In one breath
The corticospinal tract (CST) carries the motor cortex’s commands down to the spinal motor neurons, and it is the main pathway for skilled voluntary movement. Its fibres come from areas 4 and 6 and from the sensory cortex, funnel through the posterior limb of the internal capsule and pass through the medullary pyramid, where about 80% cross to form the lateral CST and 20% stay uncrossed as the anterior CST. Because every fibre is squeezed through that narrow capsule, a bleed there (usually from Charcot’s artery) cuts them all and gives contralateral hemiplegia.
Builds on: Movement and organization of the motor system · Segmental organization of the motor system · Leads to: Upper vs lower motor neuron lesions · Rubrospinal, vestibulospinal, reticulospinal and tectospinal tracts · Cortical integration and motor cortex
Classifying the descending pathways
The old split: pyramidal vs extrapyramidal
- Pyramidal tracts are the descending tracts that run through the pyramid of the medulla. In practice that means the CST, so “pyramidal tract disease” is clinical shorthand for disease that cuts CST fibres.
- Extrapyramidal tracts are the ones that bypass the pyramid: the reticulospinal, vestibulospinal, rubrospinal and tectospinal tracts.
Why "pyramidal vs extrapyramidal" misleads
- Not every CST fibre goes through the pyramid: most anterior CST fibres go around it.
- The “pyramidal signs” seen in patients do not match a pure CST lesion made in animals, so the clinical “pyramidal tract” is not the same thing as the CST.
- “Extrapyramidal” means brainstem tracts to an anatomist, but basal ganglia and cerebellar disease to a clinician.
- The basal ganglia act mainly through the CST, not through the extrapyramidal tracts.
So physiologists split the pathways another way: by where they end on the spinal motor neurons and what they do.
Lateral system pathways
- Tracts: the lateral CST and the rubrospinal tract.
- They run in the lateral funiculus and end on the lateral group of ventral-horn motor neurons.
- The lateral group supplies the distal limb muscles, so this system handles skilled voluntary movement.
Medial system pathways
- Tracts: the reticulospinal, vestibulospinal and tectospinal tracts, and the anterior CST.
- They run in the medial and anterior funiculi and end on the medial group of motor neurons.
- The medial group supplies the proximal limb and axial muscles, so this system handles posture.
| Lateral system | Medial system | |
|---|---|---|
| Funiculus | Lateral | Medial and anterior |
| Motor neurons | Lateral group | Medial group |
| Muscles | Distal limb | Proximal limb, axial |
| Controls | Skilled movement | Posture |
A slip in your book
Your book prints the heading “Medial system pathways” above the corticospinal and rubrospinal tracts as well as above the vestibulospinal ones. By its own classification the lateral CST and the rubrospinal tract are lateral-system tracts; of the corticospinal fibres only the anterior CST is medial.
Corticospinal tracts
Two tracts, not one
- CST, pyramidal tract and upper motor neuron are often used as if they meant the same thing. Strictly, they don’t.
- The corticospinal pathway has two parts:
| Tract | Share of fibres | Main job |
|---|---|---|
| Lateral CST (LCST) | about 80% | Skilled voluntary movement |
| Anterior (ventral) CST | about 20% | Posture |
- The LCST is the single most important pathway for skilled voluntary activity.
Origin
- The classic source is the primary motor cortex (area 4), especially the giant Betz cells of layer V of the precentral gyrus.
- Betz cells cannot be the whole story: there are only about 30,000 of them, but the tract has about 1 million axons. The rest come from:
- the premotor cortex (lateral part of area 6);
- the supplementary motor area (medial part of area 6);
- the primary somatosensory cortex (areas 3, 1, 2);
- the parietal cortex (areas 5, 7).
| Source | Share of CST fibres |
|---|---|
| Area 4 | 30% |
| Area 6 | 30% |
| Sensory cortex | 40% |
- So the motor cortex gives 60% of the fibres and the sensory cortex 40%.
Course
- Corona radiata: just below the cortex, fibres from all these areas converge like the spokes of a wheel.
- Internal capsule: they gather into the posterior limb of the internal capsule, a narrow passage.
- Midbrain: they descend in the cerebral peduncle, on the ventral side of the brainstem.
- Medulla: they pass through the medullary pyramid.
- Crossing: just after the pyramid, about 80% cross to the opposite side and run down the lateral funiculus as the lateral CST. Its fibres make monosynaptic contact with anterior horn cells.
- The remaining 20% do not cross. They run down the anterior funiculus on the same side as the anterior CST, and cross only in the cord segment where they end, through interneurons.
Worth knowing, though not in your pages: the crossing is called the pyramidal decussation, at the junction of medulla and cord. In the pons the fibres run through the basilar part, split into bundles by the pontine nuclei.
