muscle physiology

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<ul><li> 1. MUSCLE PHYSIOLOGY INDIAN DENTAL ACADEMY Leader in continuing dental education </li></ul><p> 2. TYPES OF MUSCLES SKELETAL MUSCLE SMOOTH MUSCLE MUSCLE RECEPTORS REFLEXES NEUROTROPHISM 3. Muscles are broadly classified into 3 types Skeletal muscle Smooth muscle Cardiac muscle Muscle contains: 75% water 20% protein 5% organic and inorganic compounds 4. 40 % of the body is skeletal muscle 10 % is smooth and cardiac muscle. 40% of adult body weight 50% of childs body weight 5. FASCIA Connective tissue that encases the muscles Functions of fascia Protect muscle fibers Attach muscle to bone Provide structure for network of nerves and blood/lymph vessels 6. Layers of fascia Epimysium Surface of muscle Tapers at ends to form tendon Perimysium Divides muscle fibers into bundles or fascicles Endomysium Surrounds single muscle fibers 7. SKELETAL MUSCLE All skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter. In most skeletal muscles, each fiber extends the entire length of the muscle. Each fiber is usually innervated by only one nerve ending, located near the middle of the fiber, except for about 2 % of the fibers 8. Sarcolemma- it is the cell membrane of the muscle fiber. consists of true cell membrane - plasma membrane, outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils At the end of the muscle fiber sarcolemma fuses with a tendon fiber 9. Sarcoplasm- The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, phosphate, and multiple protein enzymes Large numbers of mitochondria that lie parallel to the myofibrils are present that provide ATP 10. Sarcoplasmic Reticulum - surrounding the myofibrils of each muscle fiber is an extensive reticulum called the sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in controlling muscle contraction. 11. Myofibrils Each muscle fiber contains several hundred to several thousand myofibrils Each myofibril is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that cause the actual muscle contraction. 12. 13. The thick filaments are myosin and the thin filaments are actin. myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands. 14. I bands- isotropic to polarized light. light bands contain only actin filaments A bands- anisotropic to polarized light. dark bands contain myosin and ends of the actin filaments 15. Z disc - composed of filamentous proteins, passes cross wise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another across the muscle fiber giving them a striated appearance. The ends of the actin filaments are attached to the Z disc. Filaments extend in both directions to interdigitate with the myosin filaments 16. Sarcomere The portion of the myofibril that lies between two successive Z discs When the muscle fiber is contracted, the length of the sarcomere is about 2 micrometers. The actin filaments completely overlap the myosin filaments and the tips of the actin filaments begin to overlap 17. Steps In Muscle Contraction Excitation Action potential Neurotransmitter release Muscle fiber depolarization - Ca++ release from sarcoplasmic reticulum Coupling Ca++ binds to troponin-tropomyosin complex 18. Contraction Ca++ binding moves troponin-tropomyosin complex Myosin heads attach to actin Crossbridge cycling Relaxation Ca++ removed from troponin-tropomyosin complex Cross bridge detachment Ca++ pumped into SR active transport 19. General Mechanism of Muscle Contraction Initiation and execution of muscle contraction occur in the following steps 1. An action potential travels along a motor nerve to its endings on muscle fibers. 2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine. 20. 3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple acetylcholine gated channels through protein molecules floating in the membrane. 4. This allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This initiates an action potential at the membrane. 5. The action potential travels along the muscle fiber membrane 21. 6. The action potential depolarizes the muscle membrane, it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum. 7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process. 22. 8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump, this removal of calcium ions from the myofibrils causes the muscle contraction to cease. 23. Muscle Action Potential Resting membrane potential: about -80 to -90 millivolts Duration of action potential: 1 to 5 milliseconds Velocity of conduction: 3 to 5 m/sec- 24. ACTION POTENTIAL rapid changes in the membrane potential that spread rapidly along the fiber membrane. Resting Stage- the membrane potential before the action potential begins. The membrane is polarized during this stage because of the -90 millivolts negative membrane potential that is present. 25. Depolarization Stage membrane becomes very permeable to sodium ions, allowing diffusion of the ions. Repolarization Stage.- after a few 10,000ths of a second the sodium channels begin to close and the potassium channels open more than normal. Rapid diffusion of potassium ions to the exterior re- establishes the normal negative resting membrane potential. 26. Voltage-Gated Sodium and Potassium Channels 27. At rest, virtually all of the voltage-gated channels are closed, potassium and sodium can only slowly move across the membrane, through the passive "leak" channels The first thing that occurs when a depolarizing graded potential reaches the threshold is that the voltage gated Na+ channels begin to open and Na+ influx into the cell exceeds K+ efflux out of the cell 28. Two things happen next: 1. As the membrane depolarizes further and the cell becomes positive inside and negative outside, the flow of Na+ will decrease. 2) Even more importantly, the voltage- gated Na+ channels close When the inactivation gates close, Na+ influx stops and the repolarizing phase takes place. 29. Next, the voltage gated K+ channels are activated at the time the action potential reaches its peak. At this time, both concentration and electrical gradients favour the movement of K+ out of the cell. These channels are also inactivated with time but not until after the efflux of K+ has returned the membrane potential to, or below the resting level (after hyperpolarization /positive afterpotential). 