Corticospinal tract

Why it matters

When you decide to lift your foot, the command leaves the motor cortex and has to reach a motor neuron in the lumbar cord before any muscle moves. The corticospinal tract is that link: upper motor neurons in the cortex send their axons down through the brainstem to lower motor neurons in the ventral horn of the cord ^[1].

Because most of its fibres cross in the medulla, the left side of the brain moves the right side of the body. Once the tract is in the cord it has already crossed, so damage to one side of the cord weakens the same side of the body below the damage ^{[1, 2]}.

What it is

The pathway has two neurons: an upper motor neuron with its cell body in the primary motor cortex, and a lower motor neuron in the ventral horn of the cord whose axon runs out to a skeletal muscle ^[1].

• The large cortical neurons that start it are called Betz cells; their axons form the corticobulbar tract (to the brainstem) and the corticospinal tract (to the cord) ^[1].
• In the cord it runs as two bundles: the lateral corticospinal tract, made of the fibres that crossed, and the anterior corticospinal tract, which controls the trunk ^[1].

What it does

It carries the commands for voluntary movement. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate lower motor neurons in the cranial motor nuclei or in the ventral horn of the cord ^[1].

The lateral tract serves the limbs on the opposite side of the body; the anterior tract serves the trunk, whose movements involve both sides, so it is not entirely contralateral ^[1].

How it works

Follow one command from the left cortex to a muscle in the right leg ^[1]:

1. The axon leaves the precentral gyrus and descends through the deep white matter of the cerebrum ^[1].
2. It passes between the caudate nucleus and the putamen in the internal capsule ^[1].
3. It runs through the midbrain in the cerebral peduncle and then through the pons ^[1].
4. At the border of the medulla and the cord, most fibres cross to the other side at the pyramidal decussation ^[1].
5. The crossed fibres descend in the lateral corticospinal tract to the ventral horn at the right level, where they synapse on a lower motor neuron ^[1].
6. The lower motor neuron's axon leaves in the ventral root and makes the muscle contract; the neuromuscular junction is strictly excitatory ^[1].

Lateral corticospinal tractUpper motor neurons in the left motor cortex send axons down through the internal capsule, cerebral peduncle and pons; most cross at the pyramidal decussation and descend in the right lateral cord to the lower motor neurons of right-sided limb muscles.

Step through

• Excitatory synapse (filled arrowhead)
• Inhibitory synapse (bar)
• Modulatory (open circle)
• Signal or data flow, not a synapse (dashed)
• Midline crossing (decussation)

• The neuromuscular junction is strictly excitatory, so the muscle contracts when its motor neuron fires.^[1]
• The anterior corticospinal tract (not drawn) controls the muscles of the trunk and is not entirely contralateral.^[1]

Simplified: One upper and one lower motor neuron stand for many; the synapse in the ventral horn is drawn as the end of the tract, without claiming its sign.

Text description of this diagram

1. Upper motor neurons in the left motor cortex send axons down through the internal capsule, cerebral peduncle and pons; most cross at the pyramidal decussation and descend in the right lateral cord to the lower motor neurons of right-sided limb muscles.
2. Primary motor cortex (left side, Cerebral cortex) to Internal capsule (left side, Deep brain); signal or data flow (not a synapse).
3. Internal capsule (left side, Deep brain) to Cerebral peduncle (left side, Midbrain); signal or data flow (not a synapse).
4. Cerebral peduncle (left side, Midbrain) to Pons (left side, Pons); signal or data flow (not a synapse).
5. Pons (left side, Pons) to Pyramid (left side, Medulla); signal or data flow (not a synapse).
6. Pyramid (left side, Medulla) to Lateral corticospinal (right side, Spinal cord); signal or data flow (not a synapse); crosses the midline at the pyramidal decussation.
7. Lateral corticospinal (right side, Spinal cord) to Lower motor neuron (right side, Ventral horn); signal or data flow (not a synapse); synapse in the ventral horn.
8. Lower motor neuron (right side, Ventral horn) to Limb muscle (right side, Muscle); excitatory synapse; neuromuscular junction.

