Area 6a, the premotor area, is also electrically excitable but requires more intense stimuli than area 4 to evoke movements. Stimulation of its caudal aspect produces responses that are similar to those elicited from area 4. These responses are probably produced by transmission of impulses from all of area 6a to area 4 (as they cannot be obtained after ablation of area 4). Stimulation of the rostral premotor area elicits more general movement patterns, predominantly of proximal limb musculature. The latter movements are effected via pathways other than those derived from area 4 (hence, “parapyramidal”). Very strong stimuli elicit movements from a wide area of premotor frontal and parietal cortex, and the same movements may be obtained from several widely separated points. From this it may be assumed that the premotor cortex includes several anatomically distinct subregions with different afferent and efferent connections. In general, it may be said that the motor–premotor cortex is capable of synthesizing agonist actions into an almost infinite variety of finely graded, highly differentiated patterns. These are directed by visual (area 7) and tactile (area 5) sensory information and supported by appropriate postural mechanisms.
The supplementary motor area is the most anterior portion of area 6 on the medial surface of the cerebral hemisphere (area 6b in Fig. 3-3B). Stimulation of this area may induce relatively gross ipsilateral or contralateral movements, bilateral tonic contractions of the limbs, contraversive movements of the head and eyes with tonic contraction of the contralateral arm, and sometimes inhibition of voluntary motor activity and vocal arrest.
Precisely how the motor cortex controls movements is still a controversial matter. The traditional view, based on the interpretations of Hughlings Jackson and of Sherrington, and elaborated by Denny-Brown, has been that the motor cortex is organized not in terms of individual muscles but of movements, that is, the coordinated contraction of groups of muscles. Jackson visualized a widely overlapping representation of muscle groups in the cerebral cortex on the basis of his observation that a patient could recover the use of a limb following destruction of the limb area as defined by cortical stimulation. This view was supported by Sherrington’s observations that stimulation of the cortical surface activated not solitary muscles but a combination of muscles, and always in a reciprocal fashion—that is, in a manner that maintained the expected relationship between agonists and antagonists. He also noted the inconstancy of stimulatory effects; the stimulation of a given cortical point that initiated flexion of a part on one occasion might initiate extension on another.
These interpretations must be viewed with circumspection, as must all observations based on the electrical stimulation of the surface of the cortex. It has been shown that to stimulate motor cells from the surface, the electric current has to penetrate the cortex to layer V, where these neurons are located, inevitably activating a large number of other cortical neurons. The elegant experiments of Asanuma and of Evarts and his colleagues, who stimulated the depths of the cortex with microelectrodes, demonstrated the existence of discrete zones of efferent neurons that control the contraction of individual muscles; moreover, the continued stimulation of a given efferent zone often facilitated rather than inhibited the contraction of the antagonists. These investigators have also shown that cells in the efferent zone receive afferent impulses from the particular muscle to which the efferent neurons project. When the effects of many stimulations at various depths were correlated with the exact sites of each penetration, cells that projected to a particular pool of spinal motor neurons were found to be arranged in radially aligned columns approximately 1 mm in diameter.