Chapter 9 Muscular System Three Types of Muscle Tissues Skeletal Muscle usually attached to bones under conscious control striated Cardiac Muscle wall of heart not under conscious control striated Intercalated discs Smooth Muscle walls of most viscera, blood vessels, skin not under conscious control not striated

Functions of Skeletal Muscle Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature Store nutrient reserves

Structure of a Skeletal Muscle organ of the muscular system - skeletal muscle tissue - nervous tissue - blood - connective tissues fascia-tissue that surrounds the entire muscle tendons-dense connective tissue cord that connect muscles to bones aponeuroses-sheet-like tendon that connects muscles to bones

Connective Tissue Coverings epimysium-membrane that surrounds the whole muscle perimysium-membrane that surrounds fascicles fascicles-groups of muscle fibers(cells) endomysium-membrane that that surrounds individual muscle cells muscle fibers-muscle cells myofibrils-protein fibers inside muscle cells thick and thin filaments

Skeletal Muscle Fibers sarcolemma-cell membrane of a muscle fiber sacroplasm-cytoplasm of a muscle fiber sarcoplasmic reticulum-stores calcium ions transverse tubule-carries muscle impulse from sarcolemma to searcplasmic reticulum triad pair of cisternae of sarcoplasmic reticulum transverse tubule

Skeletal Muscle Fibers Myofibril-proteins fibers actin filaments-thin filaments myosin filaments-thick filaments sarcomere-functional unit of skeletal muscle

Sarcomere I bands-thin only A bands-thick and thin H band-thick only Z lines-edge of sarcomer M line-mid-line of sarcomere zone of overlap-where thick and thin filaments overlap

Myofilaments Thin Filaments Thick Filaments composed of actin associated with troponin and tropomyosin Thick Filaments composed of myosin cross-bridges

Neuromuscular Junction also known as myoneural junction site where an axon and muscle fiber meet motor neuron-neuron that controls a muscle fiber motor end plate-area of muscle fiber that forms synapse (junction) with motor neuron

Neuromuscular Junction Synapse-area where the neuron communicates with the muscle fiber synaptic cleft-space between neuron and sarcolemma synaptic vesicles-store and release neurotransmitters in neuron neurotransmitters-chemical messengers that carry the signal across the synapse

Motor Unit single motor neuron all muscle fibers controlled by motor neuron

Stimulus for Contraction acetylcholine (ACh) nerve impulse causes release of ACh from synaptic vesicles ACh binds to ACh receptors on motor end plate generates a muscle impulse muscle impulse eventually reaches sarcoplasmic reticulum and the cisternae

Excitation Contraction Coupling muscle impulses cause sarcoplasmic reticulum to release calcium ions into cytosol calcium binds to troponin to change its shape position of tropomyosin is altered binding sites on actin are exposed actin and myosin molecules bind

Sliding Filament Model of Muscle Contraction When sarcromeres shorten, thick and thin filaments slide past one another H zones and I bands narrow Z lines move closer together

Cross-bridge Cycling myosin cross-bridge attaches to actin binding site myosin cross-bridge pulls thin filament ADP and phosphate released from myosin new ATP binds to myosin linkage between actin and myosin cross-bridge break ATP splits myosin cross-bridge goes back to original position

Relaxation acetylcholinesterase – rapidly decomposes Ach remaining in the synapse muscle impulse stops stimulus to sarcolemma and muscle fiber membrane ceases calcium moves back into sarcoplasmic reticulum myosin and actin binding prevented muscle fiber relaxes Rigor mortis-muscle contract and stay contracted as calcium leaks out and there is no ATP to break the cross bridges

Major Events of Muscle Contraction and Relaxation

Energy Sources for Contraction 1) Creatine phosphate 2) Cellular respiration creatine phosphate – stores energy that quickly converts ADP to ATP

Oxygen Supply and Cellular Respiration Oxygen is needed to completely breakdown glucose myoglobin Anaerobic Phase glycolysis occurs in cytoplasm produces little ATP Aerobic Phase citric acid cycle electron transport chain occurs in the mitochondria produces most ATP myoglobin stores extra oxygen

Oxygen Debt Oxygen debt – amount of oxygen needed by liver cells to use the accumulated lactic acid to produce glucose plus the amount muscle cells require to resynthesize ATP and creatine phosphate and restore their original conditions oxygen not available glycolysis continues pyruvic acid converted to lactic acid liver converts lactic acid to glucose Anaerobic threshold or lactic acid threshold

