Nervous system

This will serve some basic information
about nervous system

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Physiology-II
Nervous System

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Nervous System
•Nervous system is the most important
organization which control and integrates
the different body functions and maintains
the consistency of internal environment.
•A network of billions of nerve cells linked
together in a highly organized fashion to
form the rapid control center of the body.

3. Parts of CNS
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4. Organization of Nervous System
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Divisions of nervous system
(1) Central or somatic nervous system
(cerebrospinal or voluntary): responsible
for consciousness and voluntary control.
(2) Autonomic nervous system
(involuntary): generally unconscious and
not under the control of ‘will’. It reflexly
regulates the activities of the viscera.

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Functional division of nervous system
Central or somatic
(cerebrospinal)
Autonomic
Afferent Efferent Associative Efferent
Non-sensory
(unconscious,
producing reflex
action only)
Sensory
(conscious)
Voluntary Reflex
Afferent
(generally
unconscious, may
be conscious
abnormally)
Reflex
action
(only)
Unconditioned reflexConditioned reflexGeneral senses
(1) Touch
(2) Pain
(3) Temperature
Special senses
(1) Vision
(2) Hearing
(3) Taste
(4) Smell
Divisions:
Functions:
Nervous system

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Functions of Nervous System
Depending on the type of nerve impulse and its interpretation,
functions of the nervous system may be of three types:
(1) Sensory functions:
These may be either conscious or unconscious. When conscious
they are called ‘sensations’. In the autonomic nervous system,
they are usually unconscious.
(2) Motor functions:
These may be of two types:
(a) Reflex or involuntary and (b) Voluntary
In the autonomic nervous system, all motor effects are reflex.
In the central nervous system, motor effects are both reflex and
voluntary.
(3) Associated functions:
For instance, idea, memory, intelligence, etc. These are carried out
mainly by the cerebrum.

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Nerve Tissue
Consists of two types of cells:
•1. Neurons/Nerve cells: Functional, Signal
conducting cells
•2. Neuroglia/Glial cells: Supporting cells

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•The functional and structural unit
of the nervous system
•Specialized to conduct information from one part of the
body to another
Neuron

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Neuron
A typical Neuron consists of:
(1) Nerve cell body or soma
(2) Two type of process: axon and dendrites

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Neuron
Cell body/soma:
The following structures are found in the cell
body:
(a) nucleus: large, sperical
(b) neuroplasm: a cytoplasmic matrix
containing neurofibrils (fine filaments) and
nissl bodies (participate in conduction of
nerve impulses)
(c) mitochondria, golgi apparatus, ribosome,
endoplasmic reticulum, centrosome etc.

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Neuron
Axon:
•It is a process arises from cell body that carries
impulse away from it.
•The term of nerve fiber is usually refers to the
axons.
•It arises from axon hillock of the cell body.
•It is generally long with few branches (axon
terminals).
•Axis cylinder contains axoplasm.
•Axon is single but constant. If a neuron has only
one process, it will be the axon.

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Neuron
Dendrites:
•It is the process that carries impulse
towards the cell body
•It collects impulses from other neurons
and carries them towards the cell body.
•It is generally short with many branches.
•Number varies from nil to numerous.
•They are the part of receptor membrane of
the neuron.

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Neuron

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Classification of Neurons
A. Based on the number of processes that
emanate from their cell body, neurons can be
classified as:
1. Apolar (have no process)
2. Unipolar (only axon, i.e., 5
th
cranial nerve)
3. Bipolar (both axon and dendrite, i.e., retina)
4. Pseudounipolar (cell body in one side, T-shaped,
one branch of T being the dendrite and another
branch is axon, found in all spinal ganglia)
5. Multipolar (many axon and dendrites, motor neuron
of spinal cord, pyramidal cell of hippocampus &
purkinje cell of cerebellum)

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Classification of Neurons
B. According to function:
1. Sensory Neuron (Afferent):
convey information from tissues and organs into the central
nervous system.
2. Motor Neuron (Efferent):
transmit signals from the central nervous system to the effector
cells/tissue.
3. Interneurons:
connect neurons within specific regions of the central nervous
system.

