The anterior regions of the CEREBRAL CORTEX which play a role in the generation of movements of the skeletal musculature. At a gross level, motor cortex is subdivided into PRIMARY MOTOR CORTEX (area 4; approximately contiguous with the precentral gyrus—see BRODMANN’S AREAS) and NON-PRIMARY MOTOR CORTEX (all of the medial and anterior cortex involved in motor preparation and planning; premotor and SUPPLEMENTARY MOTOR CORTEX). Non-primary motor cortex is five times more plentiful than primary motor cortex in Homo sapiens; in monkeys the ratio is 1:1. The premotor cortex (PMC) is located in the lateral part of Brodmann’s area 6. The supplementary motor cortex (SMC) is located in the medial part of area 6, just anterior to the leg representation in the primary motor cortex. Recently, electrophysiological studies in non-human primates have been used to characterize a set of CINGULATE MOTOR AREAS (CMC; Brodmann’s areas 23 and 24, see CINGULATE CORTEX). Subdivisions of these regions have been suggested, based on electrophysiological and anatomical grounds. For example a recent count revealed that three areas in the CMC have been proposed, two in PMC and three in SMC. Early views of primary motor cortex suggested relatively neat, independent representations of distinct muscle groups, arranged in the classical HOMUNCULUS. Single cell recordings in this area suggest that such a view is an oversimplification; there are multiple, spatially distinct representations of the muscles which control the fingers, for example. These representations are intermingled with small representations of other muscle groups of the hand, and even more distant muscle groups such as those which control rotations about the wrist, elbow and even the shoulder. In non-primary motor cortices, these motor maps are even less precise, although current theory still suggests largely separate representations of major muscle groups, such as those related to movements of the distal musculature of the hand and finger versus the more proximal musculature of the upper arm and shoulder. Other early views of the non-primary motor cortices include the notion that they influence motor activity exclusively through projects to the appropriate motor representations in primary motor cortex.
This position has been refuted by numerous anatomical studies showing direct connections of SMC and PMC to motor neurons in the spinal cord. Of course, one difficulty in establishing what the different motor cortices do is that most of the data has been based on recordings of the activity of single neurons in each of the regions. Identifying that a particular neuron changes its activity before or during the movement of a particular muscle group does not provide unambiguous information about what movement-related parameters that cell may be influencing (or being influenced by). Electrophysiological studies tend to be driven by the theoretical perspective of the particular electrophysiologist: early studies have empathised the coding of relatively low-level movement parameters such as isometric force at a single joint. Later work has examined cell activity in relation to movement direction and amplitude in space. Advances of electrophysiological technology allowed for recording more complex movements in three dimensions rather than, for example, rotations about a single joint. Unfortunately complete consensus on what movement-related parameters are coded in a given region of the motor cortex has yet to be reached. Contemporary studies of premotor cortices by physiological psychologists have tended to focus on how these regions use sensory information to guide the control of actions such as reaching and grasping, and perhaps play a role in the understanding of the meaning of actions performed by others. Studies of the more medial structures of the SMC tend to focus on potential contributions in learning new movements, movement complexity (rather loosely defined) and the integration of movements made by two limbs simultaneously. Cingulate motor cortices are still being described in terms of their connectivity with other brain regions and in their exact number and location in non-human primates. Study of the organisation of the motor cortices has depended rather heavily on electrophysiology and neuroanatomy; neuropsychological contributions have been fewer, perhaps because lesions in humans seldom respect small functional boundaries. Nevertheless, the advance of recent non-invasive techniques should allow for investigations with increasing spatial and temporal resolution to be performed using human subjects. To date, imaging studies are providing evidence which contradicts some of the simple descriptions that have been attached to the different motor cortices, such as a unique role of the SMC in the preparation of movements. Many brain regions are activated in human subjects while they prepare a series of finger movements, including primary motor cortex, parietal cortex, and all three of the non-primary motor cortical zones described above. Nevertheless, since human subjects are capable of following complex instructions as well as producing many different patterns of movements to various instructional sets, these technologies will undoubtedly advance our understanding of the functions of the motor cortices in the future.
DAVID P.CAREY
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