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Chapter 48
神經元、突觸與神經訊號傳遞
Neurons, Synapses, and Signaling
Outline

神經系統簡介

神經系統主要構成元素

The Structure of Neurons
 Neurons
 Glial Cells

Neuronal Signaling
 Resting Potential (Ion Channels and Pumps)
 Action Potential
 Neural Communication at Synapses
國立交通大學生物科技學系 柯立偉老師
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※ 神經系統簡介 ※
•
•
Nervous systems process
information in three stages:
sensory input, integration, and
motor output
Many animals have a complex
nervous system that consists of
 A 中樞神經系統 (central
nervous system, CNS) where
integration takes place; this
includes the brain and a nerve
cord
 A 周邊神經系統 (peripheral
nervous system, PNS), which
carries information into and out
of the CNS
 The 神經元 (neurons) of the
PNS, when bundled together,
form 神經 (nerves)
Sensory input
輸入
Sensor
Integration
整合
感官受器
Motor output
輸出
Effector
Peripheral
nervous
system (PNS)
周邊神經系統
Central
nervous
system (CNS)
中樞神經系統
Figure 48.3
Summary of information processing
※ 神經系統主要構成元素 ※
• Sensory neuron 感覺神經元
Sensors detect external stimuli and internal conditions and
transmit information along sensory neurons
• Interneuron 連絡神經元
Sensory information is sent to the brain or ganglia, where
interneurons integrate the information
• Motor neuron 運動神經元
Motor output leaves the brain or ganglia via motor
neurons, which trigger muscle or gland activity
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※ The Structure of Neurons ※ (CH48-1)
 The Structure of Neurons:
 Neurons:
• The basic signaling units, are distinguished by their form, function,
location, and interconnectivity within the nervous system.
• Neurons take in information, make a “decision” about it following some
relatively simple rules, and then, by changes in their activity levels, pass
it along to other neurons.
• Close relations to the morphological or structural, specializations of
neurons
 Glial cells (also called Neuroglial Cell; 膠質細胞):
• The other type of cell in the nervous system
• More numerous than neurons, more than half of the brain’s volume
• Non-neuronal cell, supportive function to brain
Structure of Neurons
(input)
postsynaptic
粒腺體
synapse
細胞核
Soma
哺乳動物
胞器
組織器官
核糖體
蛋白質
presynaptic (output)
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細胞質
Synapse dendrites
3
Structure of Neurons
Dendrites樹突
Presynaptic cell
(output)
Stimulus
粒線體
內質網
細胞核
Synapse突觸
Signal
direction
高基式體
Axon 軸突
核醣體
Cell body
(Soma)
Synaptic terminals
Synaptic
terminals
Neurotransmitter
Postsynaptic cell
(input)
Figure 48.4 Neuron structure and organization
Information across the synapse in the form of
chemical messengers called neurotransmitters.
Structure of Neurons
 As with most cells, the cell body contains metabolic (新陳代謝)
machinery that maintains the neuron.
 Machinery includes a nucleus(細胞核), endoplasmic reticulum (內質
網), ribosomes(核醣體), mitochondria(粒線體), Golgi apparatus(高爾
基體), and other intracellular organelles(胞器).
 Surrounded by the neuronal membrane (細胞膜) composed of a lipid
bilayer (雙層脂質), and suspended in cytoplasm (細胞質)
Lipids (fatty material) are not water
soluble (不溶於水), and thus form a
barrier (屏障) to watery soluble
materials (水溶性物質, ex. sodium,
potassium ions, proteins, and other
molecules)
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Structure of Neurons
 Neuron’s organelles, including its nucleus,
are in the cell body.
Dendrites
Stimulus
 The dendrites, highly branched extensions
that receive signals from other neurons. Nucleus
 The axon is typically a much longer
extension that transmits signals to other
cells at synapses.
 The cone-shaped base of an axon is called
the axon hillock.
Axon hillock
Presynaptic
cell
Cell
body
Axon
Signal
 A synapse is a junction between an axon
direction
and another cell.
Synapse
 The synaptic terminal of one axon passes
information across the synapse in the form
of chemical messengers called
neurotransmitters.
