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Muscle Lecture #10 Ch 10 Muscle Muse 10/31/12 An Introduction to Muscle Tissue Muscle Tissue A primary tissue type, divided into Skeletal muscle Cardiac muscle Smooth muscle Functions of Skeletal Muscles Produce skeletal movement Maintain body position Support soft tissues Guard openings Maintain body temperature Store nutrient reserves Skeletal Muscle Structures Muscle tissue (muscle cells or fibers) Connective tissues Nerves Blood vessels Skeletal Muscle Structures Organization of Connective Tissues Muscles have three layers of connective tissues Epimysium: – exterior collagen layer – connected to deep fascia – Separates muscle from surrounding tissues Perimysium: (not to be confused with paramecium) – surrounds muscle fiber bundles (fascicles) – contains blood vessel and nerve supply to fascicles Endomysium: – surrounds individual muscle cells (muscle fibers) – contains capillaries and nerve fibers contacting muscle cells – contains myosatellite cells (stem cells) that repair damage Skeletal Muscle Structures Figure 10–1 The Organization of Skeletal Muscles. Epimysium Bone Epimysium Perimysium Endomysium Tendon Muscle fiber in middle of a fascicle (b) Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) (a) Perimysium Fascicle Muscle fiber Figure 9.1 Skeletal Muscle Structures Organization of Connective Tissues Muscle attachments Endomysium, perimysium, and epimysium come together: – at ends of muscles – to form connective tissue attachment to bone matrix – i.e., tendon (bundle) or aponeurosis (sheet) Skeletal Muscle Structures Nerves Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord) Blood Vessels Muscles have extensive vascular systems that Supply large amounts of oxygen Supply nutrients Carry away wastes Skeletal Muscle: Attachments Muscles attach: Directly—epimysium of muscle is fused to the periosteum of bone or perichondrium of cartilage Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis Skeletal Muscle Fibers Are very long Develop through fusion of mesodermal cells (myoblasts) Become very large Contain hundreds of nuclei Skeletal Muscle Fibers Figure 10–2 The Formation of a Multinucleate Skeletal Muscle Fiber. Skeletal Muscle Fibers Figure 10–2a The Formation of a Multinucleate Skeletal Muscle Fiber. Skeletal Muscle Fibers Figure 10–2b The Formation of a Multinucleate Skeletal Muscle Fiber. Skeletal Muscle Fibers Internal Organization of Muscle Fibers The sarcolemma The cell membrane of a muscle fiber (cell) Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended afrom the cut end of the fiber. Skeletal Muscle Fibers Internal Organization of Muscle Fibers Transverse tubules (T tubules) Transmit action potential through cell Allow entire muscle fiber to contract simultaneously Have same properties as sarcolemma Skeletal Muscle Fibers Internal Organization of Muscle Fibers Myofibrils Lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments (myofilaments) Myofilaments are responsible for muscle contraction Types of myofilaments: – thin filaments: » made of the protein actin – thick filaments: » made of the protein myosin Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcoplasmic reticulum (SR) A membranous structure surrounding each myofibril Helps transmit action potential to myofibril Similar in structure to smooth endoplasmic reticulum Forms chambers (terminal cisternae) attached to T tubules Skeletal Muscle Fibers Internal Organization of Muscle Fibers Triad Is formed by one T tubule and two terminal cisternae Cisternae: – concentrate Ca2+ (via ion pumps) – release Ca2+ into sarcomeres to begin muscle contraction Skeletal Muscle Fibers Figure 10–3 The Structure of a Skeletal Muscle Fiber. Show video excitation coupling Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres The contractile units of muscle Structural units of myofibrils Form visible patterns within myofibrils Muscle striations A striped or striated pattern within myofibrils: – alternating dark, thick filaments (A bands) and light, thin filaments (I bands) Sarcomere Smallest contractile unit (functional unit) of a muscle fiber The region of a myofibril between two successive Z discs Composed of thick and thin myofilaments made of contractile proteins Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres M Lines and Z Lines: – M line: » the center of the A band » at midline of sarcomere – Z lines: » the centers of the I bands » at two ends of sarcomere Thin (actin) filament Thick (myosin) filament Z disc I band H zone A band Sarcomere Z disc I band M line (c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Sarcomere