Motor cortex and the corticospinal tract !

Concept
Exam priority: high

The motor cortex plans voluntary movement and sends the command down the corticospinal tract, which crosses in the lower brainstem.

Updated 2026-11-01

Also called: Pyramidal tract, Primary motor cortex

See it in 3D: Motor cortex · White matter · Midbrain · Pons · Medulla oblongata · Spinal cord

Key points

The primary motor cortex holds a map of the body: leg on the medial surface (anterior cerebral artery), arm and face laterally (middle cerebral artery).
Corticospinal fibres cross at the medullary pyramids: a lesion above the crossing weakens the opposite side, a cord lesion the same side.
A small infarct in the internal capsule causes pure motor hemiparesis of face, arm and leg on the opposite side.
Corticobulbar input to most cranial motor nuclei is bilateral, but the lower face receives it from the opposite cortex only: a central facial palsy spares the forehead.
The lateral tract (crossed) serves distal limbs and individual finger movement; the anterior tract (uncrossed) serves the trunk.
Supplementary motor area plans sequences; premotor cortex guides movements by sensory cues; Broca area holds the motor programs of speech.

What it is

Voluntary movement starts in premotor and supplementary motor areas, which plan, and in the primary motor cortex of the precentral gyrus, which issues the command. Its axons descend through the posterior limb of the internal capsule, the cerebral peduncles, the pons and the medullary pyramids. About 80–90% cross at the pyramidal decussation to form the lateral corticospinal tract, which controls fine distal movement; the rest descend uncrossed in the anterior tract to trunk muscles.

Key facts

Giant Betz cells of layer V supply only a small fraction (about 3%) of corticospinal fibres; most come from smaller neurons of motor, premotor and somatosensory cortex.
Most corticospinal axons end on interneurons, but in primates many end directly on motor neurons of the hand, allowing independent finger movement.
Other descending tracts (rubrospinal, reticulospinal, vestibulospinal, tectospinal) handle posture and gross movement.
Route: corona radiata → internal capsule → cerebral peduncle (midbrain) → basilar pons → pyramid (medulla) → decussation → lateral column of the cord.

Where the lesion is, by the weakness pattern

Lesion site	Weakness	Clue
Motor cortex, middle cerebral artery	Opposite face and arm more than leg	Aphasia if the dominant hemisphere
Motor cortex, anterior cerebral artery	Opposite leg more than arm	Behaviour change, urinary incontinence
Internal capsule (lacunar)	Opposite face, arm and leg equally	Pure motor, no cortical signs
Medial medulla	Opposite arm and leg	Tongue deviates to the side of the lesion (XII)
Lateral spinal cord (hemisection)	Same side, below the lesion	Pain and temperature lost on the opposite side

Clinical notes

Right after a stroke the weak limbs may be flaccid with low reflexes; spasticity and hyperreflexia develop over days to weeks.
A central facial palsy from stroke spares the forehead; a peripheral one (Bell palsy) weakens the whole half of the face.
Amyotrophic lateral sclerosis damages both upper and lower motor neurons: spasticity together with wasting and fasciculations.

Test yourself

Think of the answer, then tap the question to see it.

Why does a left middle cerebral artery stroke weaken the right face and arm more than the right leg?

The middle cerebral artery supplies the lateral motor cortex, where face and arm are mapped. The leg area lies on the medial surface, supplied by the anterior cerebral artery.

A small infarct in the posterior limb of the right internal capsule causes what deficit, and why is it so complete?

Equal weakness of the left face, arm and leg. Corticospinal and corticobulbar fibres are packed tightly there, so a small lesion catches all of them.

Why does a stroke spare the forehead while Bell palsy does not?

The facial nucleus neurons for the forehead receive input from both hemispheres, so the intact cortex still moves it. Bell palsy damages the facial nerve itself, the final common path, so the whole half of the face is weak.

A lesion of the right lateral corticospinal tract at C6 weakens which side, and why?

The right side below the lesion: in the cord the tract has already crossed, at the medullary pyramids.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 14.3 Motor Responses (OpenStax (Rice University), 2022); Neuroanatomy, Precentral Gyrus (StatPearls (NCBI Bookshelf), 2023); Neuroanatomy, Internal Capsule (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 56). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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