Why it is built this way

The primary motor cortex is laid out as a map of the body, the motor homunculus, so each part of the precentral gyrus starts the fibres for one part of the body; the neurons for the feet and lower legs are on the medial wall ^[1].

When it does not behave

A lesion of one half of the cord takes away voluntary movement on the same side below the lesion, because the lateral tract has already crossed. Together with the loss of pain and temperature on the other side, this is the Brown-Séquard pattern; a published case report describes it after a tumour inside the cord ^{[2, 5]}.

The numbers

How long the trip takes depends on fibre length and diameter. For myelinated fibres, conduction velocity is roughly proportional to diameter ^[6]:

Conduction velocity of a myelinated fibre (Hursh)^[6]

$$v \approx k\, D, \qquad k \approx 6\ \tfrac{\mathrm{m/s}}{\mu\mathrm{m}}$$

For myelinated nerve fibres, conduction velocity grows in proportion to the fibre’s outside diameter. Hursh measured a ratio of about 6 metres per second for every micrometre.

Variables of Conduction velocity of a myelinated fibre (Hursh)

Symbol	Meaning	Unit
$D$	fibre diameter (outside the myelin)	µm
$v$	conduction velocity	m/s

Valid when

• Myelinated peripheral fibres of the cat, where it was measured
• Using it for central fibres, or for human nerves, is an approximation
• Not for unmyelinated fibres, where speed grows more slowly with diameter

Worked example^[6]

A 10 µm fibre conducts at about 60 m/s by this ratio.

D = 10 µm → v = 60 m/s

fibre diameter (outside the myelin) (µm)

conduction velocity: 60 m/s

Conduction time along a fibre^[6]

$$t = \frac{L}{v} = \frac{L}{k\, D}$$

The time a signal needs to travel along a fibre is its length divided by its speed; with Hursh’s ratio, the speed comes from the fibre diameter. Synaptic delays add to this; the signal journey lab adds them up for a whole pathway.

Variables of Conduction time along a fibre

Symbol	Meaning	Unit
$L$	fibre length	m
$D$	fibre diameter	µm
$t$	conduction time	ms

Valid when

• Assumes one constant diameter along the whole fibre
• Ignores synaptic and neuromuscular delays
• Inherits the limits of Hursh’s ratio (myelinated fibres, measured in cat)

Worked example^[6]

One metre of 10 µm myelinated fibre takes about 16.7 ms.

L = 1 m, D = 10 µm → t = 16.7 ms

fibre length (m)fibre diameter (µm)

conduction time: 16.67 ms

The signal journey lab adds the pieces up for a whole route, cortex to muscle, including synaptic delays, and labels every number with where it came from ^[6].

How we see and measure it

On this site you can see where the tract runs in each cord cross-section. The outlines come from a white-matter atlas made by digitising the tract map in Gray's Anatomy, registering it to an MRI template of the cord at the C4 level and propagating it up and down the cord ^{[3, 4]}.

Frontier

Reading the motor cortex directly, and writing to the cord below an injury, are both active research. Each entry shows its evidence tier and what it does not show ^{[7, 8]}.

• 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

Common misconceptions

Check yourself

Where do most corticospinal fibres cross the midline?

At the pyramidal decussation, at the border between the medulla and the spinal cord ^[1].

A lesion cuts the left half of the cord at T10. Which leg is weak?

The left leg: the lateral corticospinal tract crossed in the medulla, so below T10 its left-side fibres serve the left side of the body ^{[1, 2]}.

Why is the anterior corticospinal tract not entirely contralateral?

It controls the muscles of the trunk, and trunk movements involve both sides of the body ^[1].

Read next

• Primary motor cortex, where the pathway starts.
• The pathway in 3D, routed on the body model.
• Lesion explorer, to cut it and see what is lost.
• Signal journey, to time the trip.