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Neuroglia/Glial cells
•These are the non-excitable supporting
connective tissues present in nervous
system, called neuroglia or glial cells.
•There are two major types of glial cells in
the vertebrate nervous system:
-microglia
-macroglia.

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Microglia
•Specialized immune cells
that act as the
macrophages of the CNS
Glial cell: Microglia

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Glial cell: Macroglia
Macroglia:
There are three types of macroglia:
1.Oligodendrocytes
2.Schwann cells
3.Astrocytes

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Neuroglia/Glial cells

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Macroglia: Oligodendrocytes &
Schwan cells
•Oligodendrocytes
and Schwann cells
are involved in
myelin formation
around axons in the
CNS and peripheral
nervous system,
respectively.

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Macroglia: Astrocytes
Astrocytes, which are found throughout the brain, are of
two subtypes.
(1) Fibrous astrocytes, which contain many intermediate
filaments, are found primarily in white matter.
(2) Protoplasmic astrocytes are found in gray matter and
have a granular cytoplasm.
•Both types send processes to blood vessels, where they
induce capillaries to form the tight junctions making up
the blood–brain barrier (BBB).
•They also send processes that envelop synapses and
the surface of nerve cells.

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Myelinogenesis
•So long as the nerve fiber is within the gray matter, it
remains naked.
•As soon as it enters the white matter, it gets the first
covering, the myelin sheath.
•The process of formation of myelin sheath is called
myelinogenesis of neuron.
•Schwann cell forms the protective covering around the
axon and called myelin sheath.
•In the central nervous system, there is no neurolemma
and myelination takes place.

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Myelinogenesis

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Myelination
•Schwann cell starts to develop in embryo and continues to increase
the wrapping around the axon through childhood.
•This development increases the thickness of the wrapping which
peaks in adolescence.
•The schwann cell contains typical organelles and cell membrane
structure.
•The axon is first enveloped completely by Schwann cell membrane.
•The Schwann cell membrane then gives several turns leaving the
axon surrounded by many concentric layers.
•The cytoplasm disappears from the concentric layers leaving only
the cell membrane.
•This outer wrapping of schwann cell is called neurolema.
•The inner lyning is made up of cell membrane of schwann cell and
called the myelin sheath.

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Excitation and Conduction
•Nerve cells have a low threshold for excitation.
•The stimulus may be
- electrical
- chemical, or
- mechanical.
•Two types of physicochemical disturbances are
produced:
(a) local, nonpropagated potentials or electrotonic
potentials; and
(b) propagated potentials, the action potentials (or
nerve impulses).
•These are the main language of the nervous system.

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Excitation and Conduction
•Action potentials are produced due to changes in the
conduction of ions across the cell membrane.
•The electrical events in neurons are rapid, measured in
milliseconds (ms); and the potential changes are
measured in millivolts (mV).
•Conduction is an active, self-propagating process, and
the impulse moves along the nerve at a constant
amplitude and velocity.

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Action Potential
•When two electrodes are connected through a suitable
amplifier and placed on the surface of a single axon, no
potential difference is observed.
•However, if one electrode is inserted into the interior of
the cell, a constant potential difference is observed, with
the inside negative relative to the outside of the cell at
rest.
•A membrane potential results from separation of positive
and negative charges across the cell membrane.
•In neurons, the resting membrane potential is usually
about –70 mV, which is close to the equilibrium potential
for K+.

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Action Potential
•The resting membrane potential represents an
equilibrium situation at which driving force of ions
across the membrane is equal.
•In resting cell the surface is positively charged and
the interior is negatively charged.
•The concentration of K
+
is much higher inside than
outside the cell
•The concentration of Na
+
is much lower inside than
outside the cell.
•When the surface is stimulated and the permeability
is increased, the change rises to threshold level and
impulse is generated.
•The depolarization of the membrane is the first
step of the manifestation of an impulse.

