Study topic · about 9 minutes

How the brain controls movement: upper and lower motor neurons

Every movement you choose starts as a plan in the frontal lobe. The primary motor cortex sends the command down the corticospinal tract, which crosses to the opposite side at the bottom of the brainstem. Its fibers are upper motor neurons: in the spinal cord they hand the command to lower motor neurons, which make the muscles contract. Two loops, through the basal ganglia and the cerebellum, help start, scale and smooth every movement.

See the corticospinal tract in 3D

From idea to action: planning a movement

Say you decide to pick up a glass of water. The prefrontal cortex sets the goal, and the parietal lobe works out where the glass and your hand are. Planning areas just in front of the motor strip then choose and order the steps: reach, open the hand, grip, lift. These are the premotor cortex on the outer surface, partly in the caudal middle frontal gyrus, and the supplementary motor area on the inner surface of the superior frontal gyrus.

The final commands come from the primary motor cortex (M1), in the precentral gyrus just in front of the central sulcus. Its large output cells, including giant Betz cells, send their axons down to the brainstem and spinal cord.

Along the strip runs a map of the body, the motor homunculus (Latin for "little man"). The neurosurgeon Wilder Penfield mapped it between the 1930s and 1950s by gently stimulating the cortex of awake patients during epilepsy surgery. The leg and foot sit at the top, where the strip folds onto the inner surface of the brain in the paracentral lobule. Below come the trunk, arm and hand, then the face, lips and tongue.

Parts that need fine control, like the hand and the mouth, get the most room. Because the map is spread out, a small stroke can weaken just the hand or just the face. The leg area is fed by the anterior cerebral artery, and the face and arm areas by the middle cerebral artery. Read more in blood supply of the brain.

See the motor homunculus in 3D

The corticospinal tract: why each side moves the opposite side

Commands leave the motor cortex through the corticospinal tract, also called the pyramidal tract. It carries roughly one million axons on each side, some of them among the longest in the body. On the way down, it passes four landmarks:

1. The internal capsule. Here, in a narrow band of white matter between the thalamus and the basal ganglia, the fibers are packed tightly. A small stroke here can weaken the whole opposite side: face, arm and leg.
2. The brainstem. The tract runs down the front of the midbrain, in the cerebral peduncles, and through the pons.
3. The pyramids. On the front of the medulla, the fibers form two ridges called the pyramids. At the bottom of the medulla, most of them (about 75 to 90 percent) cross to the other side: the pyramidal decussation.
4. The spinal cord. The crossed fibers form the lateral corticospinal tract, which controls the limbs, above all the hands and fingers. The few uncrossed fibers form the anterior corticospinal tract for the trunk; most of them cross lower down.

In the spinal cord, the fibers hand the command to lower motor neurons, directly or through interneurons. Because most fibers crossed, a stroke in the left hemisphere weakens the right arm and leg.

Corticobulbar fibers: commands for the head

Beside the tract run the corticobulbar fibers ("bulb" is an old name for the brainstem). They end on the motor nuclei of the cranial nerves, which move the jaw, face, tongue and throat. Most of these nuclei get commands from both hemispheres.

The face is the classic exception. The lower face gets its commands mainly from the opposite hemisphere, but the forehead gets them from both. So after a stroke, the opposite lower face droops while the forehead still wrinkles: a central facial palsy. Damage to the facial nerve itself, as in Bell's palsy, weakens the whole side of the face, forehead included.

Upper and lower motor neurons

Doctors divide the motor pathway into two links. Upper motor neurons start in the brain and end in the brainstem or spinal cord. Lower motor neurons sit in the anterior horn of the spinal cord's gray matter and in the motor nuclei of the cranial nerves. Their axons run out through the nerves and release acetylcholine onto the muscles.

Every command, from the brain or from a reflex, must pass through lower motor neurons, so Charles Sherrington called them the "final common path". Damage to each link causes its own pattern of signs:

Why the difference? Upper motor neurons also keep the spinal reflex circuits in check. Damage them, and the circuits below overreact, so muscles stiffen and reflexes become brisk. Damage lower motor neurons, and the muscle loses its nerve supply: it goes floppy, its reflexes fade and it wastes away. Strokes, spinal cord injuries and multiple sclerosis hit upper motor neurons; polio and nerve injuries hit lower ones.

In the knee-jerk reflex, a tap below the kneecap stretches the thigh muscle. A sensory neuron then excites the lower motor neuron directly in the spinal cord, and the leg kicks. This reflex arc needs no upper motor neurons, so it still works, and even overreacts, when they are damaged. Break the arc at the lower motor neuron and the reflex disappears (see neurons and nerve signals).

