Motor Cortex Function

Essentially, the motor cortex generates electrical impulses for voluntary motion. These signals are transmitted to the relevant body parts and elicit movement. Situated within the cerebral cortex, this region is responsible for coordinating, regulating, and executing voluntary actions. The motor cortex belongs to a hierarchical system.

The motor cortex functions to facilitate voluntary movement. This area resides in the frontal lobe and is positioned anterior to the central sulcus; this region of the cerebral cortex involves higher-level operations. It produces electronic impulses and transmits signals to the relevant areas.

The motor cortex is separated into the primary, premotor, and supplementary. The primary motor cortex is tasked with acquiring and executing skilled, fine movements. The premotor and supplementary cortex participate in administering and selecting the correct actions.

An Introduction to the Motor Cortex

There is a hierarchy to the operation of the motor system. This complex arrangement regulates body movement. It administers unconscious, voluntary motion, including feedback and fine-tuning processes. The muscle’s activity is the final connection in the motor cortex’s highly intricate organization.

The motor cortex isn’t consistently made up of a granular cell layer. Granular cells are minute cells (smallest) and are abundant across the brain. They’re tasked with numerous processes, including facilitating memory and visual capacity.

To further illustrate this point, the primary motor cortex is agranular. This state entails that it isn’t a cell pack granular layer. Instead, the primary motor cortex consists of Betz cells. (discussed in more detail below) This is relevant as the function of the motor cortex requires pyramidal Betz cells.

Specific alpha motor neurons manage the energy required for a muscle to engage. At the same time, spinal circuits direct the multifaceted and immensely involved communication network that directs reflex actions- like lifting one’s legs to walk. Reflex actions are automatic responses or reactions of a muscle or muscle collection to an impulse incited by an afferent nerve.  

It’s relevant to remember that reflex actions engage independently of conscious thought. Within the grading of the motor system, the above-mentioned neuronal communications are the lower-level features of the hierarchy.

The third and fourth features of the motor system hierarchy produce voluntary movement. These features or system levels belong to the motor cortex and the association cortex. Therefore, the function of the motor cortex is to manage voluntary action.   

Voluntary performance requires knowledge, and there are various facets of motor movement. These include movement preparation, synchronicity, spatial cognizance, autonomy, communication, execution, and transformation.

These regions belong to the cerebral cortex, and they belong to a higher level of functioning. The third and fourth-level hierarchies strategize with immediate forethought a person’s conscious actions. They choreograph a person’s desired sequence of motion. Again, the emphasis is on voluntary movement.

The Motor Cortex Function

When understanding the motor cortex function, it’s noteworthy to regard the role of evaluation, anticipation, and strategy afforded to people in real-life situations. The motor cortex assesses the appropriate bodily movements needed in any given context.

An interesting component of the evaluation capacity of the motor cortex function is the more sophisticated components of informing a person’s behavior. The motor cortices function in determining behavioral strategies illustrates the higher-level aspect and placement of this cranial area.

Environmental or real-life context involvement in determining a person’s strategy requires understanding the role of ethos, desired movement, and voluntary behavior. That is, motor neuron outputs are context-sensitive.

Another fascinating dynamic of the motor cortex is its encoding capacity. That is, if a specific area of a primate’s motor cortex is subjected to an electrical current, the consistent result is a predetermined posture or stance. Therefore, studies reveal that the motor cortex encodes stereotyped bodily positions.

The Front and Posterior Parietal Cortex

For a person to perform a voluntary movement, the frontal and posterior parietal cortex (PPC)  plays a significant role. However, this process, although thoroughly studied, remains considerably unknown. The PPC comprises several features, and these agreed definitions and distinctions have developed over time.

The prefrontal cortex and posterior frontal assess the environment through spatial awareness capacity. It enables goal-directed behavior. Research on this area initially positioned that the PPC simply instructed limbs, hands, and eyes. Today, specified PPC sectors are revealed to neuronally encode grasping and reaching actions formulated on visual cues.

The PPC has a noteworthy relationship with the prefrontal cortex. Jointly these two regions are in the highest state of interconnectedness in the motor control hierarchy. In this state, voluntary decisions surrounding motion are enabled.

The Motor Cortex Location

The motor cortex was understood to simply represent the body’s muscles. Now, the motor cortex, after decades of thorough research and study, has been discovered for its immense complexity and network system choreography.

The motor cortex resides in the brain’s frontal lobe, aligned anterior to the central sulcus. This principal sulcus, also referred to as the sulcus of Rolando, is a significant anatomical boundary or groove. It distinguishes the frontal and parietal lobes on the lateral and medial planes of the cerebral hemispheres.

The motor cortex is partitioned into the primary motor cortex, the premotor cortex, and the supplementary motor area. That is, the frontal lobe is divided into these sections. Acknowledging the frontal lobe’s role in performing voluntary movement is relevant.

