Primary motor cortex

Why it matters

Every voluntary movement of the limbs starts with a command from the primary motor cortex to a lower motor neuron in the brainstem or the cord ^[1]. Spinal cord injury, brainstem stroke and other disorders can disconnect the brain from the body; researchers record from the motor cortex of people with such paralysis to drive robotic arms and to type ^{[2, 3]}.

What it is

The primary motor cortex is located in the precentral gyrus of the frontal lobe ^[1]. It holds the upper motor neurons of the corticospinal pathway, including large cortical neurons called Betz cells that synapse with lower motor neurons in the brainstem or the cord ^[1].

What it does

It sends motor commands down two descending pathways: the corticobulbar tract to the cranial motor nuclei and the corticospinal tract to the ventral horn of the cord ^[1].

How it works

Like the primary somatosensory cortex, the primary motor cortex holds a map of the body, the motor homunculus ^[1].

• The neurons for the feet and lower legs are in the medial wall of the precentral gyrus ^[1].
• The thighs, trunk and shoulder are at the crest of the longitudinal fissure ^[1].

Single neurons are tuned to direction. In monkeys making arm movements in eight directions, most cells studied fired most for movements in one preferred direction and less and less for directions further from it ^[4].

Cosine tuning of a motor cortex neuron^[4]

$$f(\theta) = b_0 + b_1 \cos(\theta - \theta_{pref})$$

A motor cortex neuron fires most for movements in its preferred direction and less the further the movement turns away from it, following a cosine. Decoders for brain–computer interfaces build on this.

Variables of Cosine tuning of a motor cortex neuron

Symbol	Meaning	Unit
$b_0$	baseline rate	spikes/s
$b_1$	modulation depth	spikes/s
$\theta$	movement direction	°
$\theta_{pref}$	preferred direction	°
$f$	firing rate	spikes/s

Valid when

• Two-dimensional arm movements of monkeys, where it was described
• A fit to average rates over trials, not a description of single spikes
• Rates cannot go below zero, so a large b₁ with a small b₀ is not physical

Worked example^[4]

A neuron with baseline 20 and depth 10 spikes/s, moving 60° away from its preferred direction: 20 + 10 × cos 60°.

b0 = 20 spikes/s, b1 = 10 spikes/s, theta = 60 °, thetaPref = 0 ° → f = 25 spikes/s

baseline rate (spikes/s)modulation depth (spikes/s)movement direction (°)preferred direction (°)

firing rate: 25 spikes/s

When it does not behave

The command reaches a muscle only through the upper motor neuron's long axon and then the lower motor neuron ^[1]. Paralysis from spinal cord injury or brainstem stroke disconnects the brain from the body below the damage, while the motor cortex itself can still be recorded from ^[2].

The numbers

In the 1982 recordings, 74.6 % of the arm-related cells active in the task^[4] varied their firing in an orderly way with movement direction. The tuning was already there before the earliest change in muscle activity, which came about 80 ms^[4] before the movement began ^[4].

How we see and measure it

Brain–computer interfaces record from arrays of electrodes placed in the motor cortex. In one study, signals from an array in the motor cortex of two people with long-standing tetraplegia were decoded to control a robotic arm and hand ^[2].

Frontier

These results come from small trials with implanted arrays; each entry shows its evidence tier ^{[2, 3]}.

• T1T1 peer-reviewed human studyPeople with tetraplegia reach and grasp with a robotic arm2012-05 · as of 2026-10-02 · trial

  Signals from a 96-channel array in the motor cortex were decoded to control a robotic arm and hand. Two people with long-standing tetraplegia made three-dimensional reach and grasp movements; one drank coffee from a bottle.

  What it does not show: The arm was slower and less accurate than a natural arm, and the participants did not move their own limbs.

  Sources and details
• T1T1 peer-reviewed human studyTyping by imagined handwriting2021-05 · as of 2026-10-02 · trial

  An intracortical interface decoded attempted handwriting from motor cortex with a recurrent neural network. The participant, whose hand was paralysed by spinal cord injury, typed 90 characters per minute with 94.1% raw accuracy online.

  What it does not show: One participant with implanted arrays; it restores communication, not hand movement.

  Sources and details
• T1T1 peer-reviewed human studyA brain-spine interface lets a man with tetraplegia walk again2023-05 · as of 2026-10-02 · trial

  Two 64-electrode implants over the sensorimotor cortex read the intention to move a hip, knee or ankle; a decoder turns it into stimulation of the lumbosacral cord through an implanted paddle lead. The participant, with an incomplete cervical injury from ten years earlier, stood, walked and climbed stairs; the system stayed reliable for a year including use at home, and he regained some walking with crutches even with it switched off.

  What it does not show: One participant, with an incomplete injury and earlier stimulation training. It does not repair the cord, and it is not an approved treatment.

  Sources and details

The AI connection

Decoding is a learning problem: a model is fitted to map firing rates to intended movement. The cosine tuning model above is the classic starting point; a recurrent neural network decoded attempted handwriting from motor cortex in one participant ^{[4, 3]}.

Common misconceptions

Check yourself

In which gyrus is the primary motor cortex?

The precentral gyrus of the frontal lobe ^[1].

Where in the motor homunculus are the feet?

On the medial wall of the precentral gyrus ^[1].

What does a cosine-tuned neuron do when the movement is opposite its preferred direction?

It fires least: the cosine of the angle difference is at its minimum, so the rate falls to the baseline minus the modulation depth ^[4].

Read next

• Corticospinal tract, where its commands go.
• Brain–computer interfaces, reading it.
• BCI decoder lab.