Lecture 5-Overview A&P function-Nerve Tissue(Choy 5.8.2024).pdf

Nervous system

1. Lecture 5:
Overview of A&P Functions of
Nerve Tissue: Neurophysiology
By Ms Choy Kee Leong
PPLK
(5.8.2024)

1

2. LEARNING OBJECTIVES:
1.State the functions of neuron.

2.State the structures of a neuron, and explain their
functions.

3.Describe the composition of grey mater and white
mater.

4.Classify neurons according to structure and
function.

5.State 6 types of supporting nerve cells and their
functions.

6.Explain the action potential.

7.Explain the synaptic transmission.
2

3. NEURON
3

4. Cells Of The Brain
The brain is made up of two types of cells:
1. Neurons – nerves impulse
2. Neuroglia – support and protect neurons




4

5. 1. NEURON
(Actual Nerve cell)

5

6. 1. NEURON (Actual Nerve cell)
•Neurons are amitotic.

•If a neuron is destroyed, it cannot be replaced
because neurons DO NOT undergo mitosis.

•Function: Conducting nerve impulses.


6

7. •Neurons are electrical excitability = ability to respond
to a stimulus and convert it into an action potential.

•Stimulus = any change in the environment that strong
enough to initiate an action potential.

•Action potential (or impulse) = an electrical signal
that propagates (travels) along the surface of the
neuron’s membrane or a muscle fiber.


NEURON cont’d…….
7

8. STRUCTURES OF NEURON
Each neuron has 3 basic parts:

1. Cell body (soma)

2. Dendrites

3. Axon

8

9. Neuron
Cell body
Node of Ranvier
Dendrite
Axon
9

10. 10
Structure of neuron
(Produce
neurotransmiter
s)
10

11. CELL BODY
•Cell body known as soma.

•Contains a nucleus surrounded by cytoplasm.

•No centriole in the neuron, NO MITOSIS.
•The organelles are endoplasmic reticulum,
lysosomes, mitochondria, and Golgi apparatus.

11

12. •Cell body form the grey matter of the nervous system.

•Found at the periphery of the brain and the centre of
the spinal cord.

•Groups of cell bodies are called nuclei in the CNS and
ganglia in PNS.

•Function:
 Metabolic centre of the neuron cells.
Cont’d…
12

13. Brain
13

14. DENDRITE
•Consists of short processes and extension of cells
bodies.

•Short, tapering, highly branched, forming a tree-
shaped) processes emerge from the cell body.

•Receiving or input parts of neuron.

14

15. Cont’d……. Dendrite
•Dendrite in motor neurons (efferent) → form part of
synapses.

•Dendrite in sensory neurons (afferent) → form the
sensory receptors that respond to specific stimuli.

•Examples:
–Vision from eye receptor.
–Hearing from inner ear receptor.

•Function:
–Receive & carry incoming impulses towards cell
bodies.
15

16. DENDRITE
Cell body
Node of Ranvier
Dendrite
16

17. AXON
•Axons are extensions of cell bodies. Longer than
dendrites, as long as 100 cm.

•Long, cylindrical projection that joins the cell body at a
cone-shaped elevation called axon hillock.

•The axon terminals part called synaptic bulb.

•These terminals contain hundreds of tiny vesicles called
neurotransmitters → transmit nerve impulse away from
the cell body.

17

18. CONT’D...

•The membrane of the axon is called axolemma.

•Cytoplasm in the axon is called axoplasm.

•Function of axon:

 Carry nerve impulses away from the cell body to
the other part of the body.
18

19. AXON
Cell body
Node of Ranvier
Axons hillock
Axon
Synaptic bulb
19

20. Axons: Function

20

21. AXON AND DENDRITE

•Are cytoplasmic extensions, or processes,
that project from the cell body.
•They are sometimes referred to as nerve
fibers.
•Bundles of nerve fibers running through
the CNS are call tracts, in PNS is called
nerves.


