Precentral gyrus

Where is the precentral gyrus?

It lies just in front of the central sulcus, the groove that separates the frontal lobe from the parietal lobe. The postcentral gyrus lies on the other side of that groove. (Neuroscience Online, Overview of the Nervous System and Motor Cortex; OpenStax Behavioral Neuroscience, 10.3)

The strip follows the central sulcus, which runs from the top edge of the hemisphere near its midpoint down and forward until it almost meets the lateral fissure, the groove above the temporal lobe. (OpenStax Behavioral Neuroscience, 10.3; Neuroscience Online, Overview of the Nervous System)

On the medial surface of the hemisphere, the primary motor cortex continues into the anterior paracentral lobule. (Neuroscience Online, Motor Cortex)

What does the precentral gyrus do?

Many of its output cells are upper motor neurons. Their axons run down to the brainstem or spinal cord, and in the spinal cord lower motor neurons carry the command on to the muscles. (OpenStax A&P 2e, 13.2; StatPearls, Motor Neuron and Precentral Gyrus)

It receives input from other cortical areas, the premotor cortex among them, and from the cerebellum and basal ganglia by way of the thalamus. Its corticospinal axons reach the alpha motor neurons of the spinal cord directly. (OpenStax A&P 2e, 14.3; Neuroscience Online, Motor Cortex)

Through the lateral corticospinal tract, each side’s motor cortex controls the arm and leg of the opposite side. Corticobulbar control of the head muscles is largely bilateral: most cranial motor nuclei get fibers from both sides, and only the lower face is served from the opposite side alone. The anterior corticospinal tract, which controls the trunk muscles, acts on both sides of the spinal cord. (OpenStax Behavioral Neuroscience, 10.3; OpenStax A&P 2e, 14.3; StatPearls, Corticobulbar Tract)

In Brodmann’s map the primary motor cortex is area 4. The premotor cortex lies just in front of it, in area 6 (Neuroscience Online adds area 8), and helps choose and prepare movements. (OpenStax A&P 2e, 16.2 and 13.2; Neuroscience Online, Motor Cortex and Higher Cortical Functions: Association and Executive Processing)

How is the body mapped on the precentral gyrus?

The gyrus carries a map of the body, the motor homunculus. The leg is represented on the medial surface, in the anterior paracentral lobule; moving down the lateral surface you pass the trunk, arm and hand, and the face sits farthest out. (OpenStax A&P 2e, 14.3; Neuroscience Online, Motor Cortex)

The hands and face, which make fine, exact movements, get far more cortex than the trunk or legs, whose movements are coarser. (OpenStax A&P 2e, 14.3; Neuroscience Online, Motor Cortex)

Unlike the primary sensory areas, this cortex has no cell-packed granular layer 4. Its most distinctive layer is the output layer 5, which holds the giant Betz cells. (Neuroscience Online, Motor Cortex)

Where do signals from the precentral gyrus go?

Among the tracts that begin here are the corticobulbar tract to the brainstem and the corticospinal tract to the spinal cord, whose fibers synapse on lower motor neurons. StatPearls also lists a cortico-rubrospinal tract. (OpenStax A&P 2e, 14.3; StatPearls, Precentral Gyrus, Lateral Corticospinal Tract and Motor Neuron)

Primary motor cortex neurons supply about 30% of the fibers in the corticospinal tract. The premotor and supplementary motor areas supply about 30%, the somatosensory cortex about 30% and the posterior parietal cortex about 10%. (Neuroscience Online, Motor Cortex)

The motor cortex also feeds the cerebellum: corticopontine fibers end in the pontine nuclei, which project to the cerebellum through its middle peduncle. With input from proprioceptors, the cerebellum adjusts motor commands for shifts in body position or load. (OpenStax A&P 2e, 14.3; Neuroscience Online, Motor Cortex and Cerebellum)

Where the sources differ

Sources describe which side these tracts serve differently. OpenStax A&P 2e section 14.3 calls corticobulbar axons ipsilateral, while OpenStax Behavioral Neuroscience 10.3 and StatPearls call the tract largely bilateral. For the anterior corticospinal tract, A&P 14.3 says its axons cross at the spinal level they serve and send some branches to the same side, and Behavioral Neuroscience 10.3 says most of it serves the same side. (OpenStax A&P 2e, 14.3; OpenStax Behavioral Neuroscience, 10.3; StatPearls, Corticobulbar Tract)