Pawlina Summary Muscle PDF

Functional Considerations: Comparison of the FOLDER 11.4 Three Muscle Types (continued) 341...

Functional Considerations: Comparison of the FOLDER 11.4 Three Muscle Types (continued) 341 CHAPTER 11 Comparison of the Three Muscle Types Skeletal Cardiac Smooth Muscle Tissue Structural features Muscle cell Large, elongate cell, 10–100 m Short, narrow cell, 10–100 m in Short, elongate, fusiform in diameter, up to 100 cm in diameter, 80–100 m in length cell, 0.2–2 m in diameter, length (sartorius m.) 20–200 m in length Location Muscles of skeleton, visceral Heart, superior and inferior vena Vessels, organs, and viscera striated (e.g., tongue, cava, pulmonary veins esophagus, diaphragm) Connective tissue Epimysium, perimysium, Endomysium (subendocardial and Endomysium, sheaths, and components endomysium subpericardial connective tissue) bundles S M O O T H MU S C LE Fiber Single skeletal muscle cell Linear branched arrangement of Single smooth muscle cell several cardiac muscle cells Striation Present Present None Nucleus Many peripheral Single central, surrounded by Single central juxtanuclear region T tubules Present at A–I junction (triad: Present at Z lines (diad: with small None, well-developed sER, with two terminal cisternae), terminal cisternae), one T tubule/ many invaginations and two T tubules/sarcomere sarcomere; Purkinje fibers have vesicles similar to caveolae less number of T tubules Cell-to-cell None Intercalated discs containing Gap junctions (nexus) junctions 1. Fasciae adherentes 2. Macula adherens (desmosome) 3. Gap junctions Special features Well-developed sER and Intercalated discs Dense bodies, caveolae, and T tubules cytoplasmic vesicles Functions Type of Voluntary Involuntary Involuntary innervation Efferent Somatic Autonomic Autonomic innervation Type of “All or none” (type I and type II “All or none” rhythmic (pacemakers, Slow, partial, rhythmic, contraction fibers) conductive system of the heart) spontaneous contractions (pacemakers of stomach) Regulation of By binding of Ca2 to TnC, By binding of Ca2 to TnC, causes By phosphorylation of myosin contraction causes tropomyosin movement tropomyosin movement and light chain by myosin light and exposes myosin-binding exposes myosin-binding sites on chain kinase in the presence sites on actin filaments actin filaments of Ca2–calmodulin complex Growth and regeneration Mitosis None None (in normal condition) Present Response to Hypertrophy Hypertrophy Hypertrophy and hyperplasia demand Regeneration Limited (satellite cells and myo- None (in normal condition) Present genic cells from bone marrow) sER, smooth-surfaced endoplasmic reticulum; TnC, troponin-C. Pawlina_CH11.indd 341 9/29/14 7:01 PM 342 Muscle Tissue HISTOLOGY 101 OVERVIEW OF MUSCLE TISSUE ◗ Muscle tissue is responsible for movement of the body and its parts and for changes in the size and shape of internal organs. Muscle Tissue ◗ There are three major types of muscle tissue: skeletal, cardiac, and smooth muscle. SKELETAL MUSCLE myosin heads and actin molecules that lead to muscle ◗ Skeletal muscle cells called skeletal muscle fibers are very contraction. There are five recognizable stages of the long, cylindrical, multinucleated syncytia with diameters cycle: attachment, release, bending, force generation, from 10 to 100 m. and reattachment. ◗ Skeletal muscle fibers are held together by connective ◗ Regulation of muscle contraction involves Ca2, sarco- CHAPTER 11 tissue. Endomysium surrounds individual fibers; perimy- plasmic reticulum, and the transverse tubular system. sium surrounds a group of fibers to form a fascicle; and ◗ The sarcoplasmic reticulum forms enlarged terminal epimysium is dense connective tissue that surrounds the cisternae that serve as reservoirs for Ca2. Their plasma entire muscle. membrane contains an abundance of gated Ca2-release ◗ Three types of skeletal muscle fibers are distinguished channels (ryanodine receptors [RyR1]). based on contractile speed, enzymatic velocity, and meta- ◗ The transverse tubules (T tubules) are formed by invagi- bolic profile. The three types of fibers are red (type I, slow nations of the sarcoplasm that penetrate the muscle fiber oxidative), intermediate (type IIa, fast oxidative glyco- between adjacent terminal cisternae. They have an abun- lytic), and white (type IIb, fast glycolytic). dance of voltage-sensor proteins (dihydropyridine- ◗ The structural and functional subunit of the muscle sensitive receptors [DHSR]). fiber is the myofibril. It is composed of precisely aligned ◗ The T tubule and the two adjacent terminal cisternae are myofilaments: myosin-containing thick filaments and called a triad. Triads are located at the junction between actin-containing thin filaments. The smallest contractile A and I bands (two per each