Motor pathway (corticospinal tract)

What it normally does

• The command runs through two neurones in series. The upper motor neurone arises chiefly from the motor strip of the frontal lobe (precentral gyrus), with contributions from premotor, supplementary motor and parietal cortex, runs down through the internal capsule and brainstem and ends in the cord, mostly on interneurones rather than directly on the motor neurone. The lower motor neurone starts in the anterior horn, leaves through the nerve root and peripheral nerve, and ends on muscle. Only the lower one touches muscle — it is the final common pathway.

  Damage above the anterior horn and damage below it give opposite sign patterns. This split is the whole upper versus lower motor neurone examination, and it is why motor neurone disease, which kills both, looks so strange.

• Around 85% of corticospinal fibres cross to the other side in the lower medulla (pyramidal decussation) and run down the side of the cord as the lateral corticospinal tract; most of the small uncrossed remainder descends as the anterior corticospinal tract and crosses at segmental level. So the left cortex drives the right side of the body, but by the time the tract is in the cord it is already on the same side as the muscles it serves.

  A lesion above the medulla weakens the opposite side of the body; a lesion in the cord weakens the same side. This is how you decide brain versus cord before any scan.

• The descending motor system does not only excite. Acting through inhibitory interneurones in the cord, it holds the cord's own reflex circuits in check and damps the stretch reflex — and this restraint comes mainly from the reticulospinal and other brainstem pathways that travel alongside the corticospinal tract, not from the pyramidal fibres themselves. The spinal reflex arc is intact and ready to run without it.

  Cut the descending control and the reflex arc runs unopposed: stiff limbs (spasticity), brisk reflexes, up-going plantar. Because the restraint is largely non-pyramidal, spasticity builds over days to weeks and its severity tracks how much of the neighbouring descending traffic was destroyed — and it explains why antispasticity drugs work on the cord, not on the damaged tract.

• The lower motor neurone keeps the muscle alive. It releases acetylcholine at the neuromuscular junction and supplies trophic support that keeps muscle fibres bulky and electrically stable.

  Lose it and the muscle wastes, twitches on its own (fasciculation) and its reflex disappears — the flaccid, areflexic picture of a root lesion or motor neurone disease.

What goes wrong

• Ischaemic stroke of the internal capsule (lacunar pure motor stroke)← from “Around 85% of corticospinal fibres cross to th…”

  A small deep perforating branch off the middle cerebral artery (a lenticulostriate artery) occludes, usually through small-vessel disease of that perforator. In the internal capsule the corticospinal fibres for the whole body are packed into a few millimetres, so a tiny infarct takes out face, arm and leg together. These fibres are above the decussation, so the weakness appears on the opposite side. For the first hours to days the limb is floppy with quiet reflexes; the spasticity and brisk reflexes emerge over days to weeks as the cord circuits are released from descending control.

  Contralateral, upper motor neurone, forehead spared. Face plus arm plus leg equally weak with nothing else is a lacunar internal capsule stroke — a small lesion producing a big deficit, because the fibres are so tightly packed. In Australia, stroke strikes Aboriginal and Torres Strait Islander people at roughly twice the rate of other Australians and often a decade or more younger, so do not discount a stroke presentation on age alone.

  You would find: Sudden weakness of face, arm and leg on one side, all to the same degree, with no sensory loss, no aphasia and no visual field cut. The forehead still wrinkles, because the upper face receives fibres from both hemispheres. Days later: increased tone, brisk reflexes and an up-going plantar (Babinski sign).

• Spinal cord compression (metastasis, trauma, cervical spondylotic myelopathy)← from “Around 85% of corticospinal fibres cross to th…”

  Something presses on the cord — a vertebral metastasis, a burst fracture, a degenerate cervical disc. The corticospinal tract has already crossed, so weakness falls on the same side as the compression, and on both sides when the whole cord is squeezed. At the level of the lesion the anterior horn cells and exiting roots themselves are damaged, so that one segment loses its lower motor neurones while everything below loses its upper ones.

  Lower motor neurone signs at the level, upper motor neurone signs below it — that combination localises the lesion to one segment. A sensory level with bladder symptoms is cord compression until imaged, and whole-spine MRI is urgent; in suspected malignant cord compression, dexamethasone is started while imaging is arranged rather than after it.

  You would find: Stiff weak legs with brisk knee and ankle jerks and up-going plantars, a level on the trunk below which sensation changes, wasted hands or a single absent reflex at the level itself, and urinary hesitancy or retention. Acute severe compression can present flaccid and areflexic (spinal shock) before the upper motor neurone signs appear — a floppy leg does not exclude it.

• Motor neurone disease (amyotrophic lateral sclerosis)← from “The command runs through two neurones in serie…”

  Both neurones of the chain degenerate at the same time — the upper one in the cortex and the lower one in the anterior horn and the brainstem motor nuclei. The motor pathway bears the brunt: sensation stays normal, and the neurones for eye movement and for the pelvic sphincters (Onuf's nucleus) are characteristically spared. It is not purely a motor disease, though — up to about half of patients develop some cognitive or behavioural change, and roughly 10 to 15% meet criteria for frontotemporal dementia, which is why capacity and family support are part of the assessment.

  Upper and lower motor neurone signs in the same limb with no sensory loss is motor neurone disease until cervical cord compression has been imaged and excluded. Around 2 to 3 per 100 000 Australians are diagnosed each year and median survival from diagnosis is about 2 to 3 years — which is why early referral for multidisciplinary care, non-invasive ventilation and advance care planning changes more than any drug does.

  You would find: Painless progressive weakness, often starting in one hand or as slurred speech and choking. A wasted, visibly twitching (fasciculating) hand in an arm with brisk reflexes. Sensation normal, eye movements normal, continent. Weight loss, breathlessness lying flat and emotional lability (pseudobulbar affect) are common as it advances.

• Nerve root compression (radiculopathy) — the pure lower motor neurone lesion← from “The lower motor neurone keeps the muscle alive…”

  A prolapsed disc squeezes a nerve root. The ventral root carries the axon of the lower motor neurone as it leaves the cord and the dorsal root carries the sensory fibres back in, so compression interrupts both limbs of the reflex arc and cuts part of the muscle's trophic supply. Because most muscles draw innervation from more than one root, a single root lesion usually weakens rather than paralyses, and wasting is partial.

  Reflex absent, tone low, muscle wasted, plantar normal equals lower motor neurone. Reflex brisk, tone high, bulk preserved, plantar up equals upper motor neurone. Two traps: wasting takes weeks, so an acute upper motor neurone lesion can look flaccid on day one; and bilateral leg symptoms, saddle numbness or bladder or bowel change means cauda equina, not a single root — that is an emergency.

  You would find: Weakness confined to one myotome and rarely complete, a reflex that has simply gone (an S1 root leaves no ankle jerk), some wasting after weeks, low tone, a down-going plantar, and pain and numbness radiating in the matching dermatome.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.