| Brain: Primary motor cortex | ||
|---|---|---|
| Brodmann area 4 of human brain. | ||
| Primary motor cortex labeled as somatomotor cortex. | ||
| Latin | cortex motorius primus | |
| Gray's | subject #189 821 | |
| Part of | Precentral gyrus | |
| Artery | Anterior cerebral Middle cerebral |
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| NeuroNames | ancil-421 | |
| MeSH | Motor+Cortex | |
The primary motor cortex (or M1) is a brain region that in humans is located in the posterior portion of the frontal lobe. It works in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto alpha motor neurons, which connect to the muscles. Pre-motor areas are involved in planning actions (in concert with the basal ganglia) and refining movements based upon sensory input (this requires the cerebellum).
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The human primary motor cortex is located in the dorsal part of the precentral gyrus and the anterior bank of the central sulcus. The precentral gyrus is anterior to the postcentral gyrus from which it is separated by the central sulcus. Its anterior border is the precentral sulcus, while on the inferior side it borders to the lateral fissure (Sylvian fissure). On the medial side, it is contiguous with the paracentral lobule. This area can also be identified by Brodmann area number 4.
The internal pyramidal layer (layer V) of the precentral cortex contains giant (70-100 micrometers) pyramidal neurons (a.k.a. Betz cells), which send long axons to the contralateral motor nuclei of the cranial nerves and to the lower motor neurons in the ventral horn of the spinal cord. These axons form the corticospinal tract. The Betz cells' along with their long axons are referred to as the upper motor neuron (UMN).
There is a broadly somatotopic representation of the different body parts in the primary motor cortex in an arrangement called a motor homunculus (Latin: little person). The leg area is located close to the midline, and the head and face area located laterally on the convex side of the cerebral hemisphere (motor homunculus). The arm and hand motor area is the largest, and occupies the part of precentral gyrus, between the leg and face area. In humans, the lateral area of the primary motor cortex is arranged from top to bottom in areas that correspond to the buttocks, torso, shoulder, elbow, wrist, fingers thumb, eyelids, lips, and jaw. Interior sections of the motor area folding into the medial longitudinal fissure correspond with the legs.
These areas are not proportional to their size in the body with the lips, face parts, and hands, enjoying particularly large areas. Following amputation or paralysis, motor areas can shift to adopt new parts of the body.
In primates, the primary motor cortex is unusual in having in its anterior and posterior areas two representations of the digits and wrist.[1] The posterior areas can be activated by attention without any sensory feedback and has been suggested to be important for initiation of movements, while the anterior areas are dependent on sensory feedback.[2] It can also be activated by imaginary finger movements and listening to speech done without actual movements. This anterior representation area has been suggested to be important in executing movements involving complex sensoriomotor interactions.[3]
As the motor axons travel down through the cerebral white matter, they move closer together and form part of the posterior limb of the internal capsule.
They continue down into the brainstem, where some of them, after crossing over to the contralateral side, distribute to the cranial nerve motor nuclei. (Note: a few motor fibers synapse with lower motor neurons on the same side of the brainstem).
After crossing over to the contralateral side in the medulla oblongata (pyramidal decussation), the axons travel down the spinal cord as the lateral corticospinal tract.
Fibers that do not cross over in the brainstem travel down the separate ventral corticospinal tract, and most of them cross over to the contralateral side in the spinal cord, shortly before reaching the lower motor neurons.
Branches of the middle cerebral artery provide most of the arterial blood supply for the primary motor cortex.
The medial aspect (leg areas) is supplied by branches of the anterior cerebral artery.
The primary motor cortex receives thalamic inputs from different thalamic nuclei. Among others:
- Ventral lateral nucleus for cerebellar afferents
- Ventral anterior nucleus for basal ganglia afferents
Lesions of the precentral gyrus result in paralysis of the contralateral side of the body (facial palsy, arm-/leg monoparesis, hemiparesis) - see upper motor neuron.
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