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Primary motor cortex

Primary motor cortex

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The primary motor cortex (or M1) is a brain region that in humans is located in the posterior portion of the frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

. It
works in association with pre-motor
Brodmann area 6
- Human :Brodmann area 6 is part of the frontal cortex in the human brain. Situated just anterior to the primary motor cortex , it is composed of the premotor cortex and, medially, the supplementary motor area, or SMA...

 areas to plan and execute movements. M1 contains large neurons known as Betz cell
Betz cell
Betz cells are pyramidal cell neurons located within the fifth layer of the grey matter in the primary motor cortex, M1. They are named after Vladimir Alekseyevich Betz, who described them in his work published in 1874. These neurons are the largest in the central nervous system, sometimes reaching...

s, which send long axons
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 down the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 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
Basal ganglia
The basal ganglia are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas...

) and refining movements based upon sensory input (this requires the cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

).

Location


The human primary motor cortex is located in the dorsal
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 part of the precentral gyrus and the anterior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 bank of the central sulcus
Central sulcus
-External links:* via the Neuroscience Information Framework...

. The precentral gyrus is anterior to the postcentral gyrus
Postcentral gyrus
The lateral postcentral gyrus is a prominent structure in the parietal lobe of the human brain and an important landmark. It is the location of primary somatosensory cortex, the main sensory receptive area for the sense of touch...

 from which it is separated by the central sulcus. Its anterior border is the precentral sulcus
Precentral sulcus
The precentral sulcus lies parallel to, and in front of, the central sulcus....

, while on the inferior
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 side it borders to the lateral fissure (Sylvian fissure). On the medial
Anatomical terms of location
Standard anatomical terms of location are designations employed in science that deal with the anatomy of animals to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation...

 side, it is contiguous with the paracentral lobule
Paracentral lobule
The more medial part of the superior frontal gyrus is distinguished as the paracentral lobule . It is continuous with the precentral and postcentral gyri of the lateral surface....

. This area can also be identified by Brodmann area
Brodmann area
A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...

 number 4.

Layers


The internal pyramidal layer (layer V) of the precentral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 contains giant (70-100 micrometers) pyramidal neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

s (a.k.a. Betz cells), which send long axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s to the contralateral motor nuclei of the cranial nerves and to the lower motor neuron
Lower motor neuron
Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...

s in the ventral horn of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

. These axons form the corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

. The Betz cells along with their long axons are referred to as the upper motor neuron
Upper motor neuron
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle...

 (UMN).

Homunculus: the "little person"


There is a broadly somatotopic representation of the different body parts in the primary motor cortex in an arrangement called a motor homunculus
Homunculus
Homunculus is a term used, generally, in various fields of study to refer to any representation of a human being. Historically, it referred specifically to the concept of a miniature though fully formed human body, for example, in the studies of alchemy and preformationism...

 (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
Homunculus
Homunculus is a term used, generally, in various fields of study to refer to any representation of a human being. Historically, it referred specifically to the concept of a miniature though fully formed human body, for example, in the studies of alchemy and preformationism...

). 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
Medial longitudinal fissure
The great longitudinal fissure is the deep groove which separates the two hemispheres of the vertebrate brain....

 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.

Two representational areas


In primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, the primary motor cortex is unusual in having in its anterior and posterior areas two representations of the digits and wrist. 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. 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.

Pathway


As the motor axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s travel down through the cerebral white matter
White matter
White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...

, they move closer together and form part of the posterior limb of the internal capsule
Internal capsule
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons....

.

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
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

 with lower motor neuron
Lower motor neuron
Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...

s on the same side of the brainstem).

After crossing over to the contralateral side in the medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

 (pyramidal decussation
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

), the axons travel down the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

 as the lateral corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

.

Fibers that do not cross over in the brainstem travel down the separate ventral corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

, and most of them cross over to the contralateral side in the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

, shortly before reaching the lower motor neuron
Lower motor neuron
Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...

s.

Blood supply


Branches of the middle cerebral artery
Middle cerebral artery
-External links:*...

 provide most of the arterial blood supply for the primary motor cortex
Motor cortex
Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...

.

The medial aspect (leg areas) is supplied by branches of the anterior cerebral artery.

Neural input from the thalamus


The primary motor cortex receives thalamic inputs from different thalamic nuclei. Among others:

- Ventral lateral nucleus
Ventral lateral nucleus
-Inputs and outputs:It receives neuronal inputs from the basal ganglia which includes the substantia nigra and the globus pallidus . It also has inputs from the cerebellum ....

 for cerebellar afferents

- Ventral anterior nucleus
Ventral anterior nucleus
-Inputs and outputs:The ventral anterior nucleus receives neuronal inputs from the basal ganglia which includes the substantia nigra and the globus pallidus. It also has inputs from the cerebellum. It sends out neuronal input to the supplementary motor cortex and to primary motor...

 for basal ganglia afferents

Pathology


Lesions of the precentral gyrus result in paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

 of the contralateral side of the body (facial palsy, arm-/leg monoparesis, hemiparesis
Hemiparesis
Hemiparesis is weakness on one side of the body. It is less severe than hemiplegia - the total paralysis of the arm, leg, and trunk on one side of the body. Thus, the patient can move the impaired side of his body, but with reduced muscular strength....

) - see upper motor neuron
Upper motor neuron
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle...

.

See also

  • Corticospinal tract
    Corticospinal tract
    The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....

  • Motor cortex
    Motor cortex
    Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...

  • Upper motor neuron
    Upper motor neuron
    Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle...

  • Brodmann area
    Brodmann area
    A Brodmann area is a region of the cerebral cortex defined based on its cytoarchitectonics, or structure and organization of cells.-History:...

  • List of regions in the human brain

External links