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Muscle Tissue

Muscle Tissue. Kristine Krafts, M.D. Muscle Tissue Lecture Objectives. Describe, compare and contrast the histologic structure of the three types of muscle, as seen with the light and electron microscope.

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Muscle Tissue

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  1. Muscle Tissue Kristine Krafts, M.D.

  2. Muscle Tissue Lecture Objectives Describe, compare and contrast the histologic structure of the three types of muscle, as seen with the light and electron microscope. Describe, compare, and contrast the structure and organization of the sarcomere in skeletal and cardiac muscle, including the banding pattern, transverse tubules, and sarcoplasmic reticulum. Describe the unique structural features of smooth muscle which distinguish it from skeletal and cardiac muscle.

  3. Muscle Tissue Lecture Objectives Describe the structure and function of the motor end-plate (myoneural junction). Describe the connective tissue sheaths that surround skeletal muscle. Compare the ability of the three types of muscle to regenerate.

  4. Muscle Tissue Lecture Outline Introduction Skeletal muscle Cardiac muscle Smooth muscle

  5. Muscle Tissue Lecture Outline Introduction

  6. Fun facts about muscle terminology “Sarco-” is from the Greek sarx (flesh) “Myo-” is from the Greek mys (muscle) Muscle cells are longer than they are wide, so they are also called “fibers” There are three types of muscle cells: smooth, skeletal and cardiac. Smooth muscle cells are non-striated; skeletal and cardiac muscle cells are striated.

  7. Don’t make this mistake! “Striated muscle” and “skeletal muscle” are sometimes used interchangeably. This is wrong, wrong, wrong! Skeletal muscle is striated – but so is cardiac muscle. So if you mean skeletal, say skeletal, not striated.

  8. Muscle types and activity

  9. Summary that will make sense later

  10. More summary that will make sense later

  11. Muscle Tissue Lecture Outline Introduction Skeletal muscle

  12. Skeletal muscle Transverse section Longitudinal section

  13. Where do skeletal muscle cells come from? Derived from mesenchymal cells, which give rise to myoblasts. Myoblasts: spindle-shaped. Fuse to form multinucleated myotubes which elongate. Mature skeletal muscle cells (or fibers): long, unbranched tubes with many flattened nuclei. Sarcoplasm contains a ton of mitochondria, glycogen granules and myoglobin. Mature skeletal muscle cells can’t divide.

  14. Skeletal muscle Abundant eosinophilic cytoplasm Peripheral nuclei

  15. Skeletal muscle Cells are long, unbranched tubes Can just barely see striations Peripheral, flattened nuclei

  16. Skeletal muscle Fascicle Fascicle A single muscle cell is also called a fiber. A fascicle is a bundle of single muscle cells (or fibers). Fiber

  17. Connective tissue coverings of skeletal muscle Epimysium Dense layer of connective tissue that surrounds entire muscle Perimysium Connective tissue that surrounds each fascicle Endomysium Delicate connective tissue (reticular fibers and basal lamina) that surrounds each muscle cell (fiber)

  18. Perimysium Fascicle Muscle fiber (cell) Endomysium

  19. What do muscle cells contain? Muscle cell (or muscle fiber, or myofiber)

  20. What do muscle cells contain? Muscle cells contain myofibrils. Myofibrils are composed of thin (actin) and thick (myosin) filaments.

  21. Summary: muscle fibers, myofibrils, and myofilaments One skeletal muscle cell is called a fiber (or myofiber). Each myofiber contains many myofibrils. Each myofibril demonstrates striations and a banding pattern, and is composed of overlapping myofilaments. There are two kinds of myofilaments: thin (actin) filaments and thick (myosin) filaments.

  22. A myofibril is the smallest structure within one muscle fiber that demonstrates cross striations. thin (actin) filaments Thick (myosin) filaments

  23. Bands and sarcomeres Each myofibril has repeating subunits called sarcomeres which have bands (or striations) The sarcomeresin each myofibril are aligned with those of adjacent myofibrils so their bands are also aligned. A sarcomere is defined as running from Z line to Z line.

