Corticospinal tract: from cortex to anterior horn
The corticospinal tract carries voluntary motor commands from the cortex to the spinal cord. Knowing its course lets you predict the deficit from a lesion at each level.
Chapters (10)
Trace the corticospinal tract from cortex to anterior horn and predict the deficit from a lesion in the internal capsule, brainstem or spinal cord.
The short version
- The corticospinal tract runs cortex, corona radiata, internal capsule, cerebral peduncle, basis pontis and medullary pyramid, then crosses at the pyramidal decussation.
- About 90% of fibres cross at the decussation and form the lateral corticospinal tract, which controls the limbs.
- A lesion above the decussation causes weakness on the opposite side; a lesion below it, in the cord, causes weakness on the same side.
- Lower face, arm and leg weak together with no cortical signs point to the internal capsule, the classic site of a lacunar stroke.
- A spared face favours the lower brainstem or cord, though a small capsule lesion can spare it too.
Corticospinal tract: from cortex to the pyramids
The corticospinal tract begins in the cerebral cortex, mainly the primary motor cortex in the precentral gyrus and the premotor areas. These are layer V pyramidal neurons, the upper motor neurons.[1][6]
Their axons fan down through the corona radiata and then squeeze together into the internal capsule. Below the capsule, the tract runs through the cerebral peduncle of the midbrain, the base of the pons and into the medullary pyramids.[1][2][3][12]
- Cortex: primary motor cortex and premotor areas, layer V pyramidal neurons.
- Corona radiata, then the internal capsule.
- Cerebral peduncle (midbrain), basis pontis, medullary pyramid.
The internal capsule: why a small lesion costs so much
The corticospinal fibres for the limbs run in the posterior limb of the internal capsule. The fibres for the face run in the genu and the adjacent part of the posterior limb.[2]
The fibres are packed tightly together here, in an orderly arrangement, so a tiny lesion can be very costly. This resolves the puzzle the video opens with: a pea-sized stroke in the capsule can paralyse one whole side of the body, while a much larger lesion elsewhere in the same hemisphere can leave strength almost untouched.[2]
The pyramidal decussation and the anterior horn
At the bottom of the medulla, at the pyramidal decussation, about 90% of the fibres cross the midline. They continue as the lateral corticospinal tract, which controls the limbs.[3][4]
The rest stay uncrossed in the anterior corticospinal tract, which mainly controls the axial muscles of the trunk.[4][6]
In the spinal cord, the axons synapse on the anterior horn cells (directly or through interneurons), the lower motor neurons, which then drive the muscle.[1][4]
The localisation rule and upper motor neuron signs
The rule depends on where the lesion sits relative to the pyramidal decussation.[4][7]
- A lesion above the pyramidal decussation causes weakness on the opposite side.
- A lesion below it, in the cord, causes weakness on the same side.
A corticospinal lesion produces the upper motor neuron signs: spasticity, hyperreflexia and an extensor plantar response.[5][11]
Corticospinal tract lesions by level
| Lesion | Weakness | Face |
|---|---|---|
| Cortex | Focal weakness plus cortical signs, such as aphasia or neglect | Depends on the area involved |
| Internal capsule | Opposite lower face, arm and leg together, with no cortical signs | Lower face involved, forehead spared |
| Medullary pyramid | Opposite limbs | Spared |
| Spinal cord, below the decussation | Same-side limbs, below the lesion | Spared |
A cortical lesion adds cortical signs, such as aphasia or neglect.[10] An internal capsule lesion is the classic small-vessel, lacunar stroke.[2][10]
In the face, the forehead is spared because it has input from both hemispheres.[5][9]
A medullary pyramid lesion spares the face because the facial fibres left the tract higher up.[5][8]
The face rule: capsule, brainstem or cord
If the face is involved, the lesion is at or above the facial nucleus in the pons: capsule, cortex or upper brainstem. A spared face favours the lower brainstem or cord.[5][8]
Practice question
Original · USMLE styleA 68-year-old woman is brought to the emergency department after waking with weakness on the right side. Examination shows drooping of the right lower face; the forehead wrinkles symmetrically. The right arm and leg are weak with increased tone, brisk reflexes, and an extensor plantar response. Sensation, language, visual fields, and eye movements are normal.
