muscle physiology

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  • 1. MUSCLE PHYSIOLOGY INDIAN DENTAL ACADEMY Leader in continuing dental education

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 ch