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NAME-ADIGHIJE CLAUDIA CHISOM COURSE-HISTOLOGY OF BASIC TISSUES COURSE CODE-ANA 203 DEPARTMENT- PHYSIOLOGY DIRECT ENTRY TOPIC-HISTOLOGY OF THE MUSCLE AS A TISSUE THE MUSCLE The muscle is a contractile form of tissue which organisms use to effect movement. It is composed of cells that are differentiated for the optimal use of the universal cell property called contractility. Microfilaments and associated proteins together generate the forces necessary for cellular contraction, which drives movement within certain organs and body as a whole. Almost all muscle cells are of mesodermal origin and they differentiate mainly by a gradual process of cell lengthening with simultaneous synthesis of myofibrillar proteins. MUSCLE FUNCTIONS 1) Contraction for locomotion and skeletal movement 2) Contraction for propulsion 3) Contraction for pressure regulation Muscles can however be classified based on structure and based on functions. There are two types of muscles based on structure and they are 1) Striated muscle- Here, the muscle tissue in which the contractile fibrils in the cells are aligned in parallel bundles, so that their different regions form stripes visible in a microscope. Muscles of this type are attached to the skeletons by tendons and are under voluntary controls. They are usually the skeletal muscles because it is attached mainly to the bones and skin and it is responsible for the mobility of the body and limbs. Striated muscles are dense and fibrous tissues whose primarily function is to allow the body to move by repeated contraction and relaxation. 2) UNSTRIATED MUSCLE This is a smooth thin and muscle that is not controlled voluntarily. It is a muscle tissue that contracts without conscious control, having the form of thin layers or sheets made up of spindle shaped, unstriated cells with single nuclei and found in the walls of the internal organs, such as the stomach,intestine,bladder,and blood vessels, excluding the heart. The two classifications of muscles based on functions are 1) Voluntary muscles- This is a muscle whose action is normally controlled by an individual’s will; mainly skeletal muscle composed of parallel bundles of striated muscles except the heart, whose action is normally controlled by individual volition. 2) Involuntary muscles- This is a muscle that contract without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels excluding the heart. TYPES OF MUSCLES There are mainly three mainly three types of muscles; 1) SMOOTH MUSCLES Smooth muscle is the intrinsic muscle of the internal organs and blood vessels. It is also found in the iris and ciliary body of the eye and associated with hair follicles. No striations are present in smooth muscle due to the different arrangement of actin and myosin filaments. Smooth muscles are intrinsically contractile but responsive to autonomic and hormonal stimuli. They are specialized for slow and prolonged contraction. PROPERTIES OF SMOOTH MUSCLES Smooth muscle fibres are generally arranged in bundles and sheets. Each fibre is fusiform in shape with a thicker central portion and tapered at both ends. The single nucleus is located in the central part of the fibre. Its fibres do not branch. They range enormously in size from 20 (in wall of small blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth muscle fibres lie over one another in a staggered fashion. SKELETAL MUSCLE Skeletal muscle constitutes the muscle that is attached to the skeleton and controls motor movement and posture. There are a few instances where this type of muscle is restricted i.e. soft tissues like the tongue, pharynx, diaphragm and the upper part of the oesophagus. Skeletal muscle fibre cells are actually a multinucleated syncytium formed by the fusion of individual small muscle cells or myoblasts, during development. They are filled with longitudinally arrayed subunits called myofibrils. The myofibrils are made up of the myofilaments myosin and actin. Skeletal muscle fibres however bear obvious striations, have many peripherally located nuclei, are of the same thickness throughout their length and do not branch. CARDIAC MUSCLE The cardiac muscle is a type of muscle found in the heart, and at the base of the venae cava as they enter into the heart. Cardiac muscle is intrinsically contractile but is regulated by autonomic and hormonal stimuli. They exhibit striations because it also has myosin and actin filaments arranged into sarcomeres.Generally, these striations do not appear as well defined as in skeletal muscles. Cardiac muscle cells however have only one or two nuclei which are centrally located. These muscle cells are joined together in a linear array, therefore they branch and anastomose with other fibres and they are also joined together by intercalated discs .