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NAME: OGBOGU NWAMAKA. O MATRIC NO:14/MHS01/098 DEPARTMENT: MEDICINE AND SURGERY COURSE:HISTOLOGY MUSCLE AS A TISSUE Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. This is opposed to other components or tissues in muscle such as tendons or perimysium. It is formed during embryonic development through a process known as myogenesis. Muscle being considered as a tissue can be divided into three main groups according to their structure, e.g.: Smooth muscle tissue. Skeletal muscle tissue. Cardiac (heart) muscle tissue. Smooth Muscle Tissue. Smooth muscle tissue is made up of thin-elongated muscle cells, fibres. These fibres are pointed at their ends and each has a single, large, oval nucleus. Each cell is filled with a specialised cytoplasm, the sarcoplasm and is surrounded by a thin cell membrane, the sarcolemma. Each cell has many myofibrils which lie parallel to one another in the direction of the long axis of the cell. They are not arranged in a definite striped (striated) pattern, as in skeletal muscles - hence the name smooth muscle . Smooth muscle fibres interlace to form sheets or layers of muscle tissue rather than bundles. Smooth muscle is involuntary tissue, i.e. it is not controlled by the brain. Smooth muscle forms the muscle layers in the walls of hollow organs such as the digestive tract (lower part of the oesophagus, stomach and intestines), the walls of the bladder, the uterus, various ducts of glands and the walls of blood vessels . Functions of Smooth Muscle Tissue o o Smooth muscle controls slow, involuntary movements such as the contraction of the smooth muscle tissue in the walls of the stomach and intestines. The muscle of the arteries contracts and relaxes to regulate the blood pressure and the flow of blood. Smooth Muscle Tissue Skeletal Muscle Tissue. Skeletal muscle is the most abundant tissue in the vertebrate body. These muscles are attached to and bring about the movement of the various bones of the skeleton, hence the name skeletal muscles. The whole muscle, such as the biceps, is enclosed in a sheath of connective tissue, the epimysium. This sheath folds inwards into the substance of the muscle to surround a large number of smaller bundles, the fasciculi. These fasciculi consist of still smaller bundles of elongated, cylindrical muscle cells, the fibres. Each fibre is a syncytium, i.e. a cell that have many nuclei. The nuclei are oval in shaped and are found at the periphery of the cell, just beneath the thin, elastic membrane (sarcolemma). The sarcoplasm also has many alternating light and dark bands, giving the fibre a striped or striated appearance (hence the name striated muscle). With the aid of an electron microscope it can be seen that each muscle fibre is made up of many smaller units, the myofibrils. Each myofibril consists of small protein filaments, known as actin and myosin filaments. The myosin filaments are slightly thicker and make up the dark band (or A-band). The actin filaments make up the light bands (I-bands) which are situated on either side of the dark band. The actin filaments are attached to the Z-line. This arrangement of actin and myosin filaments is known as a sacromere. During the contraction of skeletal muscle tissue, the actin filaments slide inwards between the myosin filaments. Mitochondria provide the energy for this to take place. This action causes a shortening of the sacromeres (Z-lines move closer together), which in turn causes the whole muscle fibre to contract. This can bring about a shortening of the entire muscle such as the biceps, depending on the number of muscles fibres that were stimulated. The contraction of skeletal muscle tissue is very quick and forceful. Functions of Skeletal Muscle Tissue o o Skeletal muscles function in pairs to bring about the co-ordinated movements of the limbs, trunk, jaws, eyeballs, etc. Skeletal muscles are directly involved in the breathing process. Skeletal Muscle Tissue Cardiac (Heart) Muscle Tissue. Cardiac muscle is striated muscle found only in the heart. While there are many physiological differences between cardiac and skeletal muscle, a major histological difference between these two types of striated muscle is that each cardiac muscle cell is NOT a true syncytium of several cells that runs the length of the organ, as is true of skeletal muscle cells. Rather, each cardiac muscle cell is relatively small, with respect to the organ, and has one nucleus (sometimes two). Also, each cardiac muscle cell is branched, unlike skeletal muscle fibers, and interacts, through special types of intercellular junctional complexes, with neighboring cells, forming a tissue that is appropriately described as a functional syncytium. The specialized intercellular junctional complexes that provide this tissue with its functional syncytial character are intercalated disks. Intercalated disks contain three basic types of intercellular junctions: numerous fascia adherens and desmosomes, which provide for adhesion between two cells, and numerous gap junctions, which provide for the exchange of ions between the two cells, and, therefore, a means of direct electrical communication between two cells. The functional organization of cardiac muscle tissue goes hand-hand with the organization of its innervation and nature of its control. Unlike skeletal muscle cells, individual cardiac muscle cells exhibit an inherent rythmicity. That is, an auto-generated action potential that stimulates the cell's contraction. At the tissue level, cardiac muscle inherently exhibits the rhythmicity of the cardiac muscle cell with the fastest rhythmicity. Once the action potential is generated by this cell, the impulse quickly spreads among the cells of the tissue via the gap junctions of the intercalated disks. Thus, motor nurons are not needed to generate the heart beat, and cardiac muscle is NOT organized into motor units, as is skeletal muscle. However, autonomic nerves innervate portions of the cardiac muscle of the heart to modify the rate of this inherent rhythmicity. Like skeletal muscle fibers, each cardiac muscle fiber is surrounded by an endomycium that is conspicuous in typical cardiac muscle preparations. "Bundles" of cardiac muscle fibers are grouped into fascicles by a perimycium, but no epimycium is used to cover the entire organ. Functions of Cardiac (Heart) Muscle Tissue o o Cardiac muscle tissue plays the most important role in the contraction of the atria and ventricles of the heart. It causes the rhythmical beating of the heart, circulating the blood and its contents throughout the body as a consequence. Cardiac Muscle Tissue