Motor Pathways: Cortex to Muscle

The Motor Pathway at a Glance

Voluntary movement requires a two-neuron chain from cortex to muscle. The upper motor neuron (UMN) originates in the cortex and descends to the brainstem or spinal cord. The lower motor neuron (LMN) exits the CNS, travels in a peripheral nerve, and drives muscle via the neuromuscular junction (NMJ). Click any node for detail.

① Primary Motor Cortex

Precentral gyrus (Brodmann area 4), frontal lobe. Somatotopic map — the homunculus. Hand and face have disproportionately large representation.

UMN origin layer V Betz cells

Betz cells are giant pyramidal neurons that give rise to the fastest-conducting corticospinal fibres (up to 70 m/s). The supplementary motor area (SMA, area 6) and premotor cortex also contribute fibres to descending motor tracts. Together these form the internal capsule bundle — posterior limb for limbs, genu for face.

② Internal Capsule

Fibres converge tightly here. The posterior limb carries corticospinal fibres (limbs); the genu carries corticobulbar fibres (face/tongue/throat).

UMN high-yield lesion site

Small lacunar infarcts in the internal capsule can produce a pure motor hemiplegia — dense weakness of face, arm, and leg on the contralateral side with no sensory loss. This is because the entire motor outflow is packed into a tiny area. Classic territory: lenticulostriate arteries from MCA.

③ Brainstem (Midbrain → Medulla)

Fibres pass through the cerebral peduncles, basis pontis, and converge into the medullary pyramids. Corticobulbar fibres synapse on cranial nerve motor nuclei here (CN V, VII, IX, X, XI, XII).

UMN decussation ahead

Most corticobulbar fibres are bilateral — so unilateral UMN lesions above the mid-pons spare the upper face (forehead sparing in central VII palsy, vs LMN Bell's palsy which affects the whole face). Exception: the tongue (CN XII) and lower face cross predominantly contralaterally.

④ Pyramidal Decussation (Medulla)

~85–90% of fibres cross the midline here to form the lateral corticospinal tract. The remaining ~10–15% stay ipsilateral as the anterior corticospinal tract.

crossover point

This is why a lesion above the decussation causes contralateral limb weakness, while a spinal cord lesion below causes ipsilateral weakness. The anterior corticospinal tract eventually crosses at spinal level. In Brown-Séquard syndrome (hemisection), ipsilateral UMN signs appear below the lesion alongside ipsilateral dorsal column loss and contralateral spinothalamic loss.

⑤ Lateral Corticospinal Tract (Spinal Cord)

Runs in the lateral white matter. Descends ipsilaterally below the decussation. Synapses on anterior horn cells — or on interneurons that modulate them.

UMN lateral funiculus

The lateral CST is the primary voluntary motor pathway for distal limb muscles. Cervical spinal cord injury at C5 affects arm (C5–T1 segments) and leg (lumbar segments below). The tract is somatotopically organised: cervical fibres are most medial, sacral most lateral — relevant to central cord syndrome where arms are affected more than legs.

⑥ Anterior Horn Cell (Spinal Cord)

The UMN→LMN synapse. Alpha motor neurons in the anterior (ventral) horn of the spinal grey matter. The final common pathway — all motor commands must pass through here.

LMN origin Sherrington's final common path

Alpha motor neurons (Aα fibres, 12–20 µm, 70–120 m/s) innervate extrafusal muscle fibres. Gamma motor neurons regulate muscle spindle sensitivity. A single alpha motor neuron plus all its muscle fibres = a motor unit. Small muscles (eye, hand) have low innervation ratios (fine control); large postural muscles have high ratios (power). Destruction of anterior horn cells causes LMN signs: flaccidity, wasting, fasciculations (e.g. ALS, polio, spinal muscular atrophy).

⑦ Ventral Root & Peripheral Nerve

LMN axons exit via ventral nerve roots, combine with sensory fibres in the intervertebral foramen, then travel in named peripheral nerves to target muscles.

LMN outside CNS

Nerve roots (radiculopathy) vs peripheral nerve (mononeuropathy) vs plexus (plexopathy) produce distinct patterns. Root injury: dermatomal sensory loss + myotomal weakness. Nerve injury: the distribution of that nerve. EMG/NCS and careful clinical mapping distinguish these. Regeneration is possible in PNS (Schwann cells guide regrowth) but not CNS.

⑧ Neuromuscular Junction

Terminal axon → synaptic cleft → motor end plate. Acetylcholine (ACh) released from vesicles binds nicotinic ACh receptors (nAChR) on the postsynaptic membrane to trigger muscle depolarisation.

