Based on functions muscles are classified as voluntary (e.g. skeletal) and involuntary (e.g. cardiac & visceral) muscles.
Based on microscopic appearance They are classified as ‘striated’ i.e. bearing cross striations (e.g. skeletal and cardiac) are non-striated or smooth muscles (e.g. visceral).
Based on their location in the body muscles are classified as skeletal or external muscles (e.g. muscles of limbs, abdomen, chest, back etc) and these are attached with bones. Other group is internal or visceral muscles (e.g. heart, visceral organs) which are located in the wall of these organs.
STRUCTURE: Skeletal muscle as a whole is divided into several fasciculi, each composed of a group of muscle cells. Each skeletal muscle cell is 1 to 40 mm long and 10 to100 microns in width, thus appears as long thread like and so they are called muscle fibre. Cell membrane of muscle fibre is called sarcolemma and cytoplasm is known as sarcoplasm. Sarcoplasm has numerous mitochondria (sarcosomes) and endoplasmic reticulum (sarcoplastic reticulum) and several nuclei. About 80% of sarcoplasm space is occupied by parallely arranged myofilaments and each myofilament is made up of hundreds of sarcomeres joined end to end with other. Each sarcomere is about 2.5 µ long and 1.0 µ wide microscopic structure with A-band (an-isotropic) and I-band (isotropic) alternating each other. Specific arrangement of A and I bands is responsible for giving a ‘striated’ appearance to whole muscle fibre under light microscope. This striated appearance is due to typical arrangement of two contractile proteins, actin (lighter protein occupying I band) and myosin (heavier protein occupying A band). During muscle contraction, actin and myosin protein slides over each other resulting in shortening of all sarcomeres simultaneously and thus whole of muscle fibre & muscle.