Synaptic terminals
 Information is transmitted from a
Postsynaptic
Neurotransmitter
presynaptic cell (a neuron) to a
cell
postsynaptic cell (a neuron, muscle, or
Figure 48.4 Neuron structure & organization
gland cell).
Structure of Neurons
Dendrites: after the synapse with respect to info. flow (postsynaptic)
Axon: before the synapse with respect to info. flow (presynaptic)
Most of neurons are both presynaptic and postsynaptic.
Activity within a neuron involves changes in the electrical state, but at
synapses the signal between neurons is usually mediated by chemical
transmission. (但仍有存在少數的 electrical synapse)
 Dendrites with varied and complex forms:
1. 位於小腦的cerebella Purkinje cells正面像大樹一樣茂密，側面較薄
2. spinal motor neurons或thalamus neurons：其dendrites較簡約
3. Hippocampal neuron: dendrites上有 “spines”結構
→spines是little knobs attached by small necks to the surface of
dendrites, 而synapse則座落在spines上
4. 有些neurons 其cell body上即直接有synapse (而無spines)
國立交通大學生物科技學系 柯立偉老師
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A diagrammatic representation of the soma and
dendritic tree of a Purkinje cell from the cerebellum
Figure 2.3 Soma and dendritic tree of a Purkinje cell from the cerebellum.
The Purkinje cells are arrayed in rows in the cerebellum. Each one has a large
dendritic tree that is wider in one direction than the other.
Diagrammatic ventral horn motor neuron
Figure 2.4 Ventral-horn motor
neuron.
Multipolar neurons located in
the spinal cord send their axons
out the ventral root to make
synapses on muscle fibers.
國立交通大學生物科技學系 柯立偉老師
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Structure of Neurons
 Axon: 將neuron的電信號傳出(output)到axon terminal synapse處
axon terminal有特殊造型及intracellular結構, that enable
communication via the release of neurotransmitters, (the chemical
substances that transmit the signal between neurons at chemical
synapses)
The dendrites and the axon of a neuron are extensions of
the cell body.
Their internal volume is filled with the same cytoplasm (細胞質) that
fills the cell body.
Cell body, dendrites and axon 是組成神經元的主要部分，彼此之間
主要溝通是靠electrical signaling
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Four General Morphological Classes of Neurons
Unipolar:
Figure 2.6
 Only one process，分岔形成樹突和
Axon Terminals.(出現於無脊神經系統)
Bipolar:
Dendrite
Axon
 Two processes (one axon and one
dendrite)，負責sensory processes，
出現於聽覺、視覺、嗅覺系統中。
Ex. Bipolar retinal cell (Fig. 1.13)
Multipolar:
 One axon and many dendrites.
 存在於很多神經系統中，主要是Motor and
sensory processing
 Neurons in the Brain，大部分都是這類神經
元
 Figure 2.5 亦是Multipolar Neuron.
Pseudounipolar:
 出現unipoloar神經元，但獨特的是樹突和
軸突融合在一起，
出現於Dorsal-root ganglia of the spinal
cord，為somatosensory cells…
神經系統主要構成元素(Review)
Dendrites
Axon
Cell
body
Portion
of axon
Sensory neuron
Interneurons
Motor neuron
Figure 48.5 Structural diversity of neurons
Cell bodies and dendrites are black in these diagrams; axons are red. In the sensory
neuron, unlike the other neurons here, the cell body is located partway along the axon
that conveys signals from the dendrites to the axon’s terminal branches.
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※ Glial Cells (膠質細胞) ※
 Glial cell (neuroglial cell or
glia), amount 10 times of
neurons, more than half of the
brain’s volume. Main role is
structural support to neurons.
 Glial cells existed in CNS –
central nervous system (含
brain 及 spinal cord), and
PNS – peripheral nervous
system (含sensory / motors
inputs / outputs to the brain及
spinal cord)
 Central Nervous has three
main types of glial cells:
Astrocytes, Oligodendrocytes
and Microglial Cells. (Fig. 2.7)
Glial Cells (Nerve Glue)
Peripheral nervous system
Schwann cell
Central nervous system
Myelin (髓鞘) glial
80 m
Astrocyte (星形膠質細胞)
Microglia (小膠質細胞)
Oligodendrocyte (少突膠質細胞)
Glia
Figure 48.6 Glia in the mammalian brain
The glia are labeled red, the DNA in nuclei is labeled
blur, and the dendrites of neurons are labeled green.
