Human Anatomy Part 2 - K. A. Dyubenko A. K. Kolomiytsev Yu. B. Chaykovsky 2008

Special Part
Nervous system, systema nervosum - Special Part
Cerebral cortex mantle - Functional fields of the cerebral cortex - Motor cortical areas

The primary motor area (Area 4) is located in the precentral gyrus of the frontal lobe (motor cortex) (Fig. 154). Like the primary somatosensory area, the primary motor area consists of distinct regions that control individual Muscles or Muscle groups. In the precentral gyrus, the parts of The Human Body are mapped in an inverted orientation: the FOOT, lower limb muscles, and trunk are positioned in the upper portion of the gyrus, occupying a relatively small area, whereas the face and Tongue are located at the lower end, taking up a significantly larger area (the motor "homunculus") (Fig. 155 B). The face is represented bilaterally (in both hemispheres), while the rest of the body is represented unilaterally, with the motor cortex regulating the musculature of the opposite side of the body. The cortical regions associated with the control of speech articulation or hand movements are particularly extensive. The Use of the Larynx, Lips, and tongue for speech, as well as the fingers (especially the thumb), enables movements that reach the highest level of complexity in humans. Stimulation of a specific point in the primary motor area results in the contraction of muscles, typically on the contralateral side of the body.

The premotor area (Area 6) is situated on the lateral surface of the hemisphere, anterior to the precentral gyrus (premotor cortex). The premotor area generates nerve impulses that drive specific muscle groups, arranging them into precise sequences, such as during handwriting. Thus, the premotor area controls motor skills.

The frontal eye field (Area 8) is located in the frontal lobe of the Cerebral Cortex; it corresponds to the lower portion of Area 8, which occupies approximately the middle or posterior part of the middle frontal gyrus. It is sometimes considered part of the premotor area. This field controls voluntary Eye Movements, such as searching for a word in a dictionary. Stimulation of the frontal eye field triggers a turn of the HEAD and eyes toward the side opposite to the stimulation.

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Fig. 155 A, B. Diagram of the sensory and motor homunculi (after W. Penfield, T. Rasmussen, 1950). Coronal section of the cerebral hemispheres:

A - representation of general sensibility in the postcentral gyrus cortex;

B - representation of the motor system in the precentral gyrus cortex

The speech area is also a vital component of the motor cortex. Translating spoken or written words into thought engages sensory areas—the primary auditory, auditory association, primary visual, visual association, and gnostic areas—as previously noted. Converting thoughts into speech involves the motor speech area (Area 44), or Broca's area*, which is located in the frontal lobe (inferior frontal gyrus), directly above the lateral sulcus (the operculum). From this area, nerve impulses are transmitted to the premotor areas that control the Muscles of the larynx, Pharynx, and Mouth. Impulses traveling from the premotor area to the muscles produce specific, coordinated contractions that enable us to speak. Similarly, impulses are sent from Broca's area to the primary motor area. From there, impulses reach the Respiratory Muscles, regulating the airflow passing through the vocal cords. The coordinated contraction of speech and respiratory muscles allows thoughts to be transformed into speech.

* Pierre Paul Broca (1824–1880) was a French surgeon, anatomist, and anthropologist, and a professor of clinical surgery at the University of Paris.

Broca's area (Area 44) and other speech areas are located in the left cerebral hemisphere in the vast majority of people, regardless of whether they are right- or left-handed. Damage to the sensory or motor speech areas results in: aphasia, the inability to speak; word deafness, the inability to speak words; and word blindness, the inability to understand written words. Stimulation of this region may elicit singing yet indistinct speech. If this area is stimulated while a person is talking, speech is arrested.