Types of Motor Units

A motor unit = a single motor neuron + all the muscle fibers it innervates. All fibers within one motor unit are of the same histochemical type, since a motor neuron only innervates one kind of fiber. Motor units are classified based on contraction speed and fatigue resistance into three types:

1. S units (Slow)

Innervate slow-twitch (Type I / slow oxidative) fibers
Low innervation ratio (small units) - few muscle fibers per neuron
Contract relatively slowly, generate low force, but are highly fatigue-resistant
Rely on oxidative metabolism (many mitochondria, high myoglobin)
Recruited first for sustained, low-intensity activity (e.g., maintaining posture, standing)

2. FR units (Fast, Fatigue-Resistant)

Innervate fast-twitch, fatigue-resistant (Type IIa / fast oxidative-glycolytic) fibers
Intermediate contraction speed and force
Use combined oxidative and glycolytic metabolism, giving moderate fatigue resistance
Recruited second, for moderately demanding activity (e.g., walking)

3. FF units (Fast, Fatigable)

Innervate fast-twitch, fatigable (Type IIb/IIx / fast glycolytic) fibers
High innervation ratio (large units) - many fibers per neuron, producing large, powerful contractions
Contract rapidly and forcefully but fatigue quickly (rely mainly on anaerobic glycolysis)
Recruited last, for brief, high-intensity, powerful efforts (e.g., running, jumping)

Recruitment: the Size Principle

Motor units are not recruited randomly. During a graded voluntary contraction, the nervous system follows the size principle: smaller, low-threshold S units are recruited first, then FR units, and finally FF units as the demand for force and speed increases. There is overlap between stages, but this general order holds. For example, in leg muscles: S units for standing -> FR units recruited increasingly during walking -> FF units recruited during running or jumping.

Plasticity

The fiber-type properties of a motor unit are not fixed. They are shaped by the pattern of neural activity and use. In cross-innervation experiments, if the nerve from a fast motor unit is spliced onto a previously "slow" muscle, the muscle converts to a fast phenotype, with corresponding changes in myosin heavy chain (MHC) isoform expression and myosin ATPase activity. This is why endurance vs. resistance training can shift fiber-type proportions within a muscle over time.

Source: Ganong's Review of Medical Physiology, 26th Edition - "The Motor Unit," p. 3704-3706 (see also Kandel's Principles of Neural Science, "The Motor Unit Is the Elementary Unit of Motor Control")

Note on clinical correlation: Denervation of a motor unit (lower motor neuron lesion) causes fibrillations (fine, irregular single-fiber contractions) and eventual muscle atrophy, distinct from fasciculations (visible, jerky group contractions from abnormal motor neuron discharge) - worth knowing if you're studying this alongside neuromuscular disease.

Let me know if you'd like a comparison table (fiber type, speed, force, fatigue resistance, metabolism) or how this ties into specific neuromuscular disorders (e.g., ALS, myasthenia gravis).

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