The Motor Unit and the Triad System

A motor unit is the smallest functional unit through which the nervous system controls skeletal muscle: it consists of a single motor neuron together with every muscle fibre that neuron's axon branches out to supply. Because one neuron's terminal branches synapse onto many fibres at individual neuromuscular junctions, a single nerve impulse fires every fibre of that unit together, all-or-none. Small motor units (few fibres per neuron) allow fine, graded control, as in the muscles that move the eyes or fingers; large motor units (hundreds of fibres per neuron) are used where raw force matters more than precision, as in the large limb muscles.

Inside each fibre, excitation has to be carried rapidly from the surface membrane to the deepest myofibrils, and this is the job of the triad system. The sarcolemma (the fibre's surface membrane) dips inward at regular intervals to form narrow tubes called T-tubules (transverse tubules), which penetrate the fibre and run close to the sarcoplasmic reticulum (SR) — the fibre's internal calcium store. At each T-tubule, the SR bulges out into two swollen sacs called terminal cisternae, one on either side. This assembly — one central T-tubule flanked by two terminal cisternae — is called a triad, and it sits at the junction of the A-band and I-band of the sarcomere. When an action potential sweeps down the sarcolemma, it travels inward along the T-tubules and is sensed by the triad, …