signalconverted to electrical signal
•Graded potential is producedGraded potential is produced
Magnitude is dependent upon amount of neurotransmitter Magnitude is dependent upon amount of neurotransmitter
releasedreleased
Action potential may be producedAction potential may be produced
•Either excitatory or inhibitoryEither excitatory or inhibitory
Postsynaptic PotentialsPostsynaptic Potentials
Many receptors present on postsynaptic Many receptors present on postsynaptic
membranes open ion channelsmembranes open ion channels
•Ligand-gated channelsLigand-gated channels
•Electrical signal converted to chemical signal Electrical signal converted to chemical signal
converted to electrical signalconverted to electrical signal
•Graded potential is producedGraded potential is produced
Magnitude is dependent upon amount of neurotransmitter Magnitude is dependent upon amount of neurotransmitter
releasedreleased
Action potential may be producedAction potential may be produced
•Either excitatory or inhibitoryEither excitatory or inhibitory
SYNAPSESYNAPSE
Excitatory SynapsesExcitatory Synapses
Neurotransmitter binding causes depolarizationNeurotransmitter binding causes depolarization
•Single type of channel opens in membraneSingle type of channel opens in membrane
•NaNa
++
and K and K
++
simultaneously diffuse through the simultaneously diffuse through the
membrane in opposite directionsmembrane in opposite directions
•NaNa
++
influx exceeds K influx exceeds K
++
efflux efflux
•Net depolarization occursNet depolarization occurs
•Local graded depolarization events formedLocal graded depolarization events formed
““Excitatory postsynaptic potential (EPSP)”Excitatory postsynaptic potential (EPSP)”
May trigger an action potential at axon hillockMay trigger an action potential at axon hillock
•Voltage-gated channels at hillock open, etc.Voltage-gated channels at hillock open, etc.
Excitatory SynapsesExcitatory Synapses
Neurotransmitter binding causes depolarizationNeurotransmitter binding causes depolarization
•Single type of channel opens in membraneSingle type of channel opens in membrane
•NaNa
++
and K and K
++
simultaneously diffuse through the simultaneously diffuse through the
membrane in opposite directionsmembrane in opposite directions
•NaNa
++
influx exceeds K influx exceeds K
++
efflux efflux
•Net depolarization occursNet depolarization occurs
•Local graded depolarization events formedLocal graded depolarization events formed
““Excitatory postsynaptic potential (EPSP)”Excitatory postsynaptic potential (EPSP)”
May trigger an action potential at axon hillockMay trigger an action potential at axon hillock
•Voltage-gated channels at hillock open, etc.Voltage-gated channels at hillock open, etc.
SYNAPSESYNAPSE
Inhibitory SynapsesInhibitory Synapses
Neurotransmitter binding reduces a postsynaptic Neurotransmitter binding reduces a postsynaptic
neuron’s ability to generate an action potentialneuron’s ability to generate an action potential
•Increased permeability to KIncreased permeability to K
++
and Cl and Cl
--
, not Na, not Na
++
•Postsynaptic neuron becomes less likely to firePostsynaptic neuron becomes less likely to fire
•““Inhibitory postsynaptic potential (IPSP)”Inhibitory postsynaptic potential (IPSP)”
Inhibitory SynapsesInhibitory Synapses
Neurotransmitter binding reduces a postsynaptic Neurotransmitter binding reduces a postsynaptic
neuron’s ability to generate an action potentialneuron’s ability to generate an action potential
•Increased permeability to KIncreased permeability to K
++
and Cl and Cl
--
, not Na, not Na
++
•Postsynaptic neuron becomes less likely to firePostsynaptic neuron becomes less likely to fire
•““Inhibitory postsynaptic potential (IPSP)”Inhibitory postsynaptic potential (IPSP)”
SYNAPSESYNAPSE
SummationSummation
A single ESPS cannot induce an action potentialA single ESPS cannot induce an action potential
•Requires multiple axonal termini firing in concertRequires multiple axonal termini firing in concert
