2Functions Essential function of muscle is contraction or shortening. Movement – muscle contractions lead to movement of skeletonPosture – partial contractionsStabilizing JointsHeat Production – catabolism produces heat, leading to homeostasis of body temperature
4Muscle Cells (Fibers) Skeletal and smooth muscle cells are elongated These are called fibers.New terms for cells of muscle fibers:Sarcolemma – cell membraneSarcoplasm – cytoplasmSarcoplasmic reticulum – same as ER but contains high levels of calcium
5Muscle FibersEach skeletal muscle fiber (cell) is enclosed in a connective tissue layer called endomysium.Several sheathed muscle fibers are then wrapped by a fibrous membrane called a perimysium in bundles called fascicles.Groups of fascicles are bound together and wrapped in connective tissue called epimysium.These muscles are then attached to bones by tendons or aponeuroses.
6The Man Whose Arms Exploded Connective tissues gave way because of extreme sizeNothing to keep muscle fibers bundled
7Make-up of Fibers Myofibrils – fine fibers packed close together Alternating light (I) and dark (A) bands along the length of myofibril give striated appearance.Tiny contractile units of muscle fiber called sarcomeres.Myofilaments – structural units within sarcomere2 types of myofilamentsActin – thin proteinMyosin – thick protein
9Parts of the Sarcomere Z-Disc: separates each sarcomere H-zone - space between each actin moleculeA-band – region of myosin (thick)I-band – only actin (thin)M-line – holds thick filaments togetherCross-bridges – “arms” on myosin that attach to binding sites on actin
10Skeletal Muscle Activity To contract, skeletal muscle cells are stimulated by nerve impulses sent by brain.One neuron + muscle cells it stimulates = motor unit.Impulse travels along nerve on long extensions of neuron are called axons.When they reach muscle they branch into axon terminals which join with sarcolemma.
11Skeletal Muscle Activity These junctions are called neuromuscular junctions.The nerve endings and muscle never touch – this gap is called the synaptic cleft.When nerve impulse reaches axon terminal, a neurotransmitter is released from neuron.This neurotransmitter is called acetylcholine, or ACh.
12Skeletal Muscle Activity ACh diffuses across synaptic cleft and attaches to receptors on muscle cell.Sacrolemma becomes temporarily more permeable to Na+ ions than to K+ ions (Na+ in / K+ out).This gives cell interior an excess of positive ions and changes electrical conditions of cell.Creates electrical current called an action potential (AP).Electrical impulse travels across muscle cell, leading to contraction of the cell.Cell returned to resting state by sodium-potassium pumpAcetylcholinesterase (AChE), an enzyme, breaks down Acetylcholine to prevent repeated stimulation (one impulse = one contraction)
14Neuromuscular Junction Action potential travels down Transverse Tubules (T Tubules)AP reaches SR, Ca2+ ions are released into sarcoplasmIn a relaxed cell, a protein, tropomyosin, covers the binding site of myosin.Ca2+ binds to troponin, causing tropomyosin to shift, allowing myosin to bind to actinCa2+ ions trigger binding of myosin to actin to initiate filament sliding.
15Mechanism of Muscle Contraction When activated, myosin heads attach to binding sites on thin filaments and sliding begins.Each cross-bridge attaches and detaches creating tension that pulls the thin filaments towards the center of sarcomereThis leads to cell shortening, or contractions.A) Relaxed sarcomereB) Fully contractedH Zone disappearedZ discs closerI bands nearly gone
19Contractions“All or None” Muscle Law – a muscle cell will contract to its fullest extent; never partiallyHowever, entire muscle has graded responseFrequency of nerve impulsesNumber of muscle cells stimulated at onceRigor mortis – Muscle fibers run out of ATP and SR cannot pump Ca2+ ions out of sarcoplasm.Cross bridges cannot detach and skeletal muscles become locked in contractionLasts until lysosomes can break down muscle fibers (15-25 hours later)
20Types of ContractionsMuscle twitches – single, brief, jerky contractions caused by single impulseComplete tetanus – smooth, sustained contractionIsotonic contraction – muscle shortens, tension remains same; leads to movementIsometric contraction – muscle remains the same, and tension increases; no movement
21Fast and Slow MusclesSpeed of contraction is related to muscle’s specific functionRed (Type I)muscle is slow twitch, can generate ATP quickly and contract for long periods of timeRed muscles have many mitochondria for ATPEndurance
22Fast and Slow MusclesWhite (Type IIb) muscles are fast twitch, can not make ATP quickly, and fatigue relatively rapidlyWhite muscles have few mitochondriaStrength
23Naming Muscles Action performed Direction of fibers Location Number of divisionsShapePoint of attachementPronator teresTransverse abdominisExternal obliqueBicepDeltoidSternocleidomastoid