50. The postsynaptic response at a chemical synapse depends on both the neurotransmitter released from the presynaptic neuron and the receptor present on the postsynaptic membrane. Which of the following statements about neurotransmitters is NOT correct? (A) The magnitude of synaptic transmission can be influenced by the amount of neurotransmitter released. (B) Released neurotransmitter molecules can be taken back up into the presynaptic neuron. (C) A single neurotransmitter can activate both ionotropic and metabotropic receptors, producing distinct physiological effects. (D) In the adult human brain, the amino acid gamma-aminobutyric acid (GABA) is a neurotransmitter that typically depolarizes the postsynaptic neuron, making it more likely to fire an action potential. (E) Certain gaseous molecules, such as nitric oxide (NO), can act as retrograde neurotransmitters and are not stored in synaptic vesicles before release.

13. Suppose a person begins breathing very rapidly while resting (hyperventilating), which of the following best describes how homeostasis is restored? (A) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The frontal cortex detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (B) Rapid breathing increases blood CO2 levels, thereby causing the blood pH to drop. The hypothalamus detects the change and decreases stimulation of the rib muscles and the diaphragm, thereby reducing the breathing rate. (C) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The hypothalamus detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (D) Rapid breathing increases blood CO2 levels, thereby causing the blood pH to drop. The medulla oblongata detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (E) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The medulla oblongata detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate.

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