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Chapter 4: Somatic Sensation: Spinal Mechanosensory Systems

A 25-year-old man was in an automobile accident and suffered a severe spinal cord injury. He had multiple somatic sensory and motor signs. Focusing only on mechanosensation, he had no sense of touch on his right leg and lower trunk, to the level of the umbilicus. Which of the following statements best describes the side and level of injury?

A. Right side of spinal cord at the 10th thoracic segment (T10)

B. Right, T4

C. Left, T10

D. Left, T4

Comment: The touch pathway in the spinal cord is uncrossed; it crosses in the caudal medulla. If the injury were at T4, then the sensory loss would have extended to the upper chest (see Figure 4–5).


FIGURE 4–5. The dermatomes of the body have a segmental organization. The inset illustrates dermatomal overlap. The brain and spinal cord are visible on the dorsal view (right). Note that the spinal cord ends at L1 segment. This is where cerebrospinal fluid can be withdrawn by lumbar tap (Figure 3–18B).


From which of the listed body regions does the gracile nucleus receive mechanoreceptive input?

A. Contralateral arm

B. Contralateral leg

C. Ipsilateral arm

D. Ipsilateral leg

The medial lemniscus—in the medulla, at the level where there is a fourth ventricle—receives its blood supply from which of the following arteries?

A. Posterior inferior cerebellar artery

B. Vertebral artery

C. Posterior spinal artery

D. Anterior spinal artery

Comment: At the most caudal level of the medulla, where there is little or no inferior olivary nucleus, the anterior spinal artery supplies blood to the medial lemniscus.

A physician tests vibration sense by touching a tuning fork to the body surface. Which of the following receptors mediates vibration sense?

A. Thermal receptor

B. Pacinian corpuscle

C. Ruffini’s corpuscle

D. Meissner’s corpuscle

Which of the following statements best describes the organization of dermatomes associated with adjacent dorsal roots?


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