Survey
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Download Muscle Tissue - Doctor Jade Main
Document related concepts
no text concepts found
Transcript
Muscle Tissue Muscle Tissue Classification Skeletal Muscle Cardiac Muscle Intercalated Disc Smooth Muscle Skeletal Muscle • directly or indirectly attached to bones of skeleton Functions • movement – simple-breathing to highly coordinated onesswimming • posture & body position – maintenance or stability • constant muscle contraction holds the head up • store & move substances in the body • maintains body temperature – muscle contraction requires energy; when energy is used some energy is converted to heatkeeps body temperature within the normal range – when cold shivering occurs Gross Anatomy • entire muscle is surrounded by epimysium • fuses into connective tissue sheets called fascia • groups of muscle fibers are arranged in bundles called fascicles; wrapped in connective tissue layer-perimysium – contains blood vessels & nerves • endomysium surrounds each individual muscle fiber • connective tissue layers are continuous through length of muscle • at end of muscle, collagen fibers of epi-, peri- and endomysium come together to form tendons & aponeurosis • • • • • • Microscopic Anatomy muscle cell myofibril or fiber is thin & very long Multinucleate-maybe hundreds present – arranged around periphery just beneath cell membrane sarcolemma-plasma membrane surrounds sarcoplasm or cytoplasm – contains long protein bundles called myofibrils, a great deal of glycogen and a red pigment, myoglobin Smooth endoplasmic reticulum-SR or sarcoplasmic reticulum – forms network around each myofibril and periodically expands into terminal cisternae sarcolemma has tubular infoldings called T (transverse) tubules which are associated with two terminal cisternae t tubule plus adjacent terminal cisternae is a Triad – stores & releases calcium needed for contractions – T tubules conduct action potential through the entire muscle fiber Myofibril Composition • made of myofilaments – arranged in repeating patterns – appear as striations under a microscope – two types: actin & myosin • one repeat is a sarcomere – smallest, functional unit of skeletal muscle • narrow plates called Z discs separate the sarcomeres • a sarcomere extends from one Z disc to the next Sarcomere Structure • A band • darker, middle part • myosin & actin • I Bands • lighter areas • actin only • Z disc • passes through middle of each I band • defines one sarcomere • H zone • either side of M line • M line • center of H zone Proteins in Muscle Fibers • Contractile Proteins – actin – myosin • Regulatory Proteins – tropomyosin – troponin • Structural Proteins – – – – – titin alpha actinin myomesin nebulin dystrophin Myosin MYOSIN-THICK FILAMENT contractile protein comprised of 2 subunits twisted around one another forming long coiled tail & pair of heads project toward m line Actin • contractile protein • comprises thin filaments • composed of two intertwined strands of fibrous (F) actincontractile protein • each F-actin is made up of subunits called G-Actin • each G-actin has an active site which can bind a myosin head Regulatory Proteins • Control contraction-turn it on & off • Tropomyosin – winds around actin – covers myosin binding sites preventing actinmyosin interactions • Troponin – calcium binding protein each – bound to each tropomyosin • When calcium binds to troponinchanges shapepulls tropomyosin off actinmyosin binding site exposedcrossbridges form Structural Proteins • Titin – huge elastic molecule – recoils after stretching – anchors myosin to Z-disc • Nebulin – helps anchor thin filaments to Z discs – helps stabilize thick filament • Alpha actinin – comprises z discs • Myomesin – forms M line • Dystrophin – under sarcolemma – attaches actin to membrane proteins Sliding Filament Theory • theory of how muscle contraction takes place • under microscope, during muscle contraction • H zone & I bands get smaller • H zone almost disappears • zones of overlap get larger • Z lines move closer together • width & length of A band remains constant • only make sense if thin filaments slide to center of each sarcomere • actin slides over myosin which causes sarcomere to shorten • ultimately entire muscle cell shortens Sliding Filament Theory Contraction • calcium binds to troponin tropomyosin is pulled toward actin groove • myosin binding site uncovered • myosin heads interact with actin • forming cross bridges • like hinges • myosin head pivots at its base • pulls on actin • causing it to move to center of sacromere • muscle shortens Muscle Cell Contraction • Skeletal muscles only contract when activated by motor neurons from CNS NEURON STRUCTURE • Dendrites – Receive information – Typically many • Axons – Send information – Covered with Myelin Sheath – End in Terminal Buttons Neuromuscular