30. The Neuromuscular Junction Each nerve ending makes a junction, called the neuromuscular junction, with the muscle fiber near its midpoint The action potential initiated in the muscle fiber by the nerve signal travels in both direction toward the muscle fiber ends. With the exception of about 2 % of the muscle fibers, there is only one such junction per muscle fiber. 31. Motor end plate - Branching Nerve Terminals - nerve fibers invaginate into the surface of the muscle fiber but lie outside the plasma membrane. Covered by Schwann cells that insulate it from the surrounding fluids. 32. Subneural clefts numerous small folds of the muscle membrane which increase the surface area at which the synaptic transmitter can act. Synaptic trough - invaginated membrane Synaptic cleft - space between the terminal and the fiber membrane (20 to 30 nanometers wide.) 33. Acetylcholine is stored in synaptic vesicles (300,000) which are in the terminals of a single end plate When a nerve impulse reaches the neuromuscular junction, about 125 vesicles of acetylcholine are released from the terminals into the synaptic space 34. During the action potential the calcium channels open and calcium ions to diffuse from the synaptic space to the interior of the nerve terminal. The calcium ions attract the acetylcholine vesicles, drawing them to the neural membrane The vesicles fuse with the neural membrane and empty their acetylcholine into the synaptic space by exocytosis. 35. Acetylcholine-gated ion channels, are located almost entirely near the mouths of the subneural clefts. Has 5 subunit proteins, two alpha and one each of beta, delta, and gamma proteins. After the Ach attaches a conformational change occurs that opens the channel 36. the principal effect of opening channels - allows sodium ions to pour to the inside of the fiber, carrying with them positive charges creating a local positive potential change inside the muscle fiber membrane, called the end plate potential. this end plate potential initiates an action potential that spreads along the muscle membrane and thus causes muscle contraction. 37. Most is destroyed by the enzyme acetylcholinesterase, which is attached to the spongy layer of the connective tissue that fills the synaptic space between the presynaptic nerve terminal and the postsynaptic muscle membrane. A small amount of acetylcholine diffuses out of the synaptic space and is then no longer available to act on the muscle fiber membrane. Destruction of the Released Acetylcholine 38. Safety Factor of Transmission at the Neuromuscular Junction Each impulse at the neuromuscular junction causes about 3 times as much end plate potential as that required to stimulate the muscle fiber. Stimulation of the nerve fiber at rates greater than 100 times/sec for several minutes diminishes the number of acetylcholine vesicles so that impulse fail to pass into the muscle fiber- called fatigue 39. A new action potential cannot occur in an excitable fiber as long as the membrane is still depolarized from the preceding action potential. Shortly after the action potential the sodium channels (or calcium channels, or both) become inactivated, and any amount of excitatory signal applied to these channels at this point will not open the inactivation gates. 40. Absolute Refractory Period - period during which a second action potential cannot be elicited, even with a strong stimulus. Large myelinated nerve fibers - 1/2500second. Relative Refractory Period - lasts about to as long as the absolute period. During this time, stronger than normal stimuli can excite the fiber. 41. Cause of relative refractoriness : 1. During this time, some of the sodium channels still have not been reversed from their inactivation state 2. the potassium channels are usually wide open at this time, causing greatly excess flow of positive potassium ion charges to the outside of the fiber opposing the stimulating signal 42. Excitation-Contraction Coupling Skeletal muscle fibers are so large that action potentials spreading along its surface membrane cause almost no current flow deep within the fiber. This is achieved by transmission of action potentials along transverse tubules (T tubules) 43. T tubules Very small transverse to the myofibrils. Penetrate muscle fibers from one side to the other They branch among themselves They are open to the exterior of the muscle fiber at their point of origination and so basically are internal extensions of the cell membrane. The action potential spreads along the T tubules to the interior of the muscle fiber. 44. Sarcoplasmic reticulum- Terminal cisternae Long longitudinal tubules The vesicular tubules have calcium ions in high concentration, and are released from each vesicle when an action potential occurs in the adjacent T tubule. 45. Molecular Mechanism of Muscle Contraction Sliding Filament Mechanism of Muscle Contraction. 46. Myosin Filament -The myosin filament is composed of multiple myosin molecules, each having a molecular weight of about 480,000. myosin molecule - 6 polypeptide chains- 2 heavy chains- molecular weight 200,000 4 light chains - molecular weights 20,000 each. 47. heavy chains wrap spirally around each other to form a double helix, called the tail of the myosin molecule. One end of each of these chains is folded bilaterally into a globular polypeptide structure called a myosin head. 4 light chains are also part of the myosin head. These light chains help control the function of the head during muscle contraction. 48. Myosin Filament - Made up of 200 or more individual myosin molecules. (Length -1.6 micrometers.) Tails of the myosin molecules bundle together to form the body of the filament, Heads of the molecules hang outward to the sides of the body. 49. Arm- part of the body of each myosin molecule along with the head, Cross bridges- protruding arms and heads together. It is flexible at the hinges- where the arm leaves the body of the myosin filament, where the head attaches to the arm. 50. The hinged arms allow the heads either to be extended far outward from the body of the myosin filament or to be brought close to the body. The hinged heads participate in the actual contraction process 51. There are no cross-bridge heads in the center of the myosin filament for a distance of 0.2 micrometer because the hinged arms extend away from the center. The myosin filament itself is twist...</p>