Classification
| Group | Tracts | Function |
|---|
| Pyramidal | Corticospinal, corticobulbar | Voluntary, skilled, fine movement |
| Extrapyramidal | Rubrospinal, reticulospinal, vestibulospinal, tectospinal, olivospinal | Tone, posture, equilibrium, associated movements |
Corticospinal (pyramidal) Tract — Origin
- 30% from primary motor cortex (area 4)
- 30% from premotor and supplementary motor areas (area 6)
- 40% from the sensory cortex (areas 3, 1, 2) — these fibres modulate sensory input rather than produce movement
- Giant Betz cells contribute only about 3% of the fibres — a commonly asked point
- About 1 million fibres in each tract
Course
Motor cortex → Corona radiata → Posterior limb of internal capsule (genu carries corticobulbar) → Crus cerebri of midbrain → Pons → Medullary pyramid → pyramidal decussation
| Tract | Share | Position in cord | Supplies |
|---|
| Lateral corticospinal | 80–85% (crossed) | Lateral white column | Distal limb muscles — fine movement |
| Anterior corticospinal | 15–20% (uncrossed; crosses at its segment) | Anterior white column | Axial and proximal muscles — posture |
- Corticobulbar fibres supply the cranial nerve motor nuclei; most receive bilateral supply. The exceptions are the lower face and the genioglossus, which are supplied only contralaterally — this is why an upper motor neurone lesion spares the forehead
Upper VS Lower Motor Neurone Lesion
| Feature | UMN lesion | LMN lesion |
|---|
| Paralysis | Spastic | Flaccid |
| Distribution | Groups of muscles | Individual muscles |
| Tone | ↑ (hypertonia, clasp-knife) | ↓ (hypotonia) |
| Deep reflexes | Exaggerated | Lost |
| Superficial reflexes | Lost | Lost |
| Plantar response | Extensor (Babinski positive) | Flexor or absent |
| Clonus | Present | Absent |
| Muscle wasting | Absent (only disuse atrophy, late) | Marked and early |
| Fasciculation | Absent | Present |
| Electrical reaction | Normal | Reaction of degeneration |
| Example | Stroke, spinal cord lesion | Poliomyelitis, peripheral nerve injury |
Babinski Sign
- Stroking the lateral sole from heel to toes
- Normal adult → plantar flexion of the great toe
- UMN lesion → dorsiflexion of the great toe with fanning of the others — Babinski positive
- Normally positive in infants below 1 year, before the corticospinal tract is myelinated
- It is the most reliable single sign of a pyramidal tract lesion
Motor Areas of the Cortex
| Area | Brodmann number | Function |
|---|
| Primary motor | Area 4 | Execution of fine skilled movement; contralateral, inverted motor homunculus |
| Premotor | Area 6 (lateral) | Planning; orientation of the body before movement |
| Supplementary motor | Area 6 (medial) | Bilateral and sequential complex movements |
| Frontal eye field | Area 8 | Conjugate eye movement |
| Broca area | Areas 44, 45 | Motor speech; dominant hemisphere |
- In the motor homunculus the area devoted to a part is proportional to the fineness of control, not to size — hence the enormous hand, thumb, lips and tongue
- Stimulation of area 4 causes movement, not a purposeful act
Extrapyramidal Tracts
| Tract | Origin | Function |
|---|
| Rubrospinal | Red nucleus | Facilitates flexor tone |
| Lateral vestibulospinal | Deiters nucleus | Facilitates extensor (antigravity) tone |
| Reticulospinal — pontine | Pons | Facilitates extensor tone |
| Reticulospinal — medullary | Medulla | Inhibits extensor tone |
| Tectospinal | Superior colliculus | Reflex head turning to visual and auditory stimuli |
Applied Aspects
- Internal capsule stroke — fibres are tightly packed, so a small lesion causes a dense contralateral hemiplegia with UMN facial palsy sparing the forehead
- Decerebrate rigidity — lesion between the superior and inferior colliculi → unopposed extensor facilitation → all four limbs extended
- Decorticate rigidity — lesion above the red nucleus → rubrospinal facilitation of flexors survives → upper limbs flexed, lower limbs extended
- Motor neurone disease — the rare condition showing both UMN and LMN signs together