important proteins – Actin and Myosin . The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains As the name suggests, Cardiac muscles are the muscles of heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance, cardiac muscles are striated.
They are involuntary in nature as the nervous system does not control their activities directly. Let us examine a skeletal muscle in detail to understand the structure and mechanism of contraction. Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia . Each muscle bundle contains a number of muscle fibres (Figure .
). Each Fascicle (muscle bundle) Muscle fibre (muscle cell) Sarcolemma Blood capillary Figure . Diagrammatic cross sectional view of a muscle showing muscle bundles and muscle fibres myosin. Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.
Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The thick filaments in the ‘A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line.
The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere (Figure . ). In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments.
This central part of thick filament, not overlapped by thin filaments is called the ‘H’ zone. Figure . Diagrammatic representation of (a) anatomy of a muscle