Z disc M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament (d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres Zone of overlap: – the densest, darkest area on a light micrograph – where thick and thin filaments overlap The H Band: – the area around the M line – has thick filaments but no thin filaments Skeletal Muscle Fibers Internal Organization of Muscle Fibers Sarcomeres Titin: – are strands of protein- quite elastic, like springs – reach from tips of thick filaments to the Z line – stabilize the filaments Skeletal Muscle Fibers Figure 10–4a Sarcomere Structure. Skeletal Muscle Fibers Figure 10–4b Sarcomere Structure. Skeletal Muscle Fibers Figure 10–5 Sarcomere Structure. Skeletal Muscle Fibers Figure 10–6 Levels of Functional Organization in a Skeletal Muscle. Skeletal Muscle Fibers Figure 10–6 Levels of Functional Organization in a Skeletal Muscle. Skeletal Muscle Fibers Sarcomere Function Transverse tubules encircle the sarcomere near zones of overlap Ca2+ released by SR causes thin and thick filaments to interact Skeletal Muscle Fibers Muscle Contraction Is caused by interactions of thick and thin filaments Structures of protein molecules determine interactions Skeletal Muscle Fibers Four Thin Filament Proteins F-actin (Filamentous actin) Is two twisted rows of globular G-actin The active sites on G-actin strands bind to myosin Nebulin Holds F-actin strands together Tropomyosin Is a double strand Prevents actin–myosin interaction Troponin A globular protein Binds tropomyosin to G-actin Controlled by Ca2+ Skeletal Muscle Fibers Figure 10–7a, b Thick and Thin Filaments. Skeletal Muscle Fibers Initiating Contraction Ca2+ binds to receptor on troponin molecule Troponin–tropomyosin complex changes Exposes active site of F-actin Skeletal Muscle Fibers Thick Filaments Contain twisted myosin subunits Contain titin strands that recoil after stretching The mysosin molecule Tail: – binds to other myosin molecules Head: – made of two globular protein subunits – reaches the nearest thin filament Skeletal Muscle Fibers Figure 10–7c, d Thick and Thin Filaments. Skeletal Muscle Fibers Myosin Action During contraction, myosin heads Interact with actin filaments, forming cross-bridges Pivot, producing motion Skeletal Muscle Fibers Skeletal Muscle Contraction Sliding filament theory Thin filaments of sarcomere slide toward M line, alongside thick filaments The width of A zone stays the same Z lines move closer together Skeletal Muscle Fibers Figure 10–8a Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber. Skeletal Muscle Fibers Figure 10–8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber. Skeletal Muscle Fibers Skeletal Muscle Contraction The process of contraction Neural stimulation of sarcolemma: – causes excitation–contraction coupling Cisternae of SR release Ca2+: – which triggers interaction of thick and thin filaments – consuming ATP and producing tension The Neuromuscular Junction Is the location of neural stimulation Action potential (electrical signal) Travels along nerve axon Ends at synaptic terminal Synaptic terminal: – releases neurotransmitter (acetylcholine or ACh) – into the synaptic cleft (gap between synaptic terminal and motor end plate) The Neuromuscular Junction Figure 10–10a, b Skeletal Muscle Innervation. The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation. The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation. The Neuromuscular Junction The Neurotransmitter Acetylcholine or ACh Travels across the synaptic cleft Binds to membrane receptors on sarcolemma (motor end plate) Causes sodium–ion rush into sarcoplasm Is quickly broken down by enzyme (acetylcholinesterase or AChE) The Neuromuscular Junction Figure 10–10c Skeletal Muscle Innervation. The Neuromuscular Junction Action Potential Generated by increase in sodium ions in sarcolemma Travels along the T tubules Leads to excitation–contraction coupling Excitation–contraction coupling: – action potential reaches a triad: » releasing Ca2+ » triggering contraction – requires myosin heads to be in “cocked” position: » loaded by ATP energy Show video neuromuscular junction The Neuromuscular Junction Figure 10–11 The Exposure of Active Sites. The Contraction Cycle Five Steps of the Contraction Cycle Exposure of active sites Formation of cross-bridges Pivoting of myosin heads Detachment of cross-bridges Reactivation of myosin The Contraction Cycle Figure 10–12 The Contraction Cycle. The Contraction Cycle [INSERT FIG. 10.12, step 1] Figure 10–12 The Contraction Cycle. The Contraction Cycle Figure 10–12 The Contraction Cycle. The