NMJ ACh · nAChR

AChE (acetylcholinesterase) in the cleft rapidly hydrolyses ACh. Drugs targeting the NMJ: suxamethonium (depolarising block, RSI), rocuronium/vecuronium (non-depolarising, competitive antagonists reversed by sugammadex or neostigmine). Disease: Myasthenia gravis (autoimmune IgG vs nAChR — fatigable weakness), Lambert-Eaton (autoimmune vs VGCC — proximal weakness improving with use), organophosphate poisoning (AChE inhibition → cholinergic crisis).

⑨ Skeletal Muscle

End-plate potential → muscle action potential → T-tubule depolarisation → Ca²⁺ release from sarcoplasmic reticulum → actin-myosin cross-bridge cycling → contraction.

Effector excitation-contraction coupling

The sliding filament model: myosin heads bind actin, rotate (power stroke), detach (requires ATP). Ca²⁺ binds troponin → shifts tropomyosin → exposes binding sites. Muscle disease (myopathy) causes weakness with normal reflexes initially and no UMN/LMN localising signs — EMG shows myopathic units (short-duration, polyphasic, low-amplitude). CK is elevated in destructive myopathies (rhabdomyolysis, Duchenne, inflammatory myopathy).

↑ Click any node to expand detail. Use the tabs above to explore tracts, UMN/LMN comparison, NMJ pharmacology, clinical scenarios, and a self-test quiz.

Descending Motor Tracts

Multiple tracts contribute to motor control. The corticospinal tract is the primary voluntary pathway; the others modulate posture, tone, and automatic movement.

Lateral Corticospinal Tract Primary voluntary

The dominant voluntary motor tract, carrying ~85–90% of corticospinal fibres after crossing at the pyramidal decussation in the caudal medulla.

Motor cortex (area 4/6)
→ Internal capsule (posterior limb)
→ Cerebral peduncle (midbrain)
→ Basis pontis
→ Medullary pyramids
→ DECUSSATION at caudal medulla
→ Lateral funiculus (contralateral spinal cord)
→ Anterior horn cell (synapse)
→ Ventral root → Peripheral nerve → Muscle

Controls distal limb muscles (fine hand movements especially). Somatotopy: cervical most medial, sacral most lateral. Lesion above decussation = contralateral weakness. Lesion below = ipsilateral weakness.

Anterior Corticospinal Tract Axial/proximal

The ~10–15% of fibres that do not cross at the pyramidal decussation — they descend ipsilaterally and cross at each segmental level.

Motor cortex
→ Internal capsule
→ Medullary pyramids (does NOT cross here)
→ Anterior funiculus (ipsilateral spinal cord)
→ Crosses at segmental level via anterior white commissure
→ Anterior horn cell (contralateral)

Controls axial and proximal muscles (trunk, shoulder, hip). Because it receives bilateral cortical input, midline lesions may preserve some proximal function.

Corticobulbar Tract Cranial nerve motor

Controls voluntary movement of face, tongue, jaw, pharynx, and larynx via cranial nerve motor nuclei.

Motor cortex (lower homunculus: face, tongue)
→ Internal capsule (genu)
→ Brainstem (synapses bilaterally on most CN nuclei)
→ CN V (mastication), VII (facial), IX/X (swallowing/voice), XI (SCM/trapezius), XII (tongue)

Most cranial nerve nuclei receive bilateral cortical innervation — so unilateral UMN lesions only partially affect them. Exception: lower facial nucleus and XII nucleus are predominantly contralateral. This explains upper motor neuron facial palsy: forehead spared (bilateral input to upper facial nucleus), lower face weak contralateral to lesion.

Rubrospinal Tract Lateral system

Originates in the red nucleus (midbrain tegmentum). Crosses immediately in the ventral tegmental decussation. Runs in the lateral funiculus alongside the lateral CST.

Facilitates flexor tone in the upper limbs. Rudimentary in humans — thought to partially compensate for CST lesions. Clinically less important in humans than in other mammals.

Vestibulospinal Tract Posture / tone

Two components: lateral (Deiters' nucleus → ipsilateral spinal cord, facilitates extensors) and medial (bilateral, cervical cord only, head/neck posture).

Facilitates extensor tone and antigravity muscles. After CST lesion, vestibulospinal activity is unopposed → decerebrate posturing (extension) or decorticate posturing (flexion at arms, extension at legs — depending on lesion level). Relevant in trauma: bilateral extensor posturing = poor prognostic sign.

