ANAT2241 Muscle Tissue Lecture Slides PDF

ANAT2241 Histology: Basic & Systematic Week 02: Muscle Tissue Dr. Reza Shirazi Department of Anatomy, School of Medical Sciences Faculty of Medicine & Health, UNSW Sydney [email protected] Image by Dr John Abramyan, University of Michigan Resources Recommended reading: Textbook chapt...

ANAT2241 Histology: Basic & Systematic Week 02: Muscle Tissue Dr. Reza Shirazi Department of Anatomy, School of Medical Sciences Faculty of Medicine & Health, UNSW Sydney [email protected] Image by Dr John Abramyan, University of Michigan Resources Recommended reading: Textbook chapter (Wheather's online): Muscle Tissue Textbook chapter (Wheather's online): Endocrine System Textbook chapter (Junqueira’s Basic Histology: Text and Atlas, 16e online): Muscle Tissue Textbook chapter (Junqueira’s Basic Histology: Text and Atlas, 16e online): Endocrine System University of Michigan Histology and Virtual Microscopy Learning Resources Chapman Histology https://www.youtube.com/channel/UCvSvCkHjCbHn8aGd6OPno_A/featured Learning Objectives 1. To understand the classification of muscle tissue 2. To identify skeletal, cardiac and smooth muscle on the basis of histological features 3. To understand the contraction mechanism 4. To distinguish connective tissue in association with muscle cells and muscle fascicles Muscle Tissue One of the fourth basic tissue types Functions: • Responsible for movement of the body and its parts • Responsible for changes in the size and shape of internal organs • Responsible for contraction Muscle Tissue Composed of: 1- Cells (fibers) specialized for contraction 2- Moderate to small amount of ECM Special names for the organelles in muscle tissue: • Sarcoplasm for cytoplasm • Sarcoplasmic reticulum for smooth ER • Sarcolemma for cell membrane and its external lamina • External lamina: o Basal lamina when it forms a peripheral cellular investment, as in muscle cells, adipocytes and peripheral nerve supporting cells (Schwann cells) Muscle Tissue Classification Morphologic/histologic classification: 1- Striated muscle • Skeletal muscle • Cardiac muscle 2- Smooth muscle Functional/physiologic classification: 1- Voluntary muscle • Skeletal muscle 2- Involuntary muscle • Cardiac muscle • Smooth muscle Skeletal Muscle Cells (Fibers) Skeletal muscle cells/fibers: • Long • Cylindrical • Multinucleated o Peripherally located nuclei • Dimensions: o Diameter from 10 to 100 μm o Lengths up to 30 cm (X360; H&E; Skeletal muscle) Development of Skeletal Muscle • Derived from mesenchymal cells • Differentiation of mesenchymal cells into myoblasts • Fusion of myoblasts together to form myotubes • Synthesis of myofilaments and appearance of striations • Displacement of the nuclei against the sarcolemma due to the formation of functional myofilaments Development of Skeletal Muscle • Some myoblasts remain undifferentiated to form satellite cells o Acting as stem cells and form new muscle fibers following injury Organization of a Skeletal Muscle • Epimysium: o A thick layer of a dense irregular connective tissue surrounding the entire muscle o Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle • Perimysium: o A thin layer of dense connective tissue layer that immediately surrounds each bundle/fascicle of muscle fibers • Endomysium: o A very thin layer of delicate connective tissue surrounding each muscle fiber Organization of a Skeletal Muscle (X200; H&E; A cross section of skeletal muscle) • En: endomysium • P: perimysium • E: epimysium (X480; Scanning electron micrograph of an intramuscular connective tissue was) Organization of a Skeletal Muscle (X200; Giemsa with polarized light; A rich network of capillaries in endomysium surrounding muscle fibers) Myotendinous Junction in Skeletal Muscle • Continuation of the dense collagen fibers of the tendon with those in the three connective tissue layers around muscle fibers • Forms a strong unit that allows muscle contraction to move other structures (X400; H&E; Longitudinal section of a myotendinous junction) • M: muscle fibers • T: tendon Organization Within Skeletal Muscle Fibers • Striations of alternating light and dark bands in longitudinal section: o Dark bands: A bands (anisotropic or birefringent in polarized light microscopy) o Light bands: I bands (isotropic, do not alter polarized light) (X200; H&E; Longitudinal section of skeletal muscle) • F: fibroblast nucleus • N: muscle nucleus • A: A band (dark-staining) • I: I band (light-staining) Organization Within Skeletal Muscle Fibers • Sarcoplasm filled with myofibrils (long cylindrical filament bundles) • Myofibrils: end-to-end repetitive arrangement of sarcomeres (X500; Giemsa; Longitudinal section of skeletal muscle) Ultrastructure of Myofibrils • Myofibrils: end-to-end repetitive