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| Inquiry Questions None for this chapter. Self Test 1). _________________ nerves carry impulses away from the central nervous system. a). Sensory b). Motor c). Afferent d). Association Answer: b 2). Which of the following best describes the electrical state of a neuron at rest? a). The inside of a neuron is more negatively charged than the outside. b). The outside of a neuron is more negatively charged than the inside. c). The inside and the outside of a neuron have the same electrical charge. d). K+ ions leak into a neuron at rest. Answer: a 3). A nerve impulse is initiated when a). physical disruption of the cell membrane causes some of its contents, including ions, to leak out. b). the Schwann cells move to into their new positions. c). voltage-gated channels close. d). a reversal in the polarized state of the cell causes it to reach threshold. Answer: d 4). An action potential travels from one neuron to the next across the synapse using a). calcium ions. b). Schwann cells. c). neurotransmitters. d). All of these are involved. Answer: c 5). Synapses are excitatory or inhibitory based on a). integration. b). summation. c). autonomic control. d). saltatory conduction. Answer: a 6). Modern-day fish are like early vertebrates in that the dominate part of their brains is the a). forebrain. b). midbrain. c). hindbrain. d). optic lobes. Answer: c 7). Much of the sensory, motor, and associative neural activity of the cerebrum occurs on the surface, a layer called the a). corpus callosum. b). cerebral cortex. c). limbic system. d). basal ganglia. Answer: b 8). Which is not involved in the knee-jerk reflex? a). stretching of the muscle b). motor neuron c). muscle spindle d). an interneuron Answer: d 9). The _____________ cannot be controlled by conscious thought. a). motor neurons b). somatic nervous system c). autonomic nervous system d). skeletal muscles Answer: c 10). A fight-or-flight response in the body is controlled by the a). sympathetic division of the nervous system. b). parasympathetic division of the nervous system. c). release of ACh from postganglionic neurons. d). somatic nervous system. Answer: a Test Your Visual Understanding 1). Match the following descriptions with the appropriate numbered steps on the figure. a). Na+ channels open, and Na+ flows into the cell, causing rapid depolarization. b). K+ channels open, and K+ flows out of the cell, causing rapid repolarization. c). Na+ channels are closed, and some K+ leaks out to maintain a resting potential. d). K+ continues to flow out of cell, producing a hyperpolarization of the membrane. e). Na+ channels close, and K+ channels begin to open. f). A stimulus causes a depolarization of the membrane. Answer: Step 1—c Step 2—f Step 3—a Step 4—e Step 5—b Step 6—d Apply Your Knowledge 1). The equilibrium potentials for particular ions are determined by their concentrations inside and outside of the cell and are calculated using the Nernst equation: V = 58 mV x log10 Co/Ci where Co is the concentration of the ion outside the cell and Ci is the concentration of the ion inside the cell. Using this equation, calculate the equilibrium potential across the plasma membrane assuming that it is due solely to (a) K+ and (b) Na+. The concentrations for K+ are 150 mM inside and 5 mM outside. The concentrations for Na+ are 15 mM inside and 150 mM outside. Answer: a). The equilibrium potential for K+ is V = 58 mV x log10 5 mM/150 mM or 58 x (-1.477) = –85.67 mV. b). The equilibrium potential for Na+ is V = 58 mV x log10 150 mM/15 mM or 58 x 1 or 58 mV. 2). Tetraethylammonium (TEA) is a drug that blocks voltage-gated K+ channels. What effect would TEA have on the action potentials produced by a neuron? If TEA could be applied selectively to a presynaptic neuron that releases an excitatory neurotransmitter, how would it alter the synaptic effect of that neurotransmitter on the postsynaptic cell? Answer: TEA blocks K+ channels so that they will not permit the passage of K+ out of the cell, thereby not allowing the cell to return to the resting potential. Voltage-gated Na+ channels would still be functional and Na+ would still flow into the cell but there would be no repolarization. Na+ would continue to flow into the cell until an electrochemical equilibrium was reached for Na+, which is + 60 mV. After the membrane potential reached + 60 mV, there would be no net movement of sodium but the membrane would also not be able to repolarize back to the resting membrane potential. The neuron would no longer be able to function. The effects on the postsynaptic cell would be somewhat similar if TEA were applied to the presynaptic cell. The presynaptic cell would depolarize and would continue to release neurotransmitter until it had exhausted its store of synaptic vesicles. As a result, the postsynaptic cell would be bombarded with neurotransmitters and would be stimulated continuously until the stores of presynaptic neurotransmitter were depleted. The postsynaptic cell however would recover, being able to repolarize it membrane and return to the resting membrane potential. |