Muscle Chapter 17

Movement Locomotion –Movement of animal from one location to another Repositioning –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

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 Muscle fibers (cells) –elongate cells, parallel arrangement –sarcolemma – cell membrane –sarcoplamic reticulum (SR) membraneous internal network Stores Ca 2+ linked to sarcolemma by transverse tubules

Skeletal Muscle Organization myofibrils - intracellular contractile elements –Arranged in sarcomeres (repeated tandem units) –Consist of myofilaments thick filaments (myosin) thin filaments (actin)

Thick Filament Structure Bundles of several hundred myosin molecules –intertwining tails + globular heads heads contain –actin binding sites –ATP-hydrolyzing sites form crossbridges with actin

Thin Filament Structure Actin –Primary structural protein –Spherical protein subunits connected in long, double strand –Contains myosin binding site Tropomyosin –Threadlike proteins –Normally cover myosin binding sites Troponin –Ca 2+ Binding Protein –Holds tropomyosin in place –Ca 2+ binding induces shape change that repositions tropomyosin

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 Ca 2+ 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

Excitation-Contraction Coupling T-tubules conduct APs into the cell Triggers Ca 2+ channels in SR to open Ca 2+ released into the cytosol

Smooth Muscle Contraction No striations –contractile proteins not arranged in sarcomeres –arranged in fish-net network –allows for extensive contraction, even when stretched

Smooth Muscle Excitation- Contraction Coupling Depolarization of sarcolemma opens Ca 2+ channels –Ca 2+ enters the cell from the extracellular fluid Ca 2+ binds with calmodulin in cytoplasm Ca 2+ -calmodulin binds to myosin light chain kinase –activates MLCK MLCK phosphorylates myosin –needed for myosin to bind actin Cross-bridge cycling

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

Isometric Contractions 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 Tetanus –With repeated stimulation at high frequency twitches fuse to form steady tension

Length-Tension Relationship 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

Isotonic Contraction Muscle free to shorten with stimulation –  shortening distance with  load –  shortening velocity with  load

Isotonic Contraction Contraction Force (N) –Anything that changes the state of motion for an object –Force  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  Muscle Mass

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: 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: Molluscan Catch Muscle Used to seal shells of bivalves Sustained contraction w/o fatigue Little  in O 2 consumption Two groups of neurons innervate muscle 1.Exciters - releases ACh causes release of Ca 2+ binds myosin w/o release 2.Relaxers - releases seratonin causes release of cAMP causes Ca 2+ release from myosin

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

Comparative Muscle Physiology: Insect Flight Muscle 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 ( bps) –too fast for response to individual nerve signals –multiple contractions per nerve signal

Comparative Muscle Physiology: Asynchronous Muscle Function Fibrillar muscles –attached to walls of thorax horizontal arrangement vertical arrangement –not connected to wings –distort shape of thorax

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 several wingbeats per nerve impulse

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