Chapter sixteen



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CHAPTER SIXTEEN
Answers to WHAT DID YOU LEARN?

1. The cervical enlargement contains the neurons that innervate the upper limbs. The lumbar enlargement contains the neurons that innervate the lower limbs.



  1. There are 31 pairs of spinal nerves: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal (Co).

  2. The denticulate ligaments help suspend and anchor the spinal cord within the middle of the vertebral canal, thus preventing potential lateral displacement of the spinal cord.

4. The gray matter in the spinal cord is centrally located and shaped like an H or a butterfly. Bilaterally symmetrical right and left regions of gray matter are connected in the midline by a thin, horizontal bar of gray matter, the gray commissure, which surrounds a narrow central canal. Gray matter regions on both sides are artificially separated into three projections, called horns. The peripheral white matter is organized into funiculi. The anterior funiculi are interconnected by the white commissure.

5. The anterior horns primarily house the cell bodies of somatic motor neurons. The posterior horns contain the axons of sensory neurons and the cell bodies of interneurons.



6. The posterior, lateral, and anterior funiculi are in the white matter of the spinal cord.

7. A posterior root ganglion is attached to the posterior root and each posterior root ganglion is located medial to the pedicles of the adjacent vertebrae. The cell bodies of the sensory neurons in the posterior root are located in a posterior root ganglion.



8. From superior to inferior, the principal nerve plexuses are the cervical, brachial, lumbar, and sacral plexuses.

9. The main nerves of the lumbar plexus are the femoral nerve and the obturator nerve. The main nerve of the sacral plexus is the sciatic nerve, which divides into the tibial and common fibular nerves.

10. The five steps in a reflex arc are: (1) a stimulus actives a receptor; (2) a nerve impulse travels through a sensory neuron to the CNS; (3) interneurons integrate and process information; (4) a motor neuron transmits the nerve impulse to an effector; and (5) the effector responds to the nerve impulse.

11. In a monosynaptic reflex, the afferent sensory axons synapse directly on the motor neurons, whose axons project to the effector. Interneurons do not function in this type of reflex. More complex neural pathways are observed in polysynaptic reflexes, which have a number of synapses involving interneurons within the reflex arc.

12. An example of a withdrawal reflex is quickly pulling the arm away after touching something hot.

13. The components of the cranial and spinal nerves form from neural crest cells.



14. The alar plates develop into the posterior horns of the gray matter as well as the posterior part of the gray commissure.


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