Muscular System - Learning Objectives PDF

Muscular System  Skeletal Muscle - Learning Objectives 1. Describe the five characteristics of muscle tissue. 2. Explain the five general functions of skeletal muscle. 3. Identify and describe the three connective tissue layers associated with a skeletal muscle. 4. Describe the struc...

Muscular System  Skeletal Muscle - Learning Objectives 1. Describe the five characteristics of muscle tissue. 2. Explain the five general functions of skeletal muscle. 3. Identify and describe the three connective tissue layers associated with a skeletal muscle. 4. Describe the structure and function of a tendon and an aponeurosis. 5. Explain the function of blood vessels and nerves serving a muscle. 6. Explain how a skeletal muscle fiber becomes multinucleated. 7. Describe the sarcolemma, T-tubules, and sarcoplasmic reticulum, and triad of a skeletal muscle fiber. 8. Distinguish between thick and thin filaments. 9. Explain the organization of myofibrils, myofilaments, and sarcomeres. 10. Define a motor unit and describe its distribution in a muscle and why it varies in size. 11. Describe the three components of a neuromuscular junction. 12. Describe a skeletal muscle fiber at rest.  Characteristics of Muscle Tissue 1. Contractility: exhibited when filaments slide past each other ▪Enables muscle to cause movement 2. Excitability: ability to respond to a stimulus by changing electrical membrane potential 3. Conductivity: involves sending an electrical change down the length of the cell membrane 4. Extensibility: ability to be stretched 5. Elasticity: ability to return to original length following a lengthening or shortening Functions of Skeletal Muscle Tissue 1. Body movement 2. Maintenance of posture 3. Temperature regulation 4. Storage and movement of materials 5. Support soft tissue Muscle fiber = muscle cell Skeletal muscle’s fibers striated and usually attached to bones. Muscle fibers are bundled within a fascicle ✓ A whole muscle contains many fascicles ✓ A fascicle consists of many muscle fibers  Layers of CT around muscle components: 1. Epimysium tendon Dense irregular connective tissue skeletal wrapping whole muscle muscle 2. Perimysium perimysium Dense irregular connective tissue epimysium wrapping fascicle perimysium Houses many blood vessels and fascicle nerves 3. Endomysium endomysium Areolar connective tissue wrapping muscle fiber individual fiber endomysium Delicate layer for electrical insulation, capillary support, binding Figure 10.1 of neighboring cells Muscle Attachments: Origin - fixed end of the skeletal muscle (bone, cartilage) A. Attachments of muscle to bone/skin/another muscle) origin 1. Tendon: cordlike structure of dense regular connective tissue 2. Aponeurosis: thin, flattened sheet of dense irregular tissue B. Deep fascia Dense irregular CT superficial to epimysium Separates individual muscles; binds muscles with insertion similar functions C. Superficial fascia Insertion - attachment of the Areolar and adipose CT superficial to deep fascia movable end of the Separates muscles from skin skeletal muscle to another structure Microscopic Anatomy of Skeletal Muscle 1. Sarcoplasm (cytoplasm) ▪ Has typical organelles plus contractile proteins and other specializations 2. Multiple nuclei (individual cells are multinucleated) ▪ Cell is formed in embryo when multiple myoblasts fuse ▪ Some nearby myoblasts become undifferentiated satellite cells for support and repair of muscle fibers  Internal (microscopic) Structure of Skeletal Muscle Fiber Figure 10.3 a 3. Sarcolemma (plasma membrane) ▪ Has T-tubules (transverse tubules) that extend deep into the cell ▪ Sarcolemma and its T-tubules have voltage-gated ion channels that allow for conduction of electrical signals ▪ T-tubules have voltage-sensitive calcium channels responsive to the electrical signals (action potentials) 4. Sarcoplasmic Reticulum ▪ Internal membrane complex similar to smooth ER ▪ Terminal cisternae: blind sacs of sarcoplasmic reticulum Serve as reservoirs for calcium ions Two cisternae with T-tubule in between = triad ▪ Contains calcium pumps that import calcium ▪ Contains calcium release channels Triggered by electrical signal traveling down T-tubule resulting in Figure 10.3d calcium released into sarcoplasm  5. Myofibrils Hundreds to thousands per each cell Bundles of myofilaments (contractile proteins within muscle) enclosed in sarcoplasmic reticulum a. Thick filaments - bundles of many myosin protein molecules b. Thin filaments - consist of bundles of many myosin protein molecules 1) Actin with myosin binding sites where myosin heads attach 2) Tropomyosin 3) Troponin Organization of a Sarcomere (contractile