C h a p t e r 3 4
Organization and Control of Neural Function
829
1
2
3
Postsynaptic
receptor
Ion channel
Na +
Reuptake
Diffusion
Metabolite
Part of degraded
neurotransmitter
Neurotransmitter Removal.
Precise control of synaptic function
relies on the rapid removal of the
neurotransmitter from the receptor
site. A released neurotransmitter can
(1) be taken back up into the neu-
ron through reuptake, (2) diffuse
out of the synaptic cleft, or (3) be
broken down by enzymes into inac-
tive substances or metabolites. The
action of norepinephrine is largely
terminated by the reuptake process,
in which the neurotransmitter is
taken back into the neuron in an
unchanged form and reused. It can
also be broken down by enzymes
in the synaptic cleft or in the nerve
terminals. The neurotransmitter ace-
tylcholine is rapidly broken down by
the enzyme acetylcholinesterase.
3
receptors, but bring about long-term changes that
subtly enhance or depress the action of the receptors.
Neuromodulators, such as dopamine, serotonin, acetyl-
choline, histamine, and others, may act at either presyn-
aptic or postsynaptic sites. They may act on postsynaptic
receptors to produce slower and longer-lasting changes
in membrane excitability, by enhancing or decreasing
the action of faster-acting neurotransmitter molecules.
By combining with autoreceptors on its own presynaptic
membrane, a transmitter can act as a neuromodulator by
augmenting or inhibiting further nerve activity. In some
nerves, such as the peripheral sympathetic nerves, a mes-
senger molecule can have both transmitter and modula-
tor functions. For example, norepinephrine can activate
an
α
1
-adrenergic postsynaptic receptor to produce vaso-
constriction or stimulate an
α
2
-adrenergic presynaptic
receptor to inhibit further norepinephrine release.
Neurotrophic Factors.
Neurotrophic factors, also
known as
neurotrophins
, are a family of polypeptide
growth factors that influence the proliferation, differen-
tiation, and survival of neuronal and nonneuronal cells.
Neurotrophins are secreted by axon terminals indepen-
dent of action potentials. Examples include neuron-to-
neuron trophic factors in the sequential synapses of CNS
sensory neurons. Trophic factors from target cells that
enter the axon and are necessary for the long-term sur-
vival of presynaptic neurons also have been demonstrated.
Alterations in neurotrophin levels have been implicated
in neurodegenerative disorders such as Alzheimer disease
and Huntington disease, as well as psychiatric disorders
such as depression and substance abuse.
SUMMARY CONCEPTS
■■
Neurons communicate with other neurons and
body cells through electrical signals in their
membrane called action potentials. Action
potentials are divided into three parts: (1) the
resting membrane potential, during which the
membrane is polarized (positively charged on
the outside of the membrane and negatively
charged on the inside); (2) the depolarization
phase, during which sodium channels open,
allowing rapid inflow of the charged sodium ions
that generate the electrical impulse; and (3) the
repolarization phase, during which the outflow
of potassium ions returns the membrane to its
resting potential.
■■
Neurotransmission or communication relies
on chemical messengers or neurotransmitters,
released from the presynaptic neuron, that cross
the synaptic cleft and then interact with receptors
on the postsynaptic neuron.
(continued)
(
text continued from page 827
)
