Motor Representations

Areas of neocortex also represent muscles and movements. Long before it was technically possible to record the small changes in the electrical activity of neurons in neocortex, it was possible to generate and deliver an electrical current to the surface of neocortex and observe the effects. From quite early studies, it became apparent that the electrical stimulation of locations in the frontal lobe was especially effective in evoking movements of various parts of the body and that different sites in the frontal cortex related to different movements. Parts of the frontal cortex were seen to systematically represent body movements, and a primary motor area, M1, was defined first, followed by a supplementary motor area, SMA. Ml is a striplike area that extends mediolaterally along and in the central sulcus of the human brain just rostral to the somatosensory cortex, and it represents the body from foot, trunk, hand, face, and tongue in a mediolateral sequence. Ml largely, but not precisely, corresponds to cortex defined earlier by Brodmann as area 4, but many modern depictions make M1 coextensive with a redefined area 4. SMA is rostral to the more medial part of Ml. The second motor area, M2, of rats (Fig. 3) might correspond to SMA, which is best known from studies in primates.

Detailed studies of the organization of Ml have used penetrating microelectrodes to more precisely activate small numbers of the large pyramidal cells of layer 5 at low levels of current, thereby evoking movements in only a few muscles and allowing the motor map to be seen in detail. As a result of microstimulation studies, it is now apparent that Ml and probably other motor representations are organized as a patchwork or mosaic of small regions of cortex that are devoted to specific movements, that of a single finger for example. Each type of patch of cortex typically occurs more than once but next to various other patches of motor cortex for other related movements. Such an organization may give the motor cortex great flexibility in the control of various combinations of movements.

Humans and other primates have additional motor areas in the frontal lobe that are collectively called premotor areas. They include the dorsal and ventral premotor areas just rostral to medial and lateral portions of Ml, respectively, the supplementary motor area rostral to Ml on the medial wall of the cerebral hemisphere, and other motor areas on the medial wall and dorsolateral surface of the frontal lobe. Movements can be evoked by electrical stimulation from all of these areas, and each area represents the body in a sequence across the field. To evoke movements in these premotor areas takes higher levels of current than in Ml, and the movements evoked are more complex. Movements also can be evoked by stimulating areas of the somatosensory cortex, but at high levels of current. Thus, a number of areas of cortex have motor functions. Motor and premotor areas also have sensory input, and they may also contain sensory maps.

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