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Action Potential
•After an initial slow rise, depolarisation reaches to
approximately +35mv.
•After that it reverses and begins to fall very rapidly
(repolarisation) towards the resting level (-70 mv).
•In the last part of repolarisation the rate of fall is being
abruptly slowed. This slower fall is known as negative
after-potential.
•After reaching the basal level the wave overshoots
slightly but slowly in the hyperpolarisation direction.
This is known as positive after potential.
•During hyperpolarisation, the nerve will not respond to a
second stimulus for a brief period. This period is known
as absolute refractory period.
•The whole sequence of potential changes in the nerve
following excitation is known as action potential or
membrane potential.

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Action potential wave

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Mechanism of the development of
action potential
•In resting state the nerve fibre remains in polarised
state and the membrane potential lies within -70mV.
•The inside of the nerve is negative and outside of the
nerve is positive.
•Sodium ion (Na
+
) concentration outside the membrane is
higher than that of inside the membrane.
•Potassium ion (K
+
) concentration inside the membrane is
also higher than that of outside the membrane.
•Permeability of Na
+
to membrane is increased only after
excitation and it is the first event of the action
potential.

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Mechanism of the development of
action potential
•During excitation, the permeability of Na+ is increased and
thus Na
+
concentration is increase inside the cell.
•This cause the development of positive charge inside the
membrane and negativity outside. This stage is called
depolarisation. The depolarisation curve moves up to +35 mV.
•With the increase of positivity inside, further entry of Na+ is
prevented.
•But as soon as the action potential attains the voltage approx.
+35mV, K
+
begins to come out from inside the membrane.
•As a result the inside becomes negative and outside
becomes positive again. This stage is called repolarisation
and K
+
conductance is increased to the maximum causing
hyperpolarisation.

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Mechanism of the development of
action potential

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Mechanism of conduction of the nerve impulse
•The nerve impulse is a propagated wave of depolarization.
•At resting, nerve fiber remains in polarized state, with positive
charges lined up along the outside of the membrane and negative
charges along the inside.
•As soon as the fiber is excited at a point, the polarity is
changed and for a brief period, it is actually reversed.
•This reversed polarity is due to increased permeability of Na+ to
the membrane and depolarization wave is developed.
•Positive current flows inward through the depolarized membrane
and outward through the resting membrane.
•This local depolarization current then excites the adjacent
portion of the membrane producing progressively more and more
depolarization.
•The depolarization wave travels in all directions along the entire
length of the nerve fiber.
•This type of conduction is observed in the non-myelinated nerve
fibers.

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Conduction of nerve impulse

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Mechanism of conduction of the nerve impulse
•In myelinated nerve fiber, myelin sheath acts as
insulator.
•Ions cannot pass through myelin sheath and only
nodes of Ranvier permeate ions to pass through it
more easily.
•So, the depolarization in myelinated axon jumps from
one node of Ranvier to the next. This jumping or
leaping of depolarization from node to node is known
as SALTATORY CONDUCTION .

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Conduction of nerve impulse

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Nerve fibers
•A nerve fiber is a threadlike extension of a nerve cell
and consists of an axon (microfilament + microtubule)
and myelin sheath (if present) in the nervous system.

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Properties of nerve fibers
1. Excitability: the nerve can be stimulated by a suitable stimulus,
which may be:
-mechanical
-thermal
-chemical
-electrical
After excitation, nerve impulse is generated through
depolarisation, repolarisation and hyperpolarisation.
Excitability depends upon the following factors:
(a) Strength of stimulus
(b) Duration of stimulus
(c) Direction of the current
(d) Frequency of stimulus
(e) Injury

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Properties of nerve fibers
2. Conductivity: conductivity shows the following characteristics:
(i)Impulse is propagated along a nerve in both directions [but
under normal conditions the nerve impulse travels in one
direction only-in the motor nerve towards the responding organ;
in sensory nerve toward the center]
(ii) The nerve impulse is propagated with a definite speed. The
conduction velocity depends upon the diameter of the nerve
fibers, the thicker fibers showing higher velocity. The velocity also
depends on the myelination and on temperature.

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Properties of nerve fibers
3. All-or-none law:
-If the stimulus be adequate, a single nerve will always give a
maximum response
-If the strength or duration of the stimulus be further increased, no
alteration in the response will take place.
4. Refractory period:
When the nerve fiber is once excited, it will not respond to a second
stimulus for a brief period. This period is called refractory period.
5. Summation:
In a nerve fiber summation of two submaximal stimuli is possible.
6. Adaptation:
-The nerve fiber quickly adapts itself. Due to this adaptation there is
no excitation during the passage of a constant current.
-Only when the strength of the current is suddenly altered or the
current is made or broken excitation takes place.

