A Schematic of Skeletal Muscle

The diagram below shows the organization of a typical muscle (i.e. the biceps) from a gross anatomical view, down to the microscopic anatomy of the individual units of muscle contraction. Note the organization of the muscle: contractile protein filaments are bundled into myofibrils; bundles of myofibrils are contained in each individual muscle fibre (cell); muscle fibres are bundled to form fascicles and the fascicles are bundled to form the muscle organ. The light and dark striations seen with the light microscope are caused by the very orderly arrangement of actin and myosin filaments within each myofibril and the orderly arrangement of the myofibrils within the muscle cell (fibre). Each line or set of lines is given a letter name, which is the accepted way to refer to the specifics of these striations.

Also shown are the microscopic anatomical changes that occur to allow contraction. When there is a stimulus for contraction (you think to move your arm), the actin filaments are pulled along the myosin filaments (a process requiring ATP - actin and myosin filaments are labelled in the second diagram), and the Z discs are pulled closer together, causing the sarcomere to shorten. Although the difference in length produced by one sarcomere contracting is negligible, when a few thousand along the length of the muscle do so, there is considerable shortening of the muscle, allowing the desired effect (your arm moves).

This diagram compares the striations seen on the muscle fibre to the arrangement of the actin and myosin filaments which account for these striations. The proper name for each band of striations is shown for both the myofibril and the sarcomere. Note that the I band crosses two sarcomeres, so it is only labelled on the myofibril section (½ of the I band is in each of two sarcomeres).

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Latest page revision: April 4, 2000.