Contraction Cycle Figure 10–12 The Contraction Cycle. The Contraction Cycle Figure 10–12 The Contraction Cycle. The Contraction Cycle Figure 10–12 The Contraction Cycle. The Contraction Cycle Fiber Shortening As sarcomeres shorten, muscle pulls together, producing tension Contraction Duration Depends on Duration of neural stimulus Number of free calcium ions in sarcoplasm Availability of ATP The Contraction Cycle Figure 10–13 Shortening during a Contraction. The Contraction Cycle Relaxation Ca2+ concentrations fall Ca2+ detaches from troponin Active sites are re-covered by tropomyosin Sarcomeres remain contracted Rigor Mortis A fixed muscular contraction after death Caused when Ion pumps cease to function; ran out of ATP Calcium builds up in the sarcoplasm The Contraction Cycle Tension Production The all–or–none principle As a whole, a muscle fiber is either contracted or relaxed Tension of a Single Muscle Fiber Depends on The number of pivoting cross-bridges The fiber’s resting length at the time of stimulation The frequency of stimulation Need more force? Recruit more fibers Tension Production Tension of a Single Muscle Fiber Length–tension relationship Number of pivoting cross-bridges depends on: – amount of overlap between thick and thin fibers Optimum overlap produces greatest amount of tension: – too much or too little reduces efficiency Normal resting sarcomere length: – is 75% to 130% of optimal length Tension Production Figure 10–14 The Effect of Sarcomere Length on Active Tension. Tension Production Tension of a Single Muscle Fiber Frequency of stimulation A single neural stimulation produces: – a single contraction or twitch – which lasts about 7–100 msec. Sustained muscular contractions: – require many repeated stimuli Tension Production Three Phases of Twitch Latent period before contraction The action potential moves through sarcolemma Causing Ca2+ release Contraction phase Calcium ions bind Tension builds to peak Relaxation phase Ca2+ levels fall Active sites are covered Tension falls to resting levels Control of Muscle Tension Tension Production Figure 10–15 The Development of Tension in a Twitch. Tension Production Figure 10–15 The Development of Tension in a Twitch. Tension Production Treppe A stair-step increase in twitch tension Repeated stimulations immediately after relaxation phase Stimulus frequency <50/second Causes a series of contractions with increasing tension Tension Production Tension of a Single Muscle Fiber Wave summation Increasing tension or summation of twitches Repeated stimulations before the end of relaxation phase: – stimulus frequency >50/second Causes increasing tension or summation of twitches Tension Production Tension of a Single Muscle Fiber Incomplete tetanus Twitches reach maximum tension If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension Complete Tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction Tension Production Figure 10–16 Effects of Repeated Stimulations. Tension Production Figure 10–16 Effects of Repeated Stimulations. Tension Production Tension Produced by Whole Skeletal Muscles Depends on Internal tension produced by muscle fibers External tension exerted by muscle fibers on elastic extracellular fibers Total number of muscle fibers stimulated Tension Production Tension Produced by Whole Skeletal Muscles Motor units in a skeletal muscle Contain hundreds of muscle fibers That contract at the same time Controlled by a single motor neuron Tension Production Tension Produced by Whole Skeletal Muscles Recruitment (multiple motor unit summation) In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated Maximum tension Achieved when all motor units reach tetanus Can be sustained only a very short time Tension Production Figure 10–17 The Arrangement and Activity of Motor Units in a Skeletal Muscle. Tension Production Figure 10–17 The Arrangement and Activity of Motor Units in a Skeletal Muscle. Tension Production Tension Produced by Whole Skeletal Muscles Sustained tension Less than maximum tension Allows motor units rest in rotation Muscle tone The normal tension and firmness of a muscle at rest Muscle units actively maintain body position, without motion Increasing muscle tone increases metabolic energy used, even at rest Tension Production Two Types of Skeletal Muscle Tension Isotonic contraction Isometric contraction Tension Production Two Types of Skeletal Muscle Tension Isotonic Contraction Skeletal muscle changes length: – resulting in motion If muscle tension > load (resistance): – muscle shortens (concentric contraction) If muscle tension < load (resistance): – muscle