Hundreds or thousands of EPSPs act togetherHundreds or thousands of EPSPs act together
•““Summation”Summation”
Two types of summationTwo types of summation
•Temporal summationTemporal summation
One or more neurons transmit in rapid successionOne or more neurons transmit in rapid succession
•Spatial summationSpatial summation
Simultaneous stimulation by numerous termini from one Simultaneous stimulation by numerous termini from one
or more neuronsor more neurons
•(Both EPSPs and IPSPs summate)(Both EPSPs and IPSPs summate)
SummationSummation
A single ESPS cannot induce an action potentialA single ESPS cannot induce an action potential
•Requires multiple axonal termini firing in concertRequires multiple axonal termini firing in concert
Hundreds or thousands of EPSPs act togetherHundreds or thousands of EPSPs act together
•““Summation”Summation”
Two types of summationTwo types of summation
•Temporal summationTemporal summation
One or more neurons transmit in rapid successionOne or more neurons transmit in rapid succession
•Spatial summationSpatial summation
Simultaneous stimulation by numerous termini from one Simultaneous stimulation by numerous termini from one
or more neuronsor more neurons
•(Both EPSPs and IPSPs summate)(Both EPSPs and IPSPs summate)
SYNAPSESYNAPSE
Synaptic PotentiationSynaptic Potentiation
Repeated or continuous use of a synapse Repeated or continuous use of a synapse
enhances presynaptic neuron’s ability to exciteenhances presynaptic neuron’s ability to excite
•Larger postsynaptic potentials producedLarger postsynaptic potentials produced
•““Synaptic potentiation”Synaptic potentiation”
Greater [Ca++] inside presynaptic terminalsGreater [Ca++] inside presynaptic terminals
More neurotransmitter releasedMore neurotransmitter released
Larger EPSPs producedLarger EPSPs produced
Synaptic PotentiationSynaptic Potentiation
Repeated or continuous use of a synapse Repeated or continuous use of a synapse
enhances presynaptic neuron’s ability to exciteenhances presynaptic neuron’s ability to excite
•Larger postsynaptic potentials producedLarger postsynaptic potentials produced
•““Synaptic potentiation”Synaptic potentiation”
Greater [Ca++] inside presynaptic terminalsGreater [Ca++] inside presynaptic terminals
More neurotransmitter releasedMore neurotransmitter released
Larger EPSPs producedLarger EPSPs produced
SYNAPSESYNAPSE
Presynaptic InhibitionPresynaptic Inhibition
Release of excitatory neurotransmitter can be Release of excitatory neurotransmitter can be
inhibited by activity of another neuroninhibited by activity of another neuron
•Less neurotransmitter released and boundLess neurotransmitter released and bound
Presynaptic InhibitionPresynaptic Inhibition
Release of excitatory neurotransmitter can be Release of excitatory neurotransmitter can be
inhibited by activity of another neuroninhibited by activity of another neuron
•Less neurotransmitter released and boundLess neurotransmitter released and bound
NEUROTRANSMITTERNEUROTRANSMITTER
SS
More than fifty neurotransmitters identifiedMore than fifty neurotransmitters identified
Most neurons make two or moreMost neurons make two or more
•Can be released singly or togetherCan be released singly or together
Classification by StructureClassification by Structure
Acetylcholine (ACh)Acetylcholine (ACh)
Biogenic aminesBiogenic amines
Amino acidsAmino acids
PeptidesPeptides
ATPATP
Dissolved gasesDissolved gases
Classification by FunctionClassification by Function
Excitatory/InhibitoryExcitatory/Inhibitory
Direct/IndirectDirect/Indirect
SS
More than fifty neurotransmitters identifiedMore than fifty neurotransmitters identified
Most neurons make two or moreMost neurons make two or more
•Can be released singly or togetherCan be released singly or together
Classification by StructureClassification by Structure
Acetylcholine (ACh)Acetylcholine (ACh)
Biogenic aminesBiogenic amines
Amino acidsAmino acids