Junction • communication between muscles & nerves occurs at neuromuscular junction • each branch of a motor nerve fiber ends in a synaptic knob • nestled in a depression on sarcolemmamotor end plate (MEP) • exhibits many junctional folds • contains receptors Neuromuscular Juncion Neuromuscular Junction • cells do not touch • separated by a tiny gap-synaptic cleft • synaptic knobs contain vesicles of acetylcholine-ACH • neurotransmitter • the cleft & sarcolemma contain ACHE or acetylcholinesterase – Breaks down ACH Excitation Contraction Coupling • Transfer of an impulse from somatic motor neuron to muscle cell is excitation contraction coupling • 4 steps • ACH release • Activation of ACH receptors • Production of Muscle Action Potential • Termination of ACH activity STEP 1 ACH release • action potential reaches synaptic terminal • opens calcium gates • calcium enters neuron causing synaptic vesicles to fuse with cell membrane which releases ACH via exocytosis into synaptic cleft • ACH diffuses across cleft STEP 2 Activation of ACH Receptors • ACH binds to receptors on motor end plate • opens sodium gates • sodium rushes into sarcoplasm STEP 3 Production of Muscle Action Potential • positive charges of sodium accumulate • membrane potential of cell moves toward zero • as concentration of sodium increases threshold is reached • muscle cell depolarizes • Action potential begins and spreads in all directions • invaginates at T tubules • muscle cell contracts STEP 4 Termination of ACH Activity • influx of calcium continues until acetylcholinesterase degrades ACH removing it from receptors • component parts are recycled • calcium is pumped back into the SR • muscle cell relaxes Muscle Cell Contraction • • • • • arrival of action potential releases ACH into cleft binds to receptors sodium rushes into cell causes an Action Potential in muscle cell Muscle Cell Contraction • action potential is propagated across entire membrane • when reaches t tubuletravels down t tubules • t tubules & terminal cisternae of sarcoplasmic reticulum form a triad • triad releases calcium from sarcoplasmic reticulum Muscle Cell Contraction • calcium binds to troponin • changes its shape • tropomyosin swings away from active site • exposes myosin binding sites on actin • cross-bridges form • initiates contraction • effect of calcium is instantaneous • contraction cycle begins Contraction Cycle Steps • 1. ATP Hydrolysis • 2. Attachment of Myosin to Actin forming CrossBridges • 3. Power Stroke • 4. Detachment of Myosin from Actin Step 1- ATP Hydrolysis • each myosin head must have an ATP bound to it to initiate contraction • head contains myosin ATPase hydrolyzes ATPADP + Pi & energy • ADP & Pi still attached to myosin head Steps 2 & 3-Attachment of Myosin to Actin & Power Stroke • energized myosin binds to exposed active site on actin forming a crossbridge • myosin releases ADP & phosphate • flexes into a bent, low energy position bringing the thin filament with it • the power stroke Step 4-Detachment of Myosin From Actin • at end of power stroke myosin remains attached to actin until nyosin binds another ATP • upon binding more ATP, myosin releases actin and it is ready to begin the process again by hydrolyzing the ATP • each cycle shortens the sarcomere ~10 nm • each myosin head continues to attach, pivot & detach as long as calcium & ATP are available • • • • • • • • • • Relaxation duration of muscle contractions depend on duration of stimulus at neuromuscular junction ACH does not last long-chewed up by ACHE contraction continues only if more action potentials arrive at synaptic terminal in rapid succession muscle fiber & sarcoplasm return to normal or relax in two ways – active transport of calcium across cell membrane into extracellular fluid – active transport of calcium into the sarcoplasmic reticulum • more important way almost as soon as calcium is released-SR begins to absorb calcium from surrounding sarcoplasm here calcium binds to calsequestrin & is stored until stimulated again as calcium in sarcoplasm decreases, calcium detaches from troponin causing it to return to its original position recovering active sites with tropomyosin once contraction has ended sarcomere does not automatically return to its original length Sacromeres actively shorten but there is no active mechanism to reverse the process combination of elastic forces, opposing muscle contractions and gravity return muscle to its uncontracted state Tension Production • • • • • • • muscle cells contract & shorten causing them to pull on collagen fibers generates tension collagen fibers resist building tension – as muscle continues to pull on collagen fibersfibers transmit force and pull on something else what happens depends on what fibers are attached to and how muscle cells are arranged muscles are attached to at least 2 different structures – usually bone & occasionally soft tissue as muscle contracts, one attachment movesinsertion other attachment remains stationary origin developing tension pulls object toward source of tension Tension Production • tension produced by an individual muscle fiber varies • depends on • resting length of fiber at time of stimulation – determines amount of overlap between thin & thick filaments • frequency of stimulation – effects internal calcium concentration • number of muscle fibers stimulated in one muscle Length-Tension Relationship • • • • • • • • • • amount of tension depends on how stretched or contracted it was prior to being stimulated length-tension relationship amount of tension produced by a muscle is related to number of cross bridges formed number of cross bridges that can form depends on degree of overlap between thick & thin filaments only myosin heads in zone of overlap can bind to active sites on actin &produce tension Sarcomeres work most efficiently in an optimal range of lengths Outside optimal rangemuscle cannot produce as much tension optimal range is range where maximum number of cross bridges can formmaking most tension when sarcomeres are short thick filaments are jammed up against Z line – cross bridges form but myosin heads cannot pivotno tension production sarcomeres with length longer than optimal range has reduced zone of overlapless cross bridges can formless tension Frequency of Stimulation • Increasing the number of nerve impulses to the muscles will keep ACH being released • which will keep calcium being released • which will keep cross bridges forming • which will keep the muscle contracting • which will cause the development of more tension Muscle Twitch • • • • • • • • • • one above threshold stimulus to a muscle produces one contraction/relaxation cycletwitch vary in duration with type, location, temperature & environmental conditions eye twitch-7.5msec soleus (calf muscle) twitch100msec too brief to be part of normal activity to show what a twitch looks like a myogram is used twitch can be divided into three parts 1) latent period 2) contraction phase 3) relaxation phase Muscle Twitch • latent phase begins as stimulation of muscle begins-lasts 2msec • as tension rises to a peak contraction phase begins (10-100msec) • during relaxation phase tension decreases to resting levels (10-100msdc) Treppe • twitches produce no work • sending more & more stimulation to muscle in short period of time results in changes to initial twitch • when skeletal muscle is stimulated for a second time immediately after a relaxation phase treppe contraction develops TREPPE • myogram tracing shows a slightly higher tension than the first tension • tension increases over first 30-50 stimulations and thereafter amount of tension remains constant • increase in tension is due to increases in calcium in sarcoplasm • stimuli are arriving so rapidly that calcium is not reabsorbed into the SR • thus there is more Ca in cytosol when the second stimulus arrives • resulting in slightly more tension production & a slightly higher tracing Wave Summation • as frequency of stimuli increase before previous twitch has ended each new twitch rides piggy back on previous one • wave summation • result of one wave of contraction being added to another • produces sustained contraction called incomplete tetanus TETANUS • at a still higher frequencymuscl e has no time to relax between stimuli • twitches fuse into a smooth, prolonged contraction called complete tetanus • Tension Production tension developed depends on number of muscle fibers involved • each muscle fiber is innervated by one motor neuron • when nerve signal approaches end of axon-it spreads to all of axon’s terminal branches & stimulates all muscle fibers supplied by them • makes all muscle fibers connected to neuron contract at same time • one nerve fiber & all muscle fibers innervated by it is one motor unit Motor Units • some motor neurons control few muscle fibers • others control hundreds • number of neurons innervating a muscle indicates how fine movement can be in that muscle • eye muscles need to have precise control – neuron to muscle in eye controls 4-6 fibers • leg muscles do not need precise control – neuron to leg muscle can control 1000-2000 muscle fibers • • • • • • MOTOR UNITS neuron firescontracts all muscle cells in one motor unit greater tension can be be generated by recruiting more motor units smooth & steady increase in muscle tension is produced by increasing number of active motor units – recruitment peak tension occurs when all motor units in a muscle contract to tetanus such powerful contractions do not last long sustained contractions are maintained by asynchronous recruitment – motor units are activated on a rotating basis – some rest & recover while others contract Tension Production & Movement • amount of tension produced in a