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Motor Cortex of the Cerebral Hemispheres

*Cortex motorius*

For medical students2 min readUpdated 2026-10-10

The motor cortex is a specialized region of the frontal lobe of the cerebral hemispheres responsible for the planning and execution of motor activity. It serves as the highest center where various excitations are integrated to produce purposeful and adaptive behavior.

LocalizationFrontal lobe of the neocortex
OrganizationSomatotopic (Penfield's homunculus)
Main functionGeneration of motor activity and behavior
Effector pathwayPyramidal system as the final pathway

General Organization of the Neocortex and Frontal Lobe

The cerebral cortex (neocortex) has a complex anatomical and functional structure. Traditionally, it is divided into five main lobes: the frontal, parietal, occipital, temporal, and insular lobes. Each of these lobes includes both projection and association areas responsible for information processing.

The frontal lobe plays a key role in the control of voluntary movements. It houses the premotor and motor (movement) areas, whose primary function is to generate and control human motor activity.

Cortical Topography: Penfield's Homunculus

A crucial principle of motor center organization is their somatotopic organization. This means that every part of the body corresponds to a strictly defined area of the cortex.

In 1950, researchers W. Penfield and T. Rasmussen mapped the sensory and motor representations in the cortex, creating the concept of the "homunculus" (little man). The main feature of this representation is that the area of the body's cortical projection is not proportional to its actual physical size.

Image size in the cortex depends directly on two factors:

This is why the cortical areas controlling the hands or facial expressions occupy significantly more space than the zones responsible for massive trunk muscles.

Functional Specialization of Motor Areas

The motor cortex is not a homogeneous structure. It is divided into several specialized regions, each contributing to motor control:

  1. Primary motor cortex. This area is responsible for the direct execution of planned movements. Focal activation of this region produces isolated (discrete) contractions of specific muscle groups on the contralateral side of the body (due to the decussation of neural pathways).
  2. Supplementary motor area. Its main task is the generation of complex motor commands. Stimulation of this zone leads to complex postural movements involving multiple muscle groups.
  3. Premotor cortex. Controls axial musculature (trunk muscles). A distinctive feature of this area is that motor responses are generated only in response to highly intense stimuli.
  4. Frontal eye fields. A highly specialized area whose stimulation produces conjugate gaze movements of both eyes, which is critical for visual orientation.

Pyramidal System and Signal Integration

For a movement to be appropriate to the current situation, the brain must process a colossal amount of information. The pyramidal system performs a critical process—the convergence of excitations from sensory systems of various modalities and biological centers.

共产 The pyramidal system functions as the final common pathway. Through it, numerous processed sensory signals are transformed into motor commands and sent out to peripheral muscles.

> Functional significance of integration: The convergence of various excitations at the level of individual neurons is not random. It strictly serves a single global goal—achieving a useful result necessary to shape structured and effective goal-directed behavior.

Mnemonic

Penfield's rule: "The cortical scale depends not on muscle size, but on functional complexity and innervation density."

Frequently asked questions

In which layer of the motor cortex are giant Betz pyramidal cells located?

Giant Betz pyramidal cells are located in the fifth layer of the cerebral cortex.

  • Internal pyramidal layer (internal pyramidal lamina; lamina V) — contains medium and large pyramidal cells, including the largest Betz cells, which are located exclusively in the precentral gyrus.
Which descending pathways originate from the motor cortex neurons of the frontal lobe?

Descending pyramidal pathways originate from the motor cortex neurons of the frontal lobe.

  • Corticospinal tract (tractus corticospinalis) — originates from the precentral gyrus cortex; signals travel to alpha-motor neurons of the spinal cord anterior horns, providing precise, discrete voluntary movements.
  • Corticobulbar / corticonuclear tract (tractus corticobulbaris / cortico-nuclearis) — formed alongside the corticospinal tract by axons of Betz cells; part of the corticobulbar tract originates from the precentral gyrus cortex.
Which Brodmann areas contain the primary and secondary motor cortex?

The primary motor cortex is located in Brodmann area 4, while the secondary motor cortex is located in areas 6, 8, and 44.

  • Primary motor area (area 4) — located in the precentral gyrus cortex and responsible for movement execution.
  • Secondary cortical motor fields (areas 6, 8, 44) — include the premotor and supplementary motor zones, located anteriorly to the precentral gyrus in the frontal lobe.
Where are the main motor centers of the cortex localized?

Motor activity generation is the primary function of the frontal lobe of the neocortex, which houses the premotor and motor areas.

What determines the size of a body part's projection in the motor cortex (Penfield's homunculus)?

The representation size is proportional not to the physical dimensions of the body part, but to its innervation density (number of motor units) and the complexity of its functions.

What is the difference between the primary motor cortex and the supplementary motor area?

The primary cortex is responsible for executing movements (causing discrete muscle movements on the contralateral side). The supplementary area generates motor commands and elicits more complex postural movements.

What is multisensory convergence in the pyramidal system?

It is the convergence of excitations from various sensory and biological modalities onto neurons of the pyramidal system, which acts as the final common pathway to output signals to the periphery and ensure goal-directed behavior.

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