Right after a stroke or spinal cord injury, the weak limbs are often floppy for days to weeks, with weak reflexes. In the spinal cord this is called spinal shock. Stiffness and brisk reflexes come later.

The basal ganglia: starting and scaling movement

Two loops fine-tune every movement. Neither sends commands directly to the spinal cord; both take in signals from the cortex and answer back through the thalamus. The first runs through the basal ganglia, a group of nuclei deep in each hemisphere.

Their entrance is the striatum, made of the caudate nucleus and the putamen. Their exit, the internal globus pallidus, works like a brake: it holds the thalamus back nearly all the time. A movement goes ahead when the brake is briefly lifted.

• The direct pathway ("go"). The striatum quiets the internal globus pallidus, so the brake lifts and the thalamus lets the chosen movement go.
• The indirect pathway ("stop"). Signals detour through the external globus pallidus and the subthalamic nucleus, which press the brake harder to hold back unwanted movements.
• Dopamine. Dopamine from the substantia nigra pars compacta boosts the go pathway and quiets the stop pathway, making movements easier to start, bigger and faster.

In Parkinson's disease, the dopamine neurons of the substantia nigra die, and many are gone before symptoms begin. The stop pathway wins: movements become slow and small (bradykinesia), muscles stiffen (rigidity) and a hand shakes at rest (resting tremor). Steps shuffle, handwriting shrinks, and later balance suffers.

In Huntington's disease, an inherited illness, neurons in the striatum die, the caudate nucleus first. The stop pathway fails, so jerky, dance-like movements called chorea break through, along with changes in mood and thinking. A small stroke in the subthalamic nucleus also weakens the brake, causing wild flinging of the opposite arm and leg (hemiballismus).

The cerebellum: coordination, timing and balance

The second loop runs through the cerebellum. It receives a copy of each command from the motor cortex, plus feedback from the muscles, joints, inner ear and eyes. It compares the plan with what actually happened and corrects the difference, like a coach. Its answers leave through the deep cerebellar nuclei and reach the motor cortex through the ventral lateral nucleus of the thalamus of the thalamus.

Cerebellar damage does not paralyze, but movements become clumsy, badly timed and unsteady: ataxia. Typical signs include:

• Intention tremor: the hand shakes more as it nears a target, as in the finger-to-nose test.
• Dysmetria: overshooting or stopping short of a target.
• Dysdiadochokinesia: trouble with fast alternating movements, like flipping a hand back and forth.
• A wide, staggering walk, jerky eye movements (nystagmus), slurred speech and slightly floppy muscles.

Each half of the cerebellum works with the same side of the body. Its wiring crosses twice: to the opposite thalamus and cortex, then back again in the corticospinal tract. The middle strip, the cerebellar vermis, steadies the trunk and walking; the side parts guide the limbs. Alcohol disturbs the cerebellum early, which is why police sobriety tests ask people to walk a straight line.

ALS: when both kinds of motor neuron die

Amyotrophic lateral sclerosis (ALS), also called motor neuron disease or Lou Gehrig's disease, destroys upper and lower motor neurons together. Its name says so. "Amyotrophic" means the muscles waste away, from the loss of lower motor neurons. "Lateral sclerosis" means scarring along the sides of the spinal cord, where the corticospinal fibers of the upper motor neurons run.

So ALS mixes both columns of the table, which is its hallmark: a wasted, twitching hand can sit on an arm with brisk reflexes. Weakness often starts in one limb, or in the muscles for speech and swallowing, then spreads over months to years. Feeling, eye movements and bladder control are usually spared.

Most cases have no known cause, and about 1 in 10 runs in families. There is no cure yet. The physicist Stephen Hawking lived with an unusually slow form for more than 50 years.

Memory tricks

In these study guides

• MCATNeurons, the nervous system and hormones
• Anatomy and physiologyThe somatic nervous system (chapter 14)
• NursingBrain regions in a neuro assessment
• Medical schoolMotor systems

Take it further in 3D

• Moving and feelingThe motor and sensory strips. A short guided lesson in the 3D app.
• The basal gangliaChoosing which movement goes. A short guided lesson in the 3D app.
• The cerebellumSmooth, balanced, well-timed movement. A short guided lesson in the 3D app.

Sources

1. Anatomy and Physiology 2e, 14.3 Motor Responses, OpenStax
2. Neuroanatomy, Upper Motor Neuron Lesion, StatPearls, NCBI Bookshelf
3. Anatomy and Physiology 2e, 13.2 The Central Nervous System, OpenStax

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