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Chapter 16: Pyramidal Motor System

A 74-year-old woman is brought to the hospital after she suffered a stroke. Several days later, a neurologic examination reveals that she is unable to perform certain types of learned, complex movements (referred to as apraxia). Which of the following regions of the cerebral cortex is most likely affected by the stroke?

A. Precentral gyrus

B. Postcentral gyrus

C. Premotor cortex (PMC)

D. Prefrontal cortex

E. Cingulate gyrus

C. The premotor areas play an important role in the programming or sequencing of responses that compose complex, learned movements. They receive significant inputs for this process from the posterior parietal lobule and, in turn, signal appropriate neurons in the brainstem and spinal cord (both flexors and extensors). Lesions of the postcentral gyrus produce a somatosensory loss. Lesions of the precentral gyrus produce paralysis. Neither lesions of the prefrontal cortex nor those of the cingulate gyrus have been reported to produce apraxia.

A patient with motor disturbances presents with the following signs and symptoms: The patient is unable to maintain a posture when the eyes are closed and the arms outstretched and is unable to exert a steady contraction; the speed of tapping of the hand is diminished; and manipulation of small objects is reduced and so are the exploratory movements. Based on these observations, a lesion of which of the following sites could account for these deficits?

A. Medial aspect of prefrontal cortex

B. Primary region of visual cortex

C. Posterior aspect of middle temporal cortex

D. Medial half of postcentral cortex

E. Far lateral aspect of precentral cortex

D. Although the postcentral gyrus is referred to as “somatosensory” cortex, it contributes many fibers to the corticospinal tract and plays an important role in motor behavior. In essence, it is the sensory feedback that is essential for motor behavior to occur. Without such sensory information, the sequences of motor responses necessary for simple tasks such as playing the piano or swinging a baseball bat would lose their smoothness and sequential characteristics; or on a simpler level, the motor behavior described in this question requires sensory feedback, and without it, motor behavior breaks down. The other choices presented as alternate answers represent regions where lesions produce entirely different kinds of deficits unrelated to those described in the stem of this question. These include pseudobulbar palsy that would be associated with lesions of the far lateral precentral gyrus, inability to ...

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