How the Brain Moves the Body

From intention to action: the motor hierarchy of cortex, basal ganglia, cerebellum, brainstem, spinal cord and motor neurons — and the difference between upper and lower motor neuron damage.

Even a simple act — picking up a cup — draws on a hierarchy of brain systems. Each level handles a different part of the problem.

Deciding and planning

The prefrontal cortex sets the goal. The premotor cortex and supplementary motor area plan the movement: which hand, what grip, in what sequence. The parietal cortex supplies the location of the cup relative to your hand.

Commanding

The primary motor cortex sends commands down the corticospinal tract. Most of its fibres cross to the other side in the lower brainstem — the pyramidal decussation — so the left hemisphere moves the right side of the body.

Refining

Two loops refine the command without directly driving muscles. The basal ganglia help select the intended action and suppress competing ones, and set its vigour. The cerebellum coordinates timing and accuracy, predicting the consequences of the movement and correcting errors.

Executing

In the spinal cord (or brainstem, for the face), motor neurons carry the final command to the muscles. One motor neuron and all the muscle fibres it controls form a motor unit. For precise movements — the eye muscles, the fingers — motor units are small; for powerful ones — the thigh — a single motor neuron may control many hundreds of fibres. Force is increased by recruiting more motor units and firing them faster.

Upper versus lower motor neurons

Doctors distinguish upper motor neurons (in the brain, sending commands down) from lower motor neurons (in the spinal cord and brainstem, connecting to muscle). Upper damage, as after a stroke, typically causes weakness with stiffness and exaggerated reflexes; lower damage causes weakness with floppiness, muscle wasting and twitching. ALS is unusual in damaging both.