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Properties of nerve fibers
7. Accommodation:
- If a stimulus even in stronger strength is applied very
slowly to a nerve, then there may have no response only
due to lack of attaining the threshold strength. This
phenomenon is called accommodation.
8. Indefatigability:
-In the nerve muscle preparation, if the nerve is
stimulated repeatedly, then after a certain period the
muscle fails to give any response but nerve is not
fatigued.

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Classification of nerve fibers
(1)Histologically:
(a) Medullated (myelinated)
(b) non-medullated (non myelinated)
(2) Functionally:
(a) motor (efferent)
(b) sensory (afferent)

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Classification of nerve fiber
(3) Chemically:
(a)Adrenergic (producing norepinephrine)
(b)Cholinergic (producing acetylcholine)
(4) According to diameter and conduction velocity:
(Thicker the fiber, higher the impulse velocity)
(a)A fiber: Further divided into α, β & γ fibers.
(b)B fiber
(c)C fiber: Further divided into sympathetic (s.C) groups
and dorsal root (d.r.C) groups.
(diameter fiber A>B>C)

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Classification of nerve fiber
(5) Numerical classification of sensory nerve fibers:
Group Ia
Group Ib
Group II
Group III
Group IV
(6) On physio-anatomic basis:
On physio-anatomic basis, the peripheral nerves can
be classified into afferent and efferent and each of
which is again subdivided as somatic and visceral.

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Synapse
•A junction that mediates information transfer from one neuron:
-To another neuron (neuro-synapses or just synapse0
-To an effector cell
•Neuromuscular synapse if muscle involved
•Neuroglandular synapse if gland involve
•Presynaptic neuron – conducts impulses toward the synapse
•Postsynaptic neuron – transmits impulses away from the synapse

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Types of synapse
According to the nature of synapse formation:
(1) Electrical synapses
(2) Chemical synapses
According to the nature of connections:
(1) Axosomatic
(2) Axodendritic
(3) Axo-axonic

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Electrical Synapses
•Pre- and postsynaptic neurons joined by gap junctions
–allow local current to flow between adjacent cells.
- Connexons: protein tubes in cell membrane.
•Rare in CNS or PNS
•Found in cardiac muscle and many types of smooth muscle.

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Chemical Synapse

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Chemical synapse
•The transfer of information across the synapse are
brought about by chemical process.
•Neurotransmitter is released from presynaptic neuron.
•NT travel the synaptic cleft and bind with the receptors
on the postsynaptic neuron which facilitates ion channels
opening and produces action potentials.

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Axosomatic synapsle
•The presynaptic terminal of the axon ends in the cell body (soma) of
the neuron.

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Axodendritic synapse
•The presynaptic fibers of any axon end in the
dendrites of the postsynaptic cell.
Axo-axonic synapse
•The presynaptic axon form synapse with the axon
of post synaptic cells.
•Rare synapse.

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Mechanism of synaptic transmission
1.Arrival of action potential on presynaptic neuron opens volage-gated
Ca
++
channels.
2. Ca
2+
influx into presynaptic terminal.
3. Ca
2+
acts as intracellular messenger stimulating synaptic vesicles to
fuse with membrane and release Neurotransmitter (NT) via exocytosis.
4. NT diffuses across synaptic cleft and binds to receptor on postsynaptic
membrane.
5. Receptor changes shape of ion channel opening it and changing
membrane potential.
6. Opening of all types of ion channels causes a localized depolarization of the
membrane and produces an excitatory postsynaptic potential (EPSP).

