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Chapter 15: Somatosensory System

A 49-year-old woman complains about loss of accuracy in identifying and localizing sensation (in particular, conscious proprioception, tactile sensation, and pressure) from the appropriate sites along her right hand and leg. A magnetic resonance imaging (MRI) scan indicates the presence of a tumor in the region of the right dorsal columns at the level of the spinal cord–medulla border. The presence of the tumor could account for both loss of sensation and loss of accuracy in identifying the sites on the limbs associated with the respective sensation because of a general property of inhibition associated with dorsal column functions. Which one or more of the following types of inhibition have been identified within the dorsal column nuclei that are disrupted by the tumor?

A. Feedforward inhibition using local interneurons only

B. Feedback inhibition using local interneurons only

C. Descending inhibition from fibers arising in the cerebral cortex only

D. Feedforward, feedback, and descending inhibition

E. Feedforward and descending inhibition only

D. To generate an excitatory focus with an inhibitory surround, 3 types of inhibition are present in the dorsal column nuclei. First-order neurons ascending in the dorsal columns make synaptic contact with different cells in the dorsal column nuclei and excite those cells. One such cell may be an inhibitory interneuron that makes synaptic contact with a neighboring dorsal column nuclear cell, thus inhibiting that cell (ie, feedforward inhibition). In addition, the dorsal column cell that is excited by the first-order neuron may make synaptic contact with another inhibitory interneuron (in addition to its classical ascending projection to the ventral posterolateral nucleus of the thalamus). This inhibitory interneuron makes synaptic contact with an adjacent dorsal column cell and inhibits that cell (ie, feedback inhibition). Finally, a descending fiber from the postcentral gyrus can make synaptic contact with inhibitory interneurons that inhibit dorsal column cells (descending inhibition).

A 57-year-old man is referred to a neurologist after he complains about difficulties in determining the directionality and orientation of movement of stimuli along his right arm. An MRI is taken of the patient, and a central nervous system (CNS) tumor is noted. Where in the regions listed below is the tumor most likely to be present?

A. Left spinal cord

B. Medial half of thalamus

C. Ventral pons

D. Internal capsule

E. Cerebral cortex

E. As a general rule, neurons that are situated in the cortex in association with any of ...

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