Primary motor cortex: characteristics and functions

The primary motor cortex is the main motor area of ​​the brain that is responsible for managing all actions related to the control of voluntary movements. It is responsible for transmitting commands to the muscles so that they are tense or contracted and a motor action occurs.

In this article we tell you what is the primary motor cortex, where is it located, what functions are assigned to it and what kinds of damage they can cause if that area of ​​the brain is damaged.

    The primary motor cortex: definition and neuroanatomical location

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

    One third of the fibers that make up the corticospinal tract originate from neural nuclei found in the primary motor cortex, axons also terminate in the motor nuclei of the brainstem cranial nerve, basal ganglia, reticular formation and red nucleus; the protrusions of the latter structure constitute the rubrospinal tract which, together with the corticospinal tract, forms the main descending lateral motor system.

    The primary motor cortex contains pyramidal cells of the cortical layer V, also called Betz cells., The higher motor neurons responsible for transmitting the commands necessary to initiate voluntary movements. As we will see below, another special feature of the primary motor cortex is that the motor responses obtained during stimulation are organized in a somatotopic fashion.

    Somatotopic structure and organization

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

    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 mirrors that of the somatosensory cortex. In fact, it is located just across the central groove. These two areas are adjacent and 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. spinal. These actions facilitate the planning and precise execution of movements, characteristic functions of the primary motor cortex.

      the functions

      The main function of the primary motor cortex is management and execution of voluntary movements, By transmitting information through the spinal cord to move different parts of the body.

      Voluntary or striated muscles are so called because an order is needed for them to produce movement, as opposed to involuntary or smooth muscles, the activity depends on the autonomic nervous system.

      Neurons distributed throughout the primary motor cortex produce a body representation model called motor homunculus. The extent of each part of the body on the cortex corresponds to the degree of motor control exerted on each of the parts represented. 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 part, controls the body below the waist. In its lateral part, it manages the muscles of the body which are above the waist. And the control it exerts through the pyramidal tract is more important on the muscles of the hand. Unlike the legs, which work in locomotion, the face, head, and hands are used to transmit signals that express emotions.

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

      I in terms of sensory functionsThe primary motor cortex is also responsible for the kinesthetic perception of movement of different parts of the body, to discriminate vibrotactile frequency or response to touch, among others. It also seems to play a role in verbal coding when processing non-semantic elements and in topographic memory of visual references.

      Disorders associated with damage to this area of ​​the brain

      Damage to the primary motor cortex can cause paralysis of the contralateral musculature. Affected muscles may become flabby at first; then, over the course of several days, the reflexes become rapid and the muscles show spasticity.

      Coarse movement control reappears after several weeks or months, but fine movements, especially of the hands, are usually lost permanently. Some less serious consequences of damage to the primary motor cortex also include: lack of coordination, inability to express oneself clearly and with difficulty in speaking, issuing late responses, etc.

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

      Bibliographical references:

      • Kakei, S., Hoffman, DS and Strick, PL (1999). Muscle and movement representations in the primary motor cortex. Science, 285 (5436), 2136-2139.
      • Rains, GD and Camps, V. (2004). Principles of human neuropsychology. Mexico: McGraw-Hill.

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