PeptidesPeptides
ATPATP
Dissolved gasesDissolved gases
Classification by FunctionClassification by Function
Excitatory/InhibitoryExcitatory/Inhibitory
Direct/IndirectDirect/Indirect
NEURAL NEURAL
INTEGRATIONINTEGRATION
Neurons function in groups, not singlyNeurons function in groups, not singly
These various components must interactThese various components must interact
Multiple levels of neural integrationMultiple levels of neural integration
INTEGRATIONINTEGRATION
Neurons function in groups, not singlyNeurons function in groups, not singly
These various components must interactThese various components must interact
Multiple levels of neural integrationMultiple levels of neural integration
NEURONAL POOLSNEURONAL POOLS
Neurons in CNS are organized into poolsNeurons in CNS are organized into pools
•Functional groupsFunctional groups
•Integrate incoming informationIntegrate incoming information
•Forward processed informationForward processed information
Neurons in CNS are organized into poolsNeurons in CNS are organized into pools
•Functional groupsFunctional groups
•Integrate incoming informationIntegrate incoming information
•Forward processed informationForward processed information
NEURONAL POOLSNEURONAL POOLS
Simple Neuronal PoolSimple Neuronal Pool
Incoming fiber branches profusely upon entering poolIncoming fiber branches profusely upon entering pool
EPSPs induced in multiple postsynaptic neuronsEPSPs induced in multiple postsynaptic neurons
EPSPs exceed threshold in some neuronsEPSPs exceed threshold in some neurons
•Mainly those with multiple synaptic contactsMainly those with multiple synaptic contacts
EPSPs do not exceed EPSPs do not exceed
threshold in some threshold in some
neuronsneurons
•Mainly those with Mainly those with
fewer synaptic contactsfewer synaptic contacts
•Some close to thresholdSome close to threshold
““Facilitated zone”Facilitated zone”
Simple Neuronal PoolSimple Neuronal Pool
Incoming fiber branches profusely upon entering poolIncoming fiber branches profusely upon entering pool
EPSPs induced in multiple postsynaptic neuronsEPSPs induced in multiple postsynaptic neurons
EPSPs exceed threshold in some neuronsEPSPs exceed threshold in some neurons
•Mainly those with multiple synaptic contactsMainly those with multiple synaptic contacts
EPSPs do not exceed EPSPs do not exceed
threshold in some threshold in some
neuronsneurons
•Mainly those with Mainly those with
fewer synaptic contactsfewer synaptic contacts
•Some close to thresholdSome close to threshold
““Facilitated zone”Facilitated zone”
TYPES OF TYPES OF
CIRCUITSCIRCUITS
Patterns of synaptic connections in neuronal Patterns of synaptic connections in neuronal
pools are called circuitspools are called circuits
•Determine neuronal pool’s functional capabilitiesDetermine neuronal pool’s functional capabilities
Four basic circuit patternsFour basic circuit patterns
•Diverging circuitsDiverging circuits
•Converging circuitsConverging circuits
•Reverberating (oscillating) circuitsReverberating (oscillating) circuits
•Parallel after-discharge circuitsParallel after-discharge circuits
CIRCUITSCIRCUITS
Patterns of synaptic connections in neuronal Patterns of synaptic connections in neuronal
pools are called circuitspools are called circuits
•Determine neuronal pool’s functional capabilitiesDetermine neuronal pool’s functional capabilities
Four basic circuit patternsFour basic circuit patterns
•Diverging circuitsDiverging circuits
•Converging circuitsConverging circuits
•Reverberating (oscillating) circuitsReverberating (oscillating) circuits
•Parallel after-discharge circuitsParallel after-discharge circuits
TYPES OF TYPES OF
CIRCUITSCIRCUITS
Diverging (Amplifying) CircuitDiverging (Amplifying) Circuit
One incoming fiber triggers responses in ever-One incoming fiber triggers responses in ever-
increasing numbers of neuronsincreasing numbers of neurons
Common in both sensory and motor systemsCommon in both sensory and motor systems
