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The Big Picture Skeletal muscle is STRIATED and VOLUNTARY Attach to bones at tendons Durable – not harmed by rough surface of bone Smaller than muscle – several can fit at a joint Built for contraction Inside the cell Each cell, many myofibrils The myofibril has alternating light/dark bands A functional unit runs from z-disc to z-disc sarcomere Inside the cell Each myofibril, 2 filaments Actin (thin) Myosin (Thick Also has “heads” I band stays put, A band shrinks during contraction Other cellular features Sarcoplasmic reticulum (SR) Like a sleeve Stores and releases Ca Multi-nucleate Stimulation Nerve runs to muscle cell, carrying signal Neurotransmitter is acetlycholine (Ach) Release of Ach opens protein channels Allows Na+ to cross membrane Creates action potential Triggers Ca release from SR Stimulation Video Contraction At rest, binding sites are covered Calcium opens them Myosin heads attach to binding sites on actin Unique structure Heads are “cocked” Automatically “uncock” after attaching to actin Contraction Myosin releases and “recocks” only with ATP input Then attaches to next site This repeats until full contraction is reached Actin and myosin slide past each other Myosin heads resemble caterpillar walking Contraction Potential Problems Not enough ATP? Cramping? Not enough calcium? No binding sites Cerebral palsy and Parkinson’s (brain), multiple sclerosis (nerves), Muscular dystrophy (muscles weakness), ALS (nerves) How do muscles get bigger? Muscles cells don’t divide…. So…. They get bigger! More protein filaments (myofibrils) increase the size and effectiveness of muscles