Muscles

About this course

Moving the skeleton is the obvious job of muscle, but not the only one. Skeletal muscles hold you upright against gravity with small, constant adjustments and keep joints from slipping out of place, and rings of skeletal muscle at the openings of the digestive and urinary tracts let you decide when to swallow or when to empty the bladder. This course explains how the body works and gives no exercise, training, health or treatment advice.

The seven units go from the whole muscle down to single proteins: three kinds of muscle, inside a skeletal muscle, the sliding filament model, from nerve signal to cross-bridge, muscles that work in pairs, muscles and levers, and fuel and fibre types. When a sarcomere shortens, neither actin nor myosin gets shorter; they slide past each other. In the 3D sarcomere, drag the Contraction slider and the sarcomere, the I band and the H zone all get smaller while the A band stays the same. The motor neuron's axon can run up to about 90 cm to reach the muscle. Contraction stops when the neuron stops releasing ACh and pumps move Ca2+ back into the sarcoplasmic reticulum.

The lever unit works out the force in a 3D arm. The forearm and the load together weigh 6.50 kg × 9.80 m/s² = 63.7 N, so the biceps pulls about 7.4 times as hard as the weight it supports, and the humerus presses down on the forearm with the difference, about 407 N. With the forearm level and 4 kg in the hand, the model shows about 468 N. The last unit separates slow oxidative fibres, which are thin, packed with mitochondria and capillaries, and coloured red by myoglobin, from the faster types.