The praecentral gyrus is a part of the brain and is home to the primary motor cortex, which is interconnected with central motor neurons and the pyramidal tracts. The area of the brain is considered the center of movement control. In lesions, irreversible movement disorders, spasticity, or paralysis often occur.
What is the praecentral gyrus?
The praecentral gyrus is located on the frontal lobe in the cerebrum and corresponds to the cerebral segment anterior to the central furrow. Literally translated, gyrus means “turn.” In humans, the praecentral gyrus has a central function in movement control. The primary motor cortex, which is irreplaceable for all movements, is located in the convolution. According to Brodmann’s classification, the praesentral gyrus is located in area four, which is known as the area gigantopyramidalis. This area is the origin of the so-called pyramidal tracts. The pyramidal tracts in humans are the central switching element for all voluntary and reflex motor activity and are connected to the motor cranial nerve nuclei. The primary motor cortex thus contains so-called motoneurons, which are considered the common starting point of the entire motocortex. The praecentral gyrus must be distinguished from the postcentral gyrus on the parietal lobe. This area contains the somatosensory cortex for processing tactile perceptions.
Anatomy and structure
In the primary motor cortex are the motoneurons, which serve as the common starting point of the motor cortex. The axons of the motoneurons there run along the spinal cord to reach the motor cranial nerve nuclei. In the spinal cord, their impulses are switched and travel to the peripheral motoneuron in the anterior horn, from where motor commands from the brain reach the associated voluntary muscles. The primary motor cortex is not complete, but the majority of it is located on the bulge immediately anterior to the central furrow. The praecentral gyrus is built with somatotopy in this part. Thus, anatomically adjacent regions are also represented side by side in the primary motor cortex. Thus, a miniaturized and upside-down representation of the human body, so to speak, lies on the praesentral gyrus.
Function and tasks
The main function of the praecentral gyrus corresponds to its function as a primary motor cortex. The primary motor cortex must be distinguished from the supplementary motor cortex and the premotor cortex. The last two cortices serve to compose sequences of movements from a fund of learned individual movements. They are also involved in the preparation of all voluntary conscious and unconscious movements. Korbinian Brodmann introduced the so-called Brodmann areas to describe brain sections. Brodmann Area 4 accounts for the primary motor cortex. The supplementary motor cortex and premotor cortex are both located in area 6. This representation of the human body in the praecentral gyrus is also called the homunculus and is used to accurately relay movement commands from the brain. Proportionally, the homunculus is distorted. Indeed, individual areas of the human body have an extremely finely tuned ability to move, in particular the hand or the speech muscles. These areas require a particularly fine representation. Other areas of the body, on the other hand, have less fine motor skills, for example the back. Areas with a higher proportion of automatic regulation also require less fine representation. An example of this is the postural and supporting muscles. The cortical areas associated with such areas are smaller than the representing sites for fine motor control. This is how the distortion of the homunculus occurs. Somatotopy is also much more coarse in the representation than, for example, in the primary sensory cortex, which corresponds to a pinpoint representation of body surfaces. The efferent descending neural pathways of the cerebral cortex, together with the tractus corticospinalis and the tractus corticonuclearis supplying the motor cranial nerve nuclei, make up the pyramidal pathway in humans.
Diseases
The praecentral gyrus gains clinical relevance primarily when it is affected by lesions. The motor areas provide a central interface between consciousness and matter.It is through the areas that people are able to react to their environment with certain intentions and in directed movements, to move around in the environment or to approach other individuals. When the motocortex completely loses its function, voluntary movements are no longer conceivable and control over one’s own body is completely lost. Patients in this situation are described by the term locked-in syndrome. Although the affected persons are fully conscious and clearly perceive their environment, they are unable to react to their environment and are accordingly locked in their own body. Locked-in syndrome is usually a result of damage to the efferent pathways on the motor cortex. Other lesions of the primary motor cortex may also be associated with severe movement limitations. Patients with ALS, for example, are affected by degenerative changes in motor neurons. Their motor nerve cells in the central nervous system break down piece by piece. Depending on whether the upper or lower central motor neuron is affected, the degenerative phenomena result in spasticity, muscle weakness or paralysis. The disease is currently incurable and can only be treated symptomatically. To diagnose lesions of the central motor neurons and thus of the primary motor cortex, the neurologist often uses reflex examination. For example, if multiple pathological reflexes of the so-called Babinski group are present, the patient is probably suffering from lesions in this area. Tumors, hemorrhages, traumatic injuries, or inflammation in the area of the praecentral gyrus also often cause movement disorders. The prognosis for treatment depends on the cause in each individual case. If motor neurons have been irreversibly destroyed, movement disorders are usually irreversible symptoms. Only in individual cases can the functions of the affected brain areas be transferred to intact brain areas through training.
