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Muscular System

Muscular System. Dr. Anderson GCIT. Function. Muscle is a collective term for tissue that causes movement of the body, or materials through the body Muscle can also serve other functions such as Stabilizing the body Producing heat. Muscle Tissue. Three main types of muscle

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Muscular System

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  1. Muscular System Dr. Anderson GCIT

  2. Function • Muscle is a collective term for tissue that causes movement of the body, or materials through the body • Muscle can also serve other functions such as • Stabilizing the body • Producing heat

  3. Muscle Tissue • Three main types of muscle • Skeletal – attaches to skeleton and enables body movement - striated, voluntary • Smooth – lines body cavities to move fluids throughout the body – smooth, involuntary • Cardiac – makes up the heart – striated, involuntary

  4. Physiology • Muscles function by converting the energy stored in ATP into mechanical energy that the muscle can use to contract– muscles only expend energy to get shorter

  5. Muscle Characteristics • Excitability – the muscle’s ability to react to a stimulus • Contractility – the ability to shorten forcibly • Extensibility – the ability to be stretched • Elasticity – the ability for a muscle to return to its pre-stretched length

  6. Skeletal Muscle Anatomy • There are roughly 640 skeletal muscles in the human body • Each one is a discrete unit (organ) of contraction, separated by membranes and served by • One major nerve • One artery • One to a few veins

  7. Muscle Connections/Separations Individual muscles are separated by sheaths of connective tissue • Epimysium – surrounds entire muscle • Perimysium and fascicles – perimysium surrounds bundles of muscle fibers (fascicles) • Endomysium – very thin sheath of tissue that surrounds each muscle fiber (cell)

  8. Muscle Sheaths

  9. Skeletal Origins and Insertions • Skeletal muscles are connected to bone in at least two places • Origin – end connected to the immovable bone • Insertion – connected to moveable side • Direct – Epimysium of muscle is fused directly to the periosteum of bone (less common) • Indirect – The connective tissue connects to bone via a tendon (more common)

  10. Direct vs. Indirect Muscle Attachment Direct Attachment (to skull) Indirect Attachments

  11. Muscle Fiber Anatomy • Sarcolemma – cell membrane of muscle cell • Sarcoplasm – cytoplasm of muscle cell • Glycosomes – granules of stored glycogen • Myoglobin – red pigment that can store O2 in muscles

  12. Myofibrils • Complex organelle composed of bundles of myofilaments in a muscle fiber (appear banded in skeletal muscle)

  13. Sarcomere • The smallest contractile unit of the muscle • Composed of protein filaments (myofilaments) that shorten the muscle upon stimulation • Here’s where it gets complicated…..

  14. Thick and Thin Filaments • Thick filaments (myosin) – contain globular myosin “heads” that will bond to thin filaments during muscle contraction • Thin filaments – composed of: • Actin – the structural protein that provides tensile strength • Troponin and Tropomyosin -proteins that regulate muscle contraction • Titin– allows for extensibility and elasticity during and after contraction

  15. Ultrastucture of Muscle Fibrils Thick Filament (Myosin) Titin Thin Filament (Actin)

  16. Sarcomere Components

  17. Muscle Contraction • Thick filament heads and thin filaments link and form cross-bridges • The heads swivel and pull the opposing thin filaments of the sarcomere together • Helpful video! • http://www.youtube.com/watch?v=ELyoJZom5N0

  18. Regulatory Proteins • Tropomyosin – bound within the actin filaments and block myosin-binding sites (when muscle is relaxed) • Troponin – helps stabilize actin and binds Ca+ ions

  19. Sarcoplasmic Reticulum • Specialized smooth endoplasmic reticulum (sER) that surrounds each myofibril • Most run longitudinally along the length of the cell • However, between the thick filaments and the Z disc, they form pairs of terminal cisternae • This organelle is critical in that it stores and releases Ca+ ions, which are the triggers for muscle contraction

  20. T- Tubules • An invagination of the sarcolemma – located between the terminal cisternae in each sarcomere • The structure of 2 Terminal cisternae divided by t-tubules is called a TRIAD • Why is all of the surface area in triads necessary?

  21. Triads Triad

  22. The Sliding Filament Model (Sum-up) • Thin filaments are “captured” by the myosin heads of the thick filaments by forming cross-bridges • As the myosin heads swivel, they ratchet the thin (actin) filaments towards the H-line • This shortens the sarcomeres, and in turn the entire muscle, causing movement

  23. Muscle Flexing • In order for muscles to flex: • They must be stimulated by nerves (or chemicals) • An action potential must propagate (travel) along the sarcolemma • Ca+ ions must increase in concentration, causing the filaments to slide past one another

  24. Muscle Innervation (Motor Nerves) • Motor nerves from the spinal cord innervate the muscle to communicate with the muscle fibers • The fibers do not touch but come exceedingly close (1-2 nm apart) forming the synaptic cleft

  25. Synaptic Cleft (Neuromuscular Junction) • From the nerve terminal, a neurotransmitter called acetylcholine (ACh) is released • ACh is then picked up by receptors on the sarcolemma

