Human Anatomy and Physiology Chapter 9 Muscular System 9-1

Chapter 9 Muscular System Three Types of Muscle Tissues -All designed to contract Cardiac Muscle wall of heart not under conscious control striated Skeletal Muscle usually attached to bones under conscious control striated Smooth Muscle walls of most viscera, blood vessels, skin not under conscious control not striated 9-2

Structure of a Skeletal Muscle organ of the muscular system skeletal muscle tissue nervous tissue blood connective tissues fascia- tissue surrounds each muscle tendon- attach muscle to bone aponeuroses- attach muscle to muscle 9-3

Structure of a Skeletal Muscle muscle is surrounded by epimysium muscle is divided into fasicles by perimysium Fasicle is divided into muscle fibers (cells) by endomysium 9-4

Skeletal Muscle Fiber sarcolemma surrounds muscle fiber (muscle cell) and contains many nuclei, mitochondria, and myofibrils sarcoplasmic reticulum contains calcium ions muscle fiber is made of many myofibrils are made of actin (thin) and myosin (thick) are divided into sections called sarcomeres 9-5

Myofilaments Thin Filaments Thick Filaments composed of actin (protein) associated with troponin and tropomyosin Thick Filaments composed of myosin (protein) cross-bridges 9-7

Sarcomere Sarcomeres are the ‘active unit’ of the cell I band A band H zone Z line M line 9-6

Neuromuscular Junction site where axon (end of nerve) and muscle fiber communicate motor neuron motor end plate synaptic cleft synaptic vesicles neurotransmitters 9-8

Motor Unit group of muscle fibers attached to a single motor neuron all muscle fibers controlled by motor neuron muscle fibers act as group 9-9

Stimulus for Contraction acetylcholine (ACh) neurotransmitter nerve impulse causes release of acetylcholine from synaptic vesicles binds to acetylcholine receptors on motor end plate generates a muscle impulse muscle impulse eventually reaches sarcoplasmic reticulum 9-10

Excitation Contraction Coupling muscle impulses cause sarcoplasmic reticulum to release calcium ions into cytosol calcium binds to troponin-tropomyosin complex to change its shape position of t-t complex is altered binding sites on actin exposed myosin binds to actin 9-11

Cross-bridge Cycling myosin cross-bridge pulls actin 9-13 myosin cross-bridge pulls actin ADP and phosphate released from myosin new ATP binds to myosin linkage between actin and myosin cross-bridge break ATP splits 12. stimulus stopped, calcium pumped out, t-t complex recovers binding sites; or process keeps cycling (back to step 5 further down the actin filament) myosin cross-bridge goes back to original position

Sliding Filament Theory When sarcomeres shorten, thick and thin filaments slide past one another H zones and I bands get narrower Z lines move closer together 9-12

Relaxation motor neuron impulse stops acetylcholinesterase released – breaks down acetylcholine calcium moves back into sarcoplasmic reticulum t-t complex covers binding site myosin and actin binding prevented 9-14

Energy Sources for Contraction 1) Cellular respiration [ATP Production] 2) Creatine Phosphate creatine phosphate – stores energy that quickly converts ADP to ATP 9-15

Oxygen Supply and Cellular Respiration Aerobic Phase use oxygen citric acid cycle electron transport chain produces most ATP (36) myoglobin stores extra oxygen Anaerobic Phase no oxygen glycolysis produces little ATP (2) 9-16

Anaerobic threshold Anaerobic threshold- point at which there is not enough O2 and cell transitions to anaerobic respiration oxygen not available glycolysis continues pyruvic acid is converted to lactic acid liver converts lactic acid to glucose 9-17

Muscle Fatigue inability to contract commonly caused from lack of oxygen/lungs at capacity or decreased blood flow anaerobic respiration decrease in available ATP accumulation of lactic acid ion imbalances cramp – sustained, involuntary contraction 9-18

Heat Production by-product of cellular respiration muscle cells are major source of body heat blood transports heat throughout body 9-19

Muscular Responses Threshold Stimulus minimal strength required to cause contraction Recording a Muscle Contraction twitch- single contraction latent period period of contraction period of relaxation refractory period all-or-none response 9-20

Summation process by which individual twitches combine produces sustained contractions can lead to tetanic contractions –sustained contraction that lacks relaxation 9-21

Recruitment of Motor Units recruitment - increase in the number of motor units activated (causes stronger contraction) whole muscle composed of many motor units as intensity of stimulation increases, recruitment of motor units continues until all motor units are activated 9-22

Sustained Contractions smaller motor units recruited first larger motor units recruited later produces smooth movements muscle tone – continuous state of partial contraction 9-23

Types of Contractions isotonic – muscle contracts and changes length isometric – muscle contracts but does not change length eccentric – lengthening contraction concentric – shortening contraction 9-24

Fast and Slow Twitch Muscle Fibers Slow-twitch fibers (type I) always oxidative resistant to fatigue red fibers most myoglobin good blood supply posture muscles Fast-twitch fatigue-resistant fibers (type IIb) intermediate fibers oxidative intermediate amount of myoglobin pink to red in color Fast-twitch glycolytic fibers (type II) white fibers (less myoglobin) poorer blood supply susceptible to fatigue 9-25

Smooth Muscle Fibers Compared to skeletal muscle fibers shorter single nucleus elongated with tapering ends no striations myofilaments randomly organized lack transverse tubules sarcoplasmic reticula not well developed 9-26

Types of Smooth Muscle Visceral Smooth Muscle Multiunit Smooth Muscle single-unit smooth muscle sheets of muscle fibers exhibit rhythmicity/ peristalsis walls of most hollow organs Multiunit Smooth Muscle fibers function separately irises of eye walls of blood vessels 9-27

Smooth Muscle Contraction Resembles skeletal muscle contraction interaction between actin and myosin both use calcium and ATP both depend on impulses Different from skeletal muscle contraction smooth muscle lacks troponin 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 9-28

Cardiac Muscle only in the heart muscle fibers joined together by intercalated discs fibers branch network of fibers contracts as a unit self-exciting and rhythmic longer refractory period than skeletal muscle 9-29

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 9-30

Life-Span Changes 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 9-65