Anatomy I Chp 10 PDF

Chapter 10: Skeletal Muscle Tissue Neuromuscular Junction What are the characteristics of muscle and what is the structure of skeletal muscle fibers?  Muscle Tissue Muscle tissue is one of the four primary...

Chapter 10: Skeletal Muscle Tissue Neuromuscular Junction What are the characteristics of muscle and what is the structure of skeletal muscle fibers?  Muscle Tissue Muscle tissue is one of the four primary tissue types – converts the chemical energy of ATP into mechanical energy muscle contraction Important functions of skeletal muscle include: – movement contractions pull on tendons to move the bones of the skeleton – maintain posture constant tension maintains body position – support soft tissues protect and support visceral organs – guard openings to the digestive/urinary systems – thermoregulation heat generation from muscle contraction – nutrient storage  Structure of Skeletal Muscle Skeletal muscles are comprised of layers muscle fibers an connective tissue: – epimysium is an exterior collagen layer covering muscle separates muscles from other tissue – perimysium is a dividing layer of connective tissue surrounding bundles of cells called a fascicle allows for blood vessels and nerves to penetrate muscle tissue – endomysium is a thin areolar tissue layer around each muscle fiber contains capillaries, terminal axons, and myosatellite (stem) cells Endomysium, perimysium, and epimysium blend into connective tissue attachments to bone – form tendons/aponeuroses that merges into periosteum as perforating fibers tendons attach at points; aponeuroses attach broad areas Stress will break a bone before pulling the tendon/aponeurosis loose Connective Tissues of a Muscle  Muscle Fibers Skeletal muscle are long, striated muscles attached to bones containing multiple nuclei that develop via fusion of myoblasts – sarcolemma cell membrane of muscle fiber that holds sarcoplasm – stores glycogen and myoglobin – myofibrils subdivision of muscle fibers responsible for contraction – exhibits alternating light and dark striations due to overlapping arrangement of myofilaments (actin and myosin) – sarcoplasmic reticulum surrounds myofibril and forms terminal cisternae (to store and concentrate Ca2+) – transverse tubules (T tubules) narrow channels that transmit action potentials allowing whole fiber to contract simultaneously – triad is 1 T tubule and 2 terminal cisternae – sarcomere Skeletal Muscle Fibers  Skeletal Muscle Fibers Sarcolemma muscle fiber Myofibril Thin filament Thick filament Transverse tubules (T tubules) Sarcolemma Terminal cisternae sarcoplasmic reticulum Myofibril Triad  Sarcomeres Sarcomeres are the basic functional, contractile units of muscle – comprised of myofilaments (-fibrils) that form striations with muscle fibers 2 types of myofilaments: – myosin (thick) arranged in a bundle with heads directed outward in a spiral – interact with actin to form cross-bridges that pivot to produce motion – actin (thin) intertwined strands of (G) actin with an active site – Ca2+ binds to actin receptors causing a shape change in troponin-tropomyosin complex that exposes active sites » active sites bind to myosin Muscle contraction is caused by the interactions of myosin and actin filaments  Myosin and Actin Filaments Myofibril Thin filament Thick filament Z line  Sarcomeres and Striations Actin and myosin are abundant and highly organized in sarcomere of muscle tissue – M line and Z line M line is center of A band (midline of sarcomere) Z line is center of I band (ends of sarcomere) – A band dark region consisting of thick filaments – I band light region consisting of thin filaments – H band (zone) area around M line that has only thick filaments  Striations Arrangement of filaments in zone of overlap A band I band Myofibril Z line M line  Skeletal Muscle Contraction Sliding filament theory: – thin filaments of sarcomere slide towards M line in between thick filaments width of A band stays same; H and I bands get smaller In muscle contractions, sarcomeres are pulled towards center; that is, Z lines shorten the I band to produce tension -  What is the nerve-muscle relationship and what are the components of the neuromuscular junction, and its control of skeletal muscles?  Neuromuscular Junctions (Synapse) Neuromuscular junction is the location of neural stimulation; functional connection between nerve fiber and muscle cell – synaptic knob (axon terminal) swollen end of nerve fiber (contains acetylcholine - ACh) – motor end plate and junctional folds (sarcolemma) increases surface area for ACh receptors contains acetylcholinesterase (AChE) that breaks down ACh and causes relaxation – synaptic cleft gap between nerve and muscle cell Skeletal muscle must be stimulated by a nerve or it will not contract Neuromuscular Junction  Neuromuscular Junction: Neural Stimulation 1. Neural action potential reaches synaptic knob 2. Synaptic knob releases acetylcholine (ACh) into the cleft 3. Acetylcholine (ACh): - binds to Ach receptors on junctional folds of sarcolemma to propagate action potential 