Lecture 18 Chapter 50 pp 1120 1126 Vertebrate Skeletal Muscle Skeletal muscle moves bones and body has a hierarchy of smaller and smaller units Within a typical skeletal muscle is a bundle of long fibers Each fiber is a single cell with multiple nuclei Inside a muscle cell is a bundle of myofibrils containing thick and thin filaments Sarcomeres basic contractile units of muscle repeating sections Borders of sarcomeres line up in adjacent myofibrils forming striations Skeletal muscle striated muscle Thin filaments attach at Z lines thick filaments attach at M lines In resting myofibril thick and thin filaments partially overlap Edge of sarcomere thin filaments center thick filaments Muscle bundle of muscle fibers single muscle fiber myofibril sarcomere The Sliding Filament Model of Muscle Contraction Contracting muscle shortens but filaments stay the same length Filaments slide past each other like a telescope Sliding filament model thick and thin filaments ratchet past each other powered by myosin molecules Each myosin has a tail and head region Tail adheres to tails of other myosin molecules Head can bind ATP hydrolysis of bound ATP converts myosin to a high energy form that binds to actin forming a cross bridge Myosin returns to its low energy form as it pulls the thin filament toward the center of the sarcomere Cross bridge is broken when a new ATP molecule binds to the head Contraction requires repeated cycles of binding and release Thick filament contains 350 heads Each of which forms and reforms about 5 bridges per second driving the thick and thin past each other Powering repetitive contractions requires creatine phosphate and glycogen Transferring a P group from creatine phosphate to ADP synthesizes ATP Resting stores of creatine phosphate can sustain contractions for 15 seconds Breaking glycogen down into glucose can also replenish ATP stores During light moderate activity glucose is metabolized by aerobic respiration Yields enough power to sustain contractions for an hour During intense activity ATP is generated by lactic acid fermentation Very rapid but but generates much less ATP per glucose molecule sustaining contractions for only about 1 minute The Role of Calcium and Regulatory Proteins Tropomyosin regulatory protein and troponin complex additional regulatory proteins are bound to the actin strands of thin filaments At rest tropomyosin covers the myosin binding sites along the thin filament preventing actin and myosin from interacting Ca2 accumulation in the cytosol binds to the troponin complex causing tropomyosin bound along the actin strands to shift position and expose the myosin binding sites on the thin filament Therefore when Ca2 concentration rises thick thin filaments slide past each other and muscle fibers contract When Ca2 concentration falls the binding sites are covered and contraction Motor neurons trigger the release of Ca2 into the cytosol of muscle cells causing contraction This regulation of Ca2 is a multistep process Arrival of an action potential at the synaptic terminal of a motor neuron triggers release Binding of acetylcholine to receptors on the muscle fiber leads to a depolarization triggering an action potential Action potential follows foldings of the plasma membrane called transverse T stops of acetylcholine tubules These make close contact with the sarcoplasmic reticulum SR Specialized endoplasmic reticulum Action potential spreading along the T tubules triggers changes in the SR opening Ca2 channels allowing Ca2 to flow into the cytosol and bind to the troponin complex initiating muscle fiber contraction Relaxation begins as transport proteins in the SR pump Ca2 in from the cytosol When Ca2 concentration in cytosol drops to a low level regulatory proteins bound to the thin filament shift back to their starting position blocking myosin binding sites as Ca2 pumped from the cytosol accumulates in the SR Disease can cause paralysis by interfering with the excitation of skeletal muscle fibers by motor neurons In ALS motor neurons in spinal cord brainstem degenerate and muscle fibers atrophy In Myasthenia gravis the body produces antibodies to the acetylcholine receptors of skeletal muscle and the transmission between motor neurons and muscle fibers decreases Nervous Control of Muscle Tension Contraction of a whole muscle such as the bicep is graded Contraction of a single skeletal muscle is a all or none twitch Nervous system produced graded contractions by either Varying the number of muscle fibers that contract Varying the rate at which muscle fibers are stimulated Each branched motor neuron may synapse with many muscle fibers although each fiber is controlled by only one motor neuron Motor unit consists of a single motor neuron and all the muscle fibers it controls Motor neuron produces action potential all muscle fibers in motor unit contract Recruitment as more and more motor neurons controlling the muscle are activated tension developed by a muscle increases Some muscles ie ones that control posture are always partially contracted Prolonged contraction can result in muscle fatigue due to depletion of ATP and dissipation of ion gradients required for normal electrical signaling Action potentials can add together resulting in multiple twitches that add together creating greater tension When rate is so high that muscle fiber cannot relax between stimuli twitches fuse into one smooth sustained contraction called tetanus Skeletal Muscle Fibers Several distinct types of skeletal muscle fibers each for a particular set of functions Classified by source of ATP and by contraction speed Oxidative and Glycolytic Fibers Oxidative fibers rely mostly on aerobic respiration Specialized allowing them to make use of a steady energy supply Many mitochondria rich blood supply large amount of myoglobin O2 storing protein Myoglobin binds O2 more tightly than hemoglobin allowing oxidative fibers to extract O2 from the blood efficiently Glycolytic fibers Larger diameter Less myoglobin Use glycolysis as primary ATP source Fatigue more readily than oxidative fibers In poultry and fish dark meat is made up of oxidative fibers and light meat is made up of glycolytic fibers Fast Twitch and Slow Twitch Fibers Fast twitch fibers Develop tension 2 3x faster than slow twitch fibers Enable brief rapid powerful contractions Slow fibers have less sarcoplasmic reticulum and pumps Ca2 more slowly Since Ca2 remains in the cytosol longer muscle twitch in a slow fiber