Motor Cortex

Overview:

The human brain controls and executes various kinds of movements throughout the body.

The motor cortex is the cerebral cortex region responsible for the planning, control, and implementation of voluntary movements. The primary motor cortex, positioned right in front of the central sulcus, is the area that sends the most critical signal for expert movement performance. Based on the region stimulated, electrical stimulation of this location causes focused movements of muscle groups on the opposite side of the body.

In this article, the magnitude of functions that are executed by the motor cortex is explained, along with some complications that involve it.

Summary:

  • The motor cortex is frequently split into two main territories: the primary motor cortex and the nonprimary cortex
  • The primary motor cortex is characterized by the fact that distinct regions of the area are connected with motor control of various parts of the body
  • The nonprimary motor regions are only engaged in other components of movement, including such movement preparation and action selection dependent on environmental context
  • The motor cortex is divided into three sections of the frontal lobe, all of which are located directly anterior to the central sulcus
  • Whenever the primary motor cortex is injured, the individual often exhibits poor movement coordination and dexterity

Location:

The motor cortex is located in the frontal lobe, and it extends across a region of the cortex slightly anterior to the central sulcus, which extends down the side of the cerebral hemispheres. 

Structure:

The motor cortex is frequently split into two main territories: the primary motor cortex, which is located in a gyrus recognized as the precentral gyrus in front of the central sulcus, and the nonprimary motor cortex, which is located anterior to the primary motor cortex and includes two notable areas that are referred to as the premotor cortex and supplementary motor cortex (Figure 1).

Function:

Employing conscious dogs as their subjects, doctors Gustav Theodor Fritsch and Eduard Hitzig electrically activated the part of the brain presently known as the motor cortex and discovered that the activation led the dogs to act unconsciously.

Furthermore, they discovered that activating the motor cortex in various areas induced distinct muscles in the body to move.  The motor cortex was identified as the key part of the human brain engaged in the implementation and scheduling of voluntary movements as a result of this investigation.

Figure 1: Motor Cortex

The motor cortex is divided into several separate areas. The main motor cortex has been proven to be the most responsive to electrical activity, requiring the least amount of input to produce a matching muscle action. The primary motor cortex is designed in such a way that distinct regions of the area are connected with motor control of various parts of the body.

The primary motor cortex is characterized by the fact that distinct regions of the area are connected with motor control of various parts of the body, a topographic arrangement comparable to, but less exact than, that shown in the somatosensory cortex. (Know Your Brain: Motor Cortex, 2015)

The primary motor cortex comprises pyramidal nerve cells, which are big nerve cells with cell bodies that are in the shape of a triangle, and serve as the motor cortex’s major output cells. Pyramidal cell axons exit the motor cortex communicating information about such a planned motion and join one of the pyramidal system’s tracts, which include the corticospinal and corticobulbar tracts. 

The corticospinal tract transmits motor information from the cortex to the spinal cord to begin body movement, whereas the corticobulbar tract delivers motor data to the brainstem to activate cranial nerve nuclei and produce head movement, face muscle movement, and movement in the neck region.

Upper nerve cells are another name for pyramidal nerve cells in the motor cortex. They link to nerve cells known as lower motor nerve cells, which effectively innervate skeletal muscle to create movement.

Other parts of the motor cortex, defined as the nonprimary motor cortex, are located anterior to the primary motor cortex and seem to act in vital positions during motion as well. Unlike their name, the nonprimary motor regions do not serve as a backup to the primary motor cortex.

However, the nonprimary motor regions are only engaged in other components of movement, including such movement preparation and action selection dependent on environmental context.

The supplemental motor cortex and the premotor cortex are two major sections of the nonprimary motor cortex. These regions’ precise roles are not very well recognized. The supplementary motor cortex is assumed to be crucial in the implementation of movement succession, the acquisition of motor skills, and the executive control of motion, which can include things like making judgments to change to various movements associated with the received sensory input.

The premotor cortex contributes significantly (about thirty percent) to the nerve cells that reach the corticospinal tract, however, it appears to be more engaged as compared to the primary motor cortex throughout movement preparation instead of conducting the activity.

Premotor cortex nerve cells appear to play a part in the incorporation of sensory cues (– for example, the placement of an item to be gripped) into a motion to ensure adequate execution, and the choice of movements based on behavioural circumstances (for example picking up a vase to move it from the corner vs. picking up a vase to arrange flowers in it). 

