Muscle Animal Histology BIOL 241 Topic 5: Dr. Issa Al-Amri Department of Biological Sciences & Chemistry College of Arts & Sciences

Introduction Muscle or muscular tissue is one of four basic tissues. Highly specialized for contractility. Muscle classified into two types: striated and smooth. Striated muscle has two subdivisions: skeletal and cardiac muscles. In general , there are three types of muscle: Smooth, skeletal, and cardiac muscles. Muscle cells contain contractile filaments whose major components are actin and myosin.

Introduction Type of muscle

Type of muscle

Introduction Contraction occurs when the two filaments, actin and myosin interact with each other and interdigitate over a large portion of their lengths, and thus pulling the two ends of the fibers closer together. Contraction may me voluntary (skeletal muscle) or involuntary (cardiac and smooth muscle) Muscular tissue has its own characteristic terms such as: - Plasmalemma = Sarcolemma - Cytoplasm = Sarcoplasm - Endoplasmic reticulum = Sarcoplasmic reticulum - Mitochondria = Sarcosomes

Structure of skeletal muscle Skeletal muscles composed of long, cylindrical, multinucleated muscle fibers, formed by fusion of multiple myoblasts during early embryonic life. Connective tissue investments: Skeletal muscle surrounded by thin layer of dense connective tissue called epimysium. Epimysium forms tendons, which connect skeletal muscle to bone. From epimysium, connective tissue extend inward to surround each of muscle fasicles (small bundles) forming perimysium, from which, delicate network of reticular fibers envelop individual muscle fibers forming endomysium.

Structure of skeletal muscle B. Types of skeletal muscle fibers (cells): Three types of skeletal muscle: - Red (slow contraction) - White (fast contraction) - Intermediate These types differ from each other in their content of: - Myoglobin - Number of mitochondria - Concentration of enzymes - Rate of contraction Muscle fibres can undergo changes from one type to another; e.g. from red to white.

Structure of skeletal muscle Skeletal muscle fibers : Long, cylindrical, multinucleated, enveloped by external lamina and reticular fibers. Cytoplasm called sarcoplasm. Plasmalemma called the sarcolemma and forms deep tubular invaginations, or T tubules. Skeletal muscle fibers contain cylindrical collections of myofibrils that extend the entire length of the fiber.

Structure of skeletal muscle Myofibrils : composed of longitudinally arranged, cylindrical bundles of thick and thin myofilaments observed by TEM. Alignment of myofibrils results in banding pattern as alternating dark A bands and light I bands; the latter are bisected by Z discs. Sarcomere: Regular repeating region between Z disks and is the functional unit of contraction in skeletal muscle.

Structure of skeletal muscle Sarcoplasmic reticulum (SR): Modified SER surrounds myofilaments and forms meshwork around each myofibril. Regulates muscle contraction by sequestering calcium ions (leading to relaxation) or releasing calcium ions (leading to contraction). Triads: Complexes made up of narrow central T tubule flanked on each side by terminal cisternae of the SR. Located at the A-I junction. Provide uniform contraction throughout the muscle fiber.

Structure of skeletal muscle 5. Myofilaments : include thick filaments (15 nm in diameter and 1.5 µm long) and thin filaments (7 nm in diameter and 1.0 µm long). Satellite cells: Regenerative cells differentiate and fuse with one another, forming skeletal muscle cells when needed.

Structure of skeletal muscle Skeletal muscle cross-section: A bands: stain dark, contain both thin and thick filaments. I bands: lightly stained in H&E, contain only thin filaments. H bands: light regions transecting A bands; consist of thick filament only. M lines: narrow, dark regions at centers of H bands formed by several cross-connections (M-bridges). Z disks (lines): dense regions bisecting each I band. contain α-actinin and Cap Z, that bind thin filaments and anchor them to Z disks with the assistance of nebulin. Desmin: aided by plectin, anchors Z disks to each other. Peripherally located Z disks are anchored to regions of the sarcolemma, known as costameres, by vinculin and dystropin

Skeletal muscle (LM): Figure 1 : Skeletal muscle (L.S.) - nuclei (N) - connective tissue cells (CT) - myofibrils (M) - light band (I) - Z disc (Z) - dark band (A) - H zone (H) - sarcomere (S) Figure 2: Skeletal muscle (C.S.) - muscle fibres (F) - perimysium (P) - isogenous group (IG) - Perichondrium (P) - matrix (arrow) Figure 3: Skeletal muscle (C.S.) - higher mag of fig. 2 - endomysia (E) - capillaries (C) - satellite cells (SC)

Skeletal muscle (TEM): Figure 1 : Skeletal muscle (L.S.) - M disc (MD) - light band (I) - Z disc (Z) - dark band (A) - H zone (H) - sarcomere (S) Figure 2: Skeletal muscle (L.S.) - higher mag TEM - thin myofilaments (tM) - thik myofilaments (TM) - mitochondria (M) - glycogen granules (arrowhead) - triads (T)

