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  • There are three major types of muscle: (1) skeletal or striated muscle, (2) cardiac muscle, and (3) smooth or visceral muscle.

  • Skeletal muscle cells are very long, multinucleated fibers, cylindrically shaped and with diameters up to 100 μm.

  • The sarcolemma of each fiber is surrounded by an external lamina and thin connective tissue, endomysium, containing capillaries.

Organization of Skeletal Muscle Fibers
  • Groups of fibers called fascicles are surrounded by perimysium; all fascicles are enclosed within a dense connective tissue epimysium.

  • Internally, each muscle fiber is filled with myofibrils, composed of thousands of thick myosin filaments and thin actin filaments, highly organized into contractile units called sarcomeres.

  • Within sarcomeres, thick and thin filaments interdigitate; globular myosin heads project from the thick filaments toward the F-actin filaments, which are associated with tropomyosin and troponin.

  • Sarcomeres are separated by Z discs that bisect the light-staining I bands that contain mainly the thin filaments attached to α-actinin in the Z disc.

  • Between the two I bands of a sarcomere is the dark-staining A band with the thick myosin filaments; alternating light and dark bands appear as microscopic striations along the fibers.

Sarcoplasmic Reticulum & Transverse Tubule System
  • In the sarcoplasm between parallel myofibrils are mitochondria and cisternae of smooth ER, called the sarcoplasmic reticulum (SR) specialized for Ca2+ sequestration and release.

  • At each sarcomere, two terminal cisterns of SR contact a deep invagination of the sarcolemma called a transverse or T-tubule, forming a triad that triggers Ca2+ release when the sarcolemma is depolarized.

Mechanism of Contraction
  • Ca2+ binding to troponin causes tropomyosin to change shape and allow the myosin heads to bind the actin subunits, forming crossbridges between thick and thin filaments.

  • The myosin heads then pivot with ATP hydrolysis, which pulls the thin filaments along the thick filaments.

  • With Ca2+ and ATP present, a contraction cycle ensues in which myosin heads repeatedly attach, pivot, detach, and return, causing the filaments to slide past one another, shortening the sarcomere.

  • When the membrane depolarization ends, Ca2+ is again sequestered, ending contraction and allowing the sarcomeres to lengthen again as the muscle relaxes.

  • Synapses of motor axons with skeletal muscle are called MEPs, NMJs, or myoneural junctions; the neurotransmitter is acetylcholine.

  • A motor axon may form many terminal branches, each ending on an MEP of a muscle fiber; all fibers innervated by branches of that axon comprise a motor unit.

Muscle Spindles & Tendon Organs
  • These are both sensory proprioceptors in which sensory axons wrap around intrafusal fibers in small specialized fascicles or around myotendinous collagen bundles, respectively.

Muscle Fiber Types
  • Skeletal muscles contain fibers that can be physiologically classified as the three main types: (1) slow, oxidative (type I); (2) fast, intermediate oxidative-glycolytic (type IIa); and (3) fast, ...

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