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Muscle Tissue Functions of Muscle Tissue Movement Heat production Maintenance of posture Muscle contraction produces 85% of body heat Characteristics of Muscle Tissue Excitability - able to be stimulated Contractibility - able to shorten and thicken Extensibility - stretchable and extendable Elasticity - the ability to return to its original shape Muscle Tissue Types Skeletal - found attached to bone, striated, voluntary Cardiac - forms the walls of heart, striated, involuntary Smooth - found in viscera, non-striated, involuntary Gross Anatomy of Skeletal Muscle Each muscle is an organ, containing muscle, blood vessels, nerves, & connective tissue Connective Tissue Components Fascia - found under the skin, covering organs, and muscles Epimysium – surrounds entire muscle Perimysium – surrounds groups of muscle fibers called “fascicles” Endomysium – surrounds individual muscle fibers Tendons; Aponeurosis – connect muscle to bones Cell Structure Muscle Cell = Muscle Fiber Elongated, multi-nucleate, striated cells containing parallel bundles of myofibrils Sarcolemma – plasma membrane Sarcoplasm – cytoplasm containing: Myoglobin – stores oxygen Glycosomes – store starch Peripheral nuclei Sarcoplasmic reticulum – smooth E.R. – maintains calcium levels Transverse tubules – network of membranes connected to sarcolemma; penetrates deep into each contractile unit. Specialized contractile organelles Myofibrils - thread-like structures 100’s to 1000’s in each muscle fiber (cell) Actin - thin filaments containing actin protein, 2 strands of tropomyosin, & troponin Myosin - thick protein filaments composed of myosin molecules Myofibrils Actin Myosin SARCOMERE – the basic contractile unit Z discs (lines) - separate sarcomeres; anchor thin filaments A band - overlapping area of thick and thin filaments I band - contains only actin H zone - part of A band containing only myosin M line - center of H zone; anchors myosin Sliding Filament Mechanism Skeletal Muscle Contraction Muscle contraction occurs when actin and myosin are allowed to interact with each other and form crossbridges The binding sites on actin are blocked by the troponin/tropomyosin complex Calcium ions in the sarcoplasm will bind to troponin Muscle Contraction This binding will cause the troponin / tropomyosin complex to pull away from the active binding site on actin, thus allowing myosin to bind The myosin head pivots, pulling the thin filaments toward the center of the sarcomere thus shortening the sarcomere Repeated cycles of attachment, pivoting, detach and release occurs Muscle Contraction Successive interaction causes “sliding” of the filament, shortening of the sarcomere, thus shortening of the entire muscle Calcium is removed from the troponin molecule and returned to the S.R. Relaxation occurs What Role Does Calcium Play? What triggers the release of calcium? ACTION POTENTIALS The sudden change in the transmembrane potential Action Potentials Resting Membrane Potential Polarized - positive charge outside, negative charge inside Depolarized - positive charge inside, negative charge outside Repolarized - positive charge reestablished outside, negative charge inside Resting Potential/ Polarized When muscle is relaxed, the sarcolemma is polarized having a charge difference between the inside /outside of the cell When a stimulus is received opening a channel gate , Na+ ions will flow into the cell changing the polarity of the cell Depolarized The net charge of the sarcolemma becomes negative in regards to the inside of the cell which is now positive. The cell is said to be depolarized and the muscle contracted Repolarized Membrane pumps quickly restore the original status or condition The positive charge outside is reestablished once again and resting membrane potential is restored Neuromuscular Junction Each fiber is controlled by a motor neuron at a neuromuscular junction Motor neurons stimulate muscle fibers Acetylcholine (ACh ) is released into the synaptic cleft with the arrival of an action potential ACh diffuses across the cleft, binding to receptors on the motor end plate, initiating a muscle action potential Once initiated, the action potential is unstoppable and self-propagating RELAXATION Resting membrane potential is restored by: Acetylcholinesterase active transport pumps that pump Ca+2 ions back into the sarcoplasmic reticulum Calsequestrin – binds calcium Role of ATP Used to activate the myosin head in order to bind to actin After power stroke, ATP used to break the bond between actin and myosin ATP used to pump calcium back into the SR Production of ATP for muscles Direct phosphorylation Creatine phosphate couples with ADP to form ATP Provides about 15 sec of energy Glycolysis Glucose broken down anaerobically Produces lactic acid as waste product Provides about 30-60 sec of energy Aerobic Respiration Glucose broken down with oxygen Hours of energy Muscle Fatigue Insufficient oxygen Build-up of lactic acid Depletion of glycogen RECOVERY OXYGEN CONSUMPTION OXYGEN DEBT ALL - or -None Principle Muscle fibers will contract fully OR not at all once they are stimulated Threshold stimulus minimal level of stimulation needed to cause the muscle to contract Motor Units Motor units - motor neuron and all the muscle fibers it controls Number of muscle fibers in motor unit will vary The fewer the number of fibers per motor unit, the more precise the contraction The number of motor units being stimulated will determine the strength of contraction of the entire muscle Muscle Contraction Twitch contraction rapid, jerky contraction to a single stimuli phases: latent, contraction, relaxation,refractory Wave summation increase in the strength of muscle contraction due to rapid successive stimulation Tetany continuous, smooth, sustained contraction Muscle Contraction Treppe repeated stimulation following stimulation causing a staircase effect Isotonic tone or tension remains constant-muscle shortens Isometric tension increases - muscle length remains same Muscle Fiber Types Red oxidative fibers more myoglobin more capillaries more mitochondria long, slow contraction sustained energy aerobic respiration non-fatiguable fibers White glycolytic fibers less myoglobin less capillaries fewer mitochondria rapid,powerful contraction quick energy anaerobic respiration fatigue easily Benefits of Exercise Increase the size of size and strength of each fiber Increase muscle tone Increases the blood supply, thus increasing the number of red blood cells Increased respiratory and cardiovascular function Lowers blood pressure Cardiac Muscle Involuntary Intercalated discs Forms syncytium Long refractory period Long contraction rate More mitochondria than skeletal muscle Smooth Muscle Involuntary - neural & hormonal stimulation No sarcomeres - no striations Very, very long contraction rate Calmodulin - regulatory protein No tendons or aponeuroses Muscle / Bone Interaction Origin – attachment of a muscle to a stationary bone Insertion – attachment of a muscle to a movable bone Prime mover – provides major force for specific movement Antagonist – opposes prime mover Synergist – assists the prime mover (secondary muscle) Muscle / Bone Interactions Levers – rigid bar (bones) moving on fixed point Fulcrum = fixed point (joints) Effort = applied force Resistance = load Levers First class Fulcrum in center = seesaw Lifting head off chest Second class Load (resistance) in center = wheelbarrow Least common Standing on tiptoes Third class Effort in center = tweezers Biceps brachii Most common