There are other groups of nerve cells in the premotor cortex that are active when we watch somebody else perform a motion; these cells may be brought in to help us comprehend and/or copy the movements of someone else.

The motor cortex is divided into three sections of the frontal lobe, all of which are located directly anterior to the central sulcus. The primary motor cortex (Brodmann’s area 4), the premotor cortex, and the supplementary motor cortex are the three areas in question. The electrical activity of these regions causes certain parts of the body to move. 

Upon this medial surface of the brain, the primary motor cortex, or M1, is positioned on the precentral gyrus and the anterior paracentral lobule. The main motor cortex takes the lowest amount of electrical energy to trigger a movement among the three motor cortex sections. Minor motions of particular parts of the body are often elicited by low doses of short stimulation.

Premotor cortex or supplemental motor region activation demands greater current to induce movements and commonly occurs in more intricate actions than primary motor cortex activation. Longer-term stimulation in monkeys leads to the motion of a specific part of the body to a stereotyped pose or posture, irrespective of the body part’s original starting place.

As a consequence, the premotor cortex and supplementary motor regions seem to be higher-level regions that integrate complex issues of motor output and select the optimal motor strategy to accomplish desired outcomes.

The main motor cortex, like the somatosensory cortex of the postcentral gyrus, is somatotopically structured. Actions of the contralateral leg are elicited by activation of the anterior paracentral lobule. Actions from the arm, hand, arm and face are induced gradually as the stimulating electrode is shifted across the precentral gyrus from dorsomedial to ventrolateral (most laterally). 

Parts of the body that execute exact, sensitive movements, like the face and hands, have relatively large depictions contrasted to parts of the body that produce only rough, unsophisticated movement patterns, such as the waist or thighs. Somatotopic maps can also be found in the premotor cortex and supplementary motor region.

The motor cortex “homunculus” could be predicted to form since nerve cells that regulate particular muscles are crowded together in the brain. That also is, all of the nerve cells controlling the biceps muscle may be concentrated close, all of the nerve cells controlling the triceps may be grouped close, and the nerve cells controlling the soleus muscle may be grouped further apart. 

Nevertheless, electrophysiological measurements have revealed that this is not the situation. Separate muscle movements are linked to activation in various areas of the primary motor cortex. Likewise, activation of tiny parts of the primary motor cortex produces motions that necessitate the activation of multiple muscles.

As a result, the primary motor cortex homunculus doesn’t reflect specific muscle action. Instead, it appears to depict particular body component motions, which frequently necessitate the concerted response of huge groups of muscles throughout the body (Figure 2).

The motor cortex influences muscles via several descending pathways. Motor cortex output can alter some of the descending pathways.  Therefore, in combination with the corticospinal tract’s direct cortical innervation of alpha nerve cells, the accompanying cortical efferent routes regulate the other descending tracts:

  • The corticotectal tract enables the cortex to influence the tectospinal tract.
  • The corticoreticular tract enables the cortex to control the reticulospinal tracts.
  • The corticorubral tract enables the cortex to influence the rubrospinal tract.

The cortex can also impact the interpretation of the motor hierarchy’s side loops. The caudate nucleus and putamen of the basal ganglia are innervated via the corticostriate tract. Significant signals to the cerebellum are innervated by the corticopontine and corticocoolivary tracts. Lastly, cortical areas can affect other cortical regions both in a direct and indirect manner via corticocortical routes.

The majority of these routes are bidirectional. Therefore, the motor cortex gets information from various cortical areas, both in a direct and indirect way via the thalamus, as well as information from the cerebellum and basal ganglia, constantly via the thalamus.

The main motor cortex, like the rest of the neocortex, is composed of six layers. Instead of like in the primary sensory regions, the primary motor cortex is agranular, meaning it lacks a cell-packed granular layer (layer 4). Rather, the primary motor cortex’s main distinguishing layer is its descending output layer (Layer 5), which houses the massive Betz cells. 

These pyramidal cells and other primary motor cortex projecting nerve cells account for thirty percent (30%) of the fibres in the corticospinal tract. The remaining fibres come from the premotor cortex and supplementary motor region (about thirty percent), the somatosensory cortex (about thirty percent), and the posterior parietal cortex (approximately ten percent).

The primary motor cortex doesn’t seem to regulate single muscles individually, but instead specific movements or sequences of actions that involve the activation of numerous muscle groups. The strength of contraction of clusters of muscle fibres is encoded by alpha motor nerve cells in the spinal cord employing the rate code and the size principle.

