How the Brain Controls Movement
What part of the brain controls movement?
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The Short Answer
Voluntary movement is driven by the motor cortex at the back of the frontal lobe, which sends commands down the spinal cord to the muscles. But it doesn't work alone: the basal ganglia select which movement to make, the cerebellum makes it smooth and accurate, the parietal lobe tracks where your body is, and the brain stem keeps you balanced and upright.
Who Does What
| Brain region | Its role in movement |
|---|---|
| Frontal Lobe | Plans movements (premotor areas) and sends the final commands to the muscles (primary motor cortex). |
| Basal Ganglia | Selects which movement to make and suppresses competing ones, acting like a gate. |
| Cerebellum | Compares intended and actual movement and corrects errors in real time; key to learning new motor skills. |
| Parietal Lobe | Tracks the position of your body and the objects around you, and guides your hand as you reach. |
| Brain Stem | Controls posture and balance, coordinates eye and head movements, and relays commands between brain and spinal cord. |
Following One Movement
Take something as simple as reaching for a cup of tea. First, the parietal lobe combines what you see with the sense of where your arm is (proprioception) to work out where the cup is relative to your hand. Planning areas in the frontal lobe - the premotor cortex and supplementary motor area - prepare the sequence of movements. The basal ganglia then release the brakes on the chosen action while holding back others you might have made.
The primary motor cortex sends the command down the corticospinal tract. Most of its fibres cross to the other side in the lower brain stem, which is why the left side of the brain controls the right side of the body. As your hand moves, the cerebellum compares the movement you intended with sensory feedback about the one you're actually making, and adjusts it on the fly so your hand slows and opens at just the right moment. Throughout, the brain stem adjusts your posture so that leaning forward doesn't tip you over.
Not every movement needs the brain. When a doctor taps below your knee, the leg kicks because of a circuit in the spinal cord alone - the signal never has to reach the brain, which is why the reflex is so fast.
Landmark Discoveries
In 1937, neurosurgeon Wilder Penfield and Edwin Boldrey published maps made by stimulating the motor and sensory cortex of patients during brain surgery. Each body part had its own patch of cortex, with the hands, lips, and tongue taking up far more space than the trunk or legs. The distorted figure used to illustrate this map is known as the "homunculus".
In 1965, Hans Kornhuber and Lüder Deecke discovered the "readiness potential": a slow build-up of brain activity that begins up to a second or more before a voluntary movement. In 1983, Benjamin Libet reported that this build-up began several hundred milliseconds before people said they had consciously decided to move - a finding often claimed to show that free will is an illusion. Later work, including a 2012 study by Aaron Schurger and colleagues, suggests the readiness potential may partly reflect random background fluctuations in brain activity rather than a hidden decision, and the debate continues.
Understanding movement signals has led to brain-computer interfaces. In 2012, Leigh Hochberg and colleagues reported that two people with paralysis used tiny electrode arrays implanted in their motor cortex to control a robotic arm by thought; one used it to pick up a bottle and drink from it - the first time she had done so on her own in nearly 15 years.
Common Myths
"Muscle memory is stored in the muscles."
Muscles get stronger with practice, but the skill itself is stored in the brain, in circuits involving the motor cortex, basal ganglia, and cerebellum. That's why a skill can return quickly after years without practice, even when the muscles have weakened.
"Movement is controlled by the motor cortex alone."
The motor cortex sends the commands, but damage elsewhere can disrupt movement just as badly: Parkinson's affects the basal ganglia, ataxia the cerebellum, and some strokes the parietal areas that guide reaching.
"Libet proved we have no free will."
The experiments show that brain activity starts before people report deciding to move. What that activity means - and what it implies for free will - is still actively debated by neuroscientists and philosophers.
When It Goes Wrong
These conditions affect movement in different ways, depending on which part of the network is involved:
- Parkinson's Disease - A progressive movement disorder caused by the loss of dopamine-producing neurons in the midbrain, leading to tremor, stiffness, and slowed movement.
- Huntington's Disease - An inherited neurodegenerative disorder caused by a single faulty gene, leading to progressive breakdown of neurons in the basal ganglia and producing involuntary movements, cognitive decline, and psychiatric symptoms.
- Cerebellar Ataxia - A group of disorders causing uncoordinated, unsteady movement due to cerebellar dysfunction, with causes ranging from genetic disease to alcohol-related damage or stroke.
- Stroke & Aphasia - Stroke occurs when blood supply to part of the brain is interrupted; when it damages language centres, it can cause aphasia - difficulty producing or understanding language.
- Multiple Sclerosis - A chronic autoimmune condition in which the immune system attacks myelin, the insulating sheath around nerve fibres in the brain and spinal cord, disrupting communication between the brain and the rest of the body.
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Sources & Further Reading
- Brain Basics: Know Your Brain - National Institute of Neurological Disorders and Stroke
- Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation - Penfield & Boldrey (1937), Brain
- The basal ganglia: focused selection and inhibition of competing motor programs - Mink (1996), Progress in Neurobiology
- Changes in brain potential in voluntary and passive movements in humans: readiness potential and reafferent potentials (in German) - Kornhuber & Deecke (1965), Pflügers Archiv
- Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential) - Libet et al. (1983), Brain
- An accumulator model for spontaneous neural activity prior to self-initiated movement - Schurger et al. (2012), PNAS
- Reach and grasp by people with tetraplegia using a neurally controlled robotic arm - Hochberg et al. (2012), Nature