21

22. MYELIN SHEATH
•Most neurons are surrounded by a myelin sheath.

•Composed of whitish, lipid & protein (lipoprotein).

•Axon that surrounded by myelin sheath called
myelinated neurone.

•Axon not surrounded by myelin sheath called non-
myelinated neurone.

•Myelinated fibers make up white matter in the CNS.
22

23. Myelin Sheath and Neurilemma
•Whitish, fatty (protein-lipid), segmented sheath
around most long axons
•Formed by Schwann cells in the PNS
•It functions in:
• Protection of the axon
• Electrically insulating fibers from one another
• Increasing the speed of nerve impulse
transmission
•Neurilemma – remaining nucleus and cytoplasm of a
Schwann cell

23

24. MYELIN SHEATH
24

25. 25
Myelin Sheath and Neurilemma
Myelinated Axon
Unmyelinated Axon

25

26. CONTD…
•Unmyelinated regions between the myelin
segments are called the nodes of Ranvier.

•Nodes of Ranvier located in between the
myelin segments in the axon.

26

27. CONTD…
•Functions:

1. Act as an insulation (penebat) for the nerve fibers.

2. Protect axon from injury.

3. Increases the transmission rate of nerve impulses.

27

28. •Myelin sheath consists of a series of
Schwann cells arranged along the length of
the axon.

•In the CNS oligodendrocytes produce
myelin, but there is no neurilemma, which is
why fibers within the CNS do not regenerate.

•Neurilemma plays an important role in the
regeneration of nerve fibers.

CONT’D...
28

29. Common Disorders
•Multiple sclerosis:

 Disease that caused progressive destruction of
myelin sheaths of neurons in the CNS.

 Symptoms:

Heaviness or weakness in the muscles
Abnormal sensations
Double vision
Unable to breath
29

30. CLASSIFICATION OF NEURONS
Both structural and functional features are used to
classify the various neurons in the body.

1. Structural classification:
(a) Multipolar neurons
(b) Bipolar neurons
(c) Unipolar neurons

2. Functional classification:
(a) Sensory (afferent neurons)
(b) Motor (efferent neurons)
(c) Interneurons (association neurons)
30

31. (a) Multipolar neurons
•Have several dendrites + 1 axon.

•Location:
Most in the brain and spinal cord.
31

32. (b) Bipolar Neuron
•Have one main dendrite + one axon.

•Location:
Retina of the eye.
Inner ear
Olfactory area (smell).
32

33. (C) Unipolar Neurons
•Have dendrites and one axon.

•Fused together to form a continuous process.

•Begin in the embryo as bipolar neurons.

•Function:
 as sensory receptors (touch, pressure, pain, or
thermal stimuli).
33

34. 34

35. CONT’D...
2. Functional classification:
(a) Sensory (afferent neurons)
- contains sensory receptors.
- once get stimulated = forms action potential in
axons and conveyed into CNS.

(b) Motor (efferent neurons)
- convey action potential away from CNS to
effectors (muscle or glands) in PNS through
cranial & spinal nerves.

35

36. CONT’D...
(c) Interneurons (association neurons)
- between sensory & motor neurons.
- integrate incoming sensory information
from sensory to motor response.

36

37. Functional Classification Neuron



37

38. TYPE STRUCTURE FUNCTION
AFFERENT
(SENSORY)
• Dendrite - long
• Axon - short
• Cell body located in the
ganglia in PNS.
• Dendrite in PNS
• Axon extends into CNS.

• Transmit impulses
from peripheral
sense receptors to
CNS.
EFFERENT
(MOTOR)
• Dendrite - short
• Axon – long
• Dendrite & cell body located
within CNS.
• Axons extend to PNS.

