| Primary motor cortex | |
|---|---|
Brodmann area 4 of human brain.
| |
Primary motor cortex shown in green.
| |
| Details | |
| Part of | Precentral gyrus |
| Artery | Anterior cerebral Middle cerebral |
| Identifiers | |
| Latin | cortex motorius primus |
| NeuroNames | 1910 |
| NeuroLex ID | nlx_143555 |
| FMA | 224854 |
Animation. Primary motor cortex (Brodmann area 4) of the left cerebral hemisphere shown in red.
The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor cortex, the supplementary motor area, posterior parietal cortex,
and several subcortical brain regions, to plan and execute movements.
Primary motor cortex is defined anatomically as the region of cortex
that contains large neurons known as Betz cells. Betz cells, along with other cortical neurons, send long axons down the spinal cord to synapse
onto the interneuron circuitry of the spinal cord and also directly
onto the alpha motor neurons in the spinal cord which connect to the
muscles.
At the primary motor cortex, motor representation is orderly
arranged (in an inverted fashion) from the toe (at the top of the
cerebral hemisphere) to mouth (at the bottom) along a fold in the cortex
called the central sulcus.
However, some body parts may be controlled by partially overlapping
regions of cortex. Each cerebral hemisphere of the primary motor cortex
only contains a motor representation of the opposite (contralateral)
side of the body. The amount of primary motor cortex devoted to a body
part is not proportional to the absolute size of the body surface, but,
instead, to the relative density of cutaneous motor receptors on said
body part. The density of cutaneous motor receptors on the body part is
generally indicative of the necessary degree of precision of movement
required at that body part. For this reason, the human hands and face
have a much larger representation than the legs.
For the discovery of the primary motor cortex and its relationship to other motor cortical areas, see the main article on the motor cortex.
Structure
The
human primary motor cortex is located on the anterior wall of the
central sulcus. It also extends anteriorly out of the sulcus partly onto
the precentral gyrus. Anteriorly, the primary motor cortex is bordered
by a set of areas that lie on the precentral gyrus and that are
generally considered to compose the lateral premotor cortex.
Posteriorly, the primary motor cortex is bordered by the primary
somatosensory cortex, which lies on the posterior wall of the central
sulcus. Ventrally the primary motor cortex is bordered by the insular
cortex in the lateral sulcus. The primary motor cortex extends dorsally
to the top of the hemisphere and then continues onto the medial wall of
the hemisphere.
The location of the primary motor cortex is most obvious on histological examination due to the presence of the distinctive Betz cells. Layer V of the primary motor cortex contains giant (70-100 µm) pyramidal neurons which are the Betz cells. These neurons 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 a part of the corticospinal tract.
The Betz cells account for only a small percentage of the corticospinal
tract. By some measures they account for about 10% of the primary motor
cortex neurons projecting to the spinal cord or about 2-3% of the total cortical projection to the spinal cord.
Though the Betz cells do not compose the entire motor output of the
cortex, they nonetheless provide a clear marker for the primary motor
cortex. This region of cortex, characterized by the presence of Betz
cells, was termed area 4 by Brodmann.
Pathway
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.
Corticomotorneurons
Corticomotorneurons
are neurons in the primary cortex which project directly to motor
neurons in the ventral horn of the spinal cord. Axons of corticomotorneurons terminate on the spinal motor neurons of multiple muscles as well as on spinal interneurons.
They are unique to primates and it has been suggested that their
function is the adaptive control of the distal extremities (e.g. the
hands) including the relatively independent control of individual
fingers. Corticomotorneurons have so far only been found in the primary motor cortex and not in secondary motor areas.
Blood supply
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.
Function
Homunculus
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, in interior sections of the motor area folding into the medial longitudinal fissure.
The lateral, convex side 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. The arm and hand
motor area is the largest, and occupies the part of precentral gyrus
between the leg and face area.
These areas are not proportional to their size in the body with
the lips, face parts, and hands represented by particularly large areas.
Following amputation or paralysis, motor areas can shift to adopt new
parts of the body.
Neural input from the thalamus
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
Alternative maps
Map of the body in the human brain
At least two modifications to the classical somatotopic ordering of
body parts have been reported in the primary motor cortex of primates.
First, the arm representation may be organized in a core and
surround manner. In the monkey cortex, the digits of the hand are
represented in a core area at the posterior edge of the primary motor
cortex. This core area is surrounded on three sides (on the dorsal,
anterior, and ventral sides) by a representation of the more proximal
parts of the arm including the elbow and shoulder. In humans, the digit representation is surrounded dorsally, anteriorly, and ventrally, by a representation of the wrist.
A second modification of the classical somatotopic ordering of
body parts is a double representation of the digits and wrist studied
mainly in the human motor cortex. One representation lies in a posterior
region called area 4p, and the other lies in an anterior region called
area 4a. The posterior area can be activated by attention without any
sensory feedback and has been suggested to be important for initiation
of movements, while the anterior area is dependent on sensory feedback. It can also be activated by imaginary finger movements
and listening to speech while making no actual movements. This anterior
representation area has been suggested to be important in executing
movements involving complex sensoriomotor interactions.
It is possible that area 4a in humans corresponds to some parts of the
caudal premotor cortex as described in the monkey cortex.
In 2009, it was reported, that there are two evolutionary
distinct regions, an older one on the outer surface, and a new one found
in the cleft. The older one connects to the spinal motorneurons through
interneurons in the spinal cord. The newer one, found only in monkeys
and apes, connects directly to the spinal motorneurons.
