Human Physiology, 14th edition (2016)

(Tina Sui) #1

184 Chapter 7


7.4 Acetylcholine as a Neurotransmitter


When acetylcholine (ACh) binds to its receptor, it directly or
indirectly causes the opening of chemically regulated gates.
In many cases, this produces a depolarization called an
excitatory postsynaptic potential, or EPSP. In some cases,
however, ACh causes a hyperpolarization known as an
inhibitory postsynaptic potential, or IPSP.

Figure 7.24 The functional specialization of
different regions in a multipolar neuron. Integration of input
(EPSPs and IPSPs) generally occurs in the dendrites and cell
body, with the axon serving to conduct action potentials.


Integration

Impulse
conduction

Synaptic potentials
(EPSPs and IPSPs)

Action potentials
initiated

Neurotransmitter
release

Axon initial segment

Axon hillock

Node of Ranvier

Myelin sheath

Axon

Dendrites

Presynaptic
axon

Figure 7.25 Events in excitatory synaptic
transmission. The different regions of the postsynaptic neuron
are specialized, with ligand-(chemically) gated channels located
in the dendrites and cell body, and voltage-gated channels
located in the axon.

Presynaptic
neuron

Action potentials
conducted by axon

Opens voltage-gated
Ca2+ channels

Release of excitatory
neurotransmitter

Axon
terminals

Postsynaptic
neuron

Opens chemically (ligand)
gated channels

Inward diffusion of Na+
causes depolarization (EPSP)

Localized, decremental
conduction of EPSP

Opens voltage-gated Na+
and then K+ channels

Axon initial
segment

Axon

Dendrites and
cell bodies

Conduction of action potential

| CHECKPOINTS

6a. Describe the structure, locations, and functions of
gap junctions.
6b. Describe the location of neurotransmitters within
an axon and explain the relationship between
presynaptic axon activity and the amount of
neurotransmitters released.
6c. Describe the sequence of events by which action
potentials stimulate the release of neurotransmitters
from presynaptic axons.


  1. Explain how chemically regulated channels differ
    from voltage-regulated channels and the nature of
    excitatory and inhibitory postsynaptic potentials.


LEARNING OUTCOMES

After studying this section, you should be able to:


  1. Explain how ligand-gated channels produce synaptic
    potentials, using the nicotinic ACh receptor as an
    example.

  2. Explain how G-protein-coupled channels produce
    synaptic potentials, using the muscarinic ACh
    receptor as an example.

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