• Transmits impulses
from CNS to
effectors such as
muscles & glands in
periphery.
ASSOCIATION
(INTERNEURON)
• Dendrite – short.
• Axon may be short or long.
• Located entirely within CNS.
• Transmits impulses
from afferent neurons
to efferent neurons. 38

39. Three types of neuron
39

40. 2.
Neuroglia

40

41. 41
2. Neuroglia
•Cells of the brain that provide neurons with nourishment,
protection, and structural support.

•There are about 10 to 50 times more neuroglia than nerve
cells

•Glia = glue nervous tissue together.

•Make up about ½ the volume of CNS.

•Smaller than neurons, 5 – 50 times more numerous.


41

42. 2. Neuroglia cont’d…
•Does NOT generate & conduct nerve impulses.

•Give mechanical support and metabolism to the
neurons:
 Support the neurons.
 Nourish the neurons.
 Protect the neurons.
 Capable of mitosis.

42

43. 6 TYPES OF NEUROGLIA
1. Astrocytes
2. Oligodendrocytes
3. Microglia
4. Ependymal cells
5. Schwann cells
6. Satellite cells
43

44. 2. Neuroglia
1.CNS
•Astrocytes
•Oligodendrocytes
•Microglia cells
•Ependymel cells


2. Peripheral
neuroglia
•Schwan cells
•Satellite cells
44

45. 2. Neuroglia cont’d…
•Astroglia or astrocytes transport nutrients to
neurons, hold neurons in place, digest parts of
dead neurons, and regulate the blood brain barrier.
•Oligodendroglia cells provide insulation (myelin) to
neurons.
•Ependymal cells line the ventricles and secrete
cerebrospinal fluid (CSF).
•Microglia digest dead neurons and pathogens.

45

46. 46 46
46

47. 47
Types Of Neuroglia Cells
Cell type Decription Function
1. Astrocytes (CNS) Star shape
Bind by to nerves. (Blood
brain Barrier)
2. Ependymal cells (CNS) Columnar cells with cilia
Important in formation &
circulation of CSF
3. Microglia (CNS)
Small cells with long
processes
Protection; phagocytic
4. Oligodendrocytes (CNS)
Cells with long process
around axon
Form myelin sheath in CNS
5. Schwann cells (PNS)
Flat cells, wraps around an
axon
Form myelin sheath in PNS
6. Satellite cells (PNS) Similar to Schwann cells
Support nerve cell bodies
within ganglia
47

48. 1. ASTROCYTES
48

49. Cont’d...
DESCRIPTION
LOCATION • CNS
STRUCTURE • Star shaped
• Numerous radiating processes
with bulbous ends for attachment.
FUNCTION • Bind blood vessels to nerves.
• Form ‘blood-brain barrier’. (Allowing
entrance of essential nutrients,
restriction of harmful substances)
• Regulate composition of fluid
around neurons.
49

50. 2. OLIGODENDROCYTES
50

51. Cont’d...
DESCRIPTION
LOCATION • CNS
STRUCTURE • Small cells with few, but long
processes that wrap around
axons.

FUNCTION • Form myelin sheaths around
axons in CNS.


51

52. Microglia

52

53. Cont’d...
DESCRIPTION
LOCATION • CNS
STRUCTURE • Small cells with long processes.
• Modified macrophages.

FUNCTION • Protection
• Become mobile & phagocytic in
response to inflammation.


53

54. 4. EPENDYMAL CELLS
Mikrovili
54

55. Cont’d...
DESCRIPTION
LOCATION • CNS (line ventricles of brain &
central canal of spinal cord.
STRUCTURE • Columnar cells with cilia.
FUNCTION • Active role in formation and
circulation of cerebrospinal fluid
(CSF).

55

56. 5. Schwann Cells
56

57. Cont’d...
DESCRIPTION
LOCATION • PNS
STRUCTURE • Flat cells with long, flat process
that wraps around axon in PNS.
FUNCTION • Form myelin sheaths around
axons in PNS.
• Active role in nerve fiber
regeneration.