4. Action potential causes Na+ (in extracellular fluid) to travel to T tubule – Ach is removed by acetylcholinesterase (AChE) ends action potential at neuromuscular junction  Muscular Activation Muscle function is a repeating cycle of contraction and relaxation Activation encompasses: – excitation neural stimulation leads to action potentials in muscle fiber – excitation-contraction coupling action potentials on the sarcolemma activate myofilaments – contraction shortening of muscle fiber – relaxation return to resting length Muscle contraction is active; muscle relaxation is passive  Excitation Excitation is the process leading to an action potential in the muscle fiber Steps in excitation: – nerve stimulus arrives at synaptic knob causes Ca2+ to allow release of Ach – Ach diffuses across cleft and binds to receptors on sarcolemma receptors change shape and allow Na+ and K+ to cross plasma membrane – Na+/K+ movements alter resting membrane potential (-90mV) and create an action potential muscle fiber is now excited Excitation  Excitation-Contraction Coupling Excitation-contraction coupling refers to the activation of the myofilaments Steps of excitation-contraction coupling: – action potential spreads to T tubules terminal cisternae of SR release stored Ca2+ into sarcoplasm – Ca2+ binds to troponin-tropomyosin molecules of actin causing a shape change – active sites on actin are exposed actin-myosin cross bridges can now form Excitation-Contraction Coupling  Contraction Contraction refers to the development of tension in the muscle fiber Steps in contraction: – myosin head, using ATP, activates and “cocks” into extended position myosin binds to actin and a cross bridge is formed – myosin head flexes, pulling actin filament towards H zone referred to as power stroke – myosin binds to new ATP and process repeats Myosin heads contract sequentially so as to not allow actin to slide back to the resting position Contraction ← →  Relaxation Relaxation is the process of a muscle “passively” returning to resting length Steps in relaxation: – nerve stimulation stops ACh is no longer released and is broken down by AChE – Ca2+ is actively transported back into terminal cisternae Ca2+ also dissociates from troponin-tropomyosin causing a shape change – active sites on actin are blocked myosin can no longer bind to actin Relaxation  Rigor Mortis Rigor mortis is a stiffening of the body beginning 2 to 7 hours after death – deteriorating sarcoplasmic reticulum releases Ca2+ activates actin-myosin cross bridges so muscle contracts but cannot relax – relaxation requires ATP and ATP production is no longer produced after death Fibers remain contracted until myofilaments decay Rigor mortis is a temporary condition lasting about 24-96 hrs. Review of Muscle Contraction Do muscle fibers work together and what are the types of muscle contractions ?  Motor Units Motor unit is a motor neuron and all the muscle fibers it innervates; multiple motor units are dispersed within muscle 2 types of motor units: – small motor units fine control units; may contain as few as 6-10 muscle fibers per nerve fiber – eye and hand muscles – large motor units strength control units; may contain 200 or more muscle fibers per nerve fiber – gastrocnemius muscle (1000 fibers per nerve fiber) Motor units alternate to prevent fatigue (asynchronus motor unit summation) and sustain long term contraction  Motor Units Spinal cord Cell bodies of motor neurons Axons of motor neurons Motor nerve Intermingled muscle fibers from different motor units Motor unit 1 Motor unit 2 Motor unit 3 Isotonic and Isometric Contractions Muscle contraction does not always change the muscle length but tension will always develop Types of tension development: – isotonic muscle contraction develops tension while changing muscle length tension while shortening is concentric (isotonic) contraction – muscle tension > resistance tension while lengthening is eccentric (isotonic) contraction – muscle tension < resistance – isometric muscle contraction develops tension without changing length important in postural muscle function and antagonistic muscle joint stabilization Isometric and Isotonic Contractions  How do muscle fibers obtain energy to power contractions?  ATP, CP and Pyruvic Acid Sustained muscle contraction uses a lot of ATP (adenosine triphosphate) – muscles store enough energy via CP (creatine phosphate) to start contraction must manufacture more ATP as needed – mitochondria are responsible for the production of ATP Glucose is metabolized into ATP Different mechanisms synthesize ATP depending on exercise duration  ATP Generation Active muscle cells produce ATP in 2 ways: – glycolysis breaks down glucose from glycogen stored in skeletal muscles when stored ATP is exhausted – produces 2 ATP and 2 pyruvate molecules per glucose molecule; a lactic acid byproduct is also produced » lactic acid is the “feel the burn” of exercise – aerobic metabolism mitochondria utilize pyruvate, O2, and ADP to enzymatically synthesize ATP – produces net 34 ATP per glucose molecule (17 per 