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.