arrangement of sarcomeres • Sarcomere: o Repetitive functional subunit extending from Z disc to Z disc o About 2.5-μm long in resting muscle • Z disc: o A dark transverse line bisecting each I band • Characteristic pattern of transverse striations in entire muscle: o Formed by lateral registration of sarcomeres in adjacent myofibrils (X24,000; TEM; Longitudinal section of sarcomeres) • A: A band • I: I band • Z: Z disc • H: H zone • M: mitochondria Molecular Arrangement of Myofibrils/Sarcomeres • Regular arrangement of thick myofilaments (myosin) and thin myofilaments (F-actin) • Thick myofilaments (myosin): o Occupy the A band at the middle region of the sarcomere o Composed of two heavy chains (tail) and two pairs of light chains (head) • Thin myofilaments (F-actin): o Run between the thick filaments o Composed of G-actin monomer containing a binding site for myosin o Have two tightly associated regulatory proteins: § Tropomyosin § Troponin: a complex of three subunits (TnT, TnC, and TnI) q TnT: attaches to tropomyosin q TnC: binds Ca2+ q TnI: regulates the actinmyosin interaction Molecular Arrangement of Myofibrils/Sarcomeres • A bands: o Contain both the thick filaments and the overlapping portions of thin filaments • I bands: o Consist of the portions of the thin filaments that do not overlap the thick filaments in the A bands Molecular Arrangement of Myofibrils/Sarcomeres • H zone: o A lighter zone in the center of A band o Corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments • M line: o Bisecting the H zone o Containing: § Myomesin: a myosin-binding protein that holds the thick filaments in place § Creatine kinase: q catalyzes transfer of phosphate groups from phosphocreatine (a storage form of highenergy phosphate groups) to ADP q Helping to supply ATP for muscle contraction Sarcoplasmic Reticulum & Transverse Tubule System in Skeletal Muscle Fibers • Sarcoplasmic reticulum (smooth ER): o Contains pumps and other proteins for Ca2+ sequestration o Surrounds the myofibrils o Calcium release from cisternae of the sarcoplasmic reticulum through voltage-gated Ca2+ channels is triggered by membrane depolarization produced by a motor nerve Sarcoplasmic Reticulum & Transverse Tubule System in Skeletal Muscle Fibers • Transverse or T-tubules: o Tubular infoldings of the sarcolemma o Long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm o Encircle each myofibril near the aligned A- and I-band boundaries of sarcomeres o To trigger Ca2+ release from sarcoplasmic reticulum throughout the muscle fiber simultaneously and produce uniform contraction of all myofibrils Sarcoplasmic Reticulum & Transverse Tubule System in Skeletal Muscle Fibers • Triad: o Complex of a T-tubule with two terminal cisternae in longitudinal TEM sections • Allows depolarization of the sarcolemma in a T-tubule to affect the sarcoplasmic reticulum and trigger release of Ca2+ ions into cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres Mechanism of Contraction in Skeletal Muscle Fibers • Neither the thick nor the thin filaments change their length during muscle contraction • Contraction occurs as the overlapping thin and thick filaments of each sarcomere slide past one another Mechanism of Contraction in Skeletal Muscle Fibers • Contraction is induced when an action potential arrives at a synapse, the neuromuscular junction (NMJ) • Action potential is transmitted along the Ttubules to terminal cisternae of the sarcoplasmic reticulum to trigger Ca2+ release • In a resting muscle, the myosin heads cannot bind actin because the binding sites are blocked by the troponin-tropomyosin complex on the Factin filaments • Calcium ions released upon neural stimulation bind troponin, changing its shape and moving tropomyosin on the F-actin to expose the myosin-binding active sites and allow cross bridges to form • Binding actin produces a conformational change or pivot in the myosins, which pulls the thin filaments farther into the A band, toward the Z disc Mechanism of Contraction in Skeletal Muscle Fibers Mechanism of Contraction in Skeletal Muscle Fibers • Energy for the myosin head pivot that pulls actin: o Provided by hydrolysis of ATP bound to the myosin heads, after which myosin binds another ATP and detaches from actin o In the continued presence of Ca2+ and ATP, these attach-pivotdetach events occur in a repeating cycle (each lasting about 50 milliseconds) which rapidly shorten the sarcomere and contract the muscle o A single muscle contraction results from hundreds of these cycles • When the neural impulse stops and levels of free Ca2+ ions diminish: o Tropomyosin again covers the myosin-binding sites on actin and the filaments passively slide back and sarcomeres return to their relaxed length Mechanism of Contraction in