Regions of the Frontal Lobes

1. Primary Motor Cortex (M1, Brodmann area 4): The primary motor cortex is located on the precentral gyrus just rostral to the central sulcus. It is the source of cortical neurons that will project to the brainstem and spinal cord to activate neurons involved in the control of voluntary movements. It receives input from the neighboring primary somatosensory area (S1, on the postcentral gyrus) and premotor cortex, as well as from the ventral lateral nucleus of the thalamus (a relay nucleus with projections from the cerebellum). These inputs modulate the output of M1 by providing information about the positioning, timing, and coordination of voluntary movements. The output of M1 goes by way of the internal capsule to synapse in the brainstem (the projection referred to as the corticobulbar tract) or the spinal cord (the corticospinal tract). Damage to M1 will cause contralatereral motor deficits, initially a flaccid hemiplegia/hemiparesis and later a spastic hemiplegia/hemiparesis. Depending on the extent of cortical damage, these deficits may be localized to a specific region of the body or can be more widespread.