Descriptions of motor cortex often include only the primary motor area, M1. However, humans and other primates also have a number of other cortical areas that are involved in the generation of motor behavior. Recently, evidence has been presented that at least 11 motor areas can be distinguished (9 somatomotor and 2 oculomotor). Each area is architectonically different and has a unique combination of connectivity to subcortical nuclei and cortical areas. The functional relationship of these areas involves both hierarchical and parallel components. Whereas the primary motor cortex is involved mainly in the execution of voluntary movements, nonprimary motor areas, including SMA, PMD, PMV, and cingulate areas, are engaged in higher order motor processing, such as preparation, and programming of sequences of movements and coordination of bilateral and other complex movements. These areas make up a second level of motor fields that interact with M1 and act in parallel with M1 by projecting to the motor neuron pools in the spinal cord and brain stem. By projecting to the spinal cord, each of these areas has the potential to influence the generation and control of movement independently, even when M1 is lesioned. A pre-SMA and possibly other fields represent a third level of areas with motor function. These fields interact with areas of the second level but not directly with M1 and not in parallel with M1. Areas of somatosensory cortex, especially areas 3a and S2, have motor as well as sensory functions. These cortical areas interact as part of a complex motor control network that also involves the motor thalamus, the substantia nigra, the cerebellum, and the basal ganglia. The motor thalamus receives cerebellar, pallidal, and nigral inputs. Most motor behaviors probably involve the majority of cortical areas, and they likely have overlapping functions. However, there is evidence for specialization of fields so that different impairments emerge from lesions of different areas. There is much yet to learn about motor cortex, especially how the populations of neurons in each area contribute to specific types of motor behavior.

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Suggested Reading

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Kalaska, J. F., Scott, S. H., Cisek, P., and Sergio, L. E. (l997). Cortical control of reaching movements. Curr. Opin. Neurobiol. 7, 849-859.

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Rizolatti, G., Fogassi, L., and Gallese, V. (2000). Cortical mechanisms subserving object grasping and action recognition; A new view on cortical motor function. In The New Cognitive Neurosciences (M. S. Gazzaniga, Ed.), p. 539. MIT Press, Cambridge, MA.

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Wise, S. P., Boussaoud, D., Johnson, P. B., and Caminiti, R. (1997). Premotor and parietal cortex: Cortico-cortical connectivity and combinatorial computations. Annu. Rev. Neurosci. 20, 25-42.

Wu, C., Bichot, N. P., and Kaas, J. H. (2000). Converging evidence from microstimulation, architecture and connections for multiple motor areas in the frontal and cingulate cortex of prosimian primates. J. Comp. Neurol. 423, 140-177.

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