57

58. 6. SATELLITE CELLS
58

59. Cont’d…
DESCRIPTION
LOCATION • PNS
STRUCTURE • Flat cells, similar to Schwann
cells.

FUNCTION • Support nerve cell bodies within
ganglia.
• Eg. Regulate concentration of
Sodium, Potassium & CO2.

59

60. NEUROGLIA & NEURON
60

61. Action Potential
&
Nerve Impulse
61

62. Transmission of
Nerve Impulse
62

63. Action Potentials (AP)
•Only cells with excitable membranes (neurons
and muscle cells) can generate AP.
•Membrane potential = -70mV.
•AP do not decrease in strength over distance.
•Threshold stimulus is needed to generate the
AP
•An axon is capable of generating an AP.
63

64. Transmission of Nerve Impulses
1.Polarization of the neuron membrane
2.Resting membrane potential
3.Depolarization / Action potential
4.Repolarization
5.Hyperpolarization
6.Refractory period
64

65. 1. Polarization of the neuron’s membrane: Na is on
the outside & K is on the inside
•When neuron is not stimulated, its
membrane is polarized.
•Being polarized, the electrical charge
outside is positive and inside is negative.
•Na
+
is on the outside & K
+
is on the inside.
•Na
+
/ K
+
pumps on the membrane pump N
+

back outside and K
+
back inside.
•The charge of an ion inhibits the membrane
permeability.

65

66. 2. Resting potential
•When the neuron is inactive and
polarized, it is said to be at its resting
potential.
•It remains polarized until a stimulus
come along
•A resting potential of -70mV
66

67. 3. Action potential: Na
+
ions move inside
the membrane
•When neuron is stimulated, the Na
+
ion gated open
and allow Na
+
rush into the cell.
•As this happen, the neuron goes from being polarized
to depolarization.
•More positive ion is now inside the cell until the
threshold is reached.
•Each neuron has a threshold level
•Once stimulus is above the threshold, more gated Na
+

ion channel open and allow more Na
+
inside the cell
67

68. 3. Action potential: Na
+
ions move inside
the membrane (cont’d)
•This causes complete depolarization of the neuron
and an action potential is created.
•Na channel continues to open
•“All or none phenomenon” – if a stimulus does not
exceed the threshold level and cause the gates to
open, no action potential results.

68

69. 4. Repolarization: K
+
ion moves outside
& Na ion stays inside the membrane
•After inside cell is flooded with Na, the K
gated ion channel open and K moves
outside the membrane.
•Both ions restores electrical charge
balance
•Just after the K gates open, the Na gates
close
69

70. 5. Hyperpolarization : more K
+
ions are
on the outside than Na ions on the inside
•When K gated is close, the neuron has more K
on the outside than Na on the inside
•This causes the membrane potential to drop
slightly lower than the resting potential
•The membrane is said to be hyperpolarized
because it has greater potential
•After the impulse has traveled through the
neuron, the action potential is over, the cell
membrane returns to normal ( known as resting
potential)
70

71. 6. Refractory period: everything back to normal
(K
+
return inside & Na
+
returns outside)
•While neuron is busy returning to normal,
it does not respond to any incoming
stimuli.
•The neuron is back to polarized state and
stays in the resting potential until another
impulse comes along
71

72. Nerve Impulses
Figure 7.9a–b
72

73. Nerve Impulses
Figure 7.9c–d
73

74. Nerve Impulses
Figure 7.9e–f
74

75. 75
Action potential
75

76. 76
Action Potential
76

77. 77
77

78. Factors Affecting the Speed of Conductance:
Myelin, Axon Diameter, Temperature
•Impulses travel faster in myelinated
neurones → SALTATORY
CONDUCTION
- Schwann cells prevent diffusion of ions
- Flow of current between adjacent nodes of
Ranvier
- Depolarisation only at nodes of Ranvier
- Action potential jumps from node to node
78

