The columnar arrangement of cells in the sensorimotor cortex had been appreciated for many years; the wealth of radial interconnections between the cells in these columns led Lorente de Nó to suggest that these “vertical chains” of cells were the elementary functional units of the cortex. This notion received strong support from Mountcastle’s observations that all the neurons in a column receive impulses of the same sensory modality, from the same part of the body. It is still not entirely clear whether the columns contribute to a movement as units or whether individual cells within many columns are selectively activated. Both Henneman and Asanuma summarized the evidence for these disparate views.
Evarts and his colleagues also elucidated the role of cortical motor neurons in sensory evoked or planned movement. Using single-cell recording techniques, they showed that pyramidal cells fire about 60 ms prior to the onset of a movement, in a sequence determined by the required pattern and force of the movement. But other, more complex properties of the pyramidal cells were also noted. Some of them received a somatosensory input transcortically from the parietal lobe (areas 3, 1, and 2), which could be turned on or off or gated according to whether the movement was to be controlled, that is, guided, by sensory input. Many neurons of the supplementary and premotor cortices were activated before a planned movement. Thus pyramidal (area 4) motor neurons were prepared for the oncoming activation by impulses from the parietal, prefrontal, premotor, and auditory and visual areas of the cortex. This preparatory “set signal” could occur in the absence of any activity in the spinal cord and muscles. The source of the activation signal was found to be mainly in the supplementary motor cortex, which appears to be under the direct influence of the “readiness stimuli” (Bereitschaft potential) reaching it from the prefrontal areas for planned movements and from the posterior parietal cortex for motor activities initiated by sensory perceptions. There are also fibers that reach the motor area from the limbic system, presumably subserving motivation and attention. Roland has used functional cerebral blood flow measurements to follow these neural events.
Thus the prefrontal cortex, supplementary motor cortex, premotor cortex, and motor cortex are all responsive to afferent stimuli and are involved prior to, and in coordinated fashion with, a complex movement. As remarked later on, the striatopallidum and cerebellum, which project to these cortical areas, are also activated prior to or concurrently with the discharge of corticospinal neurons (see Thach and Montgomery for a critical review of the physiologic data).
Termination of the Corticospinal and Other Descending Motor Tracts
This has been studied in the monkey by interrupting the descending motor pathways in the medulla and more rostral parts of the brainstem and tracing the distribution of the degenerating elements in the spinal gray matter. On the basis of such experiments and other physiologic data, Lawrence and Kuypers proposed that the functional organization of the descending cortical and subcortical pathways is determined more by their patterns of termination and the motor capacities of the internuncial neurons upon which they terminate than by the location of their cells of origin.