The Primary Motor Cortex

The primary motor cortex (M1) plays a vital role in consolidating voluntary, skilled, and nuanced movements. Described as the powerhouse of the motor cortex, this layer of brain tissue is situated in a gyrus or ridge named the precentral gyrus. This gyrus is immediately anterior to the central sulcus.

The precentral gyrus is the incipient of a collection of motor pathways. These are the corticospinal tract, the corticobulbar tract, and the cortico-rubrospinal tract. The axons of these tracts travel across all brain regions in a complex, multifaceted way.

The primary motor cortex is in Brodmann area 4. This site transmits the predominant number of electrical impulses compared to the entire motor cortex region. Area 4 sends projections that synapses directly to motor neurons of the spinal cord.

Corticospinal fibers stem from the frontal and parietal cortex. The fibers from the primary motor cortex terminate almost exclusively in the spinal cord. The corticospinal tract is a prominent descending pathway of the central nervous system and tracks the length of the spinal cord.

Primary Motor Cortex Functions

This area is tasked with commencing directed and deliberate body movement. These movements, which are governed by intent, include facial expressions, swallowing, and the body’s extremities. The primary motor cortex directs skilled actions.

When the primary visual cortex functions under normal circumstances, electrical signals traverse the body’s center to stimulate muscles on the opposite side of the brain’s hemisphere. Therefore, the right hemisphere of the primary motor cortex determines the body’s movements on the left side. The brain operates contralaterally.

In addition, various distinct regions of Brodmann area 4 are tasked with regulating motor coordination of individual body parts. Within the primary motor cortex is a representation of every single aspect belonging to the body. However, the brain’s matter isn’t equally proportioned to each part.

This design details that more intricate, complex movements requiring precision occupy more cranial space than simple motions. Different regions in the primary motor cortex are responsible for individual body features. These cranial features are arranged in the same design as the body. That is, the arm represented in the motor cortex follows the hand.

The primary motor cortex is topographically arranged. That is, this region is a somatotropic representation. This neurological portrayal is frequently described as the motor homunculus or little man. It stretches along the precentral gyrus and administers electronic impulses directed from the premotor zone of the frontal lobe.

The Primary Motor Cortex and Pyramidal Neurons

The primary motor cortex consists predominantly of triangular-shaped axons (pyramidal neurons), the primary motor cortices’ central output cells. The axons transmit the desired motion information as signals. These impulses project into one of the tracts of the pyramidal systems.

The pyramidal neurons are projection neurons that send their electronic signals for longer distances. In layer V of the motor cortex, the pyramidal cells travel their axons down their spine and transmit messages directly to muscles.

The Betz cells are gigantic cells that belong to layer V of the primary motor cortex. They’re also referred to as pyramidal cells of Betz. These upper neurons are giant axons belonging to the central nervous system. They travel via the corticospinal tract (mentioned above), down the spinal cord, and synapse directly to anterior horn cells. The anterior horn cells synapse directly with muscles.

The Premotor Cortex

The premotor cortex resides immediately in front of the primary motor cortex. That is, it’s positioned anteriorly or rostrally to the primary motor cortex. This intricate interconnecting layer is vital to the overall functioning capacity of the motor cortex. Its distinction from the primary motor cortex is obvious- discussed below.

The upper motor neurons within the premotor cortex determine motion via an immense network of reciprocal connections. In addition, movement is relayed through projections into the corticobulbar and corticospinal pathways. These projections directly impact the lower-level motor circuitry- the spinal cord and brain stem. 

Approximately 30% of the axons in the corticospinal pathway develop from the premotor cortex. It’s widely understood that the premotor cortex employs and distributes information from neighboring cortical areas. It channels directives from these regions to elicit the required action.

The premotor cortex resides in Brodmann’s area 6. This area is positioned on the lateral plane of the cerebral hemisphere. It extends to the medial aspect of area 6 and follows the midline plane of the hemisphere. The supplementary motor area designates the midline plane- discussed below.

The Premotor Cortex Function

The premotor cortex’s role in the body’s movement is significant. The premotor cortex functions are determined according to this area’s lateral and medial planes. The functions are varied and not entirely understood. The premotor cortex regulates abstract rules to perform specific tasks.

Other functions of the premotor cortex include spatial awareness, connection to the core muscles, preparing, and planning movement. Approximately 65% of axons in the lateral premotor cortex allow reactions that connect signals with the appropriate time execution.

Essentially, the lateral premotor cortex administers movement selection. It prepares the body for deciding which movement will be executed. Instead of directly initiating action, the premotor cortex neurons encode the intention behind the actual performance.

The axons discharge an electrical impulse to the onset of external cues. Motion is governed by sensory feedback. Research where primates have had lesions to the lateral motor cortex area has revealed an impairment to perform movements from visual cues.

The medial premotor cortex, as with the lateral premotor cortex, oversees movement selection. However, the area elicits movements determined by internal cues instead of external ones. Motion sequences and postures are performed from memory, not external signals. Neurons in this area begin to discharge one to two seconds before action.