The direct connections form after birth, are dominant over the indirect
connections, and are more flexible in the circuits they can develop
which allows the post-natal learning of complex fine motor skills. "The
emergence of the 'new' M1 region during evolution of the primate lineage
is therefore likely to have been important for the enhanced manual
dexterity of the human hand."
Common misconceptions
Certain
misconceptions about the primary motor cortex are common in secondary
reviews, textbooks, and popular material. Three of the more common
misconceptions are listed here.
Segregated map of the body
One
of the most common misconceptions about the primary motor cortex is
that the map of the body is cleanly segregated. Yet it is not a map of
individuated muscles or even individuated body parts. The map contains
considerable overlap. This overlap increases in more anterior regions of
the primary motor cortex. One of the main goals in the history of work
on the motor cortex was to determine just how much the different body
parts are overlapped or segregated in the motor cortex. Researchers who
addressed this issue found that the map of the hand, arm, and shoulder
contained extensive overlap.
Studies that map the precise functional connectivity from cortical
neurons to muscles show that even a single neuron in the primary motor
cortex can influence the activity of many muscles related to many
joints.
In experiments on cats and monkeys, as animals learn complex,
coordinated movements, the map in the primary motor cortex becomes more
overlapping, evidently learning to integrate the control of many
muscles. In monkeys, when electrical stimulation is applied to the motor cortex on a behavioral timescale, it evokes complex, highly integrated movements such as reaching with the hand shaped to grasp, or bringing the hand to the mouth and opening the mouth.
This type of evidence suggests that the primary motor cortex, while
containing a rough map of the body, may participate in integrating
muscles in meaningful ways rather than in segregating the control of
individual muscle groups. It has been suggested that a deeper principle
of organization may be a map of the statistical correlations in the
behavioral repertoire, rather than a map of body parts.
To the extent that the movement repertoire breaks down partly into the
actions of separate body parts, the map contains a rough and overlapping
body arrangement.
M1 and primary motor cortex
The
term "M1" and the term "primary motor cortex" are often used
interchangeably. However, they come from different historical traditions
and refer to different divisions of cortex. Some scientists suggested
that the motor cortex could be divided into a primary motor strip that
was more posterior and a lateral premotor strip that was more anterior.
Early researchers who originally proposed this view included Campbell, Vogt and Vogt Foerster, and Fulton.
Others suggested that the motor cortex could not be divided in that
manner. Instead, in this second view, the so-called primary motor and
lateral premotor strips together composed a single cortical area termed
M1. A second motor area on the medial wall of the hemisphere was termed
M2 or the supplementary motor area. Proponents of this view included Penfield and Woolsey.
Today the distinction between the primary motor cortex and the lateral
premotor cortex is generally accepted. However, the term M1 is sometimes
mistakenly used to refer to the primary motor cortex. Strictly speaking
M1 refers to the single map that, according to some previous
researchers, encompassed both the primary motor and the lateral premotor
cortex.
Betz cells as the final common pathway
The Betz cells,
or giant pyramidal cells in the primary motor cortex, are sometimes
mistaken to be the only or main output from the cortex to the spinal
cord. This mistake is old, dating back at least to Campbell in 1905. Yet the Betz cells compose only about 2-3% of the neurons that project from the cortex to the spinal cord, and only about 10% of the neurons that project specifically from the primary motor cortex to the spinal cord. A range of cortical areas including the premotor cortex, the supplementary motor area,
and even the primary somatosensory cortex, project to the spinal cord.
Even when the Betz cells are damaged, the cortex can still communicate
to subcortical motor structures and control movement. If the primary
motor cortex with its Betz cells is damaged, a temporary paralysis
results and other cortical areas can evidently take over some of the
lost function.
Clinical significance
Lesions of the precentral gyrus result in paralysis of the contralateral side of the body (facial palsy, arm-/leg monoparesis, hemiparesis).
Movement coding
Evarts
suggested that each neuron in the motor cortex contributes to the force
in a muscle. As the neuron becomes active, it sends a signal to the
spinal cord, the signal is relayed to a motorneuron, the motorneuron
sends a signal to a muscle, and the muscle contracts. The more activity
in the motor cortex neuron, the more muscle force.
Georgopoulos and colleagues
suggested that muscle force alone was too simple a description. They
trained monkeys to reach in various directions and monitored the
activity of neurons in the motor cortex. They found that each neuron in
the motor cortex was maximally active during a specific direction of
reach, and responded less well to neighboring directions of reach. On
this basis they suggested that neurons in motor cortex, by "voting" or
pooling their influences into a "population code", could precisely specify a direction of reach.
The proposal that motor cortex neurons encode the direction of a reach became controversial. Scott and Kalaska
showed that each motor cortex neuron was better correlated with the
details of joint movement and muscle force than with the direction of
the reach. Schwartz and colleagues showed that motor cortex neurons were well correlated with the speed of the hand. Strick and colleagues
found that some neurons in motor cortex were active in association with
muscle force and some with the spatial direction of movement. Todorov
proposed that the many different correlations are the result of a
muscle controller in which many movement parameters happen to be
correlated with muscle force.
The code by which neurons in the primate motor cortex control the spinal cord, and thus movement, remains debated.
Some specific progress in understanding how motor cortex causes
movement has also been made in the rodent model. The rodent motor
cortex, like the monkey motor cortex, may contain subregions that
emphasize different common types of actions. For example, one region appears to emphasize the rhythmic control of whisking. Neurons in this region project to a specific subcortical nucleus in which a pattern generator coordinates the cyclic rhythm of the whiskers. This nucleus then projects to the muscles that control the whiskers.