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Mechanism of synaptic transmission

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Mechanism of synaptic transmission
7. Selective opening of only the smaller ions like K+ and Chloride ions,
causing hyperpolarization of the membrane and that constitutes the
inhibitory postsynaptic potential (IPSP).
9. If the EPSP exceeds threshold value, it initiates the propagated
action potential in the postsynaptic neuron or muscle action
potential (MAP) in most skeletal and cardiac muscle.
10. During the development of the EPSP, simultaneously IPSP may be
developed at the same site by incoming action potential from other
sources. The propagation of nerve impulse by EPSP is dependent
upon the intensity of the postsynaptic potential.
11. NT is quickly destroyed by enzymes or taken back up by astrocytes
or presynaptic membrane.

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Mechanism of synaptic transmission

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Properties of synapse
•Synaptic response:
☺ At the synaptic junction, impulses are received an discharged.
☺ Some times many impulses are received from different sources but
the neuron discharges its own. It may be said that the synapse not
only acts as a relay station but it may also act as an integrator.
•Law of forward conduction:
☺ An impulse is allowed to pass through a synapse in one direction
only, i.e., from the axon of one neuron to the dendrite of the next.
•Synaptic delay:
☺ The impulse while passing through a synapse takes a certain length
of time. The time between the arrival of the impulse and causing
initial depolarization is called “SYNAPTIC LATENCY”.
☺ Synaptic delay in chemical synapse is less than 0.5 millisecond.
•Seat of fatigue:
☺ The synaptic fatigue is due to exhaustion of transmitter materials
from the synaptic vesicles following REPEATED presynaptic
stimulation at faster rate.

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Properties of synapse
•Inhibition:
(A) Postsynaptic inhibition:
☺The postsynaptic inhibition is due to release of inhibitory
neurotransmitter which is capable of changing the permeability of
the postsynaptic membrane to either potassium or chloride or both
ions.
☺This change of permeability of the membrane will cause
hyperpolarization of the postsynaptic membrane and consequently,
the action potential elicited by the excitatory stimulus will fail to
occur.
(B) Presynaptic inhibition:
☺ Inhibition of a stimulatory neuron before it synapses, by inhibiting
Ca2+ entry and blocking downstream processes, preventing
neurotransmitter release, and therefore preventing the neuron
generating and EPSP post-synaptically.

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Properties of synapse
•Synaptic block:
☺The inhibitory neurotransmitter may be blocked at the synaptic
junction producing convulsion.
☺Strychnine (pesticide) blocks the inhibitory activity and causes
reduction or abolition of the IPSP in most of the synaptic junctions.
☺Tetanus toxin also has got similar effects and produces
convulsion by blocking the inhibitory neurotransmitter.
•Summation:
☺If the stimulus be subminimal, the released neurotransmitter will
produce EPSP but this EPSP will not be sufficient to produce
discharge of impulse in the postsynaptic neuron. But if a number of
subminimal stimuli be applied, their effects will be summated
together and the EPSP will be sufficient to discharge a impulse.

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Neurotransmitter
•Neurotransmitters are endogenous chemicals that
transmit signals from a neuron to a target cell across a
synapse.
•Neurotransmitters are packaged into synaptic vesicles
clustered beneath the membrane on the presynaptic side
of a synapse, and are released into the synaptic cleft,
where they bind to receptors in the membrane on the
postsynaptic side of the synapse.
•Release of neurotransmitters usually follows arrival of an
action potential at the synapse.

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Types of neurotransmitter
(A) According to the nature (chemistry) of
neurotransmitter
•Amino acids: glutamic acid, γ-aminobutyric acid
(GABA), aspartic acid, D-alanine, glycine
•Peptides: Substance P
•Monoamines and other biogenic amines:
dopamine, norepinephrine (noradrenaline; NE,
NA), epinephrine (adrenaline), histamine,
serotonin (SE, 5-HT)
•Fatty acid derivatives: Prostaglandins
•Others: acetylcholine (ACh), adenosine

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Types of neurotransmitter
(B) Based on the activity on postsynaptic neuron
•EXCITATORY (causes depolarization):
Acetylcholine, Aspartate, Dopamine, Histamine,
Norepinephrine, Epinephrine, Glutamate, Serotonin
•INHIBITORY (causes hyperpolarization): GABA,
Glycine

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Nerve endings
•A free nerve ending (FNE) is an unspecialized, afferent
nerve ending, meaning it brings information from the
body's periphery toward the brain.
•They function as cutaneous receptors and are
essentially used to detect pain.