Premotor Cortex Structure

Various subregions of the premotor cortex exhibit individual properties and supposedly accentuate different motor abilities. The nerve impulses of the premotor cortex create more complex patterns of movement than the simpler patterns produced by the primary cortex.  

Several subregions make up the premotor cortex. These areas are distinguished by their fine cytoarchitecture. In addition, they have distinct anatomical properties and connect to different parts of the brain.  

The network of the premotor cortex is varied and defined as heterogeneous. The diverse arrangement connects the separate subregions to other cranial areas. A set of acronyms summarizes the divisions of the premotor cortex: PMDr (premotor dorsal, rostral), PMDc (premotor dorsal, caudal), PMVr (premotor ventral, rostral), PMVc (premotor ventral, caudal).

It’s generally understood that the premotor cortex has sturdy afferent (input) and efferent (output) communication networks to the superior and inferior parietal cortex and prefrontal lobes. Below the cerebral cortex, the premotor cortex connects directly to the spinal cord, the striatum, and the thalamus.

The Primary Motor Cortex Versus the Premotor Cortex

Several physiological identifiers differentiate the premotor cortex from the primary motor cortex. Firstly, the primary motor cortex contains a massive number of pyramidal axons. These cells are called Betz cells (mentioned above) and are situated in layer V. Described as cytoarchitecture (the cortex viewed under a microscope), the distinction between the primary and premotor is evident.  

In comparison, the premotor cortex has only a few large pyramidal cells, which are smaller overall. Secondly, the primary motor cortex is agranular. The characteristic entails that it’s absent of a layer IV. Layer IV is entirely comprised of granular cells. The premotor cortex is dysgranular, and it’s comprised of a minimum layer IV.  

Brodmann Area, 46 of the prefrontal cortex, anterior to the premotor cortex, is entirely granular layer IV. Therefore, the premotor cortex is a physiological transition between the agranular motor cortex and the six-layered, granular prefrontal cortex.

The comparative electrical current also distinguishes the variation between the premotor cortex and the primary cortex between the adjacent cortices. The primary motor cortex has a low-intensity electrical threshold. In contrast, the premotor cortex requires more intense levels of electrical stimulation.

Consequently, higher electrical stimulation of the premotor cortex results in more complex motion patterns. In addition, the low charge threshold required to elicit movement from the primary motor cortex reveals the apparent neuronal connection from this area to the lower motor neuron from the brain stem and spinal cord.

The primary motor cortex directs movement execution according to the required time, whereas the premotor cortex reveals the firing of neurons specialized in the direction of movement. To further illustrate this point, lateral premotor neurons are stimulated at specific visual cues and prepare the body to react well before the primary motor neurons initiate necessary movement.

The Supplementary Motor Cortex

The supplementary motor area (SMA) inhabits one-third of the superior frontal gyrus, and it’s positioned posterior to it. This high-level functioning region is tasked with organizing intricate bodily movements contralaterally. This area also executes ipsilateral (same-sided) to a small degree. It’s important to note that the full functions of this area need more study

The supplementary motor cortex is aligned along the medial plane of the longitudinal fissure, positioned ventrally to the precentral gyrus. It’s immediately anterior to the cortical mapping or representation of the leg. Again, even though the functions of the supplementary motor cortex aren’t entirely understood, it’s proposed that it serves to support posture and coordination.

The premotor and supplementary cortices require higher electrical stimulation and encode complex postures, stereotyped positions, and movement patterns. Electrical stimulation reveals primarily contralateral signal transmission and somatotopic representation of the supplementary motor area.

Conclusion

The motor cortex function elicits movement from signals transferred via electrical impulses. The communication network of the motor cortex is incredibly intricate and sophisticated. The motor cortex resides within the frontal lobe and displays higher-level functioning and cognitive ability.

The motor cortex comprises three individual cortices: the primary motor cortex, the premotor cortex, and the supplementary motor cortex. The primary motor cortex involves fine, skilled motion and is mainly made from giant pyramidal neurons. The premotor and supplementary cortices are closely related anatomically to and output complex actions.

References

https://nba.uth.tmc.edu/neuroscience/m/s3/chapter03.html

https://www.sciencedirect.com/topics/neuroscience/supplementary-motor-area

https://study.com/academy/lesson/functions-of-the-premotor-cortex.html

https://pubmed.ncbi.nlm.nih.gov/31194345/#:~:text=The%20primary%20function%20of%20the,and%20the%20supplementary%20motor%20area.

https://www.simplypsychology.org/motor-cortex.html

https://en.wikipedia.org/wiki/Motor_cortex

https://www.jneurosci.org/content/38/6/1430

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179288

https://thebrain.mcgill.ca/flash/d/d_06/d_06_cr/d_06_cr_mou/d_06_cr_mou.html

Theodore T.

Theodore is a professional psychology educator with over 10 years of experience creating educational content on the internet. PracticalPsychology started as a helpful collection of psychological articles to help other students, which has expanded to a Youtube channel with over 2,000,000 subscribers and an online website with 500+ posts.