TLW 7e Chapter 28 Answers
The Nervous System
Apply Your Understanding
1. Figure 28.5a
At the synapse, a mutation in the structure of which component would affect only
whether or not a signal is transmitted, not how strongly? Explain.
Proteins in the presynaptic cell. If the neurotransmitters cannot be released by the
presynaptic cell then the impulse cannot cross the synapse and carry the impulse across
the synapse.
2. Key Biological Process: Drug Addiction (Page 593)
Describe the feelings that might be experienced by someone who goes through the four
stages related to drug addiction that are shown in the drawings below.
In panel 1, the person feels “normal.” They experience their usual levels of happiness
and stability.
In panel 2, the individual has used a drug that increases the level of neurotransmitter in
the synapse; the receptors are firing more rapidly, and they experience feelings of
elation, extreme happiness, and no sensation of being tired.
Panel 3 shows what happens as the body acclimates to the presence of excess
neurotransmitter molecules by having fewer receptors present on the neurons. The same
amount of a drug, such as cocaine, no longer gives the same happy feeling; a larger
amount must be taken to ensure the presence of even more neurotransmitters, and it must
be taken more frequently. This is physiological “addiction.”
If the person is not taking the drug, as in panel 4, then the extremely low level of
neurotransmitters that are available causes a very low level of stimulation. The person
usually feels tired, depressed, and anxious. These feelings can only be alleviated by
taking more of the drug or by going through withdrawals.
Synthesize What You Have Learned
1. When an investigator stimulates an axon by touching it with an electrode in the
middle, action potentials are generated in both directions. If instead the investigator
stimulates the axon where it meets the cell body, the action potential only goes outward
toward the axon terminals and not inward over the cell body. Can you explain what might
be going on at the junction of the axon to the cell body that would prevent further passage
of the action potential?
Axons are output channels. Nerve impulses travel outward along them, away from the
cell body, toward other neurons or to muscles or glands.
2. Botox, a derivative of the botulinum toxin that can cause fatal food poisoning, acts by
inhibiting the release of acetylcholine at the neuromuscular junction. How could treating
someone’s face with such a toxin produce desired cosmetic effects?
Botox works by muscle denervation. If the nerve to the muscle does not receive a signal
to contract then the wrinkle can’t occur.
3. Cyanide is a deadly poison that halts cellular respiration by inhibiting the
mitochondrial enzyme cytochrome c oxidase. Chronic exposure to low levels of cyanide
by those who use cassava roots as their primary food source in tropical Africa can
eventually lead to permanent paralysis, as cyanide also disables the sodium potassium
pump. Explain how this inhibition might lead to paralysis.
If the sodium-potassium pump is disabled or destroyed then an action potential cannot be
generated nor perpetuated down the axon resulting in the inability for the muscles to
move which is paralysis.