79. Conduction Velocities of Axons
•Conduction velocities vary widely among neurons
•Rate of impulse propagation is determined by:
1.Axon diameter – the larger the diameter, the faster
the impulse
2.Degree of a myelin sheath – myelination dramatically
increases impulse speed
3.Speed of conduction

80

80. Action Potential: Role of the Sodium-
Potassium Pump
•Repolarization
• Restores the resting electrical conditions of
the neuron
• Does not restore the resting ionic
conditions
• Ionic redistribution back to resting
conditions is restored by the sodium-
potassium pump

81

81. 82

82. Saltatory Conduction in Myelinated
Axons
•Myelin sheathing has bare patches of axon called nodes of
Ranvier
•Action potentials jump from node to node
Fig. 48.11
83

83. Saltatory Conduction
84

84. Factors Affecting the Speed of Conductance:
Myelin, Axon Diameter, Temperature
•Temp affects speed of conduction of
impulses
 Higher temp increases rate of diffusion of ions

•Impulses faster in an axon with larger
diameter
 Small cells / large surface area : volume ratio / ion
leakage weakens membrane
 Myelin stops ion leakage / diameter only important
for unmyelinated neurones
85

85. SYNAPES: The space where 2
neurons communicate in one
direction.

86

86. Electrical Synapses

• Are less common than chemical synapses
• Correspond to gap junctions found in
other cell types
• Contain intercellular protein channels
• Permit ion flow from one neuron to the
next
• Are found in the brain and are abundant
in embryonic tissue

87

87. Chemical Synapses

Specialized for the release and reception of
neurotransmitters
• Typically composed of two parts:
• Axonal terminal of the presynaptic
neuron, which contains synaptic vesicles
• Receptor region on the dendrite(s) or
soma of the postsynaptic neuron

88

88. Synapse
•Nerve impulse from one neuron is transmitted to the
next neuron
•Junction between the 2 neurons is called synapes
•Synapse has 3 parts:
1. Synaptic bulb/knob/button on the presynaptic
neuron
2. Synaptic cleft
3. Postsynaptic membrane of postsynaptic neuron
89

89. 90
Synapses

•A junction that transfer
information from one
neuron:
• To another neuron
• To an effector cell
•Presynaptic neuron –
conducts impulses
toward the synapse
•Postsynaptic neuron –
transmits impulses
away from the synapse

90 90

90. 91
Synapse (cont’d)
91

91. Synaptic Transmission
•Synapses is he point at which the nerve impulse
passes from one neuron to another neuron.

•In synapse, neurons communicate with other neurons
or effectors called synaptic transmission.

•Each axon terminal is separated from the next neuron
by a tiny gap called the synaptic cleft
(about 20nm (nanometer) width.

92

92. Synaptic Cleft

•Fluid-filled space separating the presynaptic
and postsynaptic neurons
• Prevent nerve impulses from directly
passing from one neuron to the next
• Transmission across the synaptic cleft:
• Is a chemical event (as opposed to an
electrical one)
• Ensures unidirectional communication
between neurons

93

93. Synaptic Cleft: Information Transfer

•Nerve impulse reaches axonal terminal of the
presynaptic neuron
•Neurotransmitter is released into the synaptic
cleft
•Neurotransmitter crosses the synaptic cleft
and binds to receptors on the postsynaptic
neuron
•Postsynaptic membrane permeability changes,
causing an excitatory or inhibitory effect

94

94. Cont’d...
•Presynaptic neuron = neuron that
sending the signal.

•Postsynaptic neuron = neuron that
receiving the message.

•There is no physical contact between
these neurons.