CIRCUITSCIRCUITS
Diverging (Amplifying) CircuitDiverging (Amplifying) Circuit
One incoming fiber triggers responses in ever-One incoming fiber triggers responses in ever-
increasing numbers of neuronsincreasing numbers of neurons
Common in both sensory and motor systemsCommon in both sensory and motor systems
TYPES OF TYPES OF
CIRCUITSCIRCUITS
Converging CircuitsConverging Circuits
Pool receives inputs from several neuronsPool receives inputs from several neurons
Circuit has “funneling” effectCircuit has “funneling” effect
Common in sensory Common in sensory
and motor systemsand motor systems
CIRCUITSCIRCUITS
Converging CircuitsConverging Circuits
Pool receives inputs from several neuronsPool receives inputs from several neurons
Circuit has “funneling” effectCircuit has “funneling” effect
Common in sensory Common in sensory
and motor systemsand motor systems
TYPES OF TYPES OF
CIRCUITSCIRCUITS
Reverberating (Oscillating) CircuitsReverberating (Oscillating) Circuits
Incoming signal travels through chain of Incoming signal travels through chain of
neuronsneurons
Each neuron makes synapses with neurons Each neuron makes synapses with neurons
upstream in the pathwayupstream in the pathway
Involved in rhythmic activities (e.g., breathing)Involved in rhythmic activities (e.g., breathing)
CIRCUITSCIRCUITS
Reverberating (Oscillating) CircuitsReverberating (Oscillating) Circuits
Incoming signal travels through chain of Incoming signal travels through chain of
neuronsneurons
Each neuron makes synapses with neurons Each neuron makes synapses with neurons
upstream in the pathwayupstream in the pathway
Involved in rhythmic activities (e.g., breathing)Involved in rhythmic activities (e.g., breathing)
TYPES OF TYPES OF
CIRCUITSCIRCUITS
Parallel After-Discharge CircuitsParallel After-Discharge Circuits
Incoming fiber stimulated parallel neuron arraysIncoming fiber stimulated parallel neuron arrays
Parallel arrays ultimately stimulate a common Parallel arrays ultimately stimulate a common
output celloutput cell
•Create prolonged burst of impulsesCreate prolonged burst of impulses
Involved in complex Involved in complex
mental processingmental processing
CIRCUITSCIRCUITS
Parallel After-Discharge CircuitsParallel After-Discharge Circuits
Incoming fiber stimulated parallel neuron arraysIncoming fiber stimulated parallel neuron arrays
Parallel arrays ultimately stimulate a common Parallel arrays ultimately stimulate a common
output celloutput cell
•Create prolonged burst of impulsesCreate prolonged burst of impulses
Involved in complex Involved in complex
mental processingmental processing
PROCESSING PROCESSING
PATTERNSPATTERNS
Serial input processingSerial input processing
•Input travels along one pathway to a specific Input travels along one pathway to a specific
destinationdestination
•All-or-nothing function of systemAll-or-nothing function of system
e.g., reflexese.g., reflexes
Parallel input processingParallel input processing
•Inputs are segregated into multiple pathwaysInputs are segregated into multiple pathways
Integrated in different CNS regionsIntegrated in different CNS regions
Different circuits do different things with inputDifferent circuits do different things with input
•Not repetitiousNot repetitious
PATTERNSPATTERNS
Serial input processingSerial input processing
•Input travels along one pathway to a specific Input travels along one pathway to a specific
destinationdestination
•All-or-nothing function of systemAll-or-nothing function of system
e.g., reflexese.g., reflexes
Parallel input processingParallel input processing
•Inputs are segregated into multiple pathwaysInputs are segregated into multiple pathways
Integrated in different CNS regionsIntegrated in different CNS regions
Different circuits do different things with inputDifferent circuits do different things with input
•Not repetitiousNot repetitious
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