Muscular tissue is mesodermal in origin. Muscles show three essential properties such as Blood vessel excitability, conductivity and contractility. The study of muscular tissues is known as myology. Muscular tissue has elongated cells celled .muscle fibers. (myocytes) which are surrounded by a connective tissue sheath. Extracellular matrix is absent. The plasma membrane of a muscle fibre is called sarcolemma The cytoplasm of a muscle fibre is called sarcoplasm, the endoplasmic reticulum, the sarco-plasmic reticulum, and the mitochondria, the Sarcosomes. The cytoplasm of a muscle fibre has several myofibrils. Each .myofibril has thick (myosin) and thin (actin) myofilaments. The regular arrangement of myosin and actin filaments is responsible for the alternate dark and light bands of a .striated muscle.. Sarcoplasm also contains ATP, phosphocreatine, glycogen and myoglobin. Muscles are of three types - skeletal, smooth and cardiac.
Skeletal (striped and voluntary) muscle: It is usually attached to skeletal structures by .tendons.. In a typical muscle such as the .biceps. muscle/skeletal muscle fibre is surrounded by a thin connective tissue sheath, the endomysium. A bundle of muscle fibres is called a fascicle. It is surrounded by a connective tissue sheath called. perimysium. A group of fascicles form a .muscle. which is surrounded by an epimysium (outer most connective tissue sheath). These connective tissue layers may extend beyond the muscle to form a chord-like tendon or sheet-like aponeurosis.
A skeletal muscle fibre: is a long, cylindrical and unbranched cell. It is a multinucleated cell with many oval nuclei characteristically in the "peripheral cytoplasm (a syncytium formed by fusion of cells). Sarcoplasm has many myofibrils which show alternate dark and light bands. So it is called striped or striated muscle.
Smooth (unstriped and involuntary muscle): It is located in the walls of the visceral organs such as blood vessels, trachea, bronchi, stomach, intestine, excretory and genital ducts, and so this is also called .visceral muscle.. As cross striations are absent, it is called .smooth muscle.. It is also found in iris and ciliary body of eye and in the skin as .arrector pili muscles. that are attached to hair follicles.
Usually smooth muscles are arranged in .layers./sheets.. A smooth muscle fibre is a spindle shaped (fusiform), uninucleate cell. Myofibrils donot show alternate dark and light bands due to irregular arrangement of actin and myosin fibres. They do not work under the conscious control, and so they are called involuntary muscles. Smooth muscles exhibit slow. and .prolonged. oontractions. They may remain contracted for long periods without fatigue (show sustained involuntary contractions called .spasms.). The contraction of smooth muscles is under the control of the autonomous nervous system.
Cardiac (striped and involuntary) muscle : The cardiac muscle is striated like the skeletal muscle (shows sarcomeres). Cardiac muscle is found in the myocardium of the heart of vertebrates. The cardiac muscle cells or the .myocardial cells are short, cylindrical, mononucleate or binucleate cells whose ends branch and form junctions with other cardiac muscle cells. Each myocardial cell is joined to adjacent myocardial cells by .electrical synapses. or gap junctions". They permit .electrical impulses. tobe conducted along the long axis of the cardiac muscle fibre. The dark lines across cardiac muscle are called intercalated discs (IDs). These discs are highly characteristic of the cardiac muscle.