unit) Myofilaments arranged in repeating units called sarcomeres Composed of overlapping thick and thin filaments Delineated at both ends by Z discs ▪ Specialized proteins perpendicular to myofilaments ▪ Anchors for thin filaments The positions of thin and thick filaments give rise to alternating I- bands and A-bands Banding Pattern of the Sarcomere 1. H zone: central portion of A band 3. I band → Light-appearing regions containing only thin Disappears with maximal muscle contraction filaments Only thick filaments present Bisected by Z disc Get smaller when muscle contracts 2. M line: middle of H zone Protein meshwork structure 4. A band → Dark-appearing region containing thick and thin filaments Attachment site for thick filaments Contains H zone and M line Makes up central region of sarcomere Innervation of Skeletal Muscle Fibers Motor unit: a motor neuron and all the muscle fibers it controls 1. Axons of motor neurons from spinal cord (or brain) innervate numerous muscle fibers 2. The number of fibers a neuron innervates varies Small motor units have less than five muscle fibers ✓ Allow for precise control of force output Large motor units have thousands of muscle fibers ✓ Allow for production of large amount of force (but not precise control) 3. Fibers of a motor unit are dispersed throughout the muscle (not just in one Figure 10.6a clustered compartment) Neuromuscular Junction Location where motor neuron innervates muscle Usually mid-region of muscle fiber Parts: 1. synaptic knob, 2. synaptic cleft, 3. motor end plate Figure 10.7a 1. Synaptic knob of motor neuron Expanded tip of the motor neuron axon Houses synaptic vesicles Small sacs filled with neurotransmitter acetylcholine (ACh) Has Ca2+ pumps in plasma membrane Establish calcium gradient, with more outside the neuron Has voltage-gated Ca2+ channels in membrane Ca2+ flows into cell (down concentration gradient) if channels open  2. Synaptic cleft ▪ Narrow fluid-filled space ▪ Separates synaptic knob from motor end plate ▪ Acetylcholinesterase resides here Enzyme that breaks down ACh molecules 3. Motor end plate ▪ Specialized region of sarcolemma with numerous folds ▪ Has many ACh receptors Plasma membrane protein channels Opened by binding of ACh + + Allow Na entry and K exit Skeletal Muscle Fibers at Rest Muscle fibers exhibit resting membrane potential (RMP) Fluid inside cell is negative compared to fluid outside cell RMP of muscle cell is about −90 mV RMP established by leak channels and Na+/K+ pumps (voltage-gated channels are closed) Figure 10.8 Physiology of Skeletal Muscle Contraction Learning Objectives 14. Explain the events that lead to release of the neurotransmitter ACh from a motor neuron. 15. Describe the steps in excitation-contraction coupling. 16. Summarize the changes that occur within a sarcomere during contraction. 17. Discuss what happens to each of the following to allow for skeletal muscle relaxation: ACh, action potential, Ca2+ concentration in sarcoplasm, and troponin-tropomyosin complex. 18. List and describe the structures associated with energy production within skeletal muscle fibers. 19. Describe how ATP is made available within skeletal muscle through myosin kinase, creatine kinase, glycolysis, and aerobic cellular respiration. 19. Explain how the means of supplying ATP is related to intensity and duration of exercise. 20. Define oxygen debt and explain why it occurs. 22. Explain the two primary criteria used to classify skeletal muscle fiber types. 23. Compare and contrast the three muscle fiber types. Overview of Events in Skeletal Muscle Contraction Figure 10.9 1. Neuromuscular Junction: Skeletal Muscle Fiber Excitation a. Calcium entry at synaptic knob Action potential travels down axon conducting segment, opens voltage-gated Ca2+ channels in the synaptic knob Ca2+ diffuses into synaptic knob Ca2+ binds to proteins on surface of synaptic vesicles b. Release of ACh from synaptic knob Vesicles merge with cell membrane at synaptic knob: exocytosis Thousands of ACh molecules released from about 300 vesicles Binding of ACh at motor end plate ACh diffuses across cleft, binds to receptors, excites skeletal muscle fiber Figure 10.10 Excitation-Contraction Coupling Stimulation of the fiber is coupled with the sliding of filaments Coupling includes: A. end-plate potential (EPP), B. muscle action potential, C. release of Ca2+ from the sarcoplasmic reticulum Figure 10.11 Excitation-Contraction Coupling (Sarcolemma, T-tubules, and Sarcoplasmic Reticulum) A. End-plate potential (EPP) ACh receptors are chemically gated channels that open when ACh binds to them Na+ diffuses into the cell through the channels (while a little K+ diffuses out) Cell