Summary

This chapter examined how the human body produces movement and how it is supported by the skeleton.

Four types of movement are shown in the human body: amoeboid movement (crawling movement of wandering cells such as leucocytes, using pseudopodia), ciliary movement (the coordinated beating of cilia lining tubes such as the trachea and the fallopian tube), flagellar movement (the whip-like beating of a flagellum, as in the swimming sperm cell), and muscular movement (produced by skeletal muscle acting on the skeleton, responsible for locomotion and most voluntary body movements).

A skeletal muscle is built as a hierarchy of fascicles, muscle fibres, myofibrils and, finally, sarcomeres -- the repeating unit bounded by two Z lines that gives striated muscle its banded appearance (the A band, I band, H zone and M line). Each sarcomere contains thin filaments (built of the protein actin together with the regulatory proteins tropomyosin and troponin) and thick filaments (built of the motor protein myosin, whose head has ATPase activity). The sliding filament theory explains contraction as the active sliding of thin filaments over stationary thick filaments, driven by the cross-bridge cycle: a nerve impulse reaching the neuromuscular junction releases acetylcholine, triggering an action potential that causes the sarcoplasmic reticulum to release calcium; calcium binds troponin C, shifting tropomyosin and exposing the myosin-binding sites on actin, so that myosin heads attach, execute a power stroke, detach on binding fresh ATP, and re-cock using the energy of ATP hydrolysis, repeating for as long as the stimulus and calcium persist. When ATP is no longer available after death, the myosin heads remain locked to actin, producing rigor mortis. …