1. Dopamine is an inhibitory neurotransmitter, found especially in the basal nuclei, that suppresses unwanted muscle contractions and contributes to smoother, more coordinated muscle action. Parkinson disease involves a dopamine deficiency. Chlorpromazine, used to treat schizophrenia, blocks dopamine receptors, thus preventing dopamine from acting and creating the same effects as a dopamine deficiency.

2. Such a change in the K⁺ gradient across the plasma membrane would cause more K⁺ to diffuse into a neuron from the extracellular fluid. This would make the cytoplasmic side of the membrane more positively charged than normal, shifting the membrane potential closer to threshold and making the neuron more excitable.

3. One phase of saltatory conduction occurs at each node of Ranvier, where voltage-regulated ion gates open, Na⁺ enters the cell, and the membrane is depolarized. Although this process is relatively slow, it boosts the nerve signal back to its original strength by letting new Na⁺ into the cell at each node. This ensures that the nerve signal is just as strong at the distal end of an axon as when it began in the trigger zone, so signal transmission is nondecremental (it does not get weaker with distance). The other phase of saltatory conduction occurs in the internodes, where Na⁺ diffuses along the intracellular side of the plasma membrane from one node to the next. This is very fast, but it is decremental (it gets weaker with distance as the Na⁺ is diluted). Saltatory conduction takes advantage of the speed of transmission that occurs along the internodes and the ability of each node to restore the nerve signal to its original strength.

4. One reason is that only the presynaptic nerve fiber releases neurotransmitters. Another is that only the postsynaptic cell has receptors that allow for a response to them. A postsynaptic dendrite or soma cannot release a neurotransmitter because it has no synaptic vesicles, and a presynaptic fiber cannot produce a voltage or metabolic response to the neurotransmitter for lack of receptors. If signals did travel freely in both directions across synapses, the nervous system would be unable to route signals to its target cells very selectively, and physiological and behavioral responses would be less coordinated.

5. The production of an action potential involves an inflow of Na⁺ into a neuron. If an anesthetic blocks Na⁺ gates, no nerve signals can be produced and no pain signals transmitted to the CNS.

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