Primary motor cortex: characteristics and functions

This part of the cerebral cortex has functions associated with movement and perception.

The primary motor cortex is the main motor area of the brain responsible for managing all actions related to the control of voluntary movements. It is responsible for transmitting orders to the muscles to tense or contract and produce the motor action.

In this article we explain what the primary motor cortex is, where it is located, what functions it is attributed and what type of alterations can originate if this cerebral area is damaged.

The primary motor cortex: definition and neuroanatomical location

The primary motor cortex is one of the main brain regions involved in motor functions. It is located in the frontal lobe and along the precentral gyrusIt is located in the frontal lobe and along the precentral gyrus, on the lateral surface of the cortex, and extends medially into the longitudinal fissure of the brain forming the anterior paracentral lobe.

One third of the fibers composing the corticospinal tract arise from neuronal nuclei found in the primary motor cortex, whose axons also terminate in the cranial nerve motor nuclei of the brain stem, the basal ganglia, the reticular formation, and the red nucleus; projections from the latter structure constitute the rubrospinal tract which, together with the corticospinal tract, form the main lateral descending motor system.

The primary motor cortex contains pyramidal cells of cortical layer V, also called Betz cells.Betz cells, the upper motor neurons responsible for transmitting the commands necessary to initiate voluntary movements. As we will see below, another peculiar feature of the primary motor cortex is that the motor responses elicited upon stimulation are organized somatotopically.

Somatotopic structure and organization

The primary motor cortex contains a topographic map of the body's muscles, in which the leg is in which the leg is represented medially, the head laterally and other parts of the body at intermediate locations. Within this map there are distinct nuclei of neurons representing different muscles. However, the areas represented are not proportional to their size in the body, and it has been shown that after amputation or paralysis, motor areas can change to adopt new body parts.

There is a spatial arrangement of motor responses in which adjacent muscles are controlled by adjacent regions of the primary motor cortex. This somatotopic map reflects that of the somatosensory cortex.. In fact, the latter is located just opposite the central sulcus. These two areas are adjacent and are connected by the cortical tissue of the paracentral lobe.

Neurons in a given area of the primary motor cortex receive proprioceptive information from a Muscle or a small group of synergistic muscles and send their output back to that muscle or group of muscles via a multisynaptic pathway through the brainstem and spinal cord. These actions facilitate the planning and precise execution of movements, functions characteristic of the primary motor cortex.

Functions

The main function of the primary motor cortex is the management and execution of voluntary movementsThe voluntary or striated muscles, by transmitting information through the spinal cord to move the different parts of the body, are so called because a command is necessary for them to produce movement.

Voluntary or striated muscles are so called because a command is necessary for them to produce movement, as opposed to involuntary or smooth muscles, whose activity depends on the autonomic nervous system.

Neurons distributed throughout the primary motor cortex produce a pattern of body representation called the homunculus. a pattern of body representation called the motor homunculus.. The extent of each body part over the cortex corresponds to the degree of motor control exercised over each of the represented parts. For example, the hands, lips and tongue are represented by large regions of the cortex, and the toes by a relatively small area.

The primary motor cortex, in its most medial portion, controls the body below the waist. In its lateral portion, it manages the muscles of the body above the waist. And the control it exerts through the pyramidal tract is greatest over the muscles of the hand. In contrast to the legs, which function in locomotion, the face, head and hands are used to transmit signals expressing emotions.

In short, the motor functions that are attributed to the primary motor cortex are: finger movements are: finger movements, swallowing, lower limb movements, voluntary control of breathing, motor imagery, control of rhythmic motor tasks, voluntary blinking, horizontal saccadic movements, movements of the lips, mouth, wrist and hands contralaterally.

Y in terms of sensory functionsThe primary motor cortex is also responsible for the kinesthetic perception of the movement of different parts of the body, for discriminating vibrotactile frequency or for the response to touch, among others. It also appears to play a role in verbal encoding during the processing of non-semantic elements and in topographical memory for visual references.

Disorders associated with damage to this area of the brain

A lesion in the primary motor cortex may cause paralysis of the contralateral musculature.. Affected muscles may become flaccid at first; then, within several days, reflexes become rapid and muscles manifest spasticity.

Control of gross movements reappears after several weeks or months, but fine movements, especially those of the hands, are usually permanently lost. Some less severe consequences of damage to the primary motor cortex also include: lack of coordination, inability to express oneself clearly and speech difficulties, delayed responses, etc.

Problems resulting from damage to the primary motor cortex, such as facial palsy, monoparalysis, monoparalysis and facial paralysis, monoparesis or hemiparesis, severely affect the quality of life.severely affect the quality of life of patients who suffer from them, often leading to an inability to carry out basic activities of daily living correctly or to communicate properly with others (due to the problems that may arise when walking or gesturing, for example).

Bibliographical references:

• Kakei, S., Hoffman, D. S., & Strick, P. L. (1999). Muscle and movement representations in the primary motor cortex. Science, 285(5436), 2136-2139.
• Rains, G. D., & Campos, V. (2004). Principles of human neuropsychology. Mexico: McGraw-Hill.