Functional Anatomy of Motor Centers
Motor function of the cortex is executed through specialized and primary motor areas. Specialized zones include the supplementary motor area and the frontal eye field, which controls voluntary eye movements.
The primary motor area is located in the precentral gyrus (gyrus precentralis). Its most critical function is forming the pyramidal tract (tractus pyramidalis), whose fibers carry impulses for conscious movements. The cytoarchitecture of this area is characterized by the presence of large Betz pyramidal cells. These neurons act as powerful integrators, forming sites of broad convergence for impulses arriving from other parts of the nervous system.
Surface Topography and Functional Asymmetry
The anatomical division of the hemispheres by surfaces is clinically vital for localizing functions:
- Lateral surface (facies lateralis): The outer aspect of the hemisphere where the precentral (motor) and postcentral (sensory) gyri are clearly visualized. Key speech centers, such as Broca's and Wernicke's areas, are also located here.
- Medial surface (facies medialis): The inner aspect of the hemisphere seen on a midsagittal section. It features structures adjacent to the corpus callosum, including the cingulate gyrus.
Historically, various approaches regarding the distribution of these centers were debated. One of these was strict localizationism, a theory proposing a rigid, fixed mapping of each function to a strictly defined, tiny patch of the cortex.
Sensory Representation (Sensory Homunculus)
The primary somatosensory cortex is located in the postcentral gyrus. Body projection here is contralateral, and body parts are represented disproportionately and upside-down.
Topography from top to bottom (medial to lateral surface):
- In the interhemispheric fissure: Genitals (most medial), foot, and leg (shin, thigh).
- Upper-lateral region: Pelvis, trunk, neck, head, and upper limb down to the hand.
- Middle and lower lateral region: Fingers (with a massive area for the thumb and index finger), eyes, nose, and face. The lips occupy the maximum cortical area due to the highest receptor density.
- Lowest region: Jaws, teeth, tongue (responsible for tactile and gustatory reception), and the intra-abdominal pharyngeal region.
Motor Representation (Motor Homunculus)
The primary motor cortex occupies the precentral gyrus and also features an inverted contralateral projection. However, the motor homunculus differs significantly from the sensory one.
Topography of the motor cortex:
- Medial surface: Foot, leg, and pelvis. Unlike the somatosensory area, genital representation is absent here.
- Upper-lateral region: Trunk, arm, and hand joints.
- Middle lateral surface: Fine motor control zone. The thumb occupies a colossal area (to ensure grasping and opposition functions), along with centers controlling the forehead, eyes, and eyelids.
- Lower-lateral region: Control of the speech apparatus and facial expression. Projections include the face, jaw, tongue (articulation), and pharynx (swallowing).
Key Organizational Principles and Convergence
Analysis of cortical homunculi reveals two main rules:
- Inversion: Lower limbs are projected to the uppermost parts of the gyri, while the head is projected to the lowest.
- Disproportion: The cortical area depends not on the physical size of the body part, but on its functional significance. In the sensory cortex, lips and tongue dominate; in the motor cortex, the hand (thumb) and facial muscles dominate, while the trunk and legs occupy minimal space.
An important mechanism of cortical function (particularly in the frontal lobes) is axonal-sensory-biological convergence. Signals of various sensory and biological modalities, along with collaterals of the pyramidal tract, converge onto the same neurons.
A striking example of complex cortical integration is Wernicke's area. Located in the posterior part of the superior temporal gyrus of the left hemisphere, it is responsible for comprehending spoken language. Clinical damage to this area leads to sensory aphasia—a condition where the patient hears sounds but loses the ability to understand the meaning of spoken words.