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MUSCULAR TISSUE Muscle Tissue: Function: Movement -integration between bones, joints, and muscles Body position -sitting, standing, posture Regulation of organ volume -sphincters regulate flow -ring-like bands of muscle -smooth muscle in stomach, bladder Movement of substances in the body -heart pumps blood –cardiac -vasoconstriction and vasodilation -release of bile and enzymes Producing heat -contraction of muscle requires energy -inefficiency of ATP production releases HEAT -heat produced helps maintain body temperature Properties of Muscle Tissue Electrical Excitability -respond to stimuli by producing action potentials -action potentials result from *auto-rhythmic signals from the tissue itself *chemical stimuli –neurotransmitters, hormones, pH Contractility -forcefully contracts when stimulated by an action potential *isometric contraction –tension no shortening *isotonic contraction –muscle shortens and movement occurs *tension on the muscle remains constant during shortening Extensibility -stretches without damage -smooth muscle stretches the most -full stomach!!!! -cardiac muscle stretches when heart full of blood -stretch on skeletal muscle relatively constant Elasticity -returns to original length and shape after contraction 3 Types: Skeletal -involved in movement and attached to skeleton -striated -voluntary -enervated by somatic nerves -some involuntary control (i.e. posture) Cardiac -heart -striated -involuntary action -auto-rhythmic -pacemaker controls rhythm *influenced by hormones and neurotransmitters -regulated by the ANS and hormones Smooth -walls of viscera (hollow organs) -blood vessels and airways -non-striated -usually involuntary -some are auto-rhythmic -regulated by ANS and hormones Organization of Skeletal Muscles Skeletal Muscle: Separate organ 100s-1000s of elongated cells called fibers CT surrounds fibers and whole muscles Nerves and blood vessels supply it CT components -fascia *general term for sheet or wrapping of CT *sheets support and surround organs *3 types –2 which relate to muscles 1.-superficial fascia *separates skin from the layers underneath *lets skin and underlying structures move independently *adipose stores triglycerides -insulates -reduction from heat loss -protection from trauma 2.-deep fascia *made of many layers that run in different directions – like plywood *very tough and resistant to force *lines the body wall and limbs *holds muscles with similar functions together *components are “interwoven” -deep fascia around the muscle blends into the tendon which mingles with the periosteum MICROSCOPIC ANATOMY OF SKELETAL MUSCLE FIBERS Basics: muscle fibers arise from cells called myoblasts myoblasts fuse creating individual muscle fibers once fusion occurs muscle cells do not undergo cell division each muscle fiber has 100 + nuclei muscle growth results from enlargement of existing fibers satellite cells are myoblasts that persist -can fuse with damaged muscle fibers -regenerate functional muscle fibers Mature muscle cells lie parallel Range from 10 to 100 micrometers in diameter Most are 100 mm long Some are as much as 30 cm long Terminology: Sarcomere -functional unit of myofibril -separated by Z discs -compartments Sarcolemma -muscle fibers plasma membrane Transverse (T) tubules -thousands of invaginations of the sarcolemma -extend from the sarcolemma to the interior of the muscle fiber -open to the outside of the fiber -filled with extra-cellular fluid -muscle action potentials propagate along sarcolemma through the T tubules -all parts of the muscle fiber can be affected by the action potential at the same time Sarcoplasm -cytoplasm of a muscle fiber Sarcoplasmic reticulum -similar to SER -stores calcium in a relaxed fiber -release of Ca+ from terminal cisterns triggers muscle contraction Myoglobin -specialized oxygen binding protein -found only in muscle cells -“likes” oxygen better than hemoglobin Mitochondria -arranged in rows -close to proteins that need ATP for contraction Myofibrils -contractile elements of skeletal muscle -2 microns in diameter -extend the entire length of the fiber -prominent striations Filaments and the Sarcomere: Smaller structures within the myofibrils Filaments -2 types -thin filaments –8 nm diameter (mostly actin) -thick filaments- 16 nm (myosin) Arranged in sarcomere compartments The functional unit of the myofibril Thick and thin filaments overlap one another Characteristic zones are characterized in electron microscopy: Z disc M line Z disc I band H Zone A band I band Sarcomere Muscle Proteins: 3 types: 1. Contractile proteins- generate force during contraction Myosin -motor protein in all 3 types of muscle -pushes or pulls –chemical energy (ATP) becomes mechanical energy -molecule shaped like 2 golf clubs twisted together -myosin tails (handles) point to M line -tails lie parallel to each other- form filament of the shaft -heads