His To Muscle

Muscular Tissue Histology

Ma. Minda Luz M. Manuguid, M.D.

Muscular tissue: General considerations  germ layer origin: Mesoderm  parenchymal cell: muscle fiber/ myocyte/ muscle cell  major function: movement  main component: contractile proteins Actin & Myosin in the form of myofibrils / microfilaments

Muscle tissue terminology  Sarcoplasm- cytoplasm of muscle cells,

matrix in which myofibrils are suspended; high in K, Mg, PO4; many mitochondria  Sarcoplasmic reticulum- endoplasmic reticulum of muscle cells Sarcolemma- cell membrane of muscle cells, with a thin polysaccharide coat; fuses with tendon fibers at the ends of muscle fibers

Contractile Proteins : Actin  Actin: 42kD

 G actin- globular subunits 5.5 ηm diameter

 F actin- two chains of

G actin wound together in a loose spiral with 36ηm periodicity

ACTIN regulatory proteins:  Tropomyosin- 2 intertwined α helices, 40ηm long, covers 7 G-actin units, bound to troponin at one end, lies in the groove of the F-actin helix, covering the Myosin-binding site when intracellular Ca is low  Troponin- complex of 3 globular subunits: TnT(tropomyosin-binding site), TnC(calcium binding site ), & TnI(inhibits Actin-Myosin interaction)  α-Actinin- binds to ends of thin filaments (in striated muscles, at the Z lines of the sarcomere)

Contractile Proteins: Myosin  Heavy Meromyosin (HMM) – “head” – protruding end – forms the cross-bridge  light Meromyosin (LMM) – “tail” – long, linear; aggregates form thick filaments

MYOSIN  Myosin- 470kD polypeptide

 Light meromyosin (LMM)- long “tail” that forms a strong lateral interaction with other LMM chains to form rod-like multi-molecular aggregates (thick filaments)  Heavy meromyosin (HMM)- “head” S1 fragment- projects radially from the axis of myosin aggregates, forming crossbridges between thick & thin filaments when the binding sites are exposed S2 fragment- tail hinge

Actin & Myosin interaction

Types of Muscle Tissue  Smooth muscle- nonstriated fusiform cells under autonomic/involuntary control; contraction is slow  Cardiac muscle- striated elongated branched tubular cells under autonomic/ involuntary control; contraction is vigorous & rhythmic  Skeletal muscle- very long cylindrical multinucleated striated cells in bundles under voluntary control; quick, forceful contractions

Types of Muscle

Smooth Muscle  Cells- elongated, fusiform, nonstriated, each enclosed by a basal lamina & a network of reticular fibers (collagen type III); 20-500μm ; central nucleus; interdigitate to form closely-packed sheets  Organelles- rudimentary sarcoplasmic reticulum; abundant perinuclear mitochondria, free ribosomes in clusters, modest Golgi apparatus

Smooth muscle

Smooth Muscle  Contractile proteins- bundles of myofilaments criss-cross obliquely, forming a lattice-like network with much actin overlap; ♦thin filaments: 5-7nm, actin & tropomyosin; ♦thick filaments: 12-16nm, myosin; ♦intermediate filaments: 10nm, vimentin & desmin (skeletin)  Dense bodies- membrane-associated & cytoplasmic- contain α-actinin; where thin & intermediate filaments insert

Smooth Muscle

 “visceral” muscle- a functional syncytium on account of numerous gap junctions between cells  Nervous control- involuntary (ANS)modifying action rather than initiating  Contraction- slow, concerted- borders of the cell become scalloped, nucleus becomes folded (like a corkscrew); can also occur as in the “sliding filament” mechanism, where thin (Actin) filaments are stable & thick (Myosin) filaments are labile

Smooth muscle locations        

walls of hollow viscera GIT Respiratory tract Genito-Urinary tract Blood vessel walls Arrectores pilorum Iris Ciliary body

