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The somatic motor system converts movement intentions into coordinated muscular actions. The hierarchy of control goes from the motor cortex (planning and execution of movement) corticospinal pathway spinal cord (reflex circuits and motor neurons) neuromuscular junction muscle skeleton. The cerebellum implements an inverse model (M¹) to proactively correct errors before sensory feedback arrives.

The motor neuron integrates excitatory and inhibitory inputs (integrate-and-fire model): when the membrane potential exceeds V_thresh, it generates an action potential that travels along the motor axon to the motor plate. Neuromuscular transmission releases ACh in quanta (synaptic vesicles) activates nicotinic channels EPP of +3040 mV always exceeds the muscle threshold contraction guaranteed under normal conditions.

The Euler-Lagrange equation describes the complete dynamics of the musculoskeletal system: inertia M(q)·q, Coriolis/centripetal forces C(q,q)·q and gravity G(q) are balanced by the muscle torque _musc. Optimal motion control minimizes jerk (derivative of acceleration) or terminal quadratic error, producing smooth bell-shaped trajectories of velocity, as observed in natural movements.

Henneman's principle (recruitment by size) states that motor units are activated in order from smallest to largest size (and strength) with increasing force required: first the slow type I (fatigue resistant, small force), then the fast type IIA, and finally the type IIX (higher force, fast fatigue). This allows for graduated and efficient control of force.

Motor nerve conduction velocity (NCV) measures the integrity of the motor axon: < 40 m/s indicates motor neuropathy. In myasthenia gravis (antibodies against ACh receptors), the EPP is reduced neuromuscular transmission fails with repeated stimulation (decrease in EMG from repetitive stimulation). In ALS (amyotrophic lateral sclerosis), upper and lower motor neurons are lost, producing a combination of spasticity (MNS) and atrophy/fasciculations (MNI).

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ID:(0, 48)