414
UNIT 3
Regulation and Integration of the Body
11
at a given time. It appears that in most cases, different neuro-
transmitters are released at different stimulation frequencies,
a restriction that avoids producing a jumble of nonsense mes-
sages. However, co-release of two neurotransmitters from the
same vesicles has been documented. Te coexistence of more
than one neurotransmitter in a single neuron makes it possible
for that cell to exert several influences rather than one discrete
effect.
Neurotransmitters are classified chemically and function-
ally.
Table 11.3
provides a detailed overview of neurotransmit-
ters, and we describe some of them here. No one expects you to
memorize this table at this point, but it will be a handy reference
for you to look back at when you read about neurotransmitters
in subsequent chapters.
Classification of Neurotransmitters
by Chemical Structure
Neurotransmitters fall into several chemical classes based on
molecular structure.
Acetylcholine
Acetylcholine (ACh)
(as
0
ĕ-til-ko
9
lēn), the first neurotransmit-
ter identified, is still the best understood because it is released at
neuromuscular junctions, which are much easier to study than
synapses buried in the CNS.
ACh is synthesized from acetic acid (as acetyl CoA) and
choline by the enzyme
choline acetyltransferase
, then trans-
ported into synaptic vesicles for later release. Once released by
the presynaptic terminal, ACh binds briefly to the postsynaptic
receptors. Ten it is released and degraded to acetic acid and
choline by the enzyme
acetylcholinesterase (AChE)
, located in
the synaptic cleF and on postsynaptic membranes. Presynaptic
terminals recapture the released choline and reuse it to synthe-
size more ACh.
ACh is released by all neurons that stimulate skeletal muscles
and by some neurons of the autonomic nervous system. ACh-
releasing neurons are also found in the CNS.
Biogenic Amines
Te
biogenic amines
(bi
0
o-jen
9
ik) include the
catecholamines
(kat
0
ĕ-kol
9
ah-mēnz), such as dopamine, norepinephrine (NE),
and epinephrine, and the
indolamines
, which include sero-
tonin and histamine.
Dopamine
and
NE
are synthesized from
the amino acid tyrosine in a common pathway consisting of
several steps. Te epinephrine-releasing cells of the brain and
adrenal medulla use the same pathway.
Serotonin
is synthesized
from the amino acid tryptophan.
Histamine
is synthesized from
the amino acid histidine.
Biogenic amine neurotransmitters are broadly distributed in
the brain, where they play a role in emotional behavior and help
regulate the biological clock. Additionally, some motor neurons
of the autonomic nervous system release catecholamines, partic-
ularly NE. Imbalances of these neurotransmitters are associated
with mental illness. ±or example, overactive dopamine signal-
ing occurs in schizophrenia. Additionally, certain psychoactive
certain kinase enzymes that promote changes resulting in more
effective responses to subsequent stimuli.
In some neurons, APs generated at the axon hillock propa-
gate back up into the dendrites. Tis current flow may alter the
effectiveness of synapses by opening voltage-gated Ca
2
1
chan-
nels, again allowing Ca
2
1
into the dendrites and promoting syn-
aptic potentiation.
Synaptic potentiation can be viewed as a learning process that
increases the efficiency of neurotransmission along a particular
pathway. Indeed, the hippocampus of the brain, which plays a
special role in memory and learning, exhibits an important type
of synaptic plasticity called
long-term potentiation
(L²P).
Presynaptic Inhibition
Events at the presynaptic membrane
can also influence postsynaptic activity.
Presynaptic inhibition
occurs when the release of excitatory neurotransmitter by one
neuron is inhibited by the activity of another neuron via an axo-
axonal synapse. More than one mechanism is involved, but the
end result is that less neurotransmitter is released and bound,
forming smaller EPSPs.
Notice that this is the opposite of what we see with synaptic
potentiation. In contrast to postsynaptic inhibition by IPSPs,
which decreases the excitability of the postsynaptic neuron,
presynaptic inhibition decreases the excitatory stimulation of
the postsynaptic neuron. In this way, presynaptic inhibition is
like a functional synaptic “pruning.”
Check Your Understanding
17.
Which ions flow through chemically gated channels to
produce IPSPs? EPSPs?
18.
What is the difference between temporal summation and
spatial summation?
For answers, see Appendix H.
Neurotransmitters
and Their Receptors
Define neurotransmitter and name several classes of
neurotransmitters.
Neurotransmitters
, along with electrical signals, are the “lan-
guage” of the nervous system—the means by which each neu-
ron communicates with others to process and send messages
to the rest of the body. Sleep, thought, rage, hunger, memory,
movement, and even your smile reflect the “doings” of these
versatile molecules. Most factors that affect synaptic transmis-
sion do so by enhancing or inhibiting neurotransmitter release
or destruction, or by blocking their binding to receptors. Just as
speech defects may hinder interpersonal communication, any-
thing that interferes with neurotransmitter activity may short-
circuit the brain’s “conversations” or internal talk (see
A Closer
Look
on pp. 418–419).
At present, more than 50 neurotransmitters or neurotrans-
mitter candidates have been identified. Although some neu-
rons produce and release only one kind of neurotransmitter,
most make two or more and may release any one or all of them
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