95

95. Termination of Neurotransmitter Effects

•Neurotransmitter bound to a postsynaptic neuron:
• Produces a continuous postsynaptic effect
• Blocks reception of additional “messages”
• Must be removed from its receptor
•Removal of neurotransmitters occurs when they:
• Are degraded by enzymes
• Are reabsorbed by astrocytes or the
presynaptic terminals
• Diffuse from the synaptic cleft

96

96. Chemical Synapse
Synaptic vesicles within synaptic bulbs contain chemical
called neurotransmitters (released when impulse reaches
synaptic bulbs)

•Neurotransmitters diffuse across synaptic cleft & bind to
receptors on postsynaptic membrane

•Eg: acetylcholine (neurotransmitter) inactivated by
cholineterase to prevent prolonged reaction



97

97. Neurotransmitters
•Are chemicals that cross the synaptic gap and
either excite or inhibit post-synaptic neurons

•Allows communication between neurons
they are like a lock and key - they must match
up

98

98. Examples Of Neurotransmitters
•ACETYLCHOLINE - muscle activity, memory and
learning, PNS, affecting the heart, stomach, liver,
sweat glands, blood vessels
•GABA (GAMMA AMINO BUTYRIC ACID) -
Central Nervous System (CNS) main amino acid
inhibitory neurotransmitter - induces sleep



99

99. Examples Of Neurotransmitters
•CATECHOLAMINES - BIOGENIC AMINES
1. EPINEPHRINE - functions as both hormone and
neurotransmitter - increases alertness and energy
2. NOREPINEPHRINE - aids in memory, learning,
and emotion - affects mood and attention
3. DOPAMINE - deals with fine motor control
(Parkinson disease sufferers have a deficit of this
chemical
4. SERATONIN - linked with mood, emotions, sleep,
consciousness, and digestion

100

100. HOW’S THE SIGNAL
AT SYNAPSE?
HOW’S THE CONDUCTIVITY?
▼
101

101. Synaptic Transmission
•Synaptic cleft (gap) of 20μm separates two
neurones at a synapse (junction of 2 neurones)
 Presynaptic membrane is at the end of a neurone
 Postsynaptic membrane is at the next neurone in
the chain

•Synaptic knob of a presynaptic neurone contains
 Neurotransmitters in small vesicles
 Mitochondria to produce ATP needed for
neurotransmitter synthesis
102

102. Aspects Of Synaptic Transmission
Summation
•Several presynaptic neurones release neurotransmitter
•Cumulative effect reaches a threshold to depolarise
postsynaptic membrane
•E.g. rod cells when they synapse with relay neurones in
the retina

Spatial summation
•Several impulses arrive at one neurone via several
synapses
•Cause sufficient depolarisation / open sufficient sodium ion
channels
•For threshold to be reached

Temporal summation
•Several impulses arrive at same neurone via same
synapse
•Threshold → action potential
103

103. Aspects Of Synaptic Transmission
Inhibition
•More inhibitory postsynaptic potentials IPSPs than
excitatory postsynaptic potentials EPSPs
•Reduces membrane potential / makes more
negative
•Hyperpolarisation of postsynaptic membrane
•Cancels effect of action potential when several
synapses
104

104. The Mechanisms Of Transmission At An
Excitatory Synapse
1.Nerve impulse reaches synaptic knob/presynaptic
membrane/neurone
2.Depolarisation opens Ca
2+
gates / calcium ions enter
3.Ca
2+
causes vesicles containing neurotransmitter to
fuse with membrane
4.Release of neurotransmitter / into synaptic cleft / by
exocytosis
5.Diffuse across synaptic cleft
6.Neurotransmitter binds to specific receptors in
postsynaptic membrane
105

105. The Mechanisms Of Transmission At An
Excitatory Synapse
7. Sodium channels open / sodium ions enter
•Depolarisation of postsynaptic membrane
•Threshold causes an action potential along
postsynaptic neurone

8. Neurotransmitter are quickly removed from the
postsynaptic membrane
•Diffuse out of the synaptic cleft
•Taken up by presynaptic membrane by endocytosis
•Enzymes break down neurotransmitters into inactive
substances
106

