OVERVIEW OF MUSCLE TISSUE

1. OVERVIEW OF MUSCLE TISSUE • Skeletal muscle tissue • attached to bones • striated and voluntary • Cardiac muscle tissue • forms wall of heart • striated and involuntary • Smooth (visceral) muscle tissue • walls of hollow organs, blood vessels • nonstriated in appearance • involuntary

2. Functions of Muscle Tissue • Produce body movements • Stabilize body positions • Storage/movement of substances within the body • bands of smooth muscle called sphincters • blood, lymph, urine, air, food and fluids, sperm • Heat production • involuntary contractions of skeletal muscle (shivering)

3. Properties of Muscle Tissue • Excitability = ability to generate electrical signals (a.p.’s) • in response to autorhythmic electrical signal in heart • in response to chemicals released from nerve cells (NTs) • Contractility • ability to contract forcefully when stimulated by act. pot. • Extensibility • ability to be stretched • can forcefully contract even when stretched • Elasticity = ability to return to original shape after being stretched

4. Skeletal Muscle: CT components • Superficial fascia= loose C.T. underlying the sub-Q layer of skin • Deep fascia = dense irregular C.T. around muscle • C.T. components of the muscle include • epimysium = surrounds whole muscle • perimysium = surrounds bundles (fascicles) of 10-100 muscle cells • endomysium = separates individual muscle cells • All these connective tissue layers extend beyond the muscle belly to form the tendon

5. Nerve and Blood Supply • Well-vascularized & highly innervated • Somatic motor neurons • axon extends from brain to group of fibers • axon branches @ muscle, each branch supplies diff. fiber • Each muscle cell supplied by at least one capillary • bring in oxygen/nutrients • remove wastes • Nerve fibers & capillaries are found in the endomysium between individual cells

6. Microscopic Anatomy of Skeletal Muscle • Muscle fibers = muscle cells • Sarcolemma = plasma membrane of muscle cell • T tubules = tiny in-folds of the sarcolemma • tunnel from surface of cell to center • open to outside of fiber  filled with interstitial fluid • allows muscle action potential to propagate to all parts of muscle fiber simultaneously • Sarcoplasm = muscle cell cytoplasm • contains a large amount of glycogen for energy production • myoglobin binds oxygen for storage

7. Microscopic Anatomy of Skeletal Muscle • Sarcoplasmic reticulum • fluid-filled system of membranous sacs • similar to endoplasmic reticulum • encircles each myofibril • terminal cisternae store Ca+2 ions in relaxed muscle • release of Ca+2 ions triggers muscle contraction • intersection w/ T-tubule forms triad

8. Microscopic Anatomy of Skeletal Muscle • Myofibrils = contractile organelles of muscle fibers • extend entire length of muscle fiber • dark & light bands give striated appearance • sarcomere = functional unit of myofibril • contain myofilaments which do not extend entire length of cell • defined as the region btwn two Z discs • thin filaments • make up I band of sarcomere • actin protein • anchored together by Z disc in middle of I band • slide relative to myosin during muscle contraction

9. Microscopic Anatomy of Skeletal Muscle • Myofibrils, ctd. • thick filaments • myosin protein • “A” band of sarcomere  dark-staining • “zone of overlap” = ends of A bands where thick/thin filaments overlap • H zone of sarcomere = region of only thick filaments • middle of A band where thin filaments do not reach • M line in center anchors myosin vertically

10. Filaments and the Sarcomere • Myofilaments do not extend entire length of muscle fiber  arranged into sarcomeres • Sarcomere = functional unit of myofibril • Thick and thin filaments overlap each other in a pattern that creates striations (light I bands and dark A bands) • I band region contains only thin filaments • Separated by Z discs

11. Muscle Proteins • Contractile proteins generate force during muscle contraction • myosin = motor protein • push/pull cell structures to produce movement • actin = main component of thin filaments • myosin binding site on each molecule • Regulatory proteins • tropomyosin • covers myosin binding sites in relaxed muscle • prevents myosin binding in absence of Ca+2 ions • troponin holds tropomyosin in place • binds calcium • moves tropomyosin & exposes myosin binding sites • Structural proteins contribute to proper alignment, elasticity and extensibility

12. Sliding Filament Mechanism Of Contraction • Myosin cross bridges pullon thin filaments • Thin filaments slide toward M-line • Z Discs come toward each other • ***H & I bands narrow*** • Sarcomeres shorten at same time muscle fiber shortens muscle shortens • Notice: Thick/thin filaments do not change in length!!

