Chapter 9 Section A Structure Molecular mechanisms of skeletal muscle contraction Mechanics of single fiber contraction Skeletal muscle energy metabolism Types of skeletal muscle fibers 9 1 STRUCTURE A overview 1 skeletal muscle a consists of alternating light and dark bands striated muscle b several nuclei c thick muscle fibers formed by myoblasts become extremely large e satellite cells d skeletal muscle differentiation is completed by birth get bigger with age repair process involving a population of undifferentiated stem cells called satellite cells in response to strain or injury they become active daughter cells differentiate into myoblasts either repair or form new muscle cells mediated increase of size of remaining muscle fibers hypertrophy f skeletal muscles are usually attached to bones by bundles of collage fibers called tendons 2 cardiac muscle a striated muscle b intercalculated discs c single nucleus d in heart regulated by ANS a several nuclei b thin muscle cells look long c not striated 3 smooth muscle B muscle 1 a number of muscle fibers bound together by connective tissue 2 generally muscle fibers are short and tendons can be very long 3 due to striations in skeletal cardiac muscle two types of filaments within cytoplasm a thick filaments composed of myosin heavy chain molecules form core b thin filaments actin each actin molecule contains binding site for myosin core of thin filament tropomyosin troponin 4 filaments are about of cylindrical bundles called myofibrils 5 sarcomere 6 myosin a one unit of the repeating pattern of thick and thin filaments b each sarcomere contains two sets of thin filaments one at each end a composed of two large polypeptide heavy chains and four smaller light chains b combine to form two globular heads and long tail c myosin molecules face opposite directions C parts of sarcomere 1 A band 7 cross bridges a formed by two globular heads b make contact with thin filament and exert force causing muscle contraction c two binding sites one for ATP one for attaching thin filament 8 the alternation dark and light bands produced by the parallel arrangement of thick and thin filaments are apparent in microscopic view of skeletal muscle a formed by thick filaments which are located in the middle of each sarcomere create wide dark band called A band a one end of each thin filament is anchored to a network of interconnecting proteins known as Z line b two successive z lines discs define the limits of one sarcomere a lies between the ends of the A bands of two adjacent sarcomeres and contains those portions of the thin filament that do not interact with thick filament b bisected by Z line 2 Z line 3 I band 4 H zone a narrow light band in middle of A band b center of H zone is known as M line c titin protein that extends from z line to m line both M line linkage between thick filaments the titin filaments act to maintain the alignment of thick filament in the middle of each sarcomere 5 each thick filament is surrounded by a hexagonal array of six thin filaments 6 each thin filament is surrounded by triangular arrangement of three thick filaments D more random skeletal facts 1 sarcoplasmic reticulum a plays important role in the activation of contraction b is muscle fiber forms sleevelike segments around each myofibril c end of each segment terminal cisternae calcium is stored here 2 tranverse tubule T tubule a lies between cisternae of adjacent segments of sarcoplasmic reticulum both surround myofibrils at region where A and I bands meet b continuous with plasma membrane 9 2 MOLECULAR MECHANISMS OF SKELETAL MUSCLE CONTRACTION A overview 1 contraction a refers to activation of the force generating sites within muscle fibers cross bridges b not necessarily shortening c following contraction muscles can relax B membrane excitation the neuromuscular junction 1 motor neurons a neurons whose axons innervate skeletal muscle fibers b when reaching a muscle axon divides into many branches each branch forming a single junction with a muscle fiber a single motor neuron innervates many muscle fibers but each muscle fiber is controlled by a branch from only one motor neuron c motor unit motor neuron plus the muscle fibers its innervates fibers do not have to be adjacent d axon terminals contain ACh e motor end plate region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon f neuromuscular junction junction of axon terminal with the motor end plate when AP gets to axon terminal depolarizes neuron causes Ca channels to open Ca diffuses into axon terminal from ECF binds to proteins enables vesicles to fuse to plasma membrane ACh diffuses from axon terminal to motor end plate binds to iontropic receptors of the nicotinic type opens ion channel in each receptor protein more Na moves into fiber at motor end plate than K moves out produces EPP g end plate potential EPP local depolarization of the motor end plate due to more NA moving in to cell then K moving out same as EPSP but magnitude is much larger because neurotransmitter is released over larger surface area more binding one EPP can depolarize the muscle plasma membrane adjacent to end plate membrane initiates AP AP in t tubules induces DHP receptors to pull open ryanodine receptor channels allows release of Ca from terminal cisternae of sarcoplasmic reticulum h every AP in a motor neuron normally produces an AP in each muscle fiber in its motor unit I ALL NEUROMUSCLAR JUNCTIONS ARE EXCITATORY j synaptic junction contains enzyme acetylcholinesterase breaks down ACh as ACh is broken down less is available to bind to receptors ion channels in the end plate close arrowhead poisoning binds to ACh receptors keeps ion channels closed not effected by acetylcholinesterase no resulting EPP b also can be blocked by inhibiting acetylcholinesterase ion channels just stay open maintain a depolarized state cannot generate AP ions will become insensitive to ACh prevents neuron from ever depolarizing again skeletal muscle will become paralyzed c ex succinylcholine agonists on ACh receptors temporarily stops muscle contraction for surgery 2 disruption of neuromuscular signaling a ex curare d some block release of ACh C excitation contraction coupling 1 excitation contraction coupling a refers to sequence of events by which an AP in the plasma membrane activates the force generating mechanisms b AP lasts 1 to 2 msec mechanical activity following an AP lasts 100 msecond or more c