  26. Electrical Gradient • Normally, the sarcolemma is polarized (more negative on the inside than the outside) • When ACh binds to it receptors, channel proteins let ions (Ca+ and K+) pass • More Ca+ ions stream into the cell than K+ ions stream out, therefore the inside of the cell becomes less negative – this is called depolarization

  27. Synaptic Cleft Arrangement

  28. Helpful Video • http://www.youtube.com/watch?v=kvMFdNw35L0

  29. Synaptic Cleft • From the nerve terminal, a neurotransmitter called acetylcholine (ACh) is released • ACh is then picked up by receptors on the sarcolemma

  30. Electrical Gradient • Normally, the sarcolemma is polarized (more negative on the inside than the outside) • When ACh binds to receptors, it opens channel proteins which lets ions (Ca+ and K+) pass • More Ca+ ions stream into the cell than K+ ions stream out, therefore the inside of the cell becomes less negative – this is called depolarization

  31. Depolarization- Successive Steps • Action potential (nerve impulse from motor neuron) reaches the muscle fiber(s) • ACh is released, opening Ca channels (ACh is quickly degraded by acetylcholinesterase 3. The “local” depolarization from the neuron (end plate depolarization) depolarizes adjacent membrane areas • Then voltage-gated ion channels allow Ca+ ions to enter 4. This action potential is the propagated through the sarcolemma, opening up more channels

  32. Depolarization- Successive Steps • 5. Voltage-dependent gates open, and more Ca+ floods through the sarcolemma • 6. The influx of Ca+ leads to muscle contraction by interacting with tropomyosin, freeing the myosin binding sites

  33. Repolarization • K+ channels open, which allows K+ to flood out of the cell, restoring the negative conditions inside • How do we restore the electrochemical balance of the cell?

  34. Sodium-Potassium Pump • ATP-dependent protein “pump” that restores the ionic concentration that was present before depolarization

  35. Excitation-Contraction Coupling • Calcium ion influx (through T-tubules and into terminal cisternae) • Calcium binds to troponin and removes the blocking action of tropomyosin, allowing myosin to bind to the thin (actin) filaments • Myosin cross-bridges with actin, leading to contraction

  36. Let’s Sum Up • http://www.youtube.com/watch?v=70DyJwwFnkU

  37. Muscle Innervation • The single motor nerve that innervates a muscle can contain hundreds of motor neurons • A motor unit is a motor neuron and all of the muscle fibers that it serves – the more motor units, the larger the force (more fibers contract) • Which muscles might have small motor units? Large motor units?

  38. Motor Unit

  39. Strength of Contraction • Increasing strength can be due to • Increased frequency of stimuli • The less time between nerve stimulation, the greater the force of flexion due to Ca being left over in the muscle • Increased number of stimulated motor units • The more motor units stimulated, the greater the force of flexion

  40. (Frequency of Stimuli) Twitch Latent – Ca floods in Contraction – Myosin heads pull and make sarcomeres shorter Relaxation – muscle activity stops as ions are replaced to normal concentrations

  41. Contraction Strength • How is the strength of a contraction controlled? • The size/number of motor neurons stimulated • Small neurons (and therefore small muscles) are most excitable and fire first • Large neurons are the least excitable and fire last (only used when strongest contractions are needed)

  42. Isotonic vs Isometric Contraction • Isotonic – muscle moves when flexed • Concentric – muscle moves and shortens • Eccentric – muscle moves while lengthening • Isometric – muscle flexes, but does not shorten or lengthen

  43. Muscle Physiology • Where do muscles get the energy to flex, and keep flexing?

  44. Muscle Metabolism • ATP!! • Provides the energy needed for cross bridge movement and detachment (of myosin heads), but short supply in most muscles • ATP-ADP-ATP cycles rapidly by: • Creatine phosphate phosphorylation • Anaerobic respiration (burning sugar without O2) • Aerobic respiration (burning sugar with O2)

  45. Creatine Phosphate • Another high-energy molecule that converts ADP to ATP by direct phosphorylation via an enzyme called creatine kinase ADP + creatine phosphate creatinekinase ATP + creatine

  46. Rigor Mortis • Lack of ATP after death causes contractures, where myosin heads cannot detach from the actin filaments, locking up the muscle until muscle proteins start to break down

  47. Length vs. Force • The strength of a muscle contraction varies drastically over its length • Strongest from 80-120% of its resting sarcomere length

  48. Velocity of Contraction • “Fast twitch” fibers – contract rapidly • Large fibers that depend on stored glycogen reserves (does not depend on delivery of nutrients) • Low myoglobin • Tires quickly • “Slow fibers” – contract slowly • Uses nutrients delivered from blood, therefore rich capillary supply • High myoglobin • High endurance

  49. Smooth Muscle • Different microstructure, but similar function as skeletal muscle • Flexing is under control of the autonomic nervous system, but also under the control of local chemistry

  50. Smooth Muscle • Lines the: • Stomach and intestines • Blood vessels • Lungs • Urinary system • Reproductive organs

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