The precentral gyrus is a prominent ridge of brain tissue in the frontal lobe of the cerebrum. It is located directly in front of the central sulcus, the groove that separates the frontal and parietal lobes. The precentral gyrus runs parallel to this sulcus on each cerebral hemisphere, defined by the precentral sulcus in front and the lateral sulcus below.
Role in Voluntary Movement
The precentral gyrus serves as the brain’s primary motor cortex, also identified as Brodmann area 4. Its principal responsibility is to govern conscious, voluntary movements of the body’s skeletal muscles. This region acts as a command center, initiating the signals required for purposeful actions. When a decision to move is made, specialized neurons within this gyrus generate electrical impulses.
These signals do not directly cause muscle contraction but start a chain of command. The impulses are sent from large neurons called pyramidal cells, specifically the giant Betz cells found in the fifth layer of the cortex. Their long axons travel down through the brain and spinal cord, forming the corticospinal tract. This pathway carries the motor commands to the lower motor neurons located in the spinal cord, which in turn activate the target muscles, resulting in physical action.
This intricate system allows for the execution of everything from simple reflexes to the highly coordinated movements required for playing a musical instrument or speaking. The precentral gyrus executes these planned movements, but the planning itself occurs in other associated motor areas.
The Motor Homunculus
The control exerted by the precentral gyrus is not random; it is organized in a somatotopic map. This map is often visualized as a distorted human figure called the motor homunculus, or “little man.” This representation illustrates how different parts of the body are mapped onto the primary motor cortex. The arrangement is orderly, with the area for the feet located medially and the regions for the hand, face, and head situated more laterally.
A striking feature of the motor homunculus is its distorted proportions. The size of each body part on this map is not related to its physical size but to the complexity and precision of the movements it can perform. Consequently, body parts that require fine-tuned motor control, such as the hands, fingers, and lips, are represented by disproportionately large areas of cortical tissue. This dedication of more neural resources allows for the intricate muscle coordination needed for tasks like writing or showing facial expressions.
In contrast, body parts that perform less dexterous, broader movements, like the trunk, hips, and legs, have much smaller representations on the cortical map. The area for the hand and fingers is particularly expansive, occupying a significant portion of the precentral gyrus between the areas for the leg and face.
Consequences of Damage
Injury to the precentral gyrus can have significant and direct consequences on motor function. Damage resulting from events like a stroke, tumor, or physical trauma disrupts the brain’s ability to send commands to specific muscles. The symptoms that arise are directly related to the location and extent of the lesion on the motor homunculus map. Because this cortex controls movement on the opposite side of the body, an injury to the precentral gyrus in one hemisphere affects the contralateral side.
A common outcome of such damage is muscle weakness, known as paresis, or complete loss of muscle function, referred to as paralysis or plegia. For example, a stroke affecting the part of the left precentral gyrus that controls the arm will lead to weakness or paralysis in the right arm. The specific location of the damage dictates the affected body part; an injury to the superior portion would impact the lower limbs, while damage to the inferior area would affect the face.
The severity of the motor deficit depends on the size of the damaged area. A small lesion might cause weakness in a single area, like the hand, while a more extensive injury could result in paralysis of an entire side of the body. The blood supply is also a factor; for instance, a blockage in the middle cerebral artery would likely affect the cortical areas representing the upper limbs and face.