General organization of the neocortex and the frontal lobe
The cerebral cortex (neocortex) has a complex anatomical and functional structure. Traditionally, it is divided into five main lobes: frontal, parietal, occipital, temporal, and insular. Each of these lobes includes both projection and association areas that ensure information processing.
A key role in the control of voluntary movements is played by the frontal lobe. It is within its boundaries that the premotor and motor (movement) areas are localized, the main function of which reduces to the formation and control of human motor activity.
Cortical topography: Penfield homunculus
The most important principle in the structure of motor centers is their somatotopic organization. This means that a strictly defined area of the cortex corresponds to each part of the body.
In 1950, researchers W. Penfield and T. Rasmussen described the sensory and motor representations in the cortex, creating the concept of the so-called "homunculus" (little man). The main feature of this representation is that the area of projection of a body part onto the cortex is not proportional to its actual physical size.
The size of the cortical image directly depends on two factors:
- Innervation density (number of receptors or involved motor units).
- Complexity of performed functions.
That is why the areas of the cortex that control the hands or facial muscles occupy significantly more space than the zones responsible for the massive muscles of the trunk.
Functional specialization of motor areas
The motor cortex is not a homogeneous structure. It is divided into several specialized areas, each of which contributes to motor control:
- Primary motor cortex. This area is responsible for the direct execution of the planned movement. Upon its focal activation, isolated (discrete) contractions of certain muscle groups on the opposite side of the body occur (due to the decussation of nerve pathways).
- Supplementary motor area. Its main task is the formation of complex motor commands. Stimulation of this zone leads to the emergence of complex postural movements involving multiple muscle groups.
- Premotor cortex. Manages the axial musculature (trunk muscles). A distinctive feature of this area is that motor reactions within it are generated only in response to very intense stimuli.
- Frontal eye fields. A highly specialized zone, upon stimulation of which conjugate movements of both eyes are observed, 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. In the pyramidal system, a crucial process takes place — the convergence (convergence) of excitations from sensory systems of various modalities and biological centers.
The pyramidal system functions as the common final pathway. It is through it that a multitude of processed sensory signals are transformed into motor commands and reach the periphery to the muscles.
> Functional meaning of integration: The merging of various excitations at the level of each individual neuron is not accidental. It is strictly subordinate to one global goal — obtaining a useful result necessary for building competent and effective goal-directed behavior of the organism.
Briefly: answers to topical questions
In which layer of the motor cortex are the 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-sized pyramidal cells, including the largest Betz cells, which are localized exclusively in the precentral gyrus.
Which descending neural pathways originate from the neurons of the motor cortex of the frontal lobe?
According to sources, pyramidal descending pathways originate from the neurons of the motor cortex of the frontal lobe.
- Corticospinal tract (tractus corticospinalis) — originates from the cortex of the precentral gyrus; the signal goes to alpha motor neurons of the ventral horns of the spinal cord and provides precise, discrete voluntary movements.
- Corticobulbar / corticonuclear tract (tractus corticobulbaris / cortico-nuclearis) — together with the corticospinal tract, is formed by axons of Betz cells; part of the corticobulbar tract originates from the cortex of the precentral gyrus.
In which Brodmann areas are the primary and secondary motor cortices localized?
The primary motor cortex is located in Brodmann's cytoarchitectonic area 4, and the secondary motor cortex is located in areas 6, 8, and 44.
- Primary motor area (area 4) — is located in the cortex of the precentral gyrus and is responsible for movement execution.
- Secondary cortical fields of the motor area (areas 6, 8, 44) — include the premotor and supplementary motor areas, located anterior to the precentral gyrus in the frontal lobe.
The homunculus rule: "The scale in the brain depends not on the size of the muscle, but on the complexity of its work and the density of innervation."
Class and exam questions
Where are the main motor centers of the cortex localized?
The formation of motor activity is the main function of the frontal lobe of the neocortex. The premotor and motor areas are located within it.
What determines the size of an organ's projection in the motor cortex (Penfield homunculus)?
The size of the representation is proportional not to the physical dimensions of the body part, but to the density of its innervation (the number of motor units) and the level of complexity of the functions it performs.
What is the difference between the primary motor cortex and the supplementary motor area?
The primary cortex is responsible for the direct execution of movement (elicits discrete muscle movements on the opposite side). The supplementary area is responsible for forming motor commands and elicits more complex postural movements.
What is multisensory convergence in the pyramidal system?
This is the process of convergence of excitations from various sensory and biological modalities on the neurons of the pyramidal system. It acts as the common final pathway for signal output to the periphery and ensures goal-directed behavior.
What else to study on this topic
- Organization of projection and association areas in other lobes of the neocortex
- Mechanisms of forming a useful result during the integration of excitations
- Physiology of the frontal eye fields and visuo-motor coordination
- The role of axial musculature and mechanisms of its control by the premotor cortex
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