The primary motor cortex (or M1) is a brain region that in humans is located in the posterior portion of the
frontal lobeThe 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- 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 cellBetz 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
axonsAn 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 cordThe 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 gangliaThe 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
cerebellumThe 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
dorsalStandard 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
anteriorStandard 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-External links:* via the Neuroscience Information Framework...
. The precentral gyrus is anterior to the
postcentral gyrusThe 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 sulcusThe precentral sulcus lies parallel to, and in front of, the central sulcus....
, while on the
inferiorStandard 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
medialStandard 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 lobuleThe 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 areaA 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
cortexThe 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 neuronA 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
axonAn 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 neuronLower 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 cordThe 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 tractThe 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 neuronUpper 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
homunculusHomunculus 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
homunculusHomunculus 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 fissureThe 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
primateA 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
axonAn 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 matterWhite 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 capsuleThe 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
synapseIn 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 neuronLower 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 oblongataThe 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 decussationThe 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 cordThe 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 tractThe 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 tractThe 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 cordThe 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 neuronLower 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
provide most of the arterial blood supply for the
primary motor cortexMotor 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-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-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
paralysisParalysis 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,
hemiparesisHemiparesis 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 neuronUpper 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
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 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 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
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