106. 107

107. 108
108

108. SYNAPSE
IMPULSE
109

109. 110

110. Action potential reaches axon terminal
▼
Neurotransmitter released from vesicles into
synaptic cleft
▼
Vesicles fuses with plasma membrane
▼
Neurotransmitter released into synaptic cleft
▼
Neurotransmitter bind to receptors on the
next post-synaptic membrane

111

111. EVENTS AT A SYNAPSE
Nerve impulse (Action Potential) arrives at a synaptic bulb of
presynaptic axon (axon terminal)
▼
Ca
2
+
channels open
▼
Ca
2
+
flows into the synaptic end bulb
▼
Ca
2
+
concentration increase
▼
Trigger synaptic vesicles to release neurotransmitter molecules
into synaptic bulb

112

112. Cont….

Neurotransmitter molecules diffuse across the
synaptic cleft
▼
Bind to neurotransmitter receptors in the
postsynaptic neuron plasma membrane
▼
Excitability of Neuron
Effectors
(muscle, glands)


113

113. Neurotransmitter is then broken down
and released by enzyme activity.
Neurotransmitter: Acetylcholine
Enzyme: Cholinesterase
▼
EXCITABILITY OF NEURON



114

114. Synaptic Transmission
115

115. Synaptic Transmission
116

116. 117

117. Thank You
118

118. Synaptic Delay
•Neurotransmitter must be released, diffuse
across the synapse, and bind to receptor
•Synaptic delay – time needed to do this (0.3-
5.0 ms)
•Synaptic delay is the rate-limiting step of
neural transmission

119

119. Postsynaptic Potentials
•Neurotransmitter receptors mediate changes in
membrane potential according to:
• The amount of neurotransmitter released
• The amount of time the neurotransmitter is
bound to receptor
•The two types of postsynaptic potentials are:
• EPSP – excitatory postsynaptic potentials
• IPSP – inhibitory postsynaptic potentials

120

120. Excitatory Postsynaptic Potentials
•EPSPs are graded potentials that can initiate
an action potential in an axon
• Use only chemically gated channels
• Na
+
and K
+
flow in opposite directions at
the same time
• Postsynaptic membranes do not generate
action potentials

121

121. Inhibitory Synapses and IPSPs
•Neurotransmitter binding to a receptor at
inhibitory synapses:
• Causes the membrane to become more
permeable to potassium and chloride ions
• Leaves the charge on the inner surface
negative
• Reduces the postsynaptic neuron’s ability
to produce an action potential

122

122. 123
123

123. Summation
•A single EPSP cannot induce an action potential
•EPSPs must summate temporally or spatially to
induce an action potential
•Temporal summation – presynaptic neurons transmit
impulses in rapid-fire order
•Spatial summation – postsynaptic neuron is
stimulated by a large number of terminals at the
same time

124

124. Neurotransmitters: Acetylcholine
•Released at the neuromuscular junction
•Synthesized and enclosed in synaptic vesicles
•Degraded by the enzyme acetylcholinesterase (AChE)
•Released by:
• All neurons that stimulate skeletal muscle
• Some neurons in the autonomic nervous
system

125

125. Functional Classification of
Neurotransmitters
•Two classifications: excitatory and inhibitory
• Excitatory neurotransmitters cause depolarizations
(e.g., glutamate)
• Inhibitory neurotransmitters cause hyperpolarizations
(e.g., GABA and glycine)
•Some neurotransmitters have both excitatory and inhibitory
effects
• Determined by the receptor type of the postsynaptic
neuron
• Example: aceytylcholine
• Excitatory at neuromuscular junctions
• Inhibitory with cardiac muscle

126

126. Neurotransmitter Receptor Mechanisms

•Direct: neurotransmitters that open ion
channels
• Promote rapid responses
• Examples: ACh and amino acids
•Indirect: neurotransmitters that act through
second messengers
• Promote long-lasting effects
• Examples: biogenic amines and peptides

127

127. ANY QUESTION?
128