sarcomere). unit of striated muscle is the sarcomere. ◗ The depolarization of the T tubule membrane triggers the ◗ The arrangement of thick and thin filaments gives rise to release of Ca2 from the terminal cisternae to initiate muscle the density differences that produce the cross-striations contraction by binding to troponin–tropomyosin complex. of the myofibril. The light-staining isotropic I band con- ◗ Muscle relaxation results from the decrease of cytosolic tains mainly thin filaments attached to both sides of the free Ca2 concentration. Z line, and the dark-staining anisotropic A band contains ◗ The neuromuscular junction (motor end plate) is the con- mainly thick filaments. tact area of the axon endings with muscle fiber. The axon ◗ Thick filaments primarily consist of myosin II molecules; terminal contains the neurotransmitter acetylcholine (ACh). thin filaments are composed of actin and two major reg- ◗ Release of ACh into the synaptic cleft of the neuromus- ulatory proteins (tropomyosin and troponin). cular junction initiates depolarization of the plasma ◗ Z lines between sarcomeres contain an actin-binding pro- membrane, which leads to muscle contraction. tein (-actinin) and Z matrix proteins. ◗ Encapsulated muscle spindles and Golgi tendon organs ◗ The actomyosin cross-bridge cycle represents a series are sensory (proprioreceptive) stretch receptors in muscles of coupled biochemical and mechanical events between and tendons. Pawlina_CH11.indd 342 9/29/14 7:01 PM  343 CHAPTER 11 CARDIAC MUSCLE ◗ Cardiac muscle is striated and has the same type and arrangement of contractile filaments as skeletal muscle. ◗ Cardiac muscle cells (cardiac myocytes) are short cylindrical cells with a centrally positioned single nucleus. They are attached to each other by intercalated discs to form a cardiac muscle fiber. ◗ The intercalated discs represent highly specialized cell-to-cell adhesion junctions containing fascia adherens, gap junctions, and maculae adherentes (desmosomes). ◗ Terminal cisternae are much smaller than in skeletal muscle and with the T tubules form diads that are located at the level of Muscle Tissue the Z line (one per sarcomere). ◗ Passage of Ca2 from the lumen of the T tubule to the sarcoplasm of a cardiac myocyte is essential to initiate the contraction cycle. ◗ Specialized cardiac conducting muscle cells (Purkinje fibers) exhibit a spontaneous rhythmic contraction. They generate and rapidly transmit action potentials to various parts of the myocardium. ◗ The autonomic nervous system regulates the rate of cardiac muscle contraction. SMOOTH MUSCLE DEVELOPMENT, REPAIR, HEALING, AND RENEWAL H I S T O L O G Y 101 ◗ Smooth muscle generally occurs as bundles or sheets of small, elongated fusiform cells (called fibers) with finely tapered ends. ◗ Myoblasts are derived from multipotential They are specialized for slow, prolonged contractions. myogenic stem cells that originate in the ◗ Smooth muscle cells possess a contractile apparatus of thin and mesoderm. Early in development, these thick filaments and a cytoskeleton of desmin and vimentin inter- cells express MyoD transcription factor, mediate filaments. Smooth muscle myosin assembles into side- which plays a key role in activation of polar myosin thick filament. muscle-specific gene expressions and dif- ◗ They do not form sarcomeres and do not exhibit striations. ferentiation of all skeletal muscle lineages. ◗ Thin filaments contain actin, tropomyosin (a smooth muscle ◗ Repair of skeletal muscle and its regenera- isoform), caldesmon, and calponin. No troponin is associated with tion can occur from multipotential myo- smooth muscle tropomyosin. genic stem cells called satellite cells. These ◗ Thin filaments are attached to cytoplasmic densities or dense cells are left over from fetal development bodies, which contain -actinin and are located throughout the and express Pax7 transcription factor. sarcoplasm and close to the sarcolemma. ◗ After muscle tissue injury, satellite cells ◗ Contraction of smooth muscle is triggered by a variety of impulses, are activated. They co-express Pax7 with including mechanical (passive stretching), electrical (depolariza- MyoD to become myogenic precursors of tion at nerve endings), and chemical (hormones acting by a second skeletal muscle cells. messenger) stimuli. ◗ Injury to cardiac muscle tissue results in ◗ Because smooth muscle cells lack T tubules, Ca2 is delivered by death of cardiac myocytes. Cardiac muscle caveolae and cytoplasmic vesicles. is repaired with fibrous connective tissue. ◗ Contraction of smooth muscle is initiated by activation of ◗ Smooth muscle cells are capable of divid- myosin light chain kinase (MLCK) by the Ca2–calmodulin ing to maintain or increase their number complex. and size. Pawlina_CH11.indd 343 9/29/14 7:01 PM