Skeletal Muscle Fibers • In the absence of ATP: o Actin-myosin cross bridges become stable, which accounts for the rigidity of skeletal muscles (rigor mortis) that occurs as mitochondrial activity stops after death Muscle Spindles & Tendon Organs • Sensory receptors • Acting as proprioceptors providing the central nervous system (CNS) with data from the musculoskeletal system Muscle Spindles & Tendon Organs Muscle spindles: • Location: among the muscle fascicles • Structure: o Encapsulated by modified perimysium, with concentric layers of flattened cells o Containing interstitial fluid and a few thin muscle fibers filled with nuclei and called intrafusal fibers o Penetrated by several sensory nerve axons wrapping around individual intrafusal fibers • Function: o Stretch detectors o Changes in length (distension) of the surrounding (extrafusal) muscle fibers caused by body movements are detected by the muscle spindles and the sensory nerves relay this information to the spinal cord o Help maintain posture and to regulate the activity of opposing muscle groups involved in motor activities such as walking (X3,600; A TEM cross section near the end of a muscle spindle) • C: capsule • MA: lightly myelinated axons • MF: intrafusal muscle fibers • SC: muscle satellite cells Muscle Spindles & Tendon Organs Golgi tendon organs: • Much smaller encapsulated structures that enclose sensory axons penetrating among the collagen bundles at the myotendinous junction • Detect changes in tension within tendons produced by muscle contraction • Act to inhibit motor nerve activity if tension becomes excessive • Because both of these proprioceptors detect increases in tension, they help regulate the amount of effort required to perform movements that call for variable amounts of muscular force Cardiac Muscle • Rather than fusing into multinucleated cells/fibers as in developing skeletal muscle fibers, cardiac muscle cells form complex junctions between interdigitating processes • Cells within one fiber often branch and join with cells in adjacent fibers • Consequently, the heart consists of tightly knit bundles of cells, interwoven in spiraling layers that provide for a characteristic wave of contraction that resembles wringing out of the heart ventricles • Usually has only one or two centrally located nucleus Organization of Cardiac Muscle • Endomysium: o Surrounding the muscle cells o a delicate sheath of connective tissue with a rich capillary network • Perimysium: o Separates bundles and layers of muscle fibers o In specific areas, forms larger masses of fibrous connective tissue comprising the “cardiac skeleton.” Intercalated discs • A unique characteristic of cardiac muscle • Transverse lines that cross the fibers at irregular intervals • Where the myocardial cells join • Represent the interfaces between adjacent cells • Consist of many junctional complexes o Transverse regions: composed of many desmosomes and fascia adherens junctions o Longitudinally oriented regions: run parallel to the myofibrils and are filled with gap junctions; § Serve as electrical synapses Cardiac Muscle • Dyads: o Junctions between terminal cisterns of sarcoplasmic reticulum and T-tubules typically involve only one structure of each type Smooth Muscle/Visceral Muscle • Specialized for slow, steady contraction under the influence of autonomic nerves and various hormones • Major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organs Smooth Muscle/Visceral Muscle • Fibers: o Fusiform, elongated, tapering, and unstriated cells o Each of which is enclosed by an external lamina and a network of type I and type III collagen fibers comprising the endomysium o Contain single elongated nucleus located centrally o Linked by numerous gap junctions o Close packing is achieved with the narrow ends of each cell adjacent to the broad parts of neighboring cells § Cross sections of smooth muscle show a range of cell diameters, with only the largest profiles containing a nucleus Smooth Muscle/Visceral Muscle • Fibers: o Have rudimentary sarcoplasmic reticulum o Lack T-tubules o Have caveolae (small plasmalemma invaginations) containing the major ion channels: § Control Ca2+ release from sarcoplasmic cisternae at myofibrils that initiates contraction Smooth Muscle/Visceral Muscle • Different organization of myofibrillar arrays of actin and myosin o Bundles of thin and thick myofilaments crisscross the sarcoplasm obliquely o Troponin is replaced with calmodulin o Tropomyosin is replaced with Ca2+sensitive myosin light-chain kinase (MLCK) • Insertion of actin myofilaments into anchoring cytoplasmic and plasmalemmaassociated dense bodies (contain αactinin and are functionally like the Z discs) • Intermediate filaments, composed of desmin, which attach to the dense bodies