Some Specialized Areas of Motor Control Found in the Human Motor Cortex

Neurosurgeons have found a few highly specialized motor regions of the human cerebral cortex (shown in Figure 55-3) that control specific motor functions.

Motor Cortex
Representation of the different muscles of the body in the motor cortex and location of other cortical areas responsible for specific types of motor movements.

These regions have been localized either by electrical stimulation or by noting the loss of motor function when destructive lesions occur in specific cortical areas. Some of the more important regions are the following.

Broca's Area and Speech. Figure 55-3 shows a premotor area labeled "word formation" lying immediately anterior to the primary motor cortex and immediately above the sylvian fissure. This region is called Broca's area. Damage to it does not prevent a person from vocalizing, but it does make it impossible for the person to speak whole words rather than uncoordinated utterances or an occasional simple word such as "no" or "yes." A closely associated cortical area also causes appropriate respiratory function, so that respiratory activation of the vocal cords can occur simultaneously with the movements of the mouth and tongue during speech. Thus, the premotor neuronal activities related to speech are highly complex.

"Voluntary" Eye Movement Field. In the premotor area immediately above Broca's area is a locus for controlling voluntary eye movements. Damage to this area prevents a person from voluntarily moving the eyes toward different objects. Instead, the eyes tend to lock involuntarily onto specific objects, an effect controlled by signals from the occipital visual cortex, as explained in Chapter 51. This frontal area also controls eyelid movements such as blinking.

Head Rotation Area. Slightly higher in the motor association area, electrical stimulation elicits head rotation. This area is closely associated with the eye movement field; it directs the head toward different objects.

Area for Hand Skills. In the premotor area immediately anterior to the primary motor cortex for the hands and fingers is a region neurosurgeons have identified as important for "hand skills." That is, when tumors or other lesions cause destruction in this area, hand movements become uncoordinated and nonpurpose-ful, a condition called motor apraxia.

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Essentials of Human Physiology

Essentials of Human Physiology

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