Part 3 · Nerve and Muscle Physiology
Same concept, other depth: Foundation →
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Henneman's size principle: small motor neurons (high input resistance) reach threshold first and innervate slow, fatigue‑resistant fibres; larger units are added for greater force. Innervation ratio ranges from a few fibres per unit (extraocular muscles) to ~1,000–2,000 (gastrocnemius). Force is graded by recruitment and rate coding (firing frequency → summation).
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