Motor Cortex and its 4 primary tracts

 

The motor cortex, located in the cerebral cortex, is the starting point for electrochemical transmissions that travel from the nerve centers to the muscles. The primary motor cortex is positioned on the precentral gyrus. The precentral gyrus is a bulging ridge that is on the back area of the frontal lobe (the biggest part of each cerebral cortex). It is bound by the central sulcus (located between the frontal and parietal lobes) and the precentral sulcus, which is in front of and parallel to the central sulcus. The primary motor cortex is located just in front of the central sulcus (Carlson).

The organization of the primary motor cortex is somatotopic. This means that the layout of the motor cortex corresponds to different body parts. The supplementary motor cortex and the premotor cortex lie adjacent to the primary motor cortex. Both are involved in arranging movements, which they carry out by interacting with the primary motor cortex. Moving, or even envisioning movement, stimulates these areas. The two auxiliary motor cortexes obtain signals from sections of the parietal and temporal cortex, informing them of what is happening and where it is occurring. For example, the cortex involved in vision is separated into two streams, dorsal and visual. The ventral stream helps answer the “what” of vision- perceiving and recognizing objects- and ends in the temporal cortex; while the dorsal steam helps answer the “where” of vision- perception of location- and ends in the parietal cortex. The parietal lobes also help answer the “how” of vision by organizing visually guided movements. This somatotopic arrangement allows different areas of the brain to control different body parts (Carlson, 2004).

The lateral group is principally involved in the management of independent limb movements; its primary job is controlling the action of the hands and fingers. When the right and left hand move in different manners at the same time, or one moves while the other stays motionless, independent limb movement is taking place (Carlson, 2004).

The corticospinal tract, which is part of the lateral group, is one of the four principal motor tracts and has two sections- lateral and ventral. It is composed of axons that come to an end in the gray matter of the spinal cord. Most of the cells that control these axons are in the primary motor cortex, but axons are also sent through the corticospinal pathway by neurons in the parietal and temporal lobes. The majority of axons in the lateral (side) corticospinal tract are formed in the areas of the primary and auxiliary motor cortexes that control the distal sections of body parts; for instance, the fingers, arms, feet, and toes. Once joined with motor neurons in the spinal cord, synapses are formed. Muscles of the distal limbs are controlled by these motor neurons. Alternatively, axons of the ventral (middle) corticospinal tract form in the upper leg areas of the primary motor cortex and descend to the proper area of the spinal cord. They then divide and propel terminal buttons into both sides of the gray matter. As a result, motor neurons are regulated and can move the muscles of the torso and upper legs (Carlson, 2004).

Another member of the lateral group, the rubrospinal tract, is the second principal motor tract. This tract starts in the red nucleus of the midbrain and obtains its key indicators from the motor cortex through the cerebellum and the corticorubral tract (which controls the movement of the face, neck, tongue, and some eye muscles). The rubrospinal tract’s axons end on motor neurons in the spinal tract; these neurons are responsible for controlling the forelimb and hindlimb muscle movement (Carlson, 2004).

The third principal motor tract, the vestibulospinal tract, is part of the ventromedial group. This group gets information from the areas of the primary motor cortex that control movement of the trunk, along with muscles close to the body (Carlson 2004). The vestibulospinal tract plays a vital role in maintaining posture, along with controlling adjustments of the head and muscles involved in balance (n.a., 2002).

Lastly, the reticulospinal tract, also a part of the ventromedial group, has many cell bodies located in nuclei of the brain stem and midbrain. The neurons control essential automatic functions, such as respiration, sneezing, coughing, and muscle tonus (a state of partial muscle contraction). Additionally, they also control voluntary actions controlled by the neocortex; walking is one example (Carlson, 2004).

In conclusion, the motor cortex controls movement in the body through different motor tracts. The four primary tracts include the corticospinal, rubrospinal, vestibulospinal, and reticulospinal tract. The corticospinal and rubrospinal tracts are in the lateral group which controls independent limb movements. The corticospinal tract controls the distal sections of body parts, along with the muscles of the torso and upper legs; while the rubrospinal tract controls the forelimb and hindlimb muscles. The vestibulospinal and reticulospinal tracts are in the ventromedial group which controls movement of the torso and muscles close to the body. The vestibulospinal tract controls posture, head adjustments, and balance; while the reticulospinal tract controls many automatic functions and voluntary neocortex functions.
Reference:

1. Brain Picture. Retrieved January 28, 2006 from http://www.emc.maricopa.edu/faculty/farabee/BIOBK/cerebrum_1.gif.

2. Carlson, Neil R. (2004). Control of Movement. In Physiology of Behavior, 8, 254-259.

3. N.a. (2002). Retrieved January 29, 2006 from http://sprojects.mmi.mcgill.ca/cns/histo/systems/motor/main.htm.