project outward in a spiral -form crossbridges Actin -thin filaments -anchoring points within the Z discs -actin filament is a twisted helix -each actin molecule has a myosin binding site -myosin head attaches to binding site -tropomyosin covers the myosin-binding site in relaxed muscle -troponin holds tropmyosin in place 2. Regulatory proteins Tropomyosin Toponin CONTRACTION AND RELAXATION OF SKELETAL MUSCLE FIBERS Sliding filament mechanism sarcoplasmic reticulum releases Ca2+ Ca2+ binds to troponin Troponin-tropomyosin complexes move away from myosin binding sites on actin When sites are free, contraction cycle begins 1. Myosin head energized by hydrolysis of ATP -ADP and phosphate remain attached 2. Energized myosin attaches to myosin-binding site on actin, releasing the phosphate group. 3. Power stroke results from the release of the phosphate group Pocket containing ADP opens ADP released from the pocket on the myosin head Myosin head rotates toward the center of the sarcomere Thin filament slides toward the M line 4. Myosin head remains attached to actin at the end of the power stroke until it binds another ATP ATP binds the pocket Myosin head detaches from actin 5. Contraction cycle repeats as long as ATP and calcium ions are available 6. Each power stroke pulls the filaments toward the M line Excitation/Contraction Coupling 1. An increase in calcium concentration starts contractions. 2. A decrease in calcium concentration stops it. 3. Calcium concentration of the cytosol of a relaxed muscle is low 4. Calcium is stored in the SR 5. Action Potential travels through the T tubules to the SR causing the release of calcium through the Ca2+ channels. 6. Calcium floods the region around the thick and thin filaments, increases 10X or more. 7. Calcium binds with the troponin causing the formation of the troponin-tropomyosin complex which allows the cycle to begin. 8. SR has Ca2+ active transport pumps to transport calcium back into the SR. Rigor Mortis: Calcium leaks out of the SR through “leaky” membranes. Initiates the excitation, contraction coupling. No ATP to detach heads from the actin. Muscles contract and stay that way until proteolytic enzymes break crossbridges. MUSCLE METABOLISM Requirements for ATP: Contraction cycle Pumping Ca2+ into SR (= relaxation) Basic cell metabolism Sources of ATP Available ATP lasts only seconds Three basic sources of ATP production 1. creatine phosphate -unique to muscle fibers -first source of ATP -synthesized from ATP when muscle is resting -creatine kinase (CK) transfers a phosphate from ATP to creatine resulting in creatine phosphate and ADP. -creatine phosphate is 3-6 times as plentiful as ATP in sarcoplasm -when ATP requirements go up, CK transfers the phosphate from creatine phosphate back to the ATP. -provides enough ATP for 15 sec. strenuous exercise -supplementation of creatine to increase atheletic performance is debatable. -While it may increase muscle mass over time, it may also turn off the body’s own synthesis of it. When available ATP is gone, more will be generated from glucose metabolism. Glucose passes easily from the blood to the muscle Glycogen is stored in muscle and can be broken down into glucose Glucose can be processed 2 ways 2. anaerobic cellular respiration glucose is broken down to pyruvate during glycolysis (cytosol) if no oxygen is available pyruvate will go to lactic acid a net 2 ATP are generated for one glucose 30-40 seconds of maximum muscle activity 3. aerobic cellular respiration oxygen is required occurs in the mitochondria slower than glycolysis results in CO2 , H2O, and about 36 ATP provides enough ATP for prolonged muscle activity as long as nutrients and oxygen are available -oxygen comes from the blood -oxygen is stored in myoglobin (in the muscle) Muscle Fatigue inability of a muscle to contract forcefully after prolonged contraction results from changes within muscle fibers central fatigue refers to tiredness that precedes muscle fatigue skeletal muscle does not fatigue uniformly contributing factors to muscle fatigue 1. inadequate release of Ca2+ from the SR 2. decline in Ca2+ in the sarcoplasm 3. depletion of creatine phosphate 4. ATP levels are NOT necessarily lower 5. insufficient oxygen 6. depletion of glycogen and other nutrients 7. build up of ADP and lactic acid 8. failure of motor neuron action potentials to release enough ACh Oxygen Consumption after Exercise 1. oxygen delivery to the muscles increases during strenuous exercise increased blood flow increased breathing effort 2. Increased breathing continues after muscle contraction has ceased 3. Oxygen consumption will remain above resting levels 4. Time frame varies 5. oxygen debt refers to the oxygen consumption taken in above resting after exercise -convert lactic acid back to glycogen stores in the liver -resynthesize creatine phosphate and ATP -replace oxygen removed from myoglobin 6. Recovery oxygen uptake is a