The premotor cortex and SMA exert what Hughlings Jackson called "the least automatic" control over voluntary motor commands. These cortical areas account for approximately 50% of the total frontal lobe motoneuron con tribution to the corticospinal tract and have specialized functions. Each of the six cortical motor areas that interact with M1 has a separate and independent set of inputs from adjacent and remote regions, as well as parallel, separate outputs to the brain stem and spinal cord.56 Table 1-1 gives an overview of their relative contributions to the corticospinal tract and their functional roles. These motor areas also interact with cortex that does not have direct spinal motoneuron connections. For example, although motorically silent prefrontal areas do not directly control a muscle contraction, they play a role in the initiation, selection, inhibition, and guidance of behavior by representational knowledge. They do this via soma-totopically arranged prefrontal to premotor, corticostriatal, corticotectal, and thalamocorti-cal connections.57
Functional imaging has revealed a somato-topic distribution of activation during upper extremity tasks in SMA, dorsal lateral premotor, and cingulate motor cortices.58 Somatotopy in the secondary sensorimotor cortices, at least for the upper extremity, may be based on a functional, rather than an anatomical repre-sentation.59 For example, the toe and foot have access to the motor program for the hand for cursive writing, even though the foot may never have practiced writing. An fMRI study that compared writing one's signature with the dominant index finger and ipsilateral big toe revealed that both actions activated the intra-parietal sulcus and premotor cortices over the convexity in the hand representation.59 The finding that one limb can manage a previously learned task from another limb may have implications for compensatory and retraining strategies after a focal brain injury.
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