Reticulospinal Tract Tone / gait / autonomic

Pontine reticulospinal (medial) facilitates extensor/axial tone. Medullary reticulospinal (lateral) inhibits extensor tone and modulates pain. Both descend bilaterally.

Important in spasticity: after CST lesion, reticulospinal tracts become disinhibited → increased tone in antigravity muscles (flexors in arm, extensors in leg = classic UMN spastic pattern). Also modulates autonomic function and respiratory drive.

UMN vs LMN: Distinguishing Features

Localising a motor lesion to upper or lower motor neuron changes both the diagnosis and the mechanism. These signs are frequently tested clinically and academically.

Upper Motor Neuron (UMN)

Tone↑ Spasticity (clasp-knife)

WeaknessPyramidal pattern*

Reflexes↑ Hyperreflexia

Babinski↑ Extensor (abnormal)

WastingMinimal/disuse only

FasciculationsAbsent

ClonusPresent

ExamplesStroke, MS, cord compression

Lower Motor Neuron (LMN)

Tone↓ Flaccidity / hypotonia

WeaknessDistribution of nerve/root

Reflexes↓ Hyporeflexia / absent

Babinski↓ Flexor (normal/absent)

WastingEarly, marked atrophy

FasciculationsPresent (denervation)

ClonusAbsent

ExamplesDisc prolapse, Guillain-Barré

* Pyramidal (UMN) Pattern of Weakness

UMN lesions produce a characteristic distribution because extensor muscles of the arm (triceps, wrist extensors) are weaker than flexors, and vice versa in the leg. This produces the classic hemiplegic posture:

ARM: shoulder adducted · elbow flexed · forearm pronated · wrist/fingers flexed
LEG: hip extended · knee extended · foot plantar-flexed and inverted (equinovarus)
→ "The arm curls up, the leg straightens and swings out" — circumduction gait

Mechanism: CST normally suppresses flexors in the arm and extensors in the leg (for fine motor control). Loss of CST input releases these from inhibition → pattern above.

Acute vs Chronic UMN Lesion

Acutely (e.g. immediately post-stroke), UMN lesions produce flaccidity and hyporeflexia — so-called spinal shock. This can confuse localisation. Over days to weeks, spasticity and hyperreflexia develop as the cord adapts. Babinski may be present immediately and is the most reliable early UMN sign.

Mixed UMN + LMN — ALS / Motor Neurone Disease

ALS destroys both anterior horn cells (LMN: wasting, fasciculations, weakness) and corticospinal tracts (UMN: spasticity, hyperreflexia, Babinski). Finding both in the same patient is the hallmark. Upper and lower limb involvement in absence of sensory signs is the clinical red flag.

Neuromuscular Junction

The NMJ converts a nerve action potential into muscle contraction. It is the site of action of several clinically important drugs and diseases encountered in prehospital and critical care practice.

Sequence of Transmission

Action potential reaches terminal bouton. Depolarisation of the axon terminal membrane.

Voltage-gated Ca²⁺ channels (VGCC) open. Ca²⁺ influx triggers vesicle fusion with presynaptic membrane.
Lambert-Eaton: autoimmune Ab vs VGCC → reduced ACh release

ACh released into synaptic cleft (200–300 Å wide). Each vesicle releases a quantum of ~10,000 ACh molecules.

ACh binds nicotinic ACh receptors (nAChR) on the postsynaptic membrane (motor end plate). Na⁺/K⁺ influx → end-plate potential (EPP).
Myasthenia gravis: autoimmune Ab vs nAChR → reduced EPP amplitude → fatigable weakness

EPP reaches threshold → muscle action potential propagates along sarcolemma and into T-tubules.

ACh hydrolysed by AChE in the synaptic cleft → choline reuptaken for ACh resynthesis. Termination of signal.
Organophosphates / nerve agents: irreversibly inhibit AChE → ACh accumulates → cholinergic crisis

T-tubule depolarisation → ryanodine receptor activation → Ca²⁺ release from sarcoplasmic reticulum → binding to troponin C.

Tropomyosin shifts → actin binding sites exposed → myosin cross-bridge cycling → contraction. Relaxation on Ca²⁺ reuptake into SR (ATP-dependent).

Pharmacology Summary

DEPOLARISING BLOCK: Suxamethonium → binds nAChR, causes fasciculations then sustained depolarisation block. Rapid offset (plasma cholinesterase). Used in RSI. Contraindicated in hyperkalaemia risk (burns >24h, denervation, prolonged immobility).

NON-DEPOLARISING BLOCK: Rocuronium/vecuronium → competitive antagonist at nAChR. No fasciculations. Reversed by sugammadex (rocuronium) or neostigmine/atropine (vecuronium).