1 pyruvate molecule created by glycolysis) » primary source of ATP for resting muscles Active muscles utilize glycolysis then aerobic metabolism and pyruvate; resting muscles utilize aerobic metabolism and burn fatty acids see pgs 317-319, 954-955 Citric Acid Cycle and Electron Transport System (ETS) Citric acid cycle functions to remove and deliver H+ from organic molecules to the ETS (via coenzymes NAD/FAD) – 2-carbon molecule (acetate) is attached to coenzyme A, forming acetyl-CoA acetyl group is removed and attached to a 4-carbon molecule forming citric acid – 1 ATP is produced for each processed acetyl group ETS functions to transfer (e-) from H+ (oxidation) creating a concentration gradient which results in the production of ATP – cytochromes pass (e-) to generate energy that pumps H+ out of mitochondrial interior diffusion of H+ powers attachment of high-energy phosphate to ADP (phosphorylation) using ATP synthase – (e-) transferred to oxygen, eventually forming water  Overview of ATP Production Glucose Glycolysis Pyruvate MITOCHONDRION Acetyl-CoA ADP Coenzyme A 4-carbon molecule ADP + phosphate Electron Citric acid transport (6-carbon molecule) system Aerobic metabolism Citric acid NAD cycle 2e- FAD  Muscle Fatigue and Oxygen Debt Fatigue causes muscles to become progressively weaker; causes include: – ATP synthesis declines as glycogen is consumed – sarcolemma and SR are damaged Ca2+ storage and release is decreased – lactic acid inhibits enzyme function, lowering pH causing decreased Ca2+ troponin binding Cori cycle (liver) converts circulating lactic acid into pyruvic acid – pyruvic acid is converted back to glucose to be stored After exercise, the body needs more O2 than usual to normalize metabolic activities resulting in oxygen debt: – replenishes O2 reserves – replenishes ATP – returns glycogen and CP to resting levels  How do the types of muscle fibers relate to muscle performance?  Muscle Fiber Types Fast-twitch fibers have enzymes for glycogen-lactic acid systems (anaerobic fermentation) – large diameter; quicker/more forceful contractions; not resistant to fatigue extraocular eye muscles, hand muscles, and biceps brachii Slow-twitch fibers have more mitochondria, myoglobin and capillaries (aerobic respiration) – small diameter; slow contractions and are resistant to fatigue gastrocnemius and postural muscles of the back Intermediate fibers are similar to fast-twitch fibers but more have capillaries; more fatigue resistant All muscles contain all fiber types; proportions are genetically determined Slow vs. Fast Fibers  Muscle Performance Muscle performance is a measure of strength and conditioning – strength depends on muscle size and number of motor units utilized – conditioning depends on: anaerobic training (weight lifting) – stimulates cell enlargement due to synthesis of more myofilaments – intense but brief periods of maximum exertion improve glycogen breakdown and glycolysis aerobic training (endurance exercise) – produces an increase in mitochondria, glycogen and density of capillaries – improves cardiovascular performance Muscle tone ensures optimal resting length, producing greatest force when muscle contracts; increasing muscle tone increases metabolic energy used, even at rest Muscle Hypertrophy vs. Muscle Atrophy Hypertrophy: – muscle growth from heavy training increases diameter of muscle fibers and number of myofibrils increases mitochondria and glycogen reserves Atrophy: – muscle shrinkage from lack of exercise reduction in diameter of muscle and number of myofibrils nominal decrease in mitochondria Muscles become flaccid when inactive for days or weeks; what you don’t use, you lose What are the structural and functional characteristics of cardiac and smooth muscle cells?  Cardiac and Smooth Muscle Cells Cardiac muscle cells are involuntary, branched with a single large nucleus and notched ends – poorly developed SR, no cisternae but wide T tubules use aerobic respiration – intercalated discs link heart cells mechanically, chemically and electrically – demonstrate automaticity Smooth muscle cells are involuntary, spindle- shaped cells with a single central nucleus – no striations, sarcomeres or Z discs SR is scanty and there are no T tubules – Ca2+ for contraction comes from extracellular fluid – disorderly arrangement of actin and myosin filaments – nerve supply is autonomic (if present)  Cardiac and Smooth Muscle Cells Cardiac Smooth Comparison of Muscle Types  SUMMARY (1 of 2) Types of muscle tissue: – skeletal, cardiac, and smooth Anatomy and functions of skeletal muscle: – sarcomeres, myofilaments, and movement Nervous control of skeletal muscle fibers: – neuromuscular junctions and action potentials Tension production in muscle fibers – length-tension relationship – motor units and iosmetric vs. isotonic contractions  SUMMARY (2 of 2) Skeletal muscle activity and energy: – glycolysis and aerobic metabolism Skeletal muscle fatigue and recovery Types of skeletal muscle fibers and performance: Structures and functions of: – cardiac muscle tissue – smooth muscle tissue