Page 455 - Fundamentals of anatomy physiology
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442 Unit 3 Control and Regulation
the greater the number of open cation channels in the up with the demand for neurotransmitter. Synaptic fatigue
postsynaptic membrane, and so the larger the depolariza- then occurs, and the synapse weakens until ACh has been
tion. If the depolarization brings an adjacent area of excit- replenished.
able membrane (such as the initial segment of an axon) to
threshold, an action potential appears in the postsynaptic Checkpoint
neuron. 1 9. Describe the general structure of a synapse.
2 0. If a synapse involves direct physical contact between
4 ACh Is Removed by AChE. The neurotransmitter’s effects
on the postsynaptic membrane are temporary, because cells, it is termed _____; if the synapse involves a
the synaptic cleft and the postsynaptic membrane con- neurotransmitter, it is termed _____.
tain the enzyme acetylcholinesterase (AChE, or cholinester- 21. What effect would blocking voltage-gated calcium ion
ase). Roughly half of the ACh released at the presynaptic channels at a cholinergic synapse have on synaptic
membrane is broken down before it reaches receptors on communication?
the postsynaptic membrane. ACh molecules that bind to 22. One pathway in the central nervous system consists of
receptor sites are generally broken down within 20 msec of three neurons, another of five neurons. If the neurons
their arrival. in the two pathways are identical, what process will
determine which pathway will transmit impulses more
12 AChE breaks down molecules of ACh (by hydrolysis) into rapidly?
acetate and choline. The choline is actively absorbed by the
axon terminal and is used to synthesize more ACh, using ac- See the blue Answers tab at the back of the book.
etate provided by coenzyme A (CoA). (Recall from Chapter 2
that coenzymes derived from vitamins are required in many 12-8 Neurotransmitters and
enzymatic reactions. p. 81) Acetate diffusing away from the
synapse can be absorbed and metabolized by the postsynaptic neuromodulators have various
cell or by other cells and tissues. functions
Synaptic Delay Learning Outcome Describe the major types of neurotransmitters and
neuromodulators, and discuss their effects on postsynaptic membranes.
A synaptic delay of 0.2–0.5 msec occurs between the arrival
of the action potential at the axon terminal and the effect on Now that we have examined the actions of acetylcholine at
the postsynaptic membrane. Most of that delay reflects the cholinergic synapses, let’s consider the actions of other neu-
time involved in calcium influx and neurotransmitter release, rotransmitters, and of neuromodulators, which change the
not in the neurotransmitter’s diffusion—the synaptic cleft is cell’s response to neurotransmitters.
narrow, and neurotransmitters can diffuse across it in very
little time. The Activities of Other Neurotransmitters
Although a delay of 0.5 msec is not very long, in that time The nervous system relies on a complex form of chemical
an action potential may travel more than 7 cm (about 3 in.) communication. Each neuron is continuously exposed to a
along a myelinated axon. When information is being passed variety of neurotransmitters. Some usually have excitatory
along a chain of interneurons in the CNS, the cumulative syn- effects, while others usually have inhibitory effects. Yet in all
aptic delay may exceed the propagation time along the axons. cases, the effects depend on the nature of the receptor rather
This is why reflexes are important for survival—they involve than the structure of the neurotransmitter. (Many drugs
only a few synapses and thus provide rapid and automatic affect the nervous system by stimulating receptors that oth-
responses to stimuli. The fewer synapses involved, the shorter erwise respond only to neurotransmitters. These drugs can
the total synaptic delay and the faster the response. The fast- have complex effects on perception, motor control, and emo-
est reflexes have just one synapse, with a sensory neuron di- tional states.)
rectly controlling a motor neuron. The muscle spindle reflexes,
discussed in Chapter 13, are arranged in this way. Major categories of neurotransmitters include biogenic
amines, amino acids, neuropeptides, dissolved gases, and a variety
Synaptic Fatigue of other compounds. Here we introduce only a few of the most
important neurotransmitters. You will encounter additional
Because ACh molecules are recycled, the axon terminal is not examples in later chapters.
totally dependent on the ACh synthesized in the cell body and
delivered by axoplasmic transport. But under intensive stimu- Norepinephrine (nor-ep-i-NEF-rin), or NE, is a neu-
lation, resynthesis and transport mechanisms may not keep
rotransmitter that is widely distributed in the brain and