Cell bodies of neurons
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Myelin Glial (髓鞘膠質細胞)
 These glia provide layers of membrane that insulate
axons
Oilgodendrocyte: provide myelin
for multiple neurons
Schawnn cell: provide myelin for
individual neuron
Main Differences between Oligodendrocyte and
Schwann Cell
 Two Types of Myelin-Producing Cells
Number of Axons
Cell Type
Location
Schwann Cell
Peripheral Nervous
System
One
Oligodendrocyte
Central Nervous
System
Many
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Myelinated by One Cell
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Astrocytes
Cultured astrocyte:
 Star-shaped, their many arms span all
around neurons.
 Large glial cells having round or
radially (放射狀) symmetric forms
 End feet: Astrocytes make contact
with blood vessels at specialization.
 Permit the astrocyte to transport
ions across the vascular wall and
to create a barrier (障礙) between
the tissues of the central nervous
system
Activated astrocytes
Astrocyte
 Functions:
 Structural support
 Metabolic support (新陳代謝)
 Transmitter reuptake and release (重複攝取和釋放)
 Regulation of ion concentration in the extracellular space
 Modulation of synaptic transmission
 Vasomodulation (血管舒縮)
 Promotion of the myelinating activity of oligodendrocytes
 Guiding neurons as they migrate to their ultimate destination
 Secreting growth factors to stimulate neuronal growth
(分泌生長因素刺激神經成長)
 Blood-brain barrier (BBB): astrocytic barrier between neuronal tissue
and blood (扮演保護CNS重要角色，許多藥物無法通過此屏障)
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What is Blood brain barrier (BBB)
 The blood brain barrier is both a physical
barrier and a system of cellular transport
mechanisms.
 It maintains homeostasis (體內平衡) by
restricting the entrances of potentially
harmful (有害的) chemicals from the
blood, and by allowing the entrance of
essential nutrients (必須營養).
 It plays a vital role in protecting the central nervous system from blood-borne (血
液輸送) agents or chemical compounds (化合物) that might unduly (不正常性)
affect neuronal activity.
 Many drugs can not across the BBB, and certain neuroactive agents, such as
dopamine (多巴胺) and norepinephrine (正腎上腺素), when placed in the blood,
do not across the BBB.
Microglia
 Small and irregularly shaped and come
Activated
into play when tissue is damaged.
 Act as phagocytes (噬菌細胞, 如白血球
等), cleaning up CNS debris (碎屑),
Resting
responsible for producing an
inflammatory (發炎反應) reaction to
insults (損傷) (Streit et al., 2004)
 May play a role in neurodegenerative
disorders (神經退化性疾病) such as
Alzheimer‘s disease, dementia (癡呆),
multiple sclerosis (硬化) and Amyotrophic
lateral sclerosis (肌萎縮性脊髓側索硬化
國立交通大學生物科技學系 柯立偉老師
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Activation of microglia cells in a tissue
section from human brain
Activated
(in diseased cerebral cortex)
Resting
Activated
(Phagocytosis)
Overview of Neurons
Presynaptic cell
(output)
Dendrites 樹突
粒線體
細胞核
Stimulus
內質網
Axon hillock
高基式體
Nucleus
Cell body
(Soma)
軸突
突觸
Signal
direction
Axon
髓鞘
核醣體
Layers of myelin
Axon
Myelin sheath
Schwann cell
Nodes of Ranvier
Synapse
Schwann
cell
Nucleus of Schwann cell
Synaptic terminals
Synaptic
terminals
Neurotransmitter
國立交通大學生物科技學系 柯立偉老師
Postsynaptic cell
(input)
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※ Neuronal Signaling ※ (CH48-2)
 The goal of neuronal processing is to
take in information,
evaluate it,
pass a signal to other neurons,
forming local and long-distance circuits and networks
 Current flow is mediated by ionic currents carried by electrically
charged atoms (ions), such as
Sodium (Na+) 鈉
Potassium (K+) 鉀
Chloride (Cl-) 氯
Neural Membrane Potential
 Neuronal membrane: a bilayer of lipid molecules (Membrane 不
溶於水,因此可控制水溶性 (Water-soluble)物質進出)
A barrier to ions, proteins及其他可溶於(內外) cellular fluid的
molecules.
