TY - CHAP M1 - Book, Section TI - Muscle Tissue A1 - Mescher, Anthony L. PY - 2021 T2 - Junqueira's Basic Histology Text and Atlas, 16e AB - Muscle Tissue SUMMARY OF KEY POINTSThere 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 FibersGroups 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 SystemIn 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 ContractionCa2+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 OrgansThese are both sensory proprioceptors in which sensory axons wrap around intrafusal fibers in small specialized fascicles or around myotendinous collagen bundles, respectively.Muscle Fiber TypesSkeletal 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, glycolytic (type IIb).Cardiac MuscleCardiac muscle fibers are also striated, but they consist of individual cylindrical cells, each containing one (or two) central nuclei and linked by adherent and gap junctions at prominent intercalated discs.Sarcomeres of cardiac muscle are organized and function similarly to those of skeletal muscle.Contraction of cardiac muscle is intrinsic at nodes of impulse-generating pacemaker muscle fibers; autonomic nerves regulate the rate of contraction.Smooth MuscleSmooth muscle fibers ... SN - PB - McGraw Hill CY - New York, NY Y2 - 2022/05/20 UR - accessmedicine.mhmedical.com/content.aspx?aid=1184200260 ER -