Human Physiology, 14th edition (2016)

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.

7. 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:

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


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