more appropriate term. - most of the lactic acid is not resynthesized into glycogen (comes from dietary CHO) - most of the lactic acid gets converted into pyruvate to fuel aerobic respiration - increased body temperature that results from exercise also increase reaction rates and thus raises the oxygen requirement. - Heart and muscles still are working above the resting rate - Tissue repair rate increases TWITCH CONTRACTION Brief contraction of all the muscle fibers in a motor unit in response to a single action potential Myogram =Record of a muscle contraction Parts of a contraction -latent period –between stimulus and the beginning of the contraction -contraction period – 10-100 msec -relaxation period –Ca2+ actively transported back into SR FREQUENCY OF SUMMATION When the second of 2 stimuli is applied after the refractory period, the muscle will respond to both stimuli. wave summation -stimuli arrive at different times causing larger contractions fused tetanus -stimulation at a rate higher than 80-100 stimuli per second -sustained contraction -addition of Ca2+ to sarcoplasm that still has Ca2+ results in a build up of Ca2+ . -peak tension is 5-10X greater than a single twitch unfused tetanus -stimulation at a rate of 20-30 /sec -muscle can only partially relax between stimuli -sustained but wavering contraction -smaller build up of Ca2+ Motor Unit Recruitment: motor units have thresholds lower thresholds are recruited (stimulated) first stronger (higher threshold) motor units added later the more effort the task requires-the more motor units are recruited thus some are active while some are relaxing precise tasks have motor units with few fibers TYPES OF SKELETAL MUSCLE FIBERS Muscle fibers are characterized by the following characteristics: 1. myoglobin content (red vs white) 2. velocity of contraction and relaxation (slow vs fast) 3. metabolic reactions to generate ATP Slow Oxidative (SO) Fibers smallest in diameter least powerful red –large amounts of myoglobin many blood capillaries large mitochondria aerobic cell respiration used to generate ATP called oxidative fibers slow –ATPase in the myosin head hydrolyzes ATP slowly compared to “fast” fibers low contraction velocity twitch contractions last 100-00 msec longer to reach peak tension resistant to fatigue capable of prolonged, sustained contractions for many hours posture and endurance activities Fast Oxidative –Glycolytic (FOG) Fibers intermediate in diameter large amounts of myoglobin –red many capillaries aerobic cellular respiration generates ATP moderately high resistance to fatigue intracellular glycogen level is high can use anaerobic glycolysis to generate ATP “fast” because ATPase in myosin heads hydrolyzes ATP 3-5X faster than “slow” fibers velocity is faster reach peak tension more quickly briefer in duration (less than 100 msec) walking, sprinting Fast Glycolytic (FG) Fibers largest in diameter highest number of fibrils can generate the most powerful contractions low myoglobin -white few capillaries large amounts of glycogen generate ATP mostly from glycolysis hydrolyze ATP rapidly contract strongly and rapidly used for intense anaerobic movement of short duration –weight lifting or throwing a ball fatigue quickly Strength training programs increase the size, strength, and glycogen content of FG fibers Muscle enlargement is due to hypertrophy of FG fibers and synthesis of muscle protein Skeletal muscle is a mixture of all 3 types SO fibers make up about ½ Postural muscles have more SO fibers Shoulders and arms –lift things have FG fibers Legs SO and FOG fibers A motor unit has only one type of fiber Types of muscles are recruited in an order of need -SO are weak and first -FOB is next -FG is last when maximal force is needed CARDIAC MUSCLE: Physical Characteristics: Similar arrangement of actin and myosin to skeletal muscle Have the same identifiable bands (I, H, A, etc.) Intercalated discs: -form connections between ends of cells -thickening of the sarcolemma -discs contain 1. desmosomes -hold fibers together 2. gap junctions -conduct muscle action potentials cardiac muscle contractions -10-15 times longer than skeletal muscle -results from prolonged Ca2+ delivery -Ca2+ enters sarcoplasm from both: 1. SR 2. extracellular fluid (ECF) Contracts in response to auto-rhythmic fiber stimulation Resting cardiac muscle -contracts and relaxes 75 times per minute (about) -continuous rhythmic activity -requires constant supply of oxygen -mitochondria are larger and more numerous -depends heavily on aerobic respiration -can use lactic acid made in skeletal muscle as an energy source for aerobic respiration during exercise SMOOTH MUSCLE: 2 Types 1. Visceral (Single Unit) Smooth Muscle Tissue Most common type Walls of arteries and hollow viscera Wrap around sheets Auto-rhythmic Fibers connect by gap junctions Process of fiber stimulation 1. One fiber stimulated by: -neurotransmitter -hormone -auto-rhythmic signal -pH 2. Muscle action potential spreads to neighboring fibers 3. Contract in unison as a single unit 4. Stimulation of one visceral muscle fiber causes contraction of many adjacent fibers. 2. Multiunit Smooth Muscle Tissue Walls of large arteries, airways, arrector pili muscles, muscles of iris and ciliary body (lens adjustment) of the eye Individual fibers Each has own motor neuron terminals Few gap junctions between neighboring fibers Stimulation of one multiunit fiber causes contraction of that fiber only Microscopic Anatomy of Smooth Muscle: Basic Characteristics: Small –30-200 microns long Thick at the middle with tapered ends Single, oval, centrally located nucleus Contain thick and thin filaments in 1:10 –15 ratios Not arranged in an orderly fashion Also contain intermediate filaments Filaments have no orderly overlap –no striations Lack transverse tubules Scanty SR for Ca2+ storage Smooth Muscle Contraction differences: Intermediate fibers attach to dense bodies (functionally similar to Z discs) Dense bodies attached to sarcolemma or dispersed through sarcoplasm Bundles of intermediate fibers stretch from one dense body to another Sliding filament mechanism generates tension that is transmitted to intermediate filaments Intermediate filaments pull on dense bodies attached to sarcolemma Muscle fiber shortens lengthwise Produces a bubble-like expansion of sarcolemma Fiber twists in a helix as it contracts, relaxes by rotating in the opposite direction Physiology of Smooth Muscle: Important differences between smooth and skeletal: Contraction starts more slowly and lasts longer Smooth can both shorten and stretch to a greater extent Process of Contraction: Increase in Ca2+ in cytosol initiates contraction Ca2+ flows into cytosol from SR (smaller amount) and extracellular fluid Since there are no transverse tubules –takes longer Ca2+ initiate contractile process at the filaments Regulation of Contraction and Relaxation: Several mechanisms involved One involves calmodulin -calmodulin binds Ca (like troponin in striated muscle) -calmodulin activates myosin light chain kinase -uses ATP to phosphorylate the myosin head -once phosphate is attached to the head it can bind actin -contraction occurs - myosin light chain kinase acts slowly –accounts for the slowness of contraction. Smooth Muscle Tone: state of continued partial contraction results from slowness of the contraction process important for maintaining steady pressure on intestinal contents and blood Stress-relaxation response: smooth muscle can stretch and still contract when stretched smooth muscle fibers initially contract within a minute the tension will decrease allows smooth muscle to have great changes in length and still contract thus even though the muscles are stretched the pressure on the contents within changes very little when the organ empties the muscle will rebound with firmness Regeneration of Muscle Tissue: Skeletal muscle: satellite cells fuse with muscle fibers to assist in growth and repair additional cells will migrate from red bone marrow to muscle to assist in repair due to muscle injury or disease limited power of regeneration fibrosis (replacement of muscle tissue with fibrous scar tissue) will occur with significant muscle damage Cardiac Muscle cardiac fibers are not repaired or replaced does not have cells comparable to satellite cells healing occurs by fibrosis athletes have enlarged hearts due to hypertrophy of the fibers due to an increased workload Smooth Muscle has greater powers of regeneration uterine muscle fibers retain their capacity for cell division (some others do as well) pericytes –stem cells associated with capillaries can form new smooth muscle fibers can also proliferate in atherosclerosis and result in a pathological condition Clinical correlations: 1. Myasthenia gravis: autoimmune disease neuromuscular junction antibodies block ACh receptors muscles become increasingly weak fatigue more easily may cease to function treat with anticholinesterase drugs 2. Muscular Dystrophy group of genetic disorders causing progressive degeneration of muscular tissue Duchene DMD most common Sex linked Gene for dystrophin is mutated Sarcolemma tears easily during contraction Damaged muscle fibers rupture and die 3. Abnormal contractions of Skeletal Muscle Spasm -sudden involuntary contraction of a single muscle in a large group of muscles Cramp -painful spasmodic contraction Tic -spasmodic twitching made involuntarily by muscles that are ordinarily under voluntary control -twitching eyelid Tremor -rhythmic, involuntary, purposeless contraction -produces quivering or shaking movement Fasciculation -involuntary brief twitch of an entire motor unit that is visible under the skin -MS or Lou Gehrig’s disease fibrillation -spontaneous contraction of a single muscle fiber not visible under the skin -can be recorded by electron myography -may signal destruction of motor neurons