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Muscle Tissue

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 ones-swimming posture & body position

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Muscle Tissue

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  1. Muscle Tissue

  2. Muscle Tissue Classification Skeletal Muscle Cardiac Muscle Intercalated Disc Smooth Muscle

  3. Skeletal Muscle • directly or indirectly attached to bones of skeleton

  4. Functions • movement • simple-breathing to highly coordinated ones-swimming • 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 heatkeeps body temperature within the normal range • when cold shivering occurs

  5. 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 • endomysiumsurrounds 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

  6. 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

  7. 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

  8. 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

  9. Proteins in Muscle Fibers • Contractile Proteins • actin • myosin • Regulatory Proteins • tropomyosin • troponin • Structural Proteins • titin • alpha actinin • myomesin • nebulin • dystrophin

  10. Myosin contractile protein comprised of 2 subunits twisted around one another forming long coiled tail & pair of heads project toward m line MYOSIN-THICK FILAMENT

  11. Actin • contractile protein • comprises thin filaments • composed of two intertwined strands of fibrous (F) actin-contractile 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

  12. Regulatory Proteins • Control contraction-turn it on & off • Tropomyosin • winds around actin • covers myosin binding sites preventing actin-myosin interactions • Troponin • calcium binding protein each • bound to each tropomyosin • When calcium binds to troponinchanges shapepulls tropomyosin off actinmyosin binding site exposedcrossbridges form

  13. 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

  14. 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

  15. Sliding Filament Theory

  16. 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 • myosinhead pivots at its base • pulls on actin • causing it to move to center of sacromere • muscle shortens

  17. Muscle Cell Contraction • Skeletal muscles only contract when activated by motor neurons from CNS

  18. NEURON STRUCTURE • Dendrites • Receive information • Typically many • Axons • Send information • Covered with Myelin Sheath • End in Terminal Buttons

  19. 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 sarcolemmamotor end plate (MEP) • exhibits many junctional folds • contains receptors

  20. Neuromuscular Juncion

  21. 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

  22. 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

  23. 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 exocytosisinto synaptic cleft • ACH diffuses across cleft

  24. STEP 2 Activation of ACH Receptors • ACH bindsto receptors on motor end plate • opens sodium gates • sodium rushes into sarcoplasm

  25. 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

  26. STEP 4 Termination of ACH Activity • influx of calcium continues until acetylcholinesterasedegrades ACH removing it from receptors • component parts are recycled • calcium is pumped back into the SR • muscle cell relaxes

  27. 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

  28. Muscle Cell Contraction • action potential is propagated across entire membrane • when reaches t tubuletravels down t tubules • t tubules & terminal cisternae of sarcoplasmic reticulum form a triad • triad releases calcium from sarcoplasmic reticulum

  29. 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

  30. Contraction Cycle Steps • 1. ATP Hydrolysis • 2. Attachment of Myosin to Actin forming Cross-Bridges • 3. Power Stroke • 4. Detachment of Myosin from Actin

  31. Step 1- ATP Hydrolysis • each myosin head must have an ATP bound to it to initiate contraction • head contains myosin ATPase hydrolyzes ATPADP + Pi & energy • ADP & Pi still attachedto myosin head

  32. Steps 2 & 3-Attachment of Myosin to Actin & Power Stroke • energized myosin binds to exposed active site on actin forming a cross-bridge • myosin releases ADP & phosphate • flexes into a bent, low energy position bringing the thin filament with it • the power stroke

  33. 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

  34. 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 intothe 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

  35. 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 fibersfibers 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 movesinsertion • other attachment remains stationaryorigin • developing tension pulls object toward source of tension

  36. 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

  37. 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 ofcross 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 rangemuscle cannot produce as much tension • optimalrange is range where maximum number of cross bridges can formmaking most tension • when sarcomeres are short thick filaments are jammedup against Z line • cross bridges form but myosin heads cannot pivotno tension production • sarcomeres with length longer than optimal range has reduced zone of overlapless cross bridges can formless tension

  38. 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

  39. Muscle Twitch • one above threshold stimulus to a muscle produces one contraction/relaxation cycle-twitch • vary in duration with type, location, temperature & environmental conditions • eye twitch-7.5msec • soleus (calf muscle) twitch- 100msec • 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

  40. Muscle Twitch • latent phase begins as stimulation of muscle begins-lasts 2msec • as tension rises to a peak contractionphase begins (10-100msec) • during relaxation phase tension decreases to resting levels (10-100msdc)

  41. 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 timeimmediately after a relaxation phase treppe contraction develops

  42. 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

  43. 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

  44. TETANUS • at a still higher frequencymuscle has no time to relax between stimuli • twitches fuse into a smooth, prolonged contraction called complete tetanus

  45. 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 onemotor unit

  46. 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

  47. MOTOR UNITS • neuron firescontracts 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 asynchronousrecruitment • motor units are activated on a rotating basis • some rest & recover while others contract

  48. 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

  49. Contraction Types • contractions types are based on pattern of tension development • Isometric • Isotonic • Concentric • Eccentric

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