lengthens (eccentric contraction) Tension Production Two Types of Skeletal Muscle Tension Isometric contraction Skeletal muscle develops tension, but is prevented from changing length Note: iso- = same, metric = measure Tension Production Figure 10–18a, b Isotonic and Isometric Contractions. Tension Production Figure 10–18c, d Isotonic and Isometric Contractions. Tension Production Resistance and Speed of Contraction Are inversely related The heavier the load (resistance) on a muscle The longer it takes for shortening to begin And the less the muscle will shorten Muscle Relaxation After contraction, a muscle fiber returns to resting length by Elastic forces Opposing muscle contractions Gravity Tension Production Figure 10–19 Load and Speed of Contraction. Tension Production Elastic Forces The pull of elastic elements (tendons and ligaments) Expands the sarcomeres to resting length Opposing Muscle Contractions Reverse the direction of the original motion Are the work of opposing skeletal muscle pairs Tension Production Gravity Can take the place of opposing muscle contraction to return a muscle to its resting state ATP and Muscle Contraction Sustained muscle contraction uses a lot of ATP energy Muscles store enough energy to start contraction Muscle fibers must manufacture more ATP as needed ATP and Muscle Contraction ATP and CP Reserves Adenosine triphosphate (ATP) The active energy molecule Creatine phosphate (CP) The storage molecule for excess ATP energy in resting muscle Energy recharges ADP to ATP Using the enzyme creatine phosphokinase (CPK or CK) When CP is used up, other mechanisms generate ATP ATP and Muscle Contraction Energy Use and Muscle Activity At peak exertion Muscles lack oxygen to support mitochondria Muscles rely on glycolysis for ATP Pyruvic acid builds up, is converted to lactic acid ATP and Muscle Contraction Figure 10–20 Muscle Metabolism. ATP and Muscle Contraction Figure 10–20a Muscle Metabolism. ATP and Muscle Contraction Figure 10–20c Muscle Metabolism. ATP and Muscle Contraction Muscle Fatigue When muscles can no longer perform a required activity, they are fatigued Results of Muscle Fatigue Depletion of metabolic reserves Damage to sarcolemma and sarcoplasmic reticulum Low pH (lactic acid) Muscle exhaustion and pain ATP and Muscle Contraction The Cori Cycle The removal and recycling of lactic acid by the liver Liver converts lactic acid to pyruvic acid Glucose is released to recharge muscle glycogen reserves Oxygen Debt After exercise or other exertion The body needs more oxygen than usual to normalize metabolic activities Resulting in heavy breathing ATP and Muscle Contraction Skeletal muscles at rest metabolize fatty acids and store glycogen During light activity, muscles generate ATP through anaerobic breakdown of carbohydrates, lipids, or amino acids At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a byproduct ATP and Muscle Contraction Muscle Performance Power The maximum amount of tension produced Endurance The amount of time an activity can be sustained Power and endurance depend on The types of muscle fibers Physical conditioning Muscle Fiber Types Three Types of Skeletal Muscle Fibers Fast fibers Slow fibers Intermediate fibers Muscle Fiber Types Three Types of Skeletal Muscle Fibers Fast fibers Contract very quickly Have large diameter, large glycogen reserves, few mitochondria Have strong contractions, fatigue quickly Muscle Fiber Types Three Types of Skeletal Muscle Fibers Slow fibers Are slow to contract, slow to fatigue Have small diameter, more mitochondria Have high oxygen supply Contain myoglobin (red pigment, binds oxygen) Muscle Fiber Types Three Types of Skeletal Muscle Fibers Intermediate fibers Are mid-sized Have low myoglobin Have more capillaries than fast fibers, slower to fatigue Muscle Fiber Types Figure 10–21 Fast versus Slow Fibers. Muscle Fiber Types Muscles and Fiber Types White muscle Mostly fast fibers (aka Fast twitch) Pale (e.g., chicken breast) Red muscle Mostly slow fibers (aka slow twitch) Dark (e.g., chicken legs) Most human muscles Mixed fibers Pink (General proportions determined by genetics) Muscle Fiber Types Muscle Hypertrophy Muscle growth from heavy training Increases diameter of muscle fibers Increases number of myofibrils Increases mitochondria, glycogen reserves Muscle Atrophy Lack of muscle activity Reduces muscle size, tone, and power Muscle Fiber Types What you don’t use, you lose Muscle tone indicates base activity in motor units of skeletal muscles Muscles become flaccid when inactive for days or weeks Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers