Functional Principles and Representation
The motor cortex does not function in isolation. To initiate any voluntary movement, its neurons gather afferent impulses from the cerebellum, basal ganglia, and thalamus. A crucial principle of this area's operation is the representation of movements rather than isolated muscles. The cortex encodes the complex functioning of muscle groups.
Furthermore, cortical activity strictly depends on the context. The discharge pattern of the same neurons changes depending on the behavioral situation, even if the same type of physical action is performed. Changing the goal instantly transforms the entire motor program.
Somatotopic organization follows the rule of functional complexity: the area of representation of any body part in the motor homunculus is proportional to the complexity and diversity of its movements, rather than its anatomical size.
Columnar organization of the cortex
Structurally, the motor cortex is divided into functional units — vertical columns. These are formed by pyramidal neurons with similar functions located one above the other.
- Column characteristics: the diameter is about 1 mm, containing several hundred pyramidal cells within.
- Function: control over the work of several muscles to ensure smooth movement in a single joint. At the same time, the principle of multiplicity exists — the contraction of the same muscle can be elicited by stimulation from different loci.
- Hierarchy: several morphological columns combine into a functional column. The aggregate of functional columns, in turn, forms the motor representation of a specific body part. Ultimately, these structures activate or inhibit the neurons of the spinal motor pool.
Types of pyramidal neurons
To transmit commands to the periphery, the cortex utilizes cells of various calibers, which is directly related to the types of spinal motor neurons.
- Large pyramidal cells (Betz cells). They are characterized by a high conduction velocity of excitation and discharge directly during the execution of movement. It is hypothesized that their activity is associated with phasic alpha motor neurons of the spinal cord, which provide rapid contractions.
- Small pyramidal neurons. They have a lower nerve impulse conduction velocity. They are characterized by a constant background firing rate. Physiologists believe they transmit signals to tonic alpha motor neurons, which maintain muscle tone and posture.
Pyramidal tract: execution of commands
The execution of the formed command is carried out via the corticospinal (pyramidal) tract—the primary pathway for voluntary motor function.
The signal originates in the primary motor cortex, where giant Betz cells are localized. Next, axons descend through the internal capsule and brainstem. In the medulla oblongata region, fiber decussation occurs, after which the pathway continues into the spinal cord.
The final station is the alpha motor neurons of the anterior horns of the spinal cord. This specific mechanism guarantees the direct transmission of commands to the skeletal musculature. The pyramidal tract is critically important for performing precise, discrete voluntary movements, especially in the distal extremities (e.g., finger movements).
Briefly: answers to topical questions
Which Brodmann cytoarchitectonic areas are part of the motor cortex?
According to Brodmann, the motor cortex includes primary and secondary cortical areas of the frontal lobe. These include:
- Primary motor cortex — area 4 (cortex of the precentral gyrus).
- Premotor and supplementary motor areas — areas 6, 8, 44.
Within the premotor cortical areas, subregions 6aα and 6aβ are also distinguished.
What are the functions of the premotor cortex and the supplementary motor area?
The functions of the premotor and supplementary motor cortex lie in movement planning and the generation of motor commands.
- Premotor cortex — serves as the highest center for planning and selection of motor programs, and also controls the axial musculature of the trunk. It acts as a repository for learned "motor patterns" that are retrieved from memory for use.
- Supplementary motor area — is responsible for the formation of motor commands; its stimulation elicits complex postural movements.
Together, these areas form the overall concept of a voluntary movement.
Through which extrapyramidal tracts does the motor cortex transmit impulses to the spinal cord?
The motor cortex is connected to subcortical structures and brainstem nuclei as part of the extrapyramidal system via the following pathways:
- with the ganglia — corticostriate pathways;
- with brainstem nuclei — corticorubral, corticonigral, and corticoreticular pathways.
The influence of the extrapyramidal system on the neurons of the anterior horns of the spinal cord is exerted via descending pathways:
- tectospinal tract (tractus tectospinalis);
- rubrospinal tract (tractus rubrospinalists);
- reticulospinal tract (tractus reticulospinalis);
- vestibulospinal tract (tractus vestibulospinalis).
What symptoms occur with an isolated lesion of the primary motor cortex (central paralysis)?
With an isolated lesion of the primary motor cortex — area 4 — a flaccid paresis develops on the contralateral side of the body.
With a combined lesion of area 4 and the adjacent premotor area along with outgoing fibers, spastic hemiparesis develops. This indicates simultaneous damage to the pyramidal tract and extrapyramidal (non-pyramidal) fibers.
To remember the roles of structures in motor function, use the following association: the cerebellum is the "navigator" (planning, correction, timing), the basal ganglia are the "volume control" (regulation of force, scale, and complex patterns), and the pyramidal tract is the "direct executor" (direct orders to muscles).
Class and exam questions
What does the area of organ representation in the motor cortex depend on?
It is proportional not to the physical size of the organ, but to the complexity and diversity of the movements it performs.
When does movement-related electrical activity arise in the cortex?
Motor potential begins to be recorded 40–50 milliseconds before the initiation of voluntary movement.
How does the motor cortex respond to changes in a behavioral task?
Changing the goal or situational context instantly restructures the overall motor program, altering the firing pattern of neurons even during identical movements.
What else to study on this topic
- Functional interaction of the cortex, thalamus, and basal ganglia
- Electrophysiology: nature and recording of the motor potential
- Spinal motor pools: phasic and tonic alpha motor neurons
- Anatomy and decussation of the corticospinal (pyramidal) tract
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