ABSTRACT
UMNs are comprised of neurons of the primary motor, premotor, and supplementary motor cortices (Figure 20.1). The premotor cortex (Brodmann area 6) occupies the frontal lobe rostral to the motor cortex, while the supplementary motor area lies on the medial side of the superior frontal gyrus, controlling patterning and initiation of motor activities. The primary motor cortex (Brodmann area 4) is arranged somatotopically (Figure 20.2), in which parts of the contralateral body are represented in a distorted fashion (motor homunculus). In this homunculus, the thumb in particular, and the hand, in general, occupies a larger area than other parts of the body. This is attributed to the relative density of the neurons associated with movements of the digits. The thumb and index finger occupy an area near the face, while the body lies superior to the head between neurons of
the shoulder and hip. The head occupies the lower part of the homunculus near the lateral cerebral (Sylvian) fissure, whereas the lower extremity is confined to the paracentral lobule, which lies rostral to the precuneus on the medial surface of the cerebral hemisphere. These neurons project to the spinal cord and brainstem via the CST and the CBT, respectively. The CST and CBT exert a wide range of influences on motor and reflex activities and sensory transmission. These effects are facilitated by their projections to the spinal and
cranial motor neurons, as well as thalamic, striatal, and other brainstem neurons.
