Size principle of motor unit recruitment (Henneman's principle) states that smaller motor units (slow-twitch type I) are recruited before larger (fast-twitch type II) units. The basis for this orderly recruitment is:

• A Smaller alpha motor neurons have higher input resistance and reach firing threshold at lower synaptic current levels ✓
• B Slow-twitch fibres are located more superficially in the muscle, receiving greater synaptic input
• C Interneurons from the cerebellum preferentially excite small motor neurons during voluntary movement
• D Slow-twitch motor units have lower conduction velocity, causing their action potentials to arrive at the neuromuscular junction first

Explanation

Henneman's size principle has a biophysical basis: smaller alpha motor neurons (innervating type I slow-twitch fibres) have smaller surface areas and therefore higher input resistance. By Ohm's law (V = IR), the same synaptic current produces a greater voltage change in a small neuron, reaching threshold at lower stimulation intensities. Larger neurons (innervating type IIb fast-twitch fibres) require greater synaptic drive. This produces orderly recruitment from oxidative fatigue-resistant to glycolytic fatigable units as force demand increases — a fundamental principle of human motor control.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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