membrane briefly becomes less negative at the end plate region EPP is local but it leads to opening of voltage-gated ion channels in the adjacent region of the sarcolemma Initiation and propagation of action potential (sarcolemma) Action potential on the sarcolemma (AP) = rapid rise (depolarization) and fall (repolarization) in the charge of the membrane EPP threshold reached by causing nearby voltage-gated Na+ channels to open Na+ diffuses into the cell through voltage-gated channels Cell depolarizes: becomes less negative, eventually becomes +30 mV This results in the opening of adjacent voltage-gated Na+ channels and more Na+ entry A chain reaction occurs as depolarization is propagated down the membrane and T- tubules Just after Na+ channels open, they close and voltage-gated K+ channels open K+ diffuses out of the cell Cell repolarizes: returns to −90mV Repolarization is then propagated down the membrane and T-tubules While the cell is depolarizing and repolarizing it is in a refractory period—unable to respond to another stimulation Events of an Action Potential at the Sarcolemma Figure 10.12 Action Potential Propagation (T-tubules & SR) AP travels down T-tubules ✓ Triggers voltage-sensitive calcium channels in T-tubule membrane to release calcium from the terminal cisternae of the sarcoplasmic reticulum Release of Ca2+ from the sarcoplasmic reticulum ✓ Ca2+ interacts with myofilaments triggering contraction (attaches to troponin) ✓ When Ca2+ binds to troponin, it triggers crossbridge cycling (troponin and tropomyosin move so actin is exposed Crossbridge Cycling - Steps Repeating steps: 1) Crossbridge formation Myosin head attaches to exposed binding site on actin 2) Power stroke Myosin head pulls thin filament toward center of sarcomere ADP and Pi released 3) Release of myosin head ATP binds to myosin head causing its release from actin 4) Reset of myosin head ATP split into ADP and Pi Provides energy to lift the myosin head readying it for the next actin binding site  Sarcomere Shortening Cycling continues as long as Ca2+ and ATP are present Results in sarcomere shortening as Z discs move closer together Narrowing of H zone and I band Thick and thin filaments remain the same length but slide past each other Skeletal Muscle Relaxation Events in muscle relaxation 1. Termination of nerve signal and no ACh release from motor neuron 2. Hydrolysis of ACh by acetylcholinesterase 3. Closure of ACh receptor causes cessation of end plate potential 4. No further action potential generation 5. Closure of calcium channels in sarcoplasmic reticulum 6. Return of Ca2+ to sarcoplasmic reticulum by pumps 7. Return of troponin to original shape 8. Return of tropomyosin blockade of actin’s myosin binding sites 9. Return of muscle to original position due to its elasticity Supplying Energy for Skeletal Muscle Metabolism 1. Abundant mitochondria in sarcoplasm for aerobic ATP production (muscle cells have little ATP in storage which is spent after about 5 seconds of intense exertion) 2. Myoglobin within cells allows storage of oxygen used for aerobic ATP production 3. Glycogen (polysaccharide) is stored for times when fuel is needed quickly Ways to generate additional ATP in skeletal muscle fiber: 1. Creatine phosphate 2. Glycolysis 3. Aerobic cellular respiration Supplying Energy for Skeletal Muscle Metabolism 1. Creatine phosphate (10-15 seconds of additional energy) Molecule containing a high-energy bond between creatine and phosphate Phosphate can be transferred to ADP to form ATP Catalyzed by creatine kinase 2. Glycolysis (occurs in cytosol): Does not require oxygen to produce ATP Glucose (from muscle’s glycogen or through blood) is converted to two pyruvate molecules 2 ATP released per glucose molecule 3. Aerobic cellular respiration (cccurs in mitochondria) Requires oxygen Pyruvate oxidized to carbon dioxide Energy used to generate ATP by oxidative phosphorylation Produces a net of 30 ATP Metabolic Processes for Generating ATP Figure 10.17 Lactate formation: Under conditions of low oxygen availability, pyruvate is converted to lactate (lactic acid) by lactate dehydrogenase. 1. Lactate can be used as fuel by skeletal muscle fiber or 2. Lactate can enter blood and be taken up by cardiac muscle or liver Lactic acid cycle - cycling of lactate to liver where it’s converted to glucose, and transport of glucose back to muscle Utilization of Energy Sources Figure 10.18 OXYGEN DEBT: Amount of additional oxygen needed after exercise to restore pre-exercise conditions in muscle. Additional oxygen required to: ✓Replace oxygen on hemoglobin and myoglobin ✓Replenish glycogen ✓Replenish ATP and creatine phosphate ✓Convert lactic acid back to glucose