  24. Myofibril, bands and sarcomere Sarcomere A-H-M and I-Z

  25. Myofibril, bands and sarcomere

  26. Skeletal muscle with A bands, I bands, Z lines

  27. Skeletal muscle with A bands, I bands, Z lines I band A band Z line

  28. What are these bands and lines composed of? A bands are composed of thick (myosin) filaments overlapping thin (actin) filaments. H bands are in the center of A bands. They lie between the free ends of the thin filaments and contain only myosin filaments. M lines bisect H bands. They are where adjacent thick filaments connect. I bands contain only thin filaments. Z lines (disks) bisect I bands. Alpha actinin attaches actin filaments to the Z line.

  29. Sarcomere and bands

  30. Sarcoplasmic reticulum • Sarcoplasmic reticulum is the smooth endoplasmic reticulum in skeletal muscle. • It is specialized to sequester & release calcium ions. • It ensheathes or surrounds each myofibril.

  31. Transverse (T) tubules At each A-I band junction, a tubular invagination of the sarcolemma termed a transverse (T) tubule penetrates the muscle fiber and lies next to the surface of myofibrils.

  32. Transverse tubules

  33. Terminal cisterna and triad • An expansion of the sarcoplasmic reticulum called a terminal cisterna lies on each side of the T tubule. • A triad is a complex of 2 terminal cisternae with a T tubule in between.

  34. Neuromuscular junction (motor end-plate) • A motor end-plate is a group of specialized synapses involving the terminal boutons of a motor axon and the sarcolemma of a skeletal muscle fiber. • The function is to transmit nerve impulses to skeletal muscle fibers to initiate contraction. There are 3 main components in a motor end-plate: • Presynaptic component • Synaptic cleft • Postsynaptic component

  35. The three parts of a neuromuscular junction Presynaptic component (the terminal bouton of the axon) Synaptic cleft Postsynaptic component (including junctional folds)

  36. Presynaptic component The presynaptic component is the terminal bouton of the axon. The myelin sheath ends before the bouton. The bouton contains mitochondria and synaptic vesicles containing acetylcholine. The presynaptic membrane of the bouton faces the synaptic cleft.

  37. Postsynaptic component The postsynaptic (muscular) component includes the postsynaptic membrane (the sarcolemma) and the underlying sarcoplasm. The postsynaptic membrane has numerous folds (junctional folds) with receptors for acetylcholine. When acetylcholine binds to the receptors, the muscle contracts.

  38. Synaptic cleft The synaptic cleft contains acetylcholinesterase which degrades the acetylcholine so that muscle contraction will stop when neural stimulation stops.

  39. Motor unit of skeletal muscle • Each motor neuron has a single axon that may terminate on a single muscle fiber or may undergo terminal branching and end on multiple muscle fibers. • A single nerve fiber and all the muscle fibers it innervates is termed a motor unit.

  40. Motor units and force of contraction Individual skeletal muscle fibers either contract all the way or not at all. To vary the force of contraction, the fibers within a muscle bundle do not all contract at the same time. The number and size of motor units controls the intensity of a muscle contraction. Muscles that require delicate movements have many, small motor units; muscles that have coarse movements have a few large motor units.

  41. Muscle spindle Sensory receptor made of modified muscle fibers (intrafusal fibers) with sensory and motor nerve fibers. Function: detect stretch of adjacent muscle fibers(extrafusal fibers);regulate nervous control of body posture and movement

  42. Muscle Tissue Lecture Outline Introduction Skeletal muscle Cardiac muscle

  43. Cardiac muscle

  44. Structure of cardiac muscle fibers Cardiac muscle cells are long, branched fibers with 1-2 plump, central nuclei. Sarcoplasm contains numerous mitochondria and lipid droplets. Arrangement of myofibrils leads to cross striations similar to skeletal muscle.

  45. Cardiac muscle: 1-2 plump nuclei and branching cells

  46. T tubules and diads in cardiac muscle Sarcoplasmic reticulum is sparse and less organized than in skeletal muscle. T tubules occur at Z line rather that at A-I junction. Diads: each T tubule is associated with a terminal portion of adjacent sarcoplasmic reticulum.

  47. Intercalated Disks Intercalated disks are complexes of several types of cell-cell junctions! • Fascia adherens anchor thin filaments. • Macula adherens (spot desmosomes) prevent cardiac muscle fibers from tearing apart during contraction. • Gap junctions allow electrical stimuli to be passed from cell to cell.

  48. Intercalated disk Macula adherens Fascia adherens Gapjunction

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