 Neuronal membrane含有ion channels及active transporters
(pumps)及receptor molecules.
 Resting membrane potential:
neuronal membrane內外的電
位差,約-70mv,可由glass pipette
electrode with fine tip,刺入
neuron細胞內量測到
此乃因Ion channels對進出Ions
之控制達成
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Ion Channels and Pump
 Ion channels: 由＂transmembrane proteins”所型成的pore (氣孔),
actual passageways (通道), 可使Ions (Na+, K+, Cl-)藉此進出
neurons. 有2類channels
1.passive (nongated): 允許特定的Ion因密度gradients而隨時進出
neurons. →open to certain ions
2.gated: can be opened or closed by electrical, chemical or physical
stimuli.
Permeability (滲透性): the extend to which a channel permits ions to
cross membrane
The membrane is more permeable to K+ than Na+, Cl-,
(稱為selectively permeable) (因neuronal membrane has many
more nongated (無柵式) K+ — selective channels than nongated
Na+ channels及 other nongated channels)
Ion Channels and Pump
Resting membrane potential
Properties of the membrane combined
with the active pumping of ions across
the neuronal membrane lead to ionic
concentration gradients (離子濃度變
化) for Na+, K+ and Cl-, and charged
proteins (A-) across the membrane.
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Ion Channels and Pump
 Active transporters: Na+/ K+ ATPase pump可主動move Na+ (K+)
出(進) membrane.
 ATP (adenosine triphosphate) 為一種energy-storing molecules,可提
供transmembrane pumps所需的fuel.
 Pumps是enzymes酵素(proteins) 構成,可以切斷chemical bond (連結) in
a ATP molecule而釋放能量以
 move 3個Na+ out of the cell及
 move 2個K+ into the cell.
 改變neuron內外濃度差,造成concentration gradients
 In resting state, neuron內有較高濃度的K+,而neuron外有較高濃度的Na+.
 1.Selective membrane permeability及
2.Transmembrane ionic concentration gradients act together to
create differences in charge across the membrane.
Ion Channels and Pump
 Resting potential process as follows:
(a) Pumps建立ionic concentration gradients 使得more Na+ outside the
neuron and more K+ inside the neuron.
(b) a的 gradients 產生一個force要將
Na+由outside neuron推入inside neuron
K+由inside neuron推出outside neuron
由High concentration
=> From inside to outside
→推進Low concentration
但因membrane is more permeable (leaky) (易滲透) to K+ than Na+,
the force of the concentration gradient pushes more K+ out of the
cell.
(c) b中因較多K+被移至neuron外,使得neuron外較 外
相斥
neuron內的電壓高(正),而產生electrical
K 相吸
gradient,而造成K+越來越難流出.
內 +
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Ion Channels and Pump
(d) b與c的2股gradients (electrical電場及ionic concentration離子場)
are in opposition to (意見相反) one another w.r.t. K+,而最後會達成
electrochemical equilibrium,即離子場將K+經nongated K+ channel
推出neuron外的force = 電場將K+留在neuron內的力
(e) d的電化平衡情況下, the small difference in charge across the
membrane 型成了resting membrane potential,內外電壓差約
- 40~ - 90mv (neuron內較外電場低)
(f) e在resting狀況下的neuron像是一個電池,含有potential energy,而可
經由active pumps改變membrane potential而產生電流(信號)
→ active Process
Overview of Neural Communication
 一個neuron與其他cells在synapses進行溝通: 接收輸入信號之
synapses位於dendrites或cell body,而送出輸出信號之synapse位
於axon terminals.
 The process of Neuronal signaling has several stages:
1.Neurons receive signals in either chemical form (如neurotransmitter
或嗅覺分子) or physical form (如skin之somatosensory之觸覺、eye之
photoreceptors之光線)
2.Stage1之信號改變postsynaptic neurons之membrane,而產生在neuron
內及週遭之電流(current flow)
3.The current flow is mediated by ionic current carried by ions (如Na+,
K+, Cl-) that are dissolved in the fluid inside and outside of neurons.