Therefore, according to the notion of the motor system’s hierarchical structure, the data provided by the motor cortex is a higher degree of abstraction than the results provided by spinal nerve cells.

The recording of the activity of these neurons while experimental animals do various motor activities has yielded clues. Generally, the primary motor cortex contains the variables that describe individual motions or simple movement patterns.

Nerve cells in the primary motor cortex fire 5-100 msec before the actual initiation of a motion. Instead of activating in response to muscle movement, these nerve cells are engaged in conveying motor orders to alpha nerve cells, which ultimately trigger the proper muscles to contract.

The power of a movement is encoded by the primary motor cortex. When carrying a tennis racket, the level of power necessary to elevate the elbow from one point to another is substantially more than when carrying a beach ball. Numerous nerve cells in the primary motor cortex contain the amount of effort needed to perform such a motion. It is important to distinguish between movement energy and muscle force. 

While a small number of nerve cells in the primary motor cortex encode singular muscle force, a greater number communicates the amount of force required for a certain action, irrespective of which individual muscles are engaged. 

Under the rate code and the size principle, alpha nerve cells interpret the instructions of motor cortex nerve cells and govern the degree of force produced by individual muscles to act.

Many nerve cells in the primary motor cortex are directed toward a certain direction of motion. For instance, when the palm is moved slightly to the right, one cell may fire powerfully, whilst another cell is suppressed when the palm is moved to the left side. 

The firing of some nerve cells is proportional to the distance travelled during a movement. A monkey was taught to move its arms to various target sites that differed in direction and distance from the centre. Several nerve cells’ firing was connected with the direction of motion, whilst other nerve cells’ firing was associated with movement distance. (Knierim, 2020)

Surprisingly, certain nerve cells were associated with the interplay of a specific distance and direction; that is, they were associated with a specific target point.

Mostly all directed motions obey a classic bell-shaped velocity-versus-distance curve. For instance, whenever the hand moves an item from one point to another (the destination), it speeds during the first half of the motion, achieves a maximum velocity around midway to the goal, and then slows down until it reaches the destination.

The firing rate of some monkey primary motor cortex nerve cells coincides with this bell-shaped speed profile, indicating that knowledge about movement speed is stored in these cells’ spike trains.

The premotor cortex transmits axons straight to the primary motor cortex and also the spinal cord. It processes more complicated, job-associated information than the primary motor cortex. High-current activation of the monkey’s premotor regions results in more complex poses than activation of the primary motor cortex.

The premotor cortex appears to be involved in the selection of acceptable motor planning for voluntary motor activity, while the main motor cortex seems to be engaged in their implementation.

Complications involving Motor Cortex:

Whenever the primary motor cortex is injured, the individual often exhibits poor movement coordination and dexterity. The individual, for instance, cannot frequently conduct fine motor motions.

The muscles of the wrists, knuckles, and hands are involved in fine motor movements. Fine motor abilities are the capacity to move each individual and coordinate motions.

Upper motor nerve cell syndrome is characterized by a change in muscle tone, either hypotonia (low/decreased muscle tone) or hypertonia (high/increased muscle tone). These disorders have the potential to impair movement and coordination. (Primary Motor Cortex Damage: Definition, Symptoms, and Treatment, 2020)

Because the muscles of a person who has had damage to the primary motor cortex often fatigue quickly, strengthening endurance will be a priority of therapy.

Conclusion:

The motor cortex is divided into three sections of the frontal lobe, all of which are located directly anterior to the central sulcus. The power of a movement is encoded by the primary motor cortex. The nonprimary motor regions are only engaged in other components of movement, including such movement preparation and action selection dependent on environmental context. The primary motor cortex doesn’t seem to regulate single muscles individually, but instead specific movements or sequences of actions that involve the activation of numerous muscle groups Whenever the primary motor cortex is injured, the individual often exhibits poor movement coordination and dexterity. The individual, for instance, cannot frequently conduct fine motor motions.

Bibliography

  • Know Your Brain: Motor Cortex. (2015, October 23). Retrieved from Neuroscientifically Challenged: https://www.neuroscientificallychallenged.com/blog/know-your-brain-motor-cortex
  • Primary Motor Cortex Damage: Definition, Symptoms, and Treatment. (2020, November 19). Retrieved from FlintRehab: https://www.flintrehab.com/primary-motor-cortex-damage/
  • Knierim, J. (2020, October 20). Chapter 3: Motor Cortex. Retrieved from Neuroscience online: https://nba.uth.tmc.edu/neuroscience/m/s3/index.htm