Answer
The precentral gyrus which contains the primary motor cortex is the most posterior gyrus of the frontal lobe. It is located just anterior to the central sulcus. Neurons in the primary motor cortex send signals to the brainstem and the spinal cord. The effect of these stimuli is to cause muscle contractions.
When one conceives the intention to contract muscles, the premotor area ( with the motor association cortex) formulates a program of muscle contractions (degree and frequency of movements) for the relevant activity--typing, dancing, speaking etc.The program is then transmitted to the primary motor area in the precentral gyrus . Thence, signals are sent to the brainstem and the spinal cord to cause muscle contractions.
Work Step by Step
There is also a motor homunculus that is a somatotopic representation of the muscles in different regions of the body. The neurons that control movements are deep in the longitudinal fissure on the medial side of the gyrus. Trunk, shoulders, and arms are controlled by neurons at the top of the gyrus. Neurons in the inferior, lateral region of the gyrus control facial regions.The motor homunculus is also a disproportionate representation of the size of body regions, This is so because the amount of cortical area given to a body area depends on number of muscles, and motor units in the area-- not on the size of the area, Therefore, face, hands, and tongue have disproportionately large cortical representations. This reflects the importance given to the fine motor control essential for speech, facial expressions, and use of hands.
The representation of muscles in motor homunculus is not point to point. Homunculus points out cortical areas responsible for motor control of certain motor regions; however, the boundaries between the cortical areas overlap.
Consequence of Decussation
Pyramidal cells of the precentral gyrus project caudally to the brain stem nuclei. Some of these fibers form the corticospinal tracts . The pyramidal decussation is formed when most of these fibers cross over in the lower medulla. Other fibers decussate more caudally, in the spinal cord . The result of the crossing over of these fibers is that each precentral gyrus controls the muscles on the contralateral side of the body