4.當一個neuron整合夠多currents from many synaptic inputs或sensory
inputs,則會產生Spikes狀的long-distance signals (action potentials),
而neuron中產生spikes的區域叫spike triggering zone.
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Overview of Neural Communication
5. Long-distance signals, action potentials, travel down the axon to its
terminals, where causes the release of neurotransmitters at synapses.
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Resting Potential
• Sodium-potassium pumps (鈉鉀幫浦) use the energy of ATP to
maintain these K+ and Na+ gradients across the plasma
membrane.
• In a mammalian neuron at resting potential, the concentration of K+ is
highest inside the cell, while the concentration of Na+ is highest
outside the cell. These concentration gradients represent chemical
potential energy.
Figure 48.7 The basis of
the membrane potential
Key
OUTSIDE
OF CELL
Na
K
Sodium-potassium pump
Potassium (K) channel
Sodium (Na) channel
INSIDE
OF CELL
Resting Potential
• The opening of ion channels in the plasma membrane converts
chemical potential to electrical potential.
• A neuron at resting potential contains many open K+ channels and
fewer open Na+ channels; K+ significantly diffuses out of the cell.
• The resulting buildup of negative charge within the neuron is the
major source of membrane potential.
Figure 48.7 The basis of
the membrane potential
OUTSIDE
OF CELL
Key
Na
K
Sodium-potassium pump
Potassium channel
Sodium channel
INSIDE
OF CELL
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※ Action Potential ※ (CH48-3)
Changes in membrane potential occur because neurons contain gated
ion channels that open or close in response to stimuli.
Hyperpolarization 超極化
•When gated K+ channels open, K+ diffuses out, making the inside of the
cell more negative and having an increase in magnitude of the membrane
potential.
Stimulus
50
Membrane potential (mV)
(a) Graded hyperpolarizations
produced by two stimuli
that increase membrane
permeability to K
0
50
Threshold
Resting
potential
Hyperpolarizations
100
0 1 2 3 4 5
Figure 48.10a
Time (msec)
Action Potential
 Depolarization 去極化
(b) Graded depolarizations
produced by two stimuli
that increase membrane
permeability to Na
Membrane potential (mV)
• A reduction in the magnitude of the membrane potential when other
types of ion channels open. For example, depolarization occurs if
gated Na+ channels open and Na+ diffuses into the cell.
0
50
100
Figure 48.10b
國立交通大學生物科技學系 柯立偉老師
Stimulus
50
Threshold
Resting
potential
Depolarizations
0 1 2 3 4 5
Time (msec)
20
Action Potential
 If a depolarization shifts the membrane potential sufficiently, it results
in a massive change in membrane voltage called an action potential.
• Action potentials have a constant magnitude, are all-or-none, and
transmit signals over long distances.
• They arise because some ion channels are voltage-gated, opening
or closing when the membrane potential passes a certain level.
Strong depolarizing stimulus
Membrane potential (mV)
(c) Action potential triggered
by a depolarization that
reaches the threshold
Figure 48.10c
50
Action
potential
0
50
Threshold
Resting
potential
100
0 1 2 3 4 5 6
Time (msec)
Generation of Action Potentials
An action potential can be considered as a series
of stages
Membrane potential
(mV)
50
Figure 48.11
Action
potential
3
0
2
4
50
1
5
Threshold
1
Resting potential
100
 At resting potential
Time
1. Most voltage-gated sodium (Na+) channels are closed; most of the
voltage-gated potassium (K+) channels are also closed.
 When an action potential is generated
2. Voltage-gated Na+ channels open first and Na+ flows into the cell which
is called the depolariztion.
3. During the rising phase, the threshold is crossed, and the membrane
potential increases.
4. During the falling phase, voltage-gated Na+ channels become
inactivated; voltage-gated K+ channels open, and K+ flows out of the
cell.
5. During the undershoot, membrane permeability to K+ is at first higher
than at rest, then voltage-gated K+ channels close and resting potential
is restored.