TEM: L. S .of skeletal muscle

TEM: C. S. of skeletal muscle

Cardiac muscle Cardiac muscle is the muscles of the heart. Muscle fibers are striated and involuntary. A. Features of cardiac muscle cells: Contract spontaneously and display a rhythmic beat, which is modified by hormonal and neural (sympathetic and parasympathetic) stimuli. Branch at their ends to form connections with adjacent cells. Contain one centrally located nucleus, or occasionally two nuclei. Contain glycogen granules, and sarcoplasm rich in myoglobin. Possess thick and thin filaments arranged in poorly defined myofibrils. Do not regenerate; injuries to cardiac muscle are repaired by the formation of fibrous connective (scar) tissue by fibroblasts.

Cardiac muscle Structural components of cardiac muscle cells: T tubules: larger than in skeletal muscle, lined by external lamina. SR: poorly defined and contributes to formation of dyads, (each consists of one T tubule and one SR). Calcium ions. Mitochondria: more abundant than in skeletal muscle. Atrial granules: present in atrial cardiac muscle cells. Intercalated disks: complex steplike junctions forming end-to-end attachments between adjacent cardiac muscle cells. Connective tissue elements. Purkinje fibers: modified cardiac muscle cells located in the bundle of His.

Cardiac muscle (LM): Figure 1 : Cardiac muscle (L.S.) - branching of fibres (arrow) - dark & light bands (arrowheads) - nucleus (N) - intercalated discs (ID) Figure 2: Cardiac muscle (L.S.) - higher mag of fig. 1 - branching of fibres (arrows) - I and A bands (arrowheads) - myofibrils (M) - capillaries (C) Figure 3: Cardiac muscle (C.S.) - cardiac muscle fibres (CM) - vascular supply (BV) - perinuclear regions (arrows) Figure 4: Cardiac muscle (C.S.) - blood vessels (BV) - endothelial nuclei (EN) - white blood cells (WBS) - perinuclear clear area (arrows)

Smooth muscle Smooth muscles are involuntary, nonstriated. Present in wall of viscera: intestine, ureters, urinary bladder, uterus, and blood vessels. Controlled by autonomic nervous system. Structure: Smooth muscles appear long, nonstraiated, fusiform cells with single nuclei situated in central part of the cell. Nucleus: appear wrinkled in the contracted muscles. Cytoplasmic organelles: Mitochondria, ER, and the Golgi complex are concentrated near the nucleus. Sacrolemmal vesicles (caveolae), present along the periphery of smooth muscle cells, SER is sparse and associated with caveolae.

Smooth muscle Filaments in smooth muscle: Contractile filaments (actin and myosin) are not organized into myofibrils. Thick filaments (composed of myosin II). Thin filaments composed of actin, caldesmon, tropomyosin, and calponin. Intermediate filaments are attached to cytoplasmic densities Cytoplasmic densities: analogous to Z disks, function as filament attachment sites. 5. Gap junctions: between cells facilitate spread of excitation.

Figure 1 : Smooth muscle (L.S.) - nuclei (N) - connective tissue (CT) Smooth muscle (LM): Figure 1 : Smooth muscle (L.S.) - nuclei (N) - connective tissue (CT) Figure 2: Smooth muscle (L.S.) - higher mag of fig. 1 - smooth muscle (sM) - capillaries (C) Figure 3: Smooth muscle (C.S.) - uterene myometrium - connective tissue (CT) - longitudinal cut (1) - transverse or cross cut (2) - oblique cut (3) - blood vessels (BV) Figure 4a: Smooth muscle (C.S.) - round nucleus surrounded by cytoplasm (asterisk) - large muscle fibres (double asterisk) - large muscle cell (triple asterisk) - sarcoplasm (arrowhead) Figure 4b: Smooth muscle (C.S.) - glandular portion (G) - LS (1), CS (2)

Smooth muscle (TEM): Figure 1 : Smooth muscle (L.S.) - external lamina (EL) - sarcolemma (SL) - caveolae (Ca) - dense bodies (DB) - intermediate microfilaments (IM) - sarcoplasm (arrows) - nucleus (N) - mitochondria (M) - capillary (C) - adherence junctions (AJ)

Smooth muscle Function: Regulation of lumen sizes in hollow organs. Rhythmic contractions of GIT. Synthesis of collagen, elastin and proteoglycans. Contraction of smooth muscle occurs by sliding filament mechanism. Contraction of smooth muscle occurs more slowly, lasts longer than contraction of skeletal muscle because rate of ATP hydrolysis is slower. Contraction of smooth muscle regulated by different mechanism from that of skeletal muscle contraction.