From nerve impulse to contraction: the sliding filaments
At rest the outside of a muscle fibre’s membrane is positive and the inside is negative, because ions are unevenly spread. The order to contract arrives from the brain or spinal cord along a motor neuron. The nerve ending meets the fibre at a neuromuscular junction, with a tiny gap between them. The nerve ending holds vesicles full of the neurotransmitter acetylcholine. When the impulse arrives, calcium ions enter the nerve ending, the vesicles empty their acetylcholine, which crosses the gap. It changes the permeability of the fibre’s membrane: sodium ions rush in and the charge reverses, which is depolarization, and the muscle contracts. A moment later the enzyme cholinesterase breaks acetylcholine into choline and acetic acid, the membrane returns to its resting state, and the fibre is ready to respond again. How does the muscle get shorter? According to the sliding-filament theory of Huxley, who compared contracted and relaxed fibres under the electron microscope, cross bridges reach from the myosin filaments to the actin filaments, with the help of calcium ions and ATP, and pull the actin filaments inwards from both sides. The Z-lines come closer and the sarcomere shortens. The filaments themselves do not get shorter: they slide past each other. When the stimulus stops, the cross bridges let go and the sarcomere returns to its length.
| Relaxed | Contracted | |
|---|---|---|
| Z-lines | far apart | close together |
| Sarcomere | long | short |
| Actin and myosin lengths | unchanged | unchanged |
| Cross bridges | released | attached and pulling |
A sarcomere model, with a grown-up. Tape two strips of cardboard, about 15 cm apart, to a table edge to stand for two Z-lines. Tape four pieces of thread to each strip, pointing inwards, as thin actin filaments, and set three drinking straws between them, side by side, as thick myosin filaments. Slide the strips together so the threads move into the spaces between the straws and nothing gets shorter. Measure the gap between the strips before and after, and write what changed and what did not.