Precentral gyrus

The precentral gyrus is a fold of cortex visible on the outer surface of the frontal lobe. It is located between the precentral sulcus in the front and the central sulcus at the back. The precentral gyrus extends from the upper edge of the brain’s hemisphere and continues onto the inner surface of the brain, where it contributes to a region called the paracentral lobule. This area is also referred to as Broadman's area 4 and is responsible for controlling voluntary movement, functioning as the primary motor area of the brain.

An organized map of the body parts, known as the motor homunculus, can be observed on the pre-central gyrus. On the outer surface, just above the lateral sulcus, we can find structures responsible for activities like swallowing and movements involving the tongue, jaw, lips, larynx, and eyes. Moving upwards, there is a significant area dedicated to finger movements, particularly the thumb, and then the hand, wrist, arms, and shoulder. The uppermost region of the precentral gyrus, along the top edge of the brain hemisphere, controls movements of the hip and knee. The primary motor areas responsible for leg and ankle movements are located on the inner surface of the cerebral hemisphere in the paracentral lobule.

It's important to note that the size of the cortex area dedicated to controlling a specific movement depends on the complexity and precision of that movement, rather than the size of the muscles involved. As a result, the trunk and leg muscles have smaller representations on the precentral gyrus compared to the intricate movements of the hands.

It's crucial to understand that the primary motor cortex doesn't plan movements or design movement patterns. Its role is to execute planned movements. After the movement planning is completed by the pre-motor area, the motor plan is sent to the primary motor cortex in the precentral gyrus for execution. The execution takes place through descending pathways such as the cortico-spinal pathway.

To understand the blood supply to the cerebral cortex, we need to discuss the anterior cerebral artery and the middle cerebral artery. These two arteries branch from the internal carotid artery and supply blood to the cerebral cortex.

The important point to note is that the anterior cerebral artery supplies the inner side of the cortex and a narrow strip of cortex on the upper-outer surface of the cerebral hemisphere. If there is a stroke affecting the anterior cerebral artery, it would impact the cortical areas in the motor homunculus responsible for controlling lower body movements, like the lower limbs.

On the other hand, the middle cerebral artery supplies a region on the upper-outer surface where the upper limbs, head, and neck area are represented. So, if a patient exhibits weakness in the right upper limb, it could be due to involvement of the left middle cerebral artery. Similarly, paralysis of the right lower limb could be caused by involvement of the left anterior cerebral artery. The reverse is also true for weakness in the left arm or leg.

This information is valuable for determining the location of a lesion from a neurological perspective.