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Structure and Function of the Motor Cortex Areas

Structure and Function of the Motor Cortex Areas

Last update: Nov 1st, 2022

The primary motor cortex (M1, or somatomotor area) occupies a large portion of the precentral gyrus (Brodmann area 4) and typically executes movements that are selected and planned by other areas of the brain.

An image showing lateral view of the right hemisphere and medial view of the left hemisphere

An image showing the Primary motor cortex, lateral view of the right hemisphere and medial view of the left hemisphere

A primary motor cortex is present in each hemisphere and both are somatotopically organized: The medial and superior portions control the muscles of the feet and legs, and the lateral portions control the muscles of the thorax, arms and head.
Each primary motor cortex controls the muscles on the opposite side of the body. Impulses exit the motor cortex via upper level neurons, which cross over at the medulla pyramids. The axons then enter the spinal cord as a part of the corticospinal tracts. Lower level neurons carry impulses from the ventral horns of the spinal cord to the skeletal muscles.
Directly anterior to the primary motor cortex (Brodmann area 6) are the premotor area (lateral) and supplementary motor area (medial).

An image showing the Primary motor cortex, Premotor cortex, lateral view of the right hemisphere and medial view of the left hemisphere

An image showing the Supplementary motor area, Primary motor cortex, Premotor cortex, lateral view of the right hemisphere and medial view of the left hemisphere

In general, the premotor area anticipates and plans movements based on input from external or sensory cues, such as vision and auditory. In contrast, the supplementary motor area selects and plans complex movements based on internal cues, such as memory.
Most of the information generated by these areas is passed to the primary motor cortex for execution and the rest travels directly down the spinal cord to the skeletal muscles.

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Brain areas responsible for body movements

Before it becomes fully active, the primary motor cortex receives input from other motor cortex areas. This information is needed to bring about coordinated, well-planned and purposeful movements.

Anterior to the primary motor cortex is the premotor cortex (lateral) and the supplementary motor area (medial), which correspond to Brodmann area 6.

In general, the premotor area anticipates and plans movements based on external sensory cues, such as responses to events that are seen or heard.
The supplementary motor area, in contrast, selects and plans complex movements involving both sides of the body based on internal cues, such as thoughts or memory.
Most of the information generated by these areas is passed to the primary motor cortex for execution and the rest travels directly down the spinal cord to the skeletal muscles.
The posterior parietal cortex (or Brodmann areas 5 and 7) also participates in the motor planning process.

An image showing the Posterior parietal cortex, Supplementary motor area, Primary motor cortex, Premotor cortex, lateral view of the right hemisphere and medial view of the left hemisphere

It receives and integrates visual, auditory, and somatosensory (sense of body) inputs from other brain areas.
From these inputs, it produces information about the position of the body parts and external objects, which is used by the premotor and supplementary cortex areas to accurately plan actions.

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The following shows how the motor cortex areas produce an action:
(1) Limb and paper location is sent to the motor cortex areas from the posterior parietal cortex.