13. How Does Contraction Begin? • Nerve impulse reaches axon terminal & synaptic vesicles release acetylcholine (ACh) • ACh diffuses across synaptic cleft & binds to receptors on the sarcolemma; Na+ channels open& Na+ rushes into cell • A muscle action potential spreads over sarcolemma and down into the transverse tubules • SR releases Ca+2 into the sarcoplasm • Ca+2 binds to troponin & causes troponin-tropomyosin complex to move & reveal myosin binding sites on actin contraction cycle begins

14. Contraction Cycle • Once Ca+2 is released from terminal cisternae & has bound troponin, myosin binding sites on actin are available for crossbridge formation • 4 steps to contraction cycle • ATP hydrolysis  puts myosin in hi-energy position • attachment of myosin to actin to form crossbridges • Release of ADP & Pi causes conformational change in myosin which pulls thin filaments toward center of sarcomere  this is the power stroke • detachment of myosin from actin when new ATP binds  mysoin returns to hi-energy conformation (this is position of myosin in resting muscle) • Cycle repeats as long as ATP is available & [Ca+2] is high near the filaments

15. Steps in the Contraction Cycle Excitation - Contraction Coupling • Refer to figure 10.7 • Notice how myosin head attaches & pulls on thin filament with energy released from ATP • All the steps that occur from the muscle action potential reaching the T tubule to contraction of the muscle fiber • Refer to figure 10.8

16. Relaxation • Acetylcholinesterase (AChE) breaks down ACh @ synaptic cleft • Muscle action potentials cease • Ca+2-release channels close • Ca+2actively resequestered by SR/ Ca+2-ATPase pump • ↓ in sarcoplasmic Ca+2 returns troponin-tropomyosin to original conformation • myosin binding sites unavailable • Contraction stops but myosin heads remain in high-energy conformation, ready for contraction cycle to begin again ***Remember: ATP is required to detach crossbridges, so resting muscle is in energized state***

17. Length-Tension Relationship • Fiber length @ onset of contration influenes contraction strength • Maximal tension develops when fiber contracts @ + 30% of its optimal length • fibers shorter/longer than optimal length result in decreased force of contraction • too long: no overlap between myosin/actin  no Xbridge formation • too short: too much overlap, thick filaments compressed by Z lines  fewer interactions btwn myson/actin • Figure 10.9 illustrates this relationship

18. Structures of NMJ Region • NMJ = synapse btwn somatic motor neuron (SMN) & skeletal muscle fiber • synapse = area of communication btwn neuron & another cell (in this case, a muscle fiber) • synaptic cleft = gap separating neuron & muscle cell • neurotransmitter (NT) = chem released by neuron into cleft which activates the muscle fiber • axon terminal = end of motor neuron • synaptic end bulbs are swellings axon terminal • contain synaptic vesicles filled w/ ACh • motor end plate = region of sarcolemma opposite axon terminal • contains ACh receptors/cation channels

19. Generation of Muscle Action Potential • Arrival of nerve impulse at nerve terminal causes release of ACh from synaptic vesicles • ACh binds to receptors on motor end plate & opens gated ion channels  Na+ rushes into muscle cell • Inside of muscle cell becomes more positive, triggering a muscle action potential that travels over the cell and down the T tubules • The release of Ca+2 from the SR is triggered and the muscle cell will shorten & generate force • Acetylcholinesterase breaks down the ACh attached to the receptors on the motor end plate so the muscle action potential will cease and the muscle cell will relax.

20. NMJ Antagonists • Botulinum toxin (C. botulinim) • blocks exocytosis of ACh  muscle cannot contract • causes flaccid paralysis • death occurs from paralysis of the diaphragm • Curare (plant poison from poison arrows) • binds ACh receptors but does not activate them • causesflaccid paralysis • used to relax skeletal muscle during surgery because does not affect heart muscle • Black widow spider venom • causes massive ACh release • spastic paralysis results  exaggerated reflexes, ↑ tone

21. Muscle MetabolismProduction of ATP in Muscle Fibers • Active muscle has high demand for ATP • required for contraction cycle • required for calcium uptake by SR • required for other metabolic reactions • Sarcoplasmic ATP only lasts for few seconds • 3 sources of ATP production within muscle • creatine phosphate • anaerobic cellular respiration • aerobic cellular respiration

22. Creatine Phosphate • Hi-energy compound found only in muscle • [Creatine phosphate] 3-6 times greater than [ATP] in muscle • Excess ATP within resting muscle used to form creatine phosphate • ATP + Cr  ADP + Cr—P (at rest) • As ATP is used during exercise, [ADP] ↑  energy transferred from Cr—P to ADP • ADP + Cr—P  ATP + Cr (in active muscle) • Sustains short, powerful bursts of maximal contraction • lasts about 15 sec • as in 100 meter dash

23. Anaerobic Cellular Respiration • Generates ATP w/o requiring O2 • When Cr—P used up, glucose broken down to form ATP • glycolysis yields 2 net ATP • pyruvate converted to lactic acid instead of entering aerobic metabolic cycles • Can provide ATP for 30 to 40 sec of maximal activity (200 meter race)

24. Aerobic Cellular Respiration • Series of reactions requiring O2 to generate ATP • If sufficient O2 is available, glc completely oxidized to yield 36 or 38 ATP • pyruvate enters mitochondria rather than being converted to lactate • 2 sources of oxygen • myoglobin • diffusion from blood • Provides ATP for prolonged activity if O2 & nutrients are available