Termination
| Lateral CST | Anterior CST | |
|---|---|---|
| Ends on | Lateral group | Medial group |
| Contact | 30% direct, 70% via interneurons | Always via interneurons |
| Crosses | In the medulla | In the cord, via interneurons |
| Muscles | Distal limb | Proximal limb, axial |
- The anterior CST ends on interneurons of its own side. Most of them cross and end on the medial group of the opposite side; a few end on the medial group of the same side.
Functions
- The motor cortex starts, plans and controls movement. The CST carries its command to the spinal interneurons and motor neurons.
- Lateral CST: skilled voluntary movements.
- Anterior CST: posture.
Effects of lesions
- Lateral CST lesion: skilled acts such as writing and painting are impaired. The subject recovers within days to weeks, because the rubrospinal tract, the other lateral-system tract, is intact.
- A pure LCST lesion is very rare in humans. Disease that hits the CST also hits the corticobulbar fibres that influence the extrapyramidal systems, so pure CST disease is not seen clinically.
- Anterior CST lesion:
- in animals, posture cannot be kept during walking or climbing;
- in humans the postural loss is slight, for two reasons: the tract is poorly developed, and the main posture pathways, the reticulospinal and vestibulospinal tracts, are still intact.
Clinical importance: the capsular lesion
- The CST can be interrupted anywhere from cortex to cord, but the internal capsule is the commonest site (a capsular lesion).
- Why there: fibres from the whole motor territory pass through a narrow tunnel in the posterior limb, so even a small lesion cuts all of them. The result is contralateral hemiplegia, because the fibres have not yet crossed.
- Why the picture is “complete”: the ascending fibre systems from the basal ganglia and cerebellum pass close to the capsule, so extrapyramidal control is damaged along with the CST. A capsular lesion is therefore called a complete upper motor neuron paralysis.
- Usual cause: rupture of Charcot’s artery, the lenticulostriate branch of the middle cerebral artery. It is called the artery of cerebral haemorrhage because it accounts for more than 60% of intracerebral haemorrhages.
Applied: capsular hemiplegia
An elderly hypertensive person suddenly loses power in the arm and leg of one side. A bleed in the opposite internal capsule has cut the CST above its crossing. The limbs show the UMN picture: spasticity, brisk tendon reflexes, lost superficial reflexes and an extensor plantar. Why each sign appears is explained in Upper vs lower motor neuron lesions.
Worth knowing, though not in your pages: the lower face of the paralysed side is weak too, but the forehead is spared, because the upper face gets corticobulbar fibres from both hemispheres. The angle of the mouth is pulled towards the healthy side.
Draw it: the corticospinal tract
- At the top, draw one cerebral hemisphere in coronal section. Mark the precentral gyrus (area 4, Betz cells), with area 6 in front and areas 3, 1, 2 behind.
- Bring fibres down from all three as a fan (the corona radiata) into the posterior limb of the internal capsule. Label it “commonest lesion site”.
- Draw the brainstem as three stacked blocks: midbrain (cerebral peduncle), pons and medulla (pyramid). Run the tract down their ventral side.
- At the lower end of the medulla, split the line. The thick limb (80%) crosses the midline and descends in the lateral funiculus as the lateral CST.
- The thin limb (20%) stays on the same side in the anterior funiculus as the anterior CST.
- Finish with a cord section. The lateral CST ends on the lateral group of ventral-horn cells (“distal muscles, skilled movement”). The anterior CST ends on an interneuron that crosses to the medial group (“axial and proximal muscles, posture”).
- Mark the midline, and write the percentages (80/20; 30% direct) beside the limbs.
Exam-answer skeleton: "Trace the corticospinal tract with a labelled diagram, and describe the effects of a lesion at the internal capsule" (long essay)
- Introduction: the main pathway for skilled voluntary movement; lateral (80%) and anterior (20%) CSTs; lateral vs medial systems in one line.
- Origin: area 4 (Betz cells, layer V) plus areas 6, 3-1-2 and 5-7; the 30,000 vs 1 million argument; the 60:40 split.
- Course: corona radiata → posterior limb of internal capsule → cerebral peduncle → pons → pyramid → 80% cross.
- Termination: lateral CST on the lateral group (30% direct); anterior CST through interneurons on the medial group.
- The labelled diagram.
- Functions: skilled movement (lateral CST), posture (anterior CST).
- Capsular lesion: why it is the commonest site (narrow tunnel, Charcot’s artery); contralateral hemiplegia; complete UMN paralysis.
- The UMN features and the basis of each, in brief (spasticity, brisk reflexes, lost superficial reflexes, Babinski).
Asked in exams
- Pyramidal (corticospinal) tract: Nov 2012, 10 marks · Nov 2014, 4 marks · Nov 2017, 10 marks · Jul 2019, 10 marks · Jan 2021, 15 marks · Apr 2021, 2 marks · Jan/Feb 2022, 10 marks · Mar 2024, MCQ