AChE INHIBITORS (therapeutic): Neostigmine (reversal), pyridostigmine (MG maintenance). Increase ACh at cleft.

ORGANOPHOSPHATES (toxic): Irreversible AChE inhibition. SLUDGE/DUMBELS syndrome. Antidote: atropine (blocks muscarinic effects) + pralidoxime if early (reactivates AChE before ageing).

Clinical Scenarios

Applying motor pathway anatomy to real presentations encountered prehospital and in emergency settings.

Scenario 1 72M — acute onset right arm and face weakness

Right-sided facial droop, arm weakness (less so leg), slurred speech. BP 192/104. Onset 40 mins ago.

ToneInitially flaccid (acute)

ReflexesMay be reduced acutely

BabinskiExtensor right → UMN sign

FaceLower face weak, forehead spared → UMN VII palsy

LocalisationLeft MCA territory → internal capsule / cortex

Key pointArm > leg weakness suggests cortical or MCA (not lacunar). Forehead sparing confirms central (UMN) VII vs Bell's palsy (whole face). Babinski is the most reliable early UMN sign in acute phase before spasticity develops.

Scenario 2 55M — progressive leg weakness, bilateral, weeks onset

Gradually worsening difficulty walking, leg stiffness. History of prostate cancer. Bladder hesitancy. No arm symptoms. Sensory level at T10.

Tone↑ Spastic in both legs

Reflexes↑ Hyperreflexia both legs

BabinskiBilateral extensor

SensationLevel at T10 (vibration/proprioception and pain/temp)

LocalisationBilateral UMN → spinal cord (T10 level)

Key pointBilateral UMN signs with sensory level = cord compression until proven otherwise. Ca. prostate → vertebral mets → extradural cord compression. Time-critical: dexamethasone, urgent MRI, neurosurgical/oncology referral.

Scenario 3 34F — ascending weakness and areflexia, recent GI illness

Three weeks post-Campylobacter gastroenteritis. Symmetrical leg weakness ascended over 5 days to arms. Tingling in feet. Facial weakness developing.

Tone↓ Flaccid, generalised

ReflexesGlobally absent (areflexia)

BabinskiAbsent / flexor

WastingMinimal (acute)

LocalisationPeripheral nerves (LMN) — generalised polyneuropathy

Key pointGuillain-Barré Syndrome (AIDP). Ascending LMN signs post-infection. Monitor respiratory function — vital capacity (not SpO₂ which is a late sign). 20-4-30 rule for ICU admission. IVIG or plasma exchange treatment. Risk of autonomic instability.

Scenario 4 Prehospital: 28M — collapse after OP pesticide exposure

Found unresponsive in agricultural shed. Miosis, bradycardia, bronchospasm, hypersalivation, incontinent, muscle fasciculations progressing to flaccid paralysis.

MechanismOrganophosphate → AChE inhibition at NMJ and muscarinic synapses

NMJ effectSustained ACh → initial fasciculations → depolarisation block → flaccid paralysis

MuscarinicSLUDGE: salivation, lacrimation, urination, defaecation, GI cramps, emesis

TreatmentAtropine (titrate to dry secretions/HR), pralidoxime if early, airway management, decontamination, RSI likely required

Key pointAtropine reverses muscarinic effects but NOT the NMJ paralysis (NMJ is nicotinic). Pralidoxime reactivates AChE before irreversible ageing (~24–48h for most OPs). SpO₂ unreliable — bronchospasm, secretions, and central apnoea all combine.

Self-Test Quiz

Ten questions covering the full pathway. Click an option to reveal the answer and explanation.

1. A patient has right-sided weakness of the lower face with forehead sparing, and right arm weakness. Where is the lesion most likely?

2. Which sign differentiates UMN from LMN lesion most reliably in the acute phase?

3. The lateral corticospinal tract controls which muscles most specifically?

4. At which level do approximately 85% of corticospinal fibres cross to the contralateral side?

5. A patient with myasthenia gravis (MG) has fatigable proximal weakness. What is the pathophysiology?

6. In organophosphate poisoning, why does atropine NOT reverse the muscle paralysis?

7. Brown-Séquard syndrome (hemisection of the spinal cord at T6) produces what motor finding?

8. Which pathway is responsible for the UMN pattern of arm weakness (flexors strong, extensors weak)?

9. A patient presents with progressive wasting and weakness of the small muscles of the hand, with brisk reflexes and an extensor plantar response. What is the most likely diagnosis?

10. What is the role of gamma motor neurons in the motor pathway?

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