Overview

Extraocular muscles≈10 fibres per motor neuron

Hand intrinsics≈100 fibres per neuron

Gastrocnemius≈1,000–2,000 fibres per neuron

When an α-motor neuron fires, every fibre in its motor unit contracts. The fibres of one unit are all the same type and are scattered across the muscle in a mosaic, not clustered together. The innervation ratio (fibres per neuron) varies with how much control is needed: ≈10 or fewer in extraocular and laryngeal muscles, ≈100 in the intrinsic hand muscles, and ≈1,000–2,000 in gastrocnemius.

Motor units come in three types. Slow (S) units have small cell bodies, low thresholds and slow type I fibres that resist fatigue. Fast fatigue-resistant (FR) units contain type IIa fibres. Fast fatigable (FF) units have large neurons, high thresholds and type IIx fibres. Henneman's size principle: as synaptic drive increases, motor neurons are recruited from smallest to largest, so force rises smoothly and the most economical units do most of the everyday work. Force is further graded by rate coding. Higher firing frequencies cause twitches to summate into unfused and then fused tetanus. All the motor neurons that supply one muscle form its motor neuron pool, a column in the ventral horn.

Function

The unit of force output and of motor control
Allows force to be graded through recruitment and rate coding

Physiology: Twitch → summation → unfused tetanus → fused tetanus as the stimulation frequency rises. Fast units fuse at higher frequencies than slow units. Electromyography (EMG) records motor unit action potentials. After denervation, surviving axons sprout to reinnervate orphaned fibres, which produces giant motor units and fibre-type grouping.

Clinical significance

Motor neuron disease (ALS)

Degeneration of upper and lower motor neurons causes mixed signs: wasting and fasciculations together with hyperreflexia. Eye and sphincter muscles are typically spared until late. EMG shows denervation with large, reinnervated motor units.

Poliomyelitis and spinal muscular atrophy

Destruction of anterior horn cells, by poliovirus or by SMN1 deficiency in spinal muscular atrophy, causes lower motor neuron paralysis with atrophy and areflexia.

Test yourself

Which fibre type is recruited first during a gradually increasing contraction (size principle)?

Type IIx (fast glycolytic)
Type IIa (fast oxidative–glycolytic)
Type I (slow oxidative)
All types are recruited at once

Answer: Type I (slow oxidative)

Henneman's size principle: small motor neurons, which supply slow, fatigue-resistant type I units, reach threshold first. Larger type IIa and then IIx units join as more force is needed.