Functional anatomy of motor centers
The motor function of the cortex is implemented through specialized and primary motor areas. Specialized zones include the supplementary motor area and the oculomotor center, which ensures voluntary eye movements.
The primary motor area is localized in the precentral gyrus (gyrus precentralis). Its most important function is the formation of the pyramidal tract (tractus pyramidalis), the fibers of which carry impulses for voluntary movements. The cytoarchitecture of this zone is characterized by the presence of large pyramidal cells. These neurons act as powerful integrators, forming hubs for the broad convergence of impulses from other parts of the nervous system.
Surface topography and functional asymmetry
The anatomical division of the hemispheres into surfaces is of great clinical importance for understanding the localization of functions:
- Lateral surface (facies lateralis): the outer part of the hemisphere where the precentral (motor) and postcentral (sensory) gyri are clearly visualized. Key speech centers—Broca's and Wernicke's areas—are also located here.
- Medial surface (facies medialis): the inner part of the hemisphere visible on a midsagittal section. It houses structures adjacent to the corpus callosum, including the cingulate gyrus.
In physiology, various approaches to the distribution of these centers have historically been considered, one of which was strict localizationism—a theory suggesting a rigid attachment of each function to a strictly defined tiny area of the cortex.
Sensory representation (Sensory homunculus)
The primary somatosensory cortex is located in the postcentral gyrus. The body projection here is contralateral. The representation of body parts is distributed unevenly and inverted "upside down".
Topography from top to bottom (from the medial to the lateral surface):
- In the interhemispheric fissure: genitalia (most medial), foot and leg (shin, thigh).
- Upper-lateral division: pelvis, trunk, neck, head, hand down to the wrist.
- Middle and lower lateral division: fingers of the hand (with a huge area for the thumb and index finger), eyes, nose, face. The maximum area here is occupied by lips due to the highest receptor density.
- Lowest division: jaws, teeth, tongue (responsible for tactile and taste reception), as well as the intra-abdominal region of the pharynx.
Motor representation (Motor homunculus)
The primary motor cortex occupies the precentral gyrus and also has an inverted contralateral projection. However, the motor homunculus has significant differences from the sensory one.
Topography of the motor cortex:
- Medial surface: foot, leg, and pelvis. Unlike the somatosensory area, the cortical representation of the genitals is absent here.
- Upper-lateral division: trunk, arm, wrist joints.
- Middle lateral surface: fine motor skills zone. A colossal area here is occupied by the thumb (to ensure gripping and opposition functions), and the control centers for the forehead, eyes, and eyelids are also located here.
- Lower-lateral division: control of the speech apparatus and facial expression. The face, jaw, tongue (articulation), and pharynx (swallowing) are projected here.
Key organization principles and convergence
The analysis of cortical homunculi allows us to distinguish two main rules:
- Inversion: lower limbs are projected into the highest parts of the gyri, and the head into the lowest.
- Disproportion: the cortical area depends not on the physical size of the organ, but on its functional significance. Lips and tongue dominate the sensory cortex, while the hand (thumb) and facial muscles dominate the motor cortex. 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, as well as collaterals of the pyramidal tract, converge on the same neurons.
A striking example of complex cortical integration is Wernicke's area. It is located in the posterior part of the superior temporal gyrus of the left hemisphere and is responsible for understanding spoken speech. Clinical damage to this area leads to sensory aphasia—a condition in which the patient hears sounds but ceases to understand the meaning of spoken words.
Briefly: answers to topical questions
Where is the motor speech center (Broca's area) located and what is its function?
The motor speech center (Broca's area) is located in the posterior part of the inferior frontal gyrus of the dominant (left) hemisphere. It is responsible for forming the detailed program of vocalization (the motor schema of speech). This center is activated during speech and internal speech ("talking to oneself" without pronouncing aloud). When providing written speech, impulses from Broca's area are directed to the motor representations of the hand in the cerebral cortex.
Which ascending pathways terminate in the postcentral gyrus?
Thalamocortical fibers, formed by axons of third-order neurons of the thalamus from ascending sensory pathways, terminate in the postcentral gyrus. Projected to it are:
- the pathway of proprioceptive and tactile sensitivity via the gracile and cuneate fasciculi, medial lemniscus, and thalamus;
- the lateral spinothalamic tract — pain and temperature sensitivity;
- the anterior spinothalamic tract — tactile sensitivity;
- sensory pathways for the head and face from the trigeminal nerve.
Imagine a little man (homunculus) who has thrown his legs over the edge of the hemisphere into the longitudinal fissure and is hanging upside down. At the same time, he has giant lips, a huge thumb, and a tongue, but a tiny torso — the size of a body part depends on how precisely the brain needs to control or sense it.
Class and exam questions
In which gyrus does the pyramidal tract begin?
The pyramidal tract originates in the precentral gyrus, where the primary motor cortex and the bodies of large pyramidal cells are located.
Is there a representation of the genitals in the motor cortex?
No, the genitals are represented only in the sensory homunculus (in the postcentral gyrus). Their representation is absent in the precentral gyrus.
What happens when Wernicke's area is damaged?
Sensory aphasia develops: the patient completely loses the ability to understand spoken speech addressed to them, although their physical hearing is preserved.
What else to study on this topic
- Features of the cytoarchitectonics of the motor cortex and types of pyramidal cells
- Mechanisms of axonal-sensory-biological convergence at the neuronal level
- Functional interaction of Broca's and Wernicke's areas
- Strict localizationism as a historical stage of cortical study
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