Essentials of Anatomy and Physiology

One important consequence of the presence of
synapses is that they ensure one-way transmission of
impulses in a living person. A nerve impulse cannot go
backward across a synapse because there is no neuro-
transmitter released by the dendrites or cell body.
Neurotransmitters can be released only by a neuron’s
axon, which does not have receptor sites for it, as does
the postsynaptic membrane. Keep this in mind when
we discuss the types of neurons later in the chapter.
An example of a neurotransmitter is acetylcholine,
which is found at neuromuscular junctions, in the
CNS, and in much of the peripheral nervous system.
Acetylcholine usually makes a postsynaptic membrane
more permeable to Na ions, which brings about
depolarization of the postsynaptic neuron. Cholin-

esteraseis the inactivator of acetylcholine. There are

many other neurotransmitters, especially in the cen-

tral nervous system. These include dopamine, GABA,

norepinephrine, glutamate, and serotonin. Each of

these neurotransmitters has its own chemical inactiva-

tor. Some neurotransmitters are reabsorbed into the

neurons that secreted them; this process is called

reuptakeand also terminates the effect of the trans-

mitter.

The complexity and variety of synapses make them

frequent targets of medications. For example, drugs

that alter mood or behavior often act on specific neu-

rotransmitters in the brain, and antihypertensive

drugs affect synapse transmission at the smooth mus-

cle of blood vessels.

The Nervous System 169

Na+

Na+

Na+

Axon of presynaptic

neuron

Vesicles of neurotransmitter Receptor site

Inactivator

(cholinesterase)

Dendrite of

postsynaptic

neuron

Inactivated

neurotransmitter

Neurotransmitter

(acetylcholine)

Mitochondrion

Figure 8–2. Impulse transmission at a synapse. The arrow indicates the direction of the

electrical impulse.

QUESTION:Is this an excitatory synapse or an inhibitory synapse? Explain your answer.