Page 453 - Fundamentals of anatomy physiology
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440 Unit 3 Control and Regulation
Electrical synapses are extremely rare in both the CNS and junctions involving skeletal muscle fibers, (2) at many synapses
PNS. They occur in some areas of the brain, including the ves- in the CNS, (3) at all neuron-to-neuron synapses in the PNS,
tibular nuclei, the eye, and in at least one pair of PNS ganglia and (4) at all neuromuscular and neuroglandular junctions in
(the ciliary ganglia). the parasympathetic division of the ANS.
12 Chemical Synapses At a cholinergic synapse between two neurons, the presyn-
aptic and postsynaptic membranes are separated by a synaptic
The situation at a chemical synapse is far more variable than cleft that averages 20 nm (0.02 μm) in width. Most of the ACh
that at an electrical synapse, because the cells are not directly in the axon terminal is packaged in synaptic vesicles, each con-
coupled. For example, an action potential that reaches an elec- taining several thousand molecules of the neurotransmitter.
trical synapse is always propagated to the next cell. But at a A single axon terminal may contain a million such vesicles.
chemical synapse, an arriving action potential may or may not
release enough neurotransmitter to bring the postsynaptic neu- &T i p s T r i c k s
ron to threshold. In addition, other factors may intervene and Cholinergic synapses are so named because the neuro
make the postsynaptic cell more or less sensitive to arriving transmitter involved is acetylcholine.
stimuli. In essence, the postsynaptic cell at a chemical synapse
is not a slave to the presynaptic neuron, and its activity can be Events at a Cholinergic Synapse
adjusted, or “tuned,” by a variety of factors.
Figure 12–16 diagrams the events that take place at a cholinergic
Chemical synapses are by far the most abundant type of syn- synapse between neurons after an action potential arrives at an
apse. Most synapses between neurons, and all communications axon terminal. For convenience, we will assume that this syn-
between neurons and other types of cells, involve chemical syn- apse is adjacent to the initial segment of the axon, an arrange-
apses. Normally, communication across a chemical synapse takes ment that is easy to illustrate.
place in only one direction: from the presynaptic membrane to
the postsynaptic membrane. 1 An Action Potential Arrives and Depolarizes the Axon Terminal.
The normal stimulus for neurotransmitter release is the de-
Acetylcholine (ACh) is the neurotransmitter that has polarization of the axon terminal by the arrival of an action
received most of our attention so far, but there are other potential.
important chemical transmitters. Based on their effects on post-
synaptic membranes, neurotransmitters are often classified as 2 Extracellular Calcium Ions Enter the Axon Terminal, Trigger-
excitatory or inhibitory. Excitatory neurotransmitters cause ing the Exocytosis of ACh. The depolarization of the axon
depolarization and promote the generation of action poten- terminal briefly opens its voltage-gated calcium channels,
tials. Inhibitory neurotransmitters cause hyperpolarization allowing calcium ions to rush in. Their arrival triggers exo-
and suppress the generation of action potentials. cytosis of ACh into the synaptic cleft. The ACh is released
in packets of roughly 3000 molecules, the average number
This classification is useful, but not always precise. of ACh molecules in a single vesicle. ACh release stops
For example, acetylcholine typically produces a depolar- very soon, because active transport mechanisms rapidly
ization in the postsynaptic membrane, but acetylcholine remove the calcium ions from the terminal cytosol. These
released at neuromuscular junctions in the heart has an ions are either pumped out of the cell or transferred into
inhibitory effect, producing a transient hyperpolarization mitochondria.
of the postsynaptic membrane. This situation highlights
an important aspect of neurotransmitter function: The ef- 3 ACh Binds to Receptors and Depolarizes the Postsynaptic
fect of a neurotransmitter on the postsynaptic membrane de- Membrane. ACh diffuses across the synaptic cleft toward
pends on the properties of the receptor, not on the nature of the receptors on the postsynaptic membrane. These ACh re-
neurotransmitter. ceptors are chemically gated ion channels. The primary
response is an increased permeability to Na+, producing
Let’s continue our discussion of chemical synapses with a depolarization in the postsynaptic membrane that lasts
a look at a synapse that releases the neurotransmitter ace- about 20 msec. (These cation channels also let potas-
tylcholine (ACh). Then we will introduce other important sium ions out of the cell, but because sodium ions are
neurotransmitters that you will encounter in later chapters. driven by a much stronger electrochemical gradient, the
net effect is a slight depolarization of the postsynaptic
Cholinergic Synapses membrane.)
Synapses that release ACh are known as cholinergic synapses. This depolarization is a graded potential. The greater
The neuromuscular junction is an example of a cholinergic the amount of ACh released at the presynaptic membrane,
synapse. p. 326 ACh is the most widespread (and best-
studied) neurotransmitter. It is released (1) at all neuromuscular
