Human Physiology, 14th edition (2016)

190 Chapter 7

with which action potentials will be produced at the axon ini-

tial segment and at each point in the axon where the impulse

is regenerated. The action potentials that begin at the initial

segment of the axon are conducted without loss of amplitude

toward the axon terminals.

Earlier in this chapter, the action potential was introduced

by describing the events that occurred when a depolarization

stimulus was artificially produced by stimulating electrodes.

Now it is apparent that EPSPs, conducted from the dendrites

and cell body, serve as the normal stimuli for the production

of action potentials at the axon hillock, and that the action

potentials at this point serve as the depolarization stimuli

for the next region, and so on. This chain of events ends at

the terminal boutons of the axon, where neurotransmitter is

released.

Acetylcholine in the PNS

Somatic motor neurons form synapses with skeletal muscle
cells (muscle fibers). At these synapses, or neuromuscular
junctions, the postsynaptic membrane of the muscle fiber is
known as a motor end plate. Therefore, the EPSPs produced
by ACh in skeletal muscle fibers are often called end-plate
potentials. This depolarization opens voltage-regulated chan-
nels that are adjacent to the end plate. Voltage-regulated chan-
nels produce action potentials in the muscle fiber, and these are
reproduced by other voltage-regulated channels along the mus-
cle plasma membrane. This conduction is analogous to con-
duction of action potentials by axons; it is significant because
action potentials produced by muscle fibers stimulate muscle
contraction (chapter 12, section 12.2).
If any stage in the process of neuromuscular transmission
is blocked, muscle weakness—sometimes leading to paralysis
and death—may result. The drug curare, for example, com-
petes with ACh for attachment to the nicotinic ACh recep-
tors and thus reduces the size of the end-plate potentials (see
table 7.5 ). This drug was first used on blow-gun darts by South
American Indians because it produced flaccid paralysis in their
victims. Clinically, curare is used in surgery as a muscle relax-
ant and in electroconvulsive shock therapy to prevent muscle
damage.
Autonomic motor neurons innervate cardiac muscle,
smooth muscles in blood vessels and visceral organs, and
glands. There are two classifications of autonomic nerves:
sympathetic and parasympathetic. Most of the parasympa-
thetic axons that innervate the effector organs use ACh as
their neurotransmitter. In some cases, these axons have an
inhibitory effect on the organs they innervate through the
binding of ACh to muscarinic ACh receptors. The action of
the vagus nerve in slowing the heart rate is an example of this
inhibitory effect. In other cases, ACh released by autonomic
neurons produces stimulatory effects as previously described.
The structures and functions of the autonomic system are
described in chapter 9.

Acetylcholine in the CNS

There are many cholinergic neurons (those that use ACh as
a neurotransmitter) in the CNS, where the axon terminals of
one neuron typically synapse with the dendrites or cell body
of another. The dendrites and cell body thus serve as the recep-
tive area of the neuron, and it is in these regions that receptor
proteins for neurotransmitters and chemically regulated gated
channels are located. Graded, local EPSPs and IPSPs spread
into the initial segment of the axon located adjacent to the axon
hillock (see fig. 7.24 ).
If the depolarization is at or above threshold by the time it
reaches the initial segment of the axon, the EPSP will stimulate
the production of action potentials, which can then regener-
ate themselves along the axon. If, however, the EPSP is below
threshold at the initial segment no action potentials will be
produced in the postsynaptic cell ( fig. 7.29 ). Gradations in the
strength of the EPSP above threshold determine the frequency

Figure 7.29 The graded nature of excitatory

postsynaptic potentials (EPSPs). Stimuli of increasing

strength produce increasing amounts of depolarization. When a

threshold level of depolarization is produced, action potentials

are generated in the axon.

Cell bodies

and dendrites Axon

Action potential

rmp

Relative amounts of

excitatory neurotransmitter

Time

–70

–50

+30

mV

Threshold

EPSPs

Membranepotential

| CHECKPOINT

8. Explain how ligand-gated channels are opened,
   using nicotinic ACh receptors as an example.
   9a. Explain how ligand-gated channels operate, using
   muscarinic ACh receptors as an example.
   9b. Describe where stimulatory and inhibitory effects
   of muscarinic ACh receptors occur and how these
   effects are produced.


9. Describe the function of acetylcholinesterase and
   discuss its physiological significance.


10. Compare the properties of EPSPs and action
    potentials, identify where in a neuron these are
    produced, and explain how EPSPs stimulate action
    potentials.