Locomotion and Movement: Muscles and the Skeleton

Four types of movement, only one of which needs muscle

Not every biological movement involves muscle at all. Amoeboid movement, seen in amoeba and certain white blood cells, relies on the continuous formation and retraction of temporary cytoplasmic extensions called pseudopodia. Ciliary movement, driven by numerous short, hair-like cilia beating in coordinated waves, moves either the surrounding fluid past a stationary cell (as in the respiratory tract's mucus-clearing cilia) or moves the whole cell through fluid. Flagellar movement, driven by one or a few longer flagella, propels individual cells, the human sperm cell's own tail being the standard example. Muscular movement, the only one of the four actually built on the contractile machinery covered below, powers the deliberate, large-scale movement of whole limbs and body parts.

The sliding filament theory: filaments don't shrink, they overlap more

A skeletal muscle fibre's contractile unit is the sarcomere, built from an ordered, repeating arrangement of thin filaments (made of the protein actin) and thick filaments (made of the protein myosin). The sliding filament theory describes contraction with a single, precise mechanical claim: neither the actin nor the myosin filaments themselves change length during contraction at all; instead, the thin actin filaments slide further inward, deeper between the thick myosin filaments, increasing their zone of overlap. This is exactly why, under a microscope, the A band (the zone spanning the full length of the myosin filaments) stays a constant length throughout contraction, while the I band (containing only actin, not overlapped by myosin) and the H zone (the central part of the A band not yet overlapped by actin) both visibly shrink as the actin slides further in, the specific, testable microscopic signature of this sliding mechanism rather than any filament actually shortening.

The molecular trigger for this sliding is calcium-controlled. At rest, the protein tropomyosin physically blocks the specific sites on the actin filament where myosin would otherwise bind. A nerve impulse arriving at the muscle triggers the release of calcium ions, which bind to troponin, a regulatory protein attached to tropomyosin, causing troponin to shift tropomyosin out of the way and expose actin's myosin-binding sites. Myosin heads then attach to these newly exposed sites and pull the actin filament inward in a repeated cycle, a cycle that itself consumes ATP: ATP binding to a myosin head causes it to release from actin, and the hydrolysis of that ATP re-energises the head to reattach further along and repeat the pulling stroke, so a continuously contracted muscle is, at the molecular level, continuously consuming ATP throughout.

The human skeleton: 206 bones in two named divisions

The adult human skeleton comprises 206 bones, split between two named divisions. The axial skeleton, totalling 80 bones, comprises the skull, the vertebral column and the rib cage, the central, longitudinal core of the body. The appendicular skeleton, totalling 126 bones, comprises the limb bones together with the shoulder (pectoral) and hip (pelvic) girdles that attach those limbs to the axial skeleton, the bones actually responsible for most of the body's own mobile, load-bearing movement.

Quick revision points

• Four movement types: amoeboid (pseudopodia), ciliary (coordinated beating of many short cilia), flagellar (one or few longer flagella, e.g. sperm), muscular (the only type built on actin-myosin contraction).
• Sliding filament theory: actin and myosin filaments do not shorten; actin slides further into the myosin, increasing overlap. A band stays constant length; I band and H zone shrink during contraction.
• Contraction trigger: nerve impulse releases calcium, which binds troponin, shifting tropomyosin to expose actin's myosin-binding sites; myosin heads then pull actin inward, a cycle powered by ATP (ATP binding detaches myosin from actin, its hydrolysis re-energises the head).
• Human skeleton: 206 bones total. Axial skeleton (80 bones): skull, vertebral column, rib cage. Appendicular skeleton (126 bones): limb bones plus the pectoral and pelvic girdles.