skeletal muscle depends on several factors • before movement is possible, tension must overcome resistance – passive force opposing movement – amount of resistance depends on object’s weight, shape, friction and other factors • when tension is greater than resistance object moves Contraction Types • contractions types are based on pattern of tension development • Isometric • Isotonic –Concentric –Eccentric Isometric Contractions • • • • tension develops with no length change in the muscle tension never exceeds resistance occurs when you begin to use a muscle occurs when you push against a locked door – cross bridges formtension rises to a peakmuscle cannot overcome resistance • Example-carrying a bag of groceries-arm muscles are contracting to hold the bag, but the arm itself is not moving • • • • • • • • • • Isotonic Contractions when tension in a muscle increases & produces a change in muscle length two types: eccentric & concentric concentric contractions – muscles shorten as it maintains tension eccentric contractions – muscle lengthens as it maintains tension Examplebicep shortens then stretches as a dumbell is curled tension on muscle remains the same in a muscle as length increases or decreases concentric is shortening (b) and eccentric is lengthening (c) To review-Isometric contraction is when a muscle is used but it does not shorten (a) Both isotonic and isometric contractions are used in normal activities Muscle Metabolism • contracting muscles use enormous amounts of ATP • one muscle fiber may have 15 billion thick filaments • during contraction each filament breaks down 2500 ATP molecules/sec Muscle Metabolism • • • • • • • • • ATP is also needed for cross bridge release to pump calcium back into SR to restore sodium & potassium levels to precontraction conditions cannot have all ATP needed for contraction before contraction begins ATP stores are depleted in 6 sec.-time enough for 8 twitches for a cell is to continue to contract more ATP must be generated muscle fiber generates ATP at same rate it is used ATP • energy used to power all activity in cells & the body • three high energy phosphate bonds • breaking off one phostphate yields about 7kcal of energy • ATP ADP + Pi + 7Kcal Ways to Acquire ATP • Creatine Phosphate • Glycolysisanerobic cellular respiration • Aerobic Cellular Respiration Ways to Acquire ATP Creatine Phosphate • at rest muscle produce more ATP than they use • excess ATP is used to make creatine phosphate (phosphocreatine) • in working muscles-creatine kinase transfers a high energy phosphate from phosphocreatine to ADP creatine + ATP • provides energy needed for short burst of intense activity • 1 minute of brisk walking or 6 second of sprinting Glycolysis • once creatine phosphate stores have been used & respiratory & cardiovascular systems cannot deliver oxygen to muscles fast enough to use aerobic respiration to produce ATP • ATP is provided by anaerobic cellular respiration-glycolysis • occurs in cytoplasm • oxidizes glucose to 2 molecules of pyruvic acid and 2 ATP molecules (net) • produces enough ATP for 30-40 seconds of maximum activity Glycolysis • without oxygen pyruvic acidlactic acid + ATP • organic acid • can lower blood pH • eventually pH changes alter functional characteristics of enzymes • muscle fibers cannot continue to contract • end resultmuscle soreness & fatigue Aerobic Respiration • after 40 seconds or so cardiopulmonary system catches up delivers oxygen to muscles fast enough for aerobic respiration • requires oxygen • occurs in mitochondria • First-TCA (tri-carboxylic acid) or Kreb’s Cycle • Second-electron transport chain or oxidative phosphorylation • starting product -pyruvic acid • muscles using aerobic respiration can contract for long periods of time • 36 molecules of ATP are produced • in exercise lasting 10 minutes or more 90% of ATP is produced aerobically Metabolism Overview Types of Muscle Fibers •slow oxidative fibers •fast glycolytic fibers •fast oxidativeglycolytic fibers Slow Oxidative Muscle Fibers • called red fibers • contain a great deal of mitochondria, blood capillaries & myoglobin-red pigment like hemoglobin which binds oxygen – provide dramatically higher oxygen supply • gives fibers dark red color – muscle dominated by slow twitch fibers is referred to as dark meat in chicken • contract slowly – require 3X as long to contract after stimulation as fast twitch fibers • fatigue resistant – specialized to contract for long periods of time – keep contracting long after fast fibers fatigue • diameters are half that of fast fibers • less dependent on anaerobic metabolism – obtain ATP via aerobic respiration – used almost constantly to maintain posture, to stand and to walk Fast Glycolytic Fibers • White fibers • Muscles appear pale – termed white muscle • contract 0.01 sec. after stimulation – 2-3X faster than slow twitch fibers • faster speed leads to faster tension development – muscles dominated by fast fibers display powerful contractions • have large diameters, densely packed myofibrils, large glycogen reserves and few mitochondria • use massive amounts of ATP • rely on anaerobic respiration – fatigue more rapidly due to lactic acid build up Fast-Oxidative Glycolytic Fibers • Intermediate fibers • combine fast twitch response with aerobic fatigue resistant metabolism • contain large amount of myoglobin & capillaries – dark red in color • get ATP by aerobic mechanisms • fast due to presence of a faster type of ATPase • moderately resistant to fatigue Thought questions: Why do chickens have white breast meat and dark leg meat? What does this say about the activities of the associated muscles? Why do ducks have dark breast meat? Muscle Composition • muscles are composed of all three fiber types • proportion of each differs from muscle to muscle • no slow twitch fibers in eye or hand muscles – need swift contractions • people with different types & levels of physical activity differ in the proportion of each fiber type • muscle performance & distribution of muscle fibers is genetically determined • proportions of different muscle fibers can change with physical conditioning Muscle Performance • rated in terms of • Power – maximum amount of tension produced • Endurance – amount of time muscle can perform particular activity • two factors determine these performance capabilities • type of muscle fiber • physical conditioning – training of that muscle Aerobic Endurance • length of time muscle can contract while supported by mitochondrial activities • determined by substrate availabilitybreak down of carbohydrates, lipids & amino acids • involves sustained low level muscle activity-jogging • training-alters characteristics of muscle fibers • fasts fibers will develop characteristics of intermediate fibers • improves performance of cardiovascular system which delivers oxygen & nutrients to muscles • does not promote hypertrophy Anaerobic Endurance • length of time muscle contraction can be supported by glycolysis & by existing ATP & creatine phosphate reserves • limited-amounts of ATP & creatine phosphate, amounts of glycogen & ability of muscles to tolerate lactic acid • improve-frequent, brief, intensive workouts – weight lifting & body building – produce muscle hypertrophy• repeated, exhaustive stimulation causes muscle fibers to develop more mitochondria, more glycolytic enzymes & more glycogen • muscle will develop more myofibrilshave more thin & thick filaments • when muscles are not usedbecome flaccid, smaller-atrophy Muscular Strength & Conditioning-Training Smooth Muscle • found in almost every organ • walls of hollow internal structures, blood vessels, stomach, intestine, gallbladder & urinary bladder • important in homeostasis • contraction changes shape of organs • generate force to move materials through the lumens of organs Smooth Muscle Structure • • • • • • • • • • • long, slender, spindle-shaped no striations, no myofibrils and no sarcomeres contains myosin & actin filaments no t-tubules SR forms loose network through sarcoplasm Actin is attached to dense bodies (like Z discs) intermediate fiber bundles are attached to dense bodies arranged so entire surface of actin is covered by myosin heads continuous line of myosin heads allows actin to slide down myosin without interruption producing tension dense bodies & intermediate filaments anchor thin filaments when sliding they slide against each other to produce contraction Smooth Muscle Contraction • dense bodies are not found in a straight line • during contraction causes cell to twist like a cork screw Types of Smooth Muscle • Multiunit types • Single-unit types or Visceral Smooth Muscle Multiunit Smooth Muscles • innervated like skeletal muscle • neural activity generates an action potential which is propagated over the sarcolemma • found-some large arteries, pulmonary air passages, piloerector muscles and the iris • cells contract or relax depending on type of neurotransmitter released Visceral Smooth Muscle • arranged in sheets or layers • adjacent cells connected by gap junctions • one muscle cell contracts electrical impulse travels to adjacent muscle cells contraction spreads in waves soon involving all cells • initial stimulus may be motor neuron • also contracts in response to chemicals, hormones, oxygen, CO2, stretching & irritation Excitation-Contraction Coupling • • • • trigger for contractioncalcium in sarcoplasm calcium enters from extracellular fluid more is released by the sarcoplasmic reticulum calcium interacts with calmodulin, a calcium binding protein which activates light chain myokinase to break down ATP • starts contraction • relaxation occurs when calcium is removed from cytosol • accomplished by a Ca-Na antiport exchange & by Ca-ATPase