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Classification of nerve ending
According to structure and function, the nerve endings may be classified as follows:
Autonomic
Nerve endings
Sensory (receptors) Motor
General Special (hearing,
vision, taste, smell)
Somatic
TelereceptorsChemicalCutaneous
Exteroceptors
Deep Visceral
Enteroceptors Proprioceptors
Autonomic
Nerve endings
Sensory (receptors) Motor
General Special (hearing,
vision, taste, smell)
Somatic
TelereceptorsChemicalCutaneous
Exteroceptors
Deep Visceral
Enteroceptors Proprioceptors

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Sensory Receptors
•Sensory receptors are a specialized structure that can be
stimulated by environmental changes as well as by
changes within the body.
•These are the terminal afferent endings that undergo
depolarization in response to specific type of physical
stimuli.
•These receptors are capable of transforming different
types of energy into nerve impulses that travel through
sensory nerve fibers toward the central nervous system.
•Their functions are generally specific, one type carrying
one kind of sensory impulse, such as of touch, heat, cold,
etc.

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Classification of sensory receptors
Sensory (receptors)
General Special (hearing, vision, taste, smell)
Cutaneous Exteroceptors Deep Visceral
Enteroceptors Proprioceptors
A. According to structure and
functions:
1. General sensory receptors:
subserving cutaneous, deep and
visceral sensations
2. Special sensory receptors:
subserving different special functions
like carrying sensations for hearing,
vision, taste, smell, etc.
B. According to form of energy they
respond to:
1. Mechanoreceptors: touch, pressure
2. Chemoreceptors: taste, smell
3. Thermal receptors: warmth, cold
4. Osmoreceptors: osmotic pressure
5. Electromagnetic: rods & cones
6. Nocieptors: pain nerve endings

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Sensations
Sensations are FEELINGS aroused by change of environment. The
different ways by which the body may be aware of its surroundings
are called SENSATIONS.
Sensory mechanism:
For each sensation the following mechanism is involved:
(a) An appropriate stimulus
(b) A specific nerve ending-selectively sensitive to that stimulus
(c) The sensory pathway-which carries the impulse to the central
nervous system
(d) The nerve center-where the impulse is finally interpreted as a
particular sensation
(e) Psychical center-where the ‘meaning’ of the sensation is
analyzed and understood.

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Classification of sensation
A. General sensations:
(a) Superficial (touch spots, heat spots, cold spots)
(b) Deep
(c) Visceral (it is sensitized only under abnormal conditions)
B. Special senses:
(a) Taste
(b) Smell
(c) Vision
(d) Hearing

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Properties of sensation
(1) Modality:
-The ability to distinguish the characteristic of a sensation from all
other sensations is known as modality.
(2) Quality:
-Quality means the nature of sensation.
-Sensations of the same modality may vary in quality. For
instance, some individuals cannot distinguish between red and
green (color blindness).
(3) Intensity:
-Stronger the stimulus, higher will be frequency and more intense
will be the sensation.
-Two sensation of same quality may differ in intensity. A warm
object delivers little energy and a hot object delivers mush energy
to the receptors.

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Properties of sensation
(4) Adaptation:
-The structure of muscles and nerve adapt to a constant stimulus.
-The frequency of impulse gradually decreases due to adaptation.
(5) Extent:
-It indicates the area from which the sensation arises.
-Depends upon the number of receptors simultaneously stimulated.
(6) Duration:
-The duration of a sensation may be shorter that that of a stimulation
due to adaptation.
-On the other hand, sensations may outlast the period of stimulation and
thereby give rise to after-sensation.
(7) Localization or projection:
-It is the ability to locate the exact spot from which the sensation arises.
-Sensations are invariably projected either to some part of our own body
or to some part of the environment.

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Reflex
•Reflex action is an involuntary (automatic) effector
response due to a sensory stimulus.
•It is the basic physiological unit of integration in the
neural activity.

