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