Muscle Fatigue inability to contract after persistent, prolonged use commonly caused from decreased blood flow ion imbalances across the sarcolemma accumulation of lactic acid (most common cause) Psychological loss of desire to continue the exercise cramp – sustained, painful, involuntary muscle contraction

Heat Production muscle cells are major source of body heat by-product of cellular respiration muscle cells are major source of body heat Helps maintain body temperature blood transports heat throughout body

Muscular Responses Threshold Stimulus minimal strength required to cause contraction Recording a Muscle Contraction myogram twitch latent period period of contraction period of relaxation refractory period all-or-none response

Length-Tension Relationship

Summation process by which individual twitches combine produces sustained contractions can lead to tetanic contractions

Recruitment of Motor Units recruitment - increase in the number of motor units activated whole muscle composed of many motor units more precise movements are produced with fewer muscle fibers within a motor unit as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated

Sustained Contractions smaller motor units (smaller diameter axons) - recruited first larger motor units (larger diameter axons) - recruited later produce smooth movements muscle tone – continuous state of partial contraction

Types of Contractions isotonic – muscle contracts and changes length concentric – shortening contraction isometric – muscle contracts but does not change length eccentric – lengthening contraction

Fast and Slow Twitch Muscle Fibers Slow-twitch fibers (type I) always oxidative resistant to fatigue red fibers most myoglobin good blood supply Many mitochondria aerobic Fast-twitch fatigue-resistant fibers (type IIa) intermediate fibers oxidative intermediate amount of myoglobin pink to red in color resistant to fatigue Fast-twitch glycolytic fibers (type IIb) white fibers (less myoglobin) poorer blood supply susceptible to fatigue Few mitochondria anaerobic

Smooth Muscle Fibers Compared to skeletal muscle fibers shorter single, centrally located nucleus elongated with tapering ends myofilaments randomly organized lack striations lack transverse tubules sarcoplasmic reticula not well developed

Types of Smooth Muscle Visceral Smooth Muscle Multiunit Smooth Muscle single-unit smooth muscle sheets of muscle fibers fibers held together by gap junctions exhibit rhythmicity exhibit peristalsis walls of most hollow organs Multiunit Smooth Muscle less organized function as separate units fibers function separately irises of eye walls of blood vessels

Smooth Muscle Contraction Resembles skeletal muscle contraction interaction between actin and myosin both use calcium and ATP both are triggered by membrane impulses Different from skeletal muscle contraction smooth muscle lacks troponin smooth muscle uses calmodulin two neurotransmitters affect smooth muscle acetlycholine and norepinephrine hormones affect smooth muscle stretching can trigger smooth muscle contraction smooth muscle slower to contract and relax smooth muscle more resistant to fatigue smooth muscle can change length without changing tautness

Cardiac Muscle located only in the heart striated muscle fibers joined together by intercalated discs Single nucleus per cells Cisternae are less developed and store less calcium transverse tubules are larger fibers branch network of fibers contracts as a unit self-exciting and rhythmic longer refractory period than skeletal muscle

Characteristics of Muscle Tissue

Skeletal Muscle Actions origin – immovable end insertion – movable end prime mover (agonist) – primarily responsible for movement synergists – assist prime mover antagonist – resist prime mover’s action and cause movement in the opposite direction

Body Movement Four Basic Components of Lever rigid bar – bones fulcrum – point on which bar moves; joint object - moved against resistance; weight force – supplies energy for movement; muscles

Levers and Movement

Life-Span Changes Changes in muscular system first begin to appear in one’s 40’s myoglobin, ATP, and creatine phosphate decline by age 80, half of muscle mass has atrophied adipose cells and connective tissues replace muscle tissue exercise helps to maintain muscle mass and function

Clinical Application Myasthenia Gravis autoimmune disorder receptors for ACh on muscle cells are attacked weak and easily fatigued muscles result difficulty swallowing and chewing ventilator needed if respiratory muscles are affected treatments include drugs that boost ACh removing thymus gland immunosuppressant drugs antibodies

Clinical Application tenanus - sustained powerful contractions of skeletal muscles throughout the body Caused by Clostridium tetani Lockjaw Tetanus shot-vaccine against the toxin