THE MOTOR CORTEX
The motor cortex is located in the precentral gyrus. Similar to the sensory cortex, the various parts of the body are represented in the cortex, with the feet at the superior aspect of the gyrus and the face on the inferior and lateral aspect (see Figure 5.45). The fa-cial area is represented in both sides; however, the rest of the body is represented on one side, with the left cortex representing the right half of the body and the right cortex the left half. Again, similar to the sen-sory cortex, the size of cortical representation is in proportion to the number of motor units going to the muscle. This, in turn, correlates with the skill with which the part is used for fine, precise, voluntary movements. For example, although the hands are small, the cortical representation is large. The lips, pharynx, and tongue required for speech also have a large representation.
The corticospinal tracts and the corticobulbar tracts originate from the motor cortex. However, there are many other areas of the brain in the parietal lobe and elsewhere that participate in motor function.
The neurons responsible for voluntary control project from the motor cortex to the nerves supplying the muscles in question via direct, pyramidal, or corti-cospinal/corticobulbar pathways. They are calledpyramidal pathways because the axons form pyra-midlike bulges, the pyramids, in the medulla. Axons of neurons originating in the cortex descend to the medulla (see Figure 5.46). As they pass near the thal-amus, all the fibers lie close together in the region of the internal capsule.The sensory tracts are also in close proximity in this region.
On reaching the medulla, most fibers of the corti-cospinal tract cross over to the opposite side, descend as the lateral corticospinal tract, and synapse with the respective motor neuron. The neurons in the cor-ticospinal tract are the upper motor neurons. Some axons descend on the same side as the anterior cor-ticospinal tract and cross over at the spinal segment where they synapse with lower motor neurons (the neurons that directly supply the muscles).
In this way, axons from one side control the muscles of the opposite side of the body. Just like the corticospinal tracts, the axons of upper motor neurons that control skeletal muscles in the head form the corticobulbartracts. They synapse with lower motor neurons thatexit via the cranial nerves.
In addition to the direct or pyramidal pathways, there are indirect or extrapyramidal motor path-ways. These tracts are complex and involve impulsesfrom the basal ganglia, limbic system, cerebellum, thalamus, and reticular formation, etc. Some tracts are the rubrospinal, tectospinal, reticulospinal, and vestibulospinal tracts.