Common questions
What is the course of the corticospinal tract?
It runs from the motor cortex through the corona radiata and internal capsule, then the cerebral peduncle, the base of the pons and the medullary pyramids. About 90% of fibres cross at the pyramidal decussation.
What happens to the fibres at the pyramidal decussation?
About 90% cross the midline and continue as the lateral corticospinal tract, which controls the limbs. The rest stay uncrossed in the anterior corticospinal tract, which mainly controls the axial muscles of the trunk.
Is weakness on the same side or the opposite side of a corticospinal lesion?
A lesion above the pyramidal decussation causes weakness on the opposite side. A lesion below it, in the cord, causes weakness on the same side.
What does an internal capsule lesion cause?
Opposite weakness of the lower face, arm and leg together, with no cortical signs. The forehead is spared because it has input from both hemispheres. This is the classic small-vessel, lacunar stroke.
Why does a spared face point away from the internal capsule?
It only favours the lower brainstem or cord, because the facial fibres leave the tract higher up. A small capsule lesion can spare the face too.
Transcript
Read the full narration, by chapter690 words
0:00The puzzle: a pea-sized stroke versus a larger lesion
A stroke the size of a pea can paralyse one whole side of the body. Yet a much larger lesion, elsewhere in the same hemisphere, can leave strength almost untouched. Why? Today: trace the corticospinal tract from cortex to anterior horn, and predict the deficit from a lesion at each
0:19Cortex: upper motor neurons and the corona radiata
level. It begins in the cerebral cortex, mainly the primary motor cortex in the precentral gyrus and the premotor areas. These are layer five pyramidal neurons, the upper motor neurons. The axons fan down through the corona radiata, then squeeze together into the internal capsule.
0:39The internal capsule: limbs, face and packed fibres
The corticospinal fibres for the limbs run in the posterior limb of the internal capsule. The fibres for the face run in the genu and the adjacent part of the posterior limb. That is why a tiny lesion here is so costly: the fibres are packed tightly together.
0:58Brainstem to the pyramidal decussation: about 90 percent cross
Below the capsule, the tract runs through the cerebral peduncle of the midbrain, the base of the pons, and into the medullary pyramids. At the bottom of the medulla, the pyramidal decussation, about 90% of the fibres cross the midline. They continue as the lateral corticospinal tract, which controls the limbs. The rest stay uncrossed in the anterior corticospinal tract, which mainly controls the axial muscles of the trunk.
1:26The spinal cord, the anterior horn cell and the localisation rule
In the spinal cord, the axons synapse on the anterior horn cells, the lower motor neurons, which then drive the muscle. Here is the localisation rule. A lesion above the pyramidal decussation causes weakness on the opposite side. A lesion below it, in the cord, causes weakness on the same side.
1:48Upper motor neuron signs
Remember the upper motor neuron signs from the upper versus lower motor neuron lesson: spasticity, hyperreflexia and an extensor plantar response.
1:58The lesions in turn: cortex, capsule, pyramid, cord, and the face rule
Take the lesions in turn. A cortical lesion gives focal weakness plus cortical signs, such as aphasia or neglect. An internal capsule lesion gives opposite weakness of the lower face, arm and leg together, with no cortical signs. This is the classic small-vessel, lacunar stroke. In the face, the forehead is spared, because it has input from both hemispheres. A lesion in the medullary pyramid gives opposite limb weakness, but the face is spared, because the facial fibres left the tract higher up. A lesion in the cord, below the decussation, gives same-side limb weakness below the lesion, and again the face is spared. Face involved means the lesion is at or above the facial nucleus in the pons: capsule, cortex or upper brainstem. Face spared favours the lower brainstem or cord, though a small capsule lesion can spare it too.