25. Muscle Fatigue • Inability to contract after prolonged activity • Factors that contribute to fatigue: • central (psychological) fatigue = feeling of tiredness & desire to stop physical activity • neuromuscular fatigue = insufficient release of ACh from motor neurons • muscular fatigue: • depletion of creatine phosphate, glycogen • inadequate release of Ca+2 from SR • insufficient oxygen • lactate accumulation • Oxygen debt = amount of O2 required to replenish ATP/Cr—P used during exercise

26. The Motor Unit • Motor unit = one somatic motor neuron & all skeletal muscle cells (fibers) it stimulates • Muscle fibers of a unit normally scattered throughout belly of muscle (not all in one location) • All fibers within one unit contract in unison • Total strength of a contraction depends on # motor units activated & size of motor units • precise movements require large # of small motor units (few fibers/unit) • large/powerful movements involve large motor units (many fibers/unit)

27. Twitch Contraction • Brief contraction of all fibers in a motor unit in response to single action potential in its motor neuron • Latent period = delay btwn stimulus & onset of contraction • Muscle a.p. moving along sarcolemma & Ca+2 is released from SR • Contraction period = period where crossbridges form and peak muscle tension develops • Relaxation period = time during which Ca+2 is actively taken up by SR pump • Refractory period = period when muscle cannot contract even if it receives an additional stimulus

28. Frequency of Stimulation • Wave summation = repeated stimuli arriving @ different times, (but before muscle has completely relaxed from previous stimulus) produces an additive effect • results in stronger contraction • Incomplete (unfused) tetanus = sustained muscle contraction that permits partial relaxation between stimuli • stimulation rate ~20-30x/sec • Complete (fused) tetanus = sustained contraction that lackspartial relaxation between stimuli • stimulation rate ~ 80-100x/sec

29. Motor Unit Recruitment • Process in which # of active motor units is increased • Generally, motor units in a whole muscle fire asynchronously • some fibers are active & others are relaxed • Asynchronous contraction delays muscle fatigue allows for sustained contraction • Produces smooth muscular contraction • not series of jerky movements • Weak motor units are recruited first • then progressively larger units recruited as needed • size of unit recruited dependent upon amt of force needed

30. TYPES OF SKELETAL MUSCLE FIBERS • Slow oxidative fibers • Least powerful of three fiber types • Large amounts of myoglobin  dark in color • Generate ATP primarily by aerobic cellular respiration • have many large mitochondria for this function • Highest resistance to fatigue of all fiber types  adapted for endurance activities • Fast oxidative-glycolytic fibers • Lots of myoglobin, but less than slow oxidative fibers • Generate ATP via aerobic respiration • High glycogen content  can also generate fair amt of ATP via anaerobic means • Fairly fatigue resistant

31. TYPES OF SKELETAL MUSCLE FIBERS • Fast glycolytic fibers • Generate quickest, most powerful contractions of all fiber types • Low myoglobin content  appear white • Lots of glycogen  primarily generate ATP via anaerobic means • Adapted for short, powerful movements  weight lifting, throwing a ball, sprinting, etc. • Fatigue quickly • Relative ratios of fiber types are genetically determined • All muscles contain the three fiber types, but distribution offibers dependent upon particular function/need of the muscle

32. CARDIAC MUSCLE TISSUE • Found only in the heart wall • fibers arranged similarly to skeletal muscle fibers but are branched rather than parallel • fibers connect to adjacent fibers by intercalated discs • desmosomes prevent cell separation during contraction • gap junctions allow a.p. to spread quickly • contractions last longer than skeletal muscle twitch due to prolonged delivery of calcium ions from sarcoplasmic reticulum and the extracellular fluid. • Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers. • This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.

33. SMOOTH MUSCLE • Nonstriated, involuntary & autorhythmic • Two types • Visceral (single unit) smooth muscle • found in walls of hollow viscera & small blood vessels; • fibers work as single unit, an a.p. stimulates all fibers to contract at once • Multiunit smooth muscle • found in large blood vessels, large airways, arrector pili muscles, eye muscles • each fiber has own neuron • fibers operate singly rather than as one unit

34. Microscopic Anatomy of Smooth Muscle • One centrally located nucleus • Thick and thin filaments are not organized into sarcomeres • Contains intermediate filaments attached to dense bodies • Small SR, but no T-tubules • Caveolae = structures containing XC Ca+2 for use by sm. musc. • Lengthwise shortening of fiber results from dense bodies pulling on filament • Twists into helix during contraction • Reverse motion during relaxation

35. Physiology of Smooth Muscle • Contraction starts slowly & lasts longer • no transverse tubules & very little SR • Ca+2 must flow in from outside • Can contract/stretch to greater extent than skeletal muscle • Regulatory protein = calmodulin • binds Ca in cytosol  facilitates myosin-actin binding • Smooth muscle tone = state of c’td partial contraction • result of prolonged presence of calcium ions • Stress-relaxation response: smooth muscle can change in length without generating tension • retains contractile ability while stretched