Motor cortex · Grey Matter

The motor cortex is the strip of the frontal lobe just in front of the central sulcus that turns a decided action into commands for muscles, each hemisphere moving the opposite side of the body.

Motor cortex. The motor cortex is the strip of the frontal lobe just in front of the central sulcus that turns a decided action into commands for muscles, each hemisphere moving the opposite side of the body.

The primary motor cortex (M1, Brodmann area 4) lies on the precentral gyrus. Its cells are laid out as a map of the body, the motor homunculus that Penfield charted by stimulating the cortex during surgery: the leg on the midline at the top, then trunk, arm and hand, and the face and tongue low on the side, with hands, lips and tongue given far more cortex than their size. Large pyramidal cells in its fifth layer (among them the Betz cells) send their axons down as the corticospinal tract, through the brainstem to the spinal cord. In front of it, the premotor cortex and the supplementary motor area (Brodmann area 6) prepare sequences and the timing of movements before M1 issues them.

The crossing is why a stroke on one side weakens the other. About 85 to 90 % of corticospinal fibres cross at the pyramidal decussation in the lower medulla.

M1 issues commands and does not refine them alone. The cerebellum corrects the movement against its prediction, and the basal ganglia select which movement starts.

The plan comes from in front. The prefrontal cortex holds the goal, the premotor areas assemble the sequence and M1 sends it, a gradient from abstract to concrete across the frontal lobe.

Questions: How do the basal ganglia let one movement start and hold the others back? Their output nuclei keep the motor thalamus under constant inhibition, a brake on every possible action. When the cortex proposes an action, the striatum's direct pathway inhibits the output nuclei for that action alone, releasing the brake so the thalamus and motor cortex can run it, while the indirect pathway reinforces the brake on the competing actions. Dopamine favours the direct pathway, so when the dopamine neurons die in Parkinson's disease the brake dominates and movements become hard to start, slow and small. What separates the strip that moves the body from the strip that feels it, and why are lips and hands so large on both maps? The central sulcus, a deep groove running across the top of each hemisphere, has the primary motor cortex on its front bank (the precentral gyrus) and the primary somatosensory cortex on its back bank (the postcentral gyrus). Penfield mapped both by stimulating patients' cortex during surgery and found two matching body maps, the homunculi, with the leg on the midline and the face low on the side. The size of each part on the map follows how finely it is controlled or felt, which is why lips, tongue and fingers take far more cortex than the trunk or the back. How does a decision in the prefrontal cortex become a movement of the hand? The frontal lobe works as a gradient from abstract at the front to concrete at the back. The prefrontal cortex holds the goal (pick up the cup), the premotor cortex and supplementary motor area choose and sequence the movements that serve it, and the primary motor cortex on the precentral gyrus issues the commands down the corticospinal tract to the spinal motor neurons. Along the way the basal ganglia release the chosen action and the cerebellum adjusts its force and timing, so the plan arrives at the muscles already corrected. What does an implant over the motor cortex read when a paralysed person intends to move? Neurons in motor cortex still fire with the intention to move even when the spinal cord or the muscles can no longer carry it out, and each neuron fires more for some directions or speeds than for others. An array recording a few hundred of them sees a pattern that changes with the intended movement, and a decoder trained while the person imagines or attempts moving maps that pattern onto a cursor's velocity, a click or, in the speech areas, the sounds a person tries to articulate. What it reads is intended movement, the output of planning, which is why these systems work best for actions the user tries to make. How does the cerebellum correct a movement before the error shows? When the motor cortex sends a command, a copy reaches the cerebellum through the pons, and the cerebellum predicts what the movement will feel like and where the limb will end up. Sensory feedback is too slow to steer a fast movement, so the cerebellum compares the command with its prediction and with early feedback, and sends corrections back through the thalamus to the motor cortex. Errors that remain are signalled by climbing fibres and change the circuit, so the next attempt is better: a throw that missed left is aimed differently the next time. Why does a stroke in the left hemisphere weaken the right side of the body? The axons of the motor cortex descend as the corticospinal tract through the internal capsule and the brainstem, and in the lower medulla about 85 to 90 % of them cross to the other side at the pyramidal decussation before running down the spinal cord. The left motor cortex therefore commands the right arm and leg, and a lesion anywhere above the crossing weakens the opposite side. Below the crossing, in the spinal cord, a lesion weakens the same side.