2:57Practice question
Your turn. A 68-year-old woman wakes with weakness on the right side. The right lower face droops, but the forehead wrinkles normally. The right arm and leg are weak, with increased tone, brisk reflexes and an extensor plantar response. Sensation, language, visual fields and eye movements are normal. Which of the following is the most likely site of the lesion? A, left lateral corticospinal tract at the fifth cervical level. B, left medullary pyramid. C, left internal capsule. D, right lateral corticospinal tract at the fifth cervical level. E, right internal capsule. Pause here and decide.
3:46Answer and explanation
A is out: a cord lesion causes same-side weakness, so the left cord would weaken the left limbs, and the face would be spared. B is out: a left pyramid lesion does weaken the right limbs, but the face is spared. D is out: the right cord tract serves the right limbs, which fits the side, but a cord lesion cannot droop the face. E is out: the right capsule would weaken the left side. So the answer is C. The left internal capsule holds the fibres for the face and limbs, so one small lesion gives right face, arm and leg weakness, with the forehead spared.
4:26Recap and next lesson: the hypothalamus by nucleus
Three things to keep. One: the tract runs cortex, internal capsule, peduncle, pons, pyramid, then crosses at the pyramidal decussation. Two: above the decussation, weakness is opposite. Below it, in the cord, weakness is on the same side. Three: face and limbs together point to the capsule, and a spared face favours the lower brainstem or cord. Next, the hypothalamus by nucleus.
Sources
Numbers in the text, e.g. [1], point here- Corticospinal tract. Wikipedia. en.wikipedia.org/wiki/Corticospinal_tract
- Internal capsule. Wikipedia. en.wikipedia.org/wiki/Internal_capsule
- Medullary pyramids (brainstem). Wikipedia. en.wikipedia.org/wiki/Medullary_pyramids_(brainstem)
- Lateral corticospinal tract. Wikipedia. en.wikipedia.org/wiki/Lateral_corticospinal_tract
- Corticobulbar tract; Facial nerve paralysis; Upper motor neuron lesion. Wikipedia. en.wikipedia.org/wiki/Corticobulbar_tract · en.wikipedia.org/wiki/Facial_nerve_paralysis · en.wikipedia.org/wiki/Upper_motor_neuron_lesion
- The corticospinal tract: review. Front Neuroanat 2026, PMC13106552. europepmc.org/article/PMC/PMC13106552
- Motor deficits and the pyramidal decussation in stroke. Eur Stroke J 2026, PMC13335639. europepmc.org/article/PMC/PMC13335639
- Medullary pyramid and pyramidal decussation infarcts. Cureus 2025, PMC12718626. europepmc.org/article/PMC/PMC12718626
- Central facial palsy and forehead sparing. PMC13435481; PMC13061271. europepmc.org/article/PMC/PMC13435481 · europepmc.org/article/PMC/PMC13061271
- Cortical and lacunar stroke syndromes. PMC12450147. europepmc.org/article/PMC/PMC12450147
- Neuroanatomy, Pyramidal Tract Lesions. StatPearls. ncbi.nlm.nih.gov/books/NBK540976/
- Cerebral peduncle compression and contralateral weakness. PMC11610721. europepmc.org/article/PMC/PMC11610721
The teaching figure of about 90 percent crossing is the textbook value; one review quotes 75 to 90 percent. All brain and cord diagrams on screen are schematic.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): Brain section, Spinal cord section. Gray's Anatomy (1918), plates 764 (the motor tract), 746 (tracts in the internal capsule) and 680 (decussation of the pyramids), Henry Vandyke Carter, public domain via Wikimedia Commons, adapted (recoloured, cropped). The glow trace, lesion markers, body and face sketches, tables, cards and all handwriting are drawn in code (original). No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-015.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24