A P 2 EXAM 1 Skeletal Muscle Functions Characteristics Muscle Functions Produce movement Maintain posture Stabilize joints Generate Heat Functional Characteristics Excitability Contractility Extensibility Elasticity stretching Generate heat maintain body temp muscle accounts for at least 40 of Contractility the ability to shorten forcefully when adequately stimulated Excitability the ability to receive and respond to a stimulation Extensibility the ability to be stretched Elasticity the ability to recoil and resume resting lengths after being body mass stretched Objective 1 Define and identify the anatomy of skeletal muscle and explain the sliding filament theory leading to muscle contraction The Skeletal Myofiber Musle Cell Multinucleated peripheral Terminally differentiated Highly plastic Load driven size alterations Increase hypertrophy Decrease atrophy Activity determined phenotype e g I IIa IIx Overall Composition of Skeletal Muscle In mammals 75 water 5 inorganic salts pigments and substrates 20 a mixture of proteins Muscle Cell Dimensions CSA In adult human being from 2000 m2 to 7500 m2 Mean and median 3000 4000 m2 range Myofibrils myofilaments Each fiber is composed of myofibrils arranged in a 3 D mosaic pattern Myofibrils account for roughly 80 of cellular volume and contain the contractile elements of the muscle cell Striations are due to a repeating series of dark A bands and light I bands Myofilaments make up the myofibrils and consist of thick and thin filaments Myofiber Ultrastructure and Molecular Composition of the Myofilaments Two types of myofilaments thick filaments composed of bundles of myosin and thin filaments composed of strands of actin Tropomyosin and troponin are regulatory proteins present in thin plasma membrane basement membrane fuses with tendon filaments Sarcolemma bone Sarcoplasm glycogen myoglobin Transverse Tubules conduction of impulse from the surface of the cell to the terminal cisternae T tubules cytoplasm contains proteins minerals fats organnelles transport system deep into fiber allows rapid are infoldings of the sarcolemma that conduct electrical impulses The sarcoplasmic reticulum SR is a smooth endoplasmic reticulum surrounding each myofibril SR membranous channel parallel to myofibril and storage site for Calcium Ca2 Myofiber Hypertrophy and Skeletal Muscle Stem Cells Satellite Cells Muscle growth and regeneration are accomplished by satellite cells Hawke et al J Appl Physiol 2001 Locus of Satellite Cells Muscle specific stem cell NCAM CD56 mAb Nuclei Mayer s hematoxylin Satellite Cells and Myonuclei Fundamental Importance Also known as stem cells muscle growth occurs due to degeneration and regeneration of satellite cells Depending on the age and muscle type 85 or 95 of the nuclear material is true myonuclei ie located inside the plasma membrane The balance 5 to 15 is satellite cells located between the basement membrane and plasma membrane Approx 200 300 nuclei per mm of fiber length contrast to many other cells in our body single nucleus Myofiber Hypertrophy and Skeletal Muscle Stem Cells Satellite Cells Mechanisms of repair regeneration hypertrophy largely dependent upon activation of muscle stem cells i e satellite cells Mauro 1961 Hawke 2001 Adams 2002 Rosenblatt 1992 Myonuclear Domain Theory Each myonucleus controls transcription and consequent protein synthesis over a finite volume of cytoplasm Allen 1999 As muscle fiber begins to grow the myonucleus can only manage a specific volume so more nuclei are needed to control satellite cells properly Additional nuclei are required to support increased fiber volume Petrella et al 2006 Primary donors of nuclei to developing muscle or adult muscle undergoing repair regeneration and growth satellite cells add additional nuclei Satellite Cells and Myonuclei Fundamental Importance summary When muscle fibers are viewed longitudinally with LM light microscope nuclear material appears to be located along the peripheral edge of the fibers for the growth and development of muscle satellite cells needed for growth for the adaptive capacity of skeletal muscle to various forms of training or disuse atrophy vs hypertrophy for the recovery from the exercise induced or traumatic injury or neuromuscular disease can be damaged by stretching or chemicals Exercise induced Ultrastructural Muscle Damage Streaming of the Z line or Disruption of the Z disc Hypertrophy vs Hyperplasia Resting myofiber myotrauma progenitor cell activation chemotaxis to injured fiber fusion to damaged myofiber regenerated myofiber with central nuclei exercise or myotrauma activates the progenitor cell which then attacks damaged muscle fibers fusion of cells causing hypertrophy to repair fiber Myostatin Blocker Impact of mutation o Mutation in the mystatin gene increases muscle mass and enhances racing performance in heterozygote dogs o Double muscling syndrome o Myostatin depresses muscle growth STRENGTH depends on cross section AND VELOCITY Muscle Cell Dimensions Length Vary considerably in human where located changes size of fibers Length of medial gastrocnemius 250 mm with fiber lengths of 35 mm 15 of muscle length Sartorius muscle 500 mm with fiber lengths of 450 mm 90 of muscle length 1 2 mm in intraocular muscles of the eye Long fibers move a longer distance than short muscle fibers shorter the fiber greater the force production longer muscles built for velocity Muscle Cell Dimensions Number From several hundreds in small muscles to over a million in large muscles such as those involved in hip flexion and knee extension First lumbrical 10 250 Medial gastrocnemius 1 033 000 Coordinated contraction and relaxation of this many individual cells during standing running walking and other activities The smaller the number the more precise movements helping with reflexes as well Myotendinous Junction junction The ends of muscle fibers connect to tendons at the myotendinous Allow the force generated by muscle fibers to be applied through the tendons to the bones to produce the desired movement Complex specialized structure Some muscularskeletal injuries associated with exercise occur at this area in athletes hard to repair the area because of complexity one t tube is part of a triad distance between z disks decreased due to myosin contracting shortening everything Muscle Cell Dimensions Sarcomeres smallest contractile unit of skeletal myofiber The sarcomere length the distance from one Z disk to the next Optimal sarcomere length
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