Muscle

Mammals have three types of muscle:

• Skeletal muscle - voluntary, multinucleate, with striations, attached by tendons to bones and causes locomotion. Contractions strong but not long-lasting

• Cardiac muscle - striated cells and branched, intercalated discs between cells, central single nuclei, only found in heart muscle. Contracts spontaneously

• Smooth muscle - spindle-shaped cells with a single nucleus, no striations, contratct slowly, involuntary. Involved with the movement of materials within the body i.e. found in walls of hollow organs.

• Skeletal muscle consists of muscle fibres, which are multinucleate and possess many mitochondria, extensive saroplasmic reticulum (storage of calcium ions) and infoldings of the cell-surface membrane
• Muscle fibres comprise myofibrils consisting of actin (thin) and myosin (thick) filaments. Each myofibril has recognisable repeating units called sarcomeres, each of which contains overlapping filaments of actin and myosin
• Contraction of the sarcomere (and muscle) involes the following:

- the release of calcium ions unblocks the binding sites on the actin filaments

- myosin heads attach to the actin forming acto-myosin bridges

- the bridges rotate, pulling the actin filaments over the myosin filaments

- ATP allows myosin to detach and the cycle of attachment-rotation-detachment is repeated