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Reflex arc
The complete pathway for a reflex action is called reflex arc. It has three parts:
(1) Afferent limb: consists of-
(a) sensory receptor
(b) afferent/sensory nerve fiber
(2) Center: consists of nerve cells where the sensory stimulus is converted into
a motor impulse. There is an interneuron in the center.
(3) Efferent limb: consists of –
(a) efferent/motor nerve fiber and its endings
(b) the effector organ/muscle

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Types of reflex arc
(1)Simple/two-neuron/monosynaptic reflex arc: Has only two
neurons, i.e., Stretch reflex.
(2)Three neuron/disynaptic reflex arc: There is a connecting
neuron between the afferent fiber and motoneuron.
(3)Polysynaptic/multisynaptic reflex arc: Consists of several
neurones, i.e., Withdrawal reflex in response to a noxious and
painful stimulation of the muscles, skin and subcutaneous tissues.
(4)Complex reflex arc: The axon of the sensory neuron, while
passing upwards, give off collaterals at different levels, each of
which may form separate reflex arcs.
(5)Asynaptic reflex arc: This reflex arc is not concerned with the
synapse or nerve cell and also known as axon reflex arc. This is
not a true reflex arc.

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Properties of reflex arc
(1) Irradiation:
☺ If the sensory stimulus be too strong, the impulse would spread on
to many neighbouring neurones in the center and produce a wider
response.
☺ For example, a weak pin prick on the finger will produce a reflex
movement of that finger only. But if the prick be too hard, the whole
hand will jerk up.
☺ Irradiation is due to transmission of the impulse through a large
number of collaterals of the afferent fibers and their interneurones.
(2) Delay:
☺There is a short interval between the application of stimulus and the
onset of reflex response. This period is called total reflex delay. This
time is lost in crossing the number of synapses in the central nervous
system.

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Properties of reflex arc
(3) Summation:
☺If the stimulus is subminimal and applied to an afferent nerve there
will be liberation of a chemical transmitter which will cause excitatory
postsynaptic potential (EPSP), but this EPSP will not be sufficient to
produce discharge of impulse from the motoneurones.
☺ If a number of subminimal stimuli be applied, their effects will be
summated together and the EPSP will be sufficient to induce the
motoneurones to discharge impulses and produce the reflex
response. This is called summation.
(4) Occlusion:
☺When a reflex contraction is produced by simultaneous
stimulation of two afferent nerves, the amount of tension (T) in
the muscle is less that the sumtotal of the tensions (t
1
+t
2
) setup in the
same muscle when the two afferent nerves are separately
stimulated (i.e., T<t
1
+t
2
).

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Properties of reflex arc
(5) Facilitation:
☺ The passage of a reflex impulse facilitates the transmission of
the next impulse (by reducing synaptic resistance).
☺ If a reflex be elicited repeatedly at proper intervals, the response
becomes progressively higher. Each subsequent stimulus seems to
exert a better effect than the previous one and makes the passage
of the next impulse easier.
(6) Inhibition:
☺In this phenomenon a stimulus diminishes or inhibits the effects of
another stimulus.
☺ For example, when the flexor muscles of a joint are stimulated the
extensor muscles are inhibited due to the inhibitory activity exerted
by the interneurones.

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Properties of reflex arc
(7) Recruitment:
☺ When muscle fibers are stimulated directly through their motor
nerve, the tension rises ‘very quickly’ to the maximum. But if
they are stimulated reflexly through a sensory nerve, the tension in
the muscle develops ‘gradually’ to the peak. After repeated
stimulation of the afferent nerves more internuncial neurons are
activated and lead to excitation of more number of motoneurons due
to recruitment property of reflex.
(8) After discharge:
☺After reflex contracton, if the stimulation is discontinued, the
muscle does not completely relax at once. It relax gradually. This
is due to the fact that the center go on discharging motor
impulses for a brief period, even after the sensory stimuli are
stopped. So, even after cessation of afferent stimulation, these
impulses travel for certain periods.

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Properties of reflex arc
(9) Fatigue:
☺ If a particular reflex be repeatedly elicited at frequent intervals, the
response becomes progressively feebler and finally disappears
altogether. This phenomenon is called fatigue.
(10) Reciprocal innervation:
☺In a reflex action when one group of muscles contracts, the
antagonistic group relaxes to some degree.

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The End