HISTOLOGY OF MUSCLE TISSUE Dr. Sangeeta Kotrannavar Assistant Professor Dept. of Anatomy, USM-KLE IMP, Belagavi Objectives Distinguish the microscopic features of • Skeletal • Cardiac • Smooth muscles Muscle • Latin musculus =little mouse (mus) • Muscle cells are known as MYOCYTES. • Myocytes are elongated so referred as muscle fibers Fleshy • Definition • Muscle is a contractile tissue which brings about movements Tendons Muscle makes up 30-35% (in women) & 40-45% (in men) of body mass Type of muscles BASED ON BASED ON BASED ON LOCATION STRIATIONS CONTROL Skeletal / Somatic STRIATED / STRIPED VOLUNTARY Smooth / Visceral UN-STRIATED / IN-VOLUNTARY UNSTRIPED Cardiac STRIATED / STRIPED IN-VOLUNTARY SKELETAL MUSCLE Skeletal muscle organization Muscles are complex structures: arranged in fascicles Muscle bundles / fascicles • Epimysium surrounds entire muscle – Dense CT that merges with tendon – Epi = outer, Mys = muscle • Perimysium surrounds muscle fascicles – Peri = around – Within a muscle fascicle are many muscle fibers • Endomysium surrounds muscle fiber – Endo = within SKELETAL MUSCLE • Each bundles contains many muscle fiber Structure of a skeletal muscle fiber • Elongated, unbranched cylindrical fibers • Length- 1 mm – 5 cm, Width – 10 mm - 100μm • Fibers have striations of dark & light bands • Many flat nuclei beneath sarcolemma • Plasma membrane = sarcolemma • Smooth endoplsmic reticulum = sarcoplasmic reticulum (SR) • Cytoplasm = sarcoplasm • Mitochondria = sarcosomes • Each muscle fiber made of long cylindrical myofibrils Structure of a myofibril • Contractile, unbranched parallel threads situated along long axis of entire length of muscle fiber • Cross striations on myofibril (under LM) are due to uniform arrangement of myofilaments within it. Arrangement of myofilaments • Consist of 2 types of myofilaments • Thick filament - protein myosin & occupy A band • Thin filament - protein actin & also tropomysin & troponin Sarcomere • I band = Isotropic - light band composed of mainly actin / thin filament and is bisected by z line • A band = Anisotropic - Dark band corresponds to the length of the myosin / thick filament • H zone – lighter zone in the center of A band • M line – connection between thick filaments in middle of H zone • Sarcomere is structural and functional contractile unit of muscle • Is defined as distance between two z lines Transverse(T)Tubules & Sarcoplasmic reticulum (SR) Transverse(T)Tubules & Sarcoplasmic reticulum (SR) • SR surrounds each myofibrils at junction of A & I band • T tubules derived from sarcolemma • Expanded terminal cisternae of SR & T tubules form triads & located at A–I junctions • Stimulus for muscle contraction carried by T tubules to every myofiber, myofibril, and SR • After stimulation, SR releases calcium ions into sarcomeres • Calcium activates the binding of actin and myosin, causing muscle contraction and shortening • After the end of stimulus, calcium is actively transported and stored in sarcoplasmic reticulum Muscle Contraction • During contraction no shortening of thick & thin filaments, but increase in degree of overlapping. • A band remains constant in width whereas I & H bands become narrow & Z lines comes closer Skeletal muscle histology Skeletal muscle Longitudinal section Transverse section Skeletal muscle histology Imp. features • Long & parallel cylindrical fibers • No branching • Cross striations are present (A & I bands) • Multinucleated flat peripheral nuclei High power, Longitudinal section Innervation of skeletal muscle • Motor fibers (60%) • Myelinated Alpha efferent - supply extrafusal muscle fibers –produces contraction • Myelinated Gamma efferent - supply intratrafusal muscle fibers –maintains muscle tone • Unmyelinated sym. Fibers – vasomotor to vessels • Sensory fibers (40%) • Muscle spindles. • Motor unit– One grey column neuron & muscle fiber supplied by it . • Motor end plate– Junction between muscle fiber & nerve terminal that supplies it . Cardiac Muscle • Has characteristics of both skeletal and smooth muscle • Functions to provide the contractile activity of the heart • Is very fatigue resistant • Activation of cardiac muscle is involuntary (like smooth muscle) Cardiac muscle • Distinct cross-striations as a result of regular arrangements of actin and myosin filaments (less prominent than skeletal muscle) • Sarcoplasmic reticulum is less well developed • T tubules are located at the Z lines and are much larger than those in skeletal muscles. • Mitochondria are larger & more abundant in the cardiac cells, which is needed for continuous function. Cardiac Muscle • Short cylindrical with branching fibers • Cross striations present • Cells joined by junctional complexes called intercalated disks • Contain one central nucleus (rarely two) Intercalated disks • Cardiac cells are joined end to end by specialized, interdigitating junctional complexes called intercalated discs (which consist of fascia adherens, desmosomes, and gap junctions). • Intercalated disks permits rapid spread of contractile stimuli from one cell to another. So act as a functional syncytium APPLIED ASPECT •Myocardial infarct--- Sudden reduction in blood supply to myocardium leads to muscle cell death. Smooth Muscle • Actin and myosin fi laments are present, but they do not show regular arrangement or striations, instead they form lattice network, and they insert into dense bodies in sarcoplasm. • Dense bodies act as Z disk, contain alpha-actinin • Numerous vesicular invaginations of cell membrane called as caveolae which act similar to T tubules • Zonula adherens binds muscle cells, whereas gap junctions provide functional coupling Smooth Muscle • Sarcoplasmic reticulum is not well developed for calcium storage • Following stimulation, calcium enters sarcoplasm from caveolae and sarcoplasmic reticulum, stimulates actin & myosin that brings the contraction • Gap junctions couple muscles and allow ionic communication between all fibers Smooth Muscle Smooth Muscle • No striations • spindle-shaped cells with tapering ends. • An elongated or ovoid single centrally placed nucleus • The cytoplasm (sarcoplasm) of each muscle fiber stains dark. Smooth Muscle; location • In intestines, uterus, ureters & other hollow organs ; smooth muscles are arranged in concentric layers around the organs. Peristaltic contractions propel contents in the organs • In the blood vessels, smooth muscle fibers are arranged in a circular pattern, where they control blood pressure by altering luminal diameters. Other contractile cells • Myoepithelial cells– Relation to gland helps in squeezing Muscular hypertrophy MCQ Regarding muscle A Skeletal muscles show branching pattern B Smooth muscle shows striation C Intercalated discs seen in smooth muscle D Cardiac muscles is under voluntary control E Skeletal muscle has single nucleus SEQ ; compare and write the features of muscles Thank you .