![]() These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscles. Both tracts are named for their origin in the cortex and their targets-either the spinal cord or the brainstem (the term “bulbar” refers to the brainstem as the bulb, or enlargement, at the top of the spinal cord). The two descending pathways traveled by the axons of Betz cells are the corticospinal tract and the corticobulbar tract. Neurons located in the primary motor cortex, named Betz cells, are large cortical neurons that synapse with lower motor neurons in the spinal cord or the brainstem. The motor output from the cortex descends into the brainstem and to the spinal cord to control the musculature through motor neurons. The “power muscles” that perform coarser movements, such as the buttock and back muscles, occupy much less space on the motor cortex. The greatest amount of cortical space is given to muscles that perform fine, agile movements, such as the muscles of the fingers and the lower face. ![]() Also, the relative space allotted for the different regions is exaggerated in muscles that have greater enervation. The hand and face are in the lateral face of the gyrus. The neurons responsible for musculature in the feet and lower legs are in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder at the crest of the longitudinal fissure. The primary motor cortex is arranged in a similar fashion to the primary somatosensory cortex, in that it has a topographical map of the body, creating a motor homunculus. The primary motor cortex receives input from several areas that aid in planning movement, and its principle output stimulates spinal cord neurons to stimulate skeletal muscle contraction. The primary motor cortex is located in the precentral gyrus of the frontal lobe. This area is responsible for controlling movements of the structures of speech production. Also, anterior to the premotor cortex and primary motor cortex is Broca’s area. There are direct connections between the frontal eye fields and the superior colliculus. The frontal eye fields are responsible for moving the eyes in response to visual stimuli. For example, these areas might prepare the body for the movements necessary to drive a car in anticipation of a traffic light changing.Īdjacent to these two regions are two specialized motor planning centers. Neurons in these areas are most active leading up to the initiation of movement. The supplemental motor area also manages sequential movements that are based on prior experience (that is, learned movements). The premotor area aids in controlling movements of the core muscles to maintain posture during movement, whereas the supplemental motor area is hypothesized to be responsible for planning and coordinating movement. The prefrontal areas project into the secondary motor cortices, which include the premotor cortex and the supplemental motor area. The regions of the frontal lobe that remain are the regions of the cortex that produce movement. These are primarily in the anterior part of the frontal lobe. One way to define the prefrontal area is any region of the frontal lobe that does not elicit movement when electrically stimulated. In generating motor responses, the executive functions of the prefrontal cortex will need to initiate actual movements. ![]() However, the muscles that are responsible for the basic process of breathing are also utilized for speech, which is entirely voluntary. One example is the ability of our breathing to switch to unconscious control while we are focused on another task. However, some aspects of the somatic system use voluntary muscles without conscious control. The term “voluntary” suggests that there is a conscious decision to make a movement. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The defining characteristic of the somatic nervous system is that it controls skeletal muscles.
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