Smooth Muscle

Smooth Muscle contraction  Influx of Calcium  Calcium complexes with Calmodulin  Ca-Calmodulin complex activates enzyme myosin light-chain kinase  Myosin light-chain kinase phosphorylates myosin light chain  Myosin interacts with Actin  Contraction

Striated Muscles: Cardiac & Skeletal Common features:  cytoplasmic cross-striations (alternating dark A & light I bands) due to the arrangement of myofilaments;  Sarcomere- smallest repetitive subunit of the contractile apparatus- consists of an A band flanked by halves of an I band; extends from one Z line (midpoint of I band) to another

Sarcomere

Striated Muscle features  T tubules - invaginations of the sarcolemma penetrating the fiber, interlacing with myofibrils  Sarcoplasmic reticulum forms a Longitudinal tubular system in close proximity to myofilaments in the cytoplasm, with Calciumrich expanded ends- cisternae

Diads & Triads Close associations between T & L tubules  Diads – in cardiac muscles – 1 T tubule & 1 SR cistern

Triads – in skeletal muscles – 1 T tubule & 2 SR cisternae

Cardiac Muscle  Cells: in tightly knit bundles; 15μm in diameter, 85-100μm long; striated, elongated, branching, each with a central pale nucleus & surrrounded by a delicate endomysium rich in capillaries;  Intercalated disks- gap junctions at the intersections between cells, areas of low resistance to ionic flow

Cardiac Muscle

Cardiac Muscle

 Organelles: numerous mitochondria, parallel arrangement; irregular sarcoplasmic reticulum (longitudinal tubular system) among the myofilaments, ending in expanded cisternae  Sarcolemma invaginates periodically at the level of the Z bands, producing numerous, large T tubules  Diad- one T tubule & one sarcoplasmic reticulum cisterna

Cardiac Muscle  innervation- autonomic nerves- involuntary  locations: ♦ myocardium ♦ origins of great vessels (Aorta, Venae Cava, Pulmonary Artery) from the heart  modified cardiac myocytes: ♦ Purkinje cells ♦ nodal cells

Cardiac Muscle

Skeletal Muscle  Cells- long cylindrical multinucleated

fibers with cytoplasmic cross-striations & peripherally oriented nuclei oriented in bundles (“fascicles”)  Connective tissue coverings: endomysium-covers individual fibers; perimysium- covers each fascicle; epimysium- covers the entire muscle  Satellite cells- small cells that may play a role in regeneration of injured muscle

Skeletal muscle

Skeletal muscle fibers  Red (slow twitch) fibers- contain myoglobinin sustained/ postural movements White (fast twitch) fibers- in muscles for rapid, precise, fine movements Intermediate fibers- intermediate features – red, but fast twitch

Skeletal Muscle  Sarcoplasmic reticulum is well-developed,

longitudinally arranged tubules ending in expanded ends called ‘cisterns’/ terminal cisterns, which contain Calsequestrin & high Calcium levels  T tubules- invaginations of the sarcolemma penetrating deep into the interior of the muscle fiber at the junction of the A & I bands, forming a grid perforated by myofibrils  Triad- association of one T tubule with two terminal cisterns of the SR (2/sarcomere)

Skeletal Muscle

Skeletal muscle  Innervation: voluntary – motor nerves exceptions : reflex arcs  Motor end-plate/myoneural junction/ neuromuscular junction  Location- attached to bonesexceptions: upper1/3, esophagus; tongue Examples : gastrocnemius, gluteus, trapezius, deltoid, soleus, frontalis sternocleidomastoid, masseter,

Skeletal Muscle

Striated muscle contraction  nerve impulse is propagated along the sarcolemma, along the invaginated T tubules to the diads / triads Ca is released from the SR cisternae to the sarcoplasm Ca binds to TnC, configurational change exposes myosin-binding sites on G actin ATPase cleaves ATP Myosin binds to Actin  muscle contraction occurs

Actin & Myosin interaction

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