Muscle Contraction Physiology

Motor units — the control quanta. A motor unit is one motor neuron plus all the muscle fibers it stimulates. When the neuron fires, all its fibers contract together. Motor units vary in size: muscles needing fine control (eye, fingers) have small units (few fibers per neuron), while muscles needing power (thigh, back) have large units (many fibers per neuron). This is a neat structure–function match — precision needs small units, force needs large ones.

Grading force by recruitment. The body increases a muscle's force by recruitment — activating more motor units (and firing them faster). A gentle grip uses few units; a maximal one recruits many. Recruitment usually adds smaller units first, then larger ones, producing smooth increases in force. This is why you can pick up a feather or a dumbbell with the same hand.

Excitation–contraction coupling. This is the bridge from "electrical" to "mechanical." When the muscle fiber's membrane is excited (by ACh at the neuromuscular junction), the electrical impulse travels along the membrane and down T-tubules into the fiber, triggering the sarcoplasmic reticulum to release calcium. Calcium then starts the cross-bridge cycle (previous section). "Coupling" means the electrical excitation is coupled to the mechanical contraction through this calcium release.

Twitch, summation, and tetanus. How a muscle responds depends on how rapidly it's stimulated:

• A single stimulus produces a twitch — one brief contract-and-relax.
• If stimuli arrive before the muscle fully relaxes, the contractions add together (summation), producing more force.
• With rapid, repeated stimulation, contractions fuse into a smooth, sustained, maximal contraction called (complete) tetanus. Normal purposeful movements rely on this smooth tetanic contraction, not single twitches.

(Note: this physiological tetanus — smooth sustained contraction — is different from the disease tetanus caused by a bacterial toxin, though the disease is named for the sustained muscle spasms it produces.)

Muscle tone. Even at rest, muscles are never completely slack. Muscle tone is a continuous, low-level contraction produced by rotating activation of a few motor units at a time. It keeps muscles firm, helps maintain posture, and keeps muscles ready to respond. Loss of tone (flaccidity) or excess tone (spasticity) are important neurological signs.

How it works

Grading a movement:

Need more force?
   → recruit more motor units (and fire faster)
   → each active neuron excites its fibers (ACh at NMJ)
   → excitation travels down T-tubules → calcium released (E-C coupling)
   → cross-bridge cycling → contraction
Rapid repeated stimulation → summation → smooth tetanus (normal movement)
At rest → a few units rotate on/off → muscle tone

Comparisons

Common confusions

• Motor unit ≠ single fiber. It's one neuron and all the fibers it controls.
• Physiologic tetanus vs the disease. Physiologic tetanus is normal smooth contraction; the disease is toxin-induced spasm.
• More force = more units (recruitment), not a "stronger" single fiber contraction (each fiber is all-or-none).
• Muscle tone is not active movement — it's a resting readiness, always present in healthy muscle.

Memory aids

• "Small units for skill, large units for lifting."
• Recruit = "call in more workers for a bigger job."
• Tetanus (good) = "too-tense-to-relax, smooth and strong."

Quick review

• A motor unit is one motor neuron and all its fibers; force is graded by recruitment (activating more/faster units).
• Excitation–contraction coupling links the electrical signal to contraction by releasing calcium via T-tubules and the sarcoplasmic reticulum.
• A single stimulus gives a twitch; rapid stimulation causes summation and smooth (physiologic) tetanus, which powers normal movement.
• Muscle tone is continuous resting partial contraction; abnormal tone (flaccid/spastic) is a neurological sign.