Chapter 12| The Nervous System

  1. Figure 12.3 Which of the following statements is false?
    1. The soma is the cell body of a nerve cell.
    2. Myelin sheath provides an insulating layer to the dendrites.
    3. Axons carry the signal from the soma to the target.
    4. Dendrites carry the signal to the soma.
      Illustration shows a neuron. The main part of the cell body, called the soma, contains the nucleus. Branch-like dendrites project from three sides of the soma. A long, thin axon projects from the fourth side. The axon branches at the end. The tip of the axon is in close proximity to dendrites of an adjacent nerve cell. The narrow space between the axon and dendrites is called the synapse. Cells called oligodendrocytes are located next to the axon. Projections from the oligodendrocytes wrap around the axon, forming a myelin sheath. The myelin sheath is not continuous, and gaps where the axon is exposed are called nodes of Ranvier.
      Figure 12.3 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.
  2. Figure 12.11 Potassium channel blockers, such as amiodarone and procainamide, which are used to treat abnormal electrical activity in the heart, called cardiac dysrhythmia, impede the movement of K+ through voltage-gated K+ channels. Which part of the action potential would you expect potassium channels to affect?
    Graph plots membrane potential in millivolts versus time. The membrane remains at the resting potential of negative 70 millivolts until a nerve impulse occurs in step 1. Some sodium channels open, and the potential begins to rapidly climb past the threshold of excitation of negative 55 millivolts, at which point all the sodium channels open. At the peak action potential, the potential begins to rapidly drop as potassium channels open and sodium channels close. As a result, the membrane repolarizes past the resting membrane potential and becomes hyperpolarized. The membrane potential then gradually returns to normal.
    Figure 12.11 The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. (5) The K+ channels close and the Na+/K+ transporter restores the resting potential.
  3. Figure 12.26 Which of the following statements is false?
    1. The parasympathetic pathway is responsible for relaxing the body, while the sympathetic pathway is responsible for preparing for an emergency.
    2. Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
    3. Slowing of the heartbeat is a parasympathetic response.
    4. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while sympathetic neurons are responsible for releasing acetylcholine.
      The autonomic nervous system is divided into sympathetic and parasympathetic systems. In the sympathetic system, the soma of the preganglionic neurons is usually located in the spine while in the parasympathetic system the soma is usually in the brainstem or sacral, at the bottom of the spine. In both systems, the preganglionic neuron releases the neurotransmitter acetylcholine into the synapse. Postganglionic neurons of the sympathetic system have somas in a sympathetic ganglion, located next to the spinal cord. Postganglionic neurons of the parasympathetic system have somas in ganglions near the target organ. Postganglionic neurons of the sympathetic system release norepinephrine into the synapse, in which the fight or flight response is activated; while postganglionic neurons of the parasympathetic system release acetylcholine or nitric oxide, in which the rest and digest response is activated.
      Figure 12.26 In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response.

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