Microanatomy of Muscle

Abstract

Skeletal muscle fibres are thread-like syncytia of fused mononuclear cells; they are connected at both ends to tendons, which in turn are connected to the skeleton, and in some instances, also to the connective tissue of the skin. The tensile strength per unit area of the tendon tissue is about 100 times greater than that of muscle tissue; hence, the cross-sectional area of the muscle is usually larger than that of the tendon. Because the weakest part of a chain determines its strength, muscle fibres must have the same cross-sectional area throughout their length. If fibres are tapered or wedge-shaped, the attachment of the tendon is staggered such that the decreasing number of myofibrils is compensated for by an increasing number of collagen fibrils running parallel to the muscle fibre. Fibres or strands of fibres working in parallel must be of the same functional length, i.e. the number of sarcomeres must be the same. These two demands — equal cross-sectional area along the fibre length and equal number of sarcomeres in fibres working together — determine the array of muscle fibres within a muscle. The basic unit is a parallelogram formed by the fibres and the tendon sheets at which they more or less obliquely insert.