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Movement • Locomotion Muscle – Movement of animal from one location to another • Repositioning Chapter 17 – Movement of animal appendages • Internal movement – Movement of gases, fluids and ingested solids through the animal Muscle Tissue • Specially designed to physically shorten (contract) – Generates mechanical force • Functions – locomotion and external movements – internal movement (circulation, digestion) – heat generation Skeletal Muscle Organization • Muscle fibers (cells) – elongate cells, parallel arrangement – sarcolemma – cell membrane – sarcoplamic reticulum (SR) Muscle Types • Skeletal Muscle – large muscle fibers (cells) – striated (banded) – voluntary control • Cardiac Muscle – striated – fibers linked by intercalated disks (electrical synapses) – self-excitation • Smooth Muscle – small tapered fibers – lack striations Skeletal Muscle Organization • myofibrils - intracellular contractile elements – Arranged in sarcomeres (repeated tandem units) – Consist of myofilaments • thick filaments (myosin) • thin filaments (actin) • membraneous internal network • Stores Ca2+ • linked to sarcolemma by transverse tubules 1 Thick Filament Structure • Bundles of several hundred myosin molecules Thin Filament Structure • Actin – Primary structural protein – Spherical protein subunits connected in long, double strand – Contains myosin binding site – intertwining tails + globular heads • heads contain – actin binding sites – ATP-hydrolyzing sites • Tropomyosin – Threadlike proteins – Normally cover myosin binding sites • Troponin – Ca2+ Binding Protein – Holds tropomyosin in place – Ca2+ binding induces shape change that repositions tropomyosin • form crossbridges with actin Skeletal Muscle Contraction: Sliding Filament Mechanism • Movement of thin filaments over thick – thick filaments are stationary; thin are dragged across thick – sarcomere shortening by increasing overlap of thick and thin filaments Crossbridge Cycling • Myosin head binds to actin • Cross bridge bends (Power Stroke) – thin filaments pulled toward center of sarcomere • Cross bridge link broken • Cross bridge ‘unbends’ and binds to next actin molecule Neural Activation of Skeletal Muscle Contraction • Excitation-Contraction Coupling – Events that link muscle excitation to muscle contraction • Excitation = Action Potential – Brief, rapid depolarization of cell membrane. • Triggers release of Ca2+ into the cytosol of the muscle fiber Neural Input • • • • • Action potential travels down axon to terminal Exocytosis of acetylcholine (ACh) ACh diffuses across cleft ACh opens ACh-gated Na+ channels Action potential propagates down sarcolemma 2 Excitation-Contraction Coupling • T-tubules conduct APs into the cell • Triggers Ca2+ channels in SR to open • Ca2+ released into the cytosol Smooth Muscle ExcitationContraction Coupling • Depolarization of sarcolemma opens Ca2+ channels – Ca2+ enters the cell from the extracellular fluid • Ca2+ binds with calmodulin in cytoplasm • Ca2+-calmodulin binds to myosin light chain kinase – activates MLCK • MLCK phosphorylates myosin – needed for myosin to bind actin • Cross-bridge cycling Isometric Contractions Smooth Muscle Contraction • No striations – contractile proteins not arranged in sarcomeres – arranged in fish-net network – allows for extensive contraction, even when stretched Whole Muscle Mechanics Types of Contractions • Isometric Contraction – muscle is prevented from shortening – contraction generates force on attachment points • Isotonic Contraction – muscle allowed to shorten upon contraction – muscle moves and object of a given mass Length-Tension Relationship Twitch – response to a single, rapid stimulus Summation – Application of stimuli in rapid succession – Muscle responds to second before fully relaxed from first – Increased tension generated • Maximum force generated is associated with starting length of muscle • Related to the overlaps of thick and thin filaments • Maximum tension generated at normal in vivo resting length – Too long - pull thick and thin filaments apart – Too short - thin filaments form opposite sides collide Tetanus – With repeated stimulation at high frequency twitches fuse to form steady tension 3 Isotonic Contraction Isotonic Contraction • Contraction Force (N) • Muscle free to shorten with stimulation – ↓ shortening distance with↑ load – ↓ shortening velocity with↑ load Comparative Muscle Physiology: Vertebrate Skeletal Fiber Types • Fast (Twitch) Fibers – – – – Low myoglobin content (white) used for rapid movements large nerve fibers w/ high velocities anaerobic activity • Slow (Tonic) Fibers – – – – High myoglobin content (red) used for low-force prolonged contractions small nerve fibers w/ low velocities aerobic activity Comparative Muscle Physiology: Molluscan Catch Muscle • • • • Used to seal shells of bivalves Sustained contraction w/o fatigue Little ↑ in O2 consumption Two groups of neurons innervate muscle 1. Exciters - releases ACh • • 2. causes release of Ca2+ binds myosin w/o release Relaxers - releases seratonin • • causes release of cAMP causes Ca2+ release from myosin – Anything that changes the state of motion for an object – Force α Cross-Sectional Area • Work (J) – Expresses forces applied to an object to set it in motion – Force × Distance – With ↑ load, ↑ mass, ↓ distance – Max work obtained at ~40% max. load – Work α Muscle Mass Comparative Muscle Physiology: Fiber Types in Tuna • Tonic Fibers – located in lateral core areas – used for “cruising” • Twitch Fibers – much of the remaining muscle mass – used for short bursts of high activity Comparative Muscle Physiology: Crustacean Chelipod Muscle • Pinnate (angular) arrangement of fibers – can have more, shorter fibers – 2x increase in force • Few or only single motor unit(s) per muscle • Multiple innervation to each muscle fibers – slow neuron - stimulates tonic activity – fast neuron - stimulates twitch activity – inhibitory neuron - restricts activity 4 Comparative Muscle Physiology: Insect Flight Muscle Comparative Muscle Physiology: Asynchronous Muscle Function • Synchonous flight muscle – moths, grasshoppers, dragonflies – slow wingbeat frequencies – each contraction is a response to a single nerve impulse • Asynchronous flight muscle – – – – flies, bees mosquitoes high wingbeat frequencies (100-1000 bps) too fast for response to individual nerve signals multiple contractions per nerve signal Comparative Muscle Physiology: Asynchronous Muscle Function • upstroke – vertical muscles contract – thorax distorted (click) – stretch horizontal muscles • induced them to contract • downstroke – horizontal muscles contract – distort thorax – stretch vertical muscles • Fibrillar muscles – attached to walls of thorax • horizontal arrangement • vertical arrangement – not connected to wings – distort shape of thorax Comparative Muscle Physiology: Electric Eels • Electric organs – modified skeletal muscle • Electrocytes – stacked in columns – Respond to signals from motor neurons – All electrocytes in column depolarize spontaneously – Up to 600 V discharge • several wingbeats per nerve impulse 5