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Key
Na
K
Action
potential
OUTSIDE OF CELL
Sodium
channel
3
0
50
2 Depolarization
4 Falling phase of the action potential
50
Membrane potential
(mV)
3 Rising phase of the action potential
Threshold
2
1
4
5
1
Resting potential
100
Time
Potassium
channel
Figure 48.11
INSIDE OF CELL
Inactivation loop
1 Resting state
5 Undershoot
Conduction of Action Potentials
• At the site where the action potential is generated, usually
the axon hillock, an electrical current depolarizes the
neighboring region of the axon membrane.
• Action potentials travel in only one direction: toward the
synaptic terminals.
• Inactivated Na+ channels behind the zone of depolarization
prevent the action potential from traveling backwards.
• The axons are insulated by a myelin sheath, which causes
an action potential’s speed to increase.
• Myelin sheaths are made by glia— oligodendrocytes in
the CNS and Schwann cells in the PNS.
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Conduction of Action Potentials
Figure 48.12
 An action potential is generated as
Na+ flows inward across the
membrane at one location.
Axon
Plasma
membrane  The depolarization of the action
Action
Action
potential
potential
11
Na
Na
K
K
22
Cytosol
Action
potential
Na
Na
potential spreads to the
neighboring region of the
membrane, reinitiating the action
potential there. To the left of this
region, the membrane is
repolarizing as K+ flows outward.
 The depolarization-repolarization
K
K
K
3
process is repeated in the next
region of the membrane. In this
way, local currents of ions across
the plasma membrane cause the
action potential to be propagated
along the length of the axon.
Action
potential
Na
K
Conduction of Action Potentials
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Axon
Myelin
sheath
Figure 48.14 Saltatory conduction
Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath
where voltage-gated Na+ channels are found.
Action potentials in myelinated axons jump between the nodes of Ranvier in a
process called saltatory conduction
國立交通大學生物科技學系 柯立偉老師
23
※ Neural Communication at Synapses ※
(CH48-4)
 Most synapses are chemical synapses. The presynaptic neuron
synthesizes and packages the neurotransmitter in synaptic vesicles
located in the synaptic terminal.
 The action potential causes the release of the neurotransmitter.
 The neurotransmitter diffuses across the synaptic cleft and is received
by the postsynaptic cell.
Presynaptic
cell
Postsynaptic cell
Figure 48.15
A chemical synapse.
1
Axon
Synaptic vesicle
Postsynaptic
containing
neurotransmitter membrane
Synaptic
cleft
Presynaptic
membrane
3
K
Ca2 2
Voltage-gated
Ca2 channel
Ligand-gated
ion channels
4
Na
Neural Communication at Synapses
 Direct synaptic transmission involves binding of neurotransmitters to
ligand-gated ion channels in the postsynaptic cell.
 Neurotransmitter binding causes ion channels to open, allows specific
ions to diffuse across the postsynaptic membrane and generating a
postsynaptic potential.
Presynaptic
cell
Postsynaptic cell
Figure 48.15
A chemical synapse.
1
Axon
Synaptic vesicle
Postsynaptic
containing
neurotransmitter membrane
Synaptic
cleft
Presynaptic
membrane
3
K
Ca2 2
Voltage-gated
Ca2 channel
國立交通大學生物科技學系 柯立偉老師
Ligand-gated
ion channels
4
Na
24
Generation of Postsynaptic Potentials
 Postsynaptic potentials fall into two categories
• Excitatory postsynaptic potentials (興奮性突觸後電位EPSPs) are
depolarizations that bring the membrane potential toward threshold.
• Inhibitory postsynaptic potentials (抑制性突觸後電位IPSPs) are
hyperpolarizations that move the membrane potential farther from
threshold
E1
E2
E1
E1
E2
Postsynaptic
neuron
Membrane potential (mV)
Figure 48.17
Terminal branch
of presynaptic
neuron
E1
E2
E2
Axon
hillock
I
I
I
I
0
Action
potential
Threshold of axon of
postsynaptic neuron
Resting
potential
Action
potential
70
E1
E1
(a) Subthreshold, no
summation
E1 E1
(b) Temporal summation
國立交通大學生物科技學系 柯立偉老師
E1  E2
(c) Spatial summation
E1
I
E1  I
(d) Spatial summation
of EPSP and IPSP
25