Muscles Contraction and Excitation PDF

Muscles Contraction and excitation By Dr Sally Anwar Sayed Learning Objectives 1. To study physiological anatomy of skeletal muscle. 2. To demonstrate basic mechanism of skeletal muscle contraction. 3. To demonstrate excitation contraction coupling. 4. To study changes tha...

Muscles Contraction and excitation By Dr Sally Anwar Sayed Learning Objectives 1. To study physiological anatomy of skeletal muscle. 2. To demonstrate basic mechanism of skeletal muscle contraction. 3. To demonstrate excitation contraction coupling. 4. To study changes that occur during and after muscle contraction. 5. To demonstrate contraction and excitation of smooth muscle. Skeletal muscles represent about 40 percent of the body weight and another 10 percent is smooth and cardiac muscle. Skeletal Muscle: Are characterized by: 1-Makes up the great mass of the somatic musculature. 2-Show a well developed cross striation. 3-Does not contract in absence of nervous stimulation. 4-Lacks of anatomic and functional connections between individual muscle fibers. 5-Generally under voluntary control. Cardiac Muscles: Are characterized by: 1-Has cross striation 2-Functionally syncytial. 3-Contracts rhythmically in absence of external nervous connection owing to the presence of myocardium pace-maker cells discharging spontaneously. Smooth Muscles: Are characterized by: 1-Lacks cross striation. 2-The type found in most of hollow viscera functionally syncytial and contains pace-maker that discharge irregularly whereas the type found in the eye and some other locations is not spontaneously active and resembles skeletal muscle. Physiological Anatomy of Skeletal Muscle Skeletal muscle is made up of individual muscle fibers that are the “building blocks” of the muscular system in the same sense that neurons are the building blocks of the nervous system. Myofibrils; Actin and Myosin Filaments: Each muscle fiber contain several hundred to several thousand myofibrils. Each myofibril in turn, lying side-by-side about 1500 myosin filaments and 3000 actin filaments. The actin and myosin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark banks. The light bands contain only actin and are called I bands. The dark bands contain myosin filament as well as ends of actin that overlap myosin and are called A bands, small projections arise from sides of myosin filament called cross -bridges. The Sarcoplasm: The fluid of sarcoplasm that suspend the myofibrils. It is rich with large of amounts of mitochondria to supply the contracting myofibrils with (ATP) needed. The Sarcoplasmic reticulum: Also in the sarcoplasm is an extensive endoplasmic reticulum called sarcoplasmic reticulum. It has a special organization which is important in control of muscle contraction. Generally the events of contraction as follows: 1-An action potential travels along a motor nerve to its ending on muscle fiber. 2-Secretion of small amount of acetylcholine (A. Ch.) transmitter. 3-Opening of A. Ch. Channels allows large quantities of sodium ions to flow into the interior of nerve fiber. This initiate action potential. 4-Travel of action potential along the muscle fiber membrane. 5-The action potential depolarize the membrane surface and also travels deeply within the fiber, hence it causes the sercoplasmic reticulum to release large quantities of calcium ions. 6-The calcium ions initiate the attractive forces between actin and myosin filaments causing them to slide together, which is the contractile process. 7-Calcium ions are pumped back into the endoplasmic reticulum until a new action potential. Molecular Basis of Contraction: The Myosin Filament: The myosin filament is made up of about 200 molecules. Tails of molecules in bundled together to form the body of the filament. Part of myosin helix extend to the side with the head forming an arm. The protruding arm and heads are called cross-bridges. T h e Acti n F i l a me n t: It i s co m p o s e d o f th r e e d i f fer en t p r ote ins : a cti n , tr op omyos in a n d tr o p o n in. M a i n ly th e a cti n f i la men t is a d o u b l e s tra n d e d F - a ctin p r o te in m o l e cule a r ra n g e d i n a h e l i x. Tr o p o myo si n Mo l e cu l e s : Th e s e m o l e cule s a r e co n n e cted l o o s e ly w ith F - a cti n s tra n d s , w ra p p e d s p i ra lly a r ou nd a cti n h e l i x. It i s b e l ie ved to l i e on top of a cti ve s ites. So th a t a ttra ction b e tw een a cti n a nd myosi n ca n not occur. Tr o p o n i n Mo l e cu l e s: It i s a co m p l ex o f th r e e p r o te in s u b u n i ts ; t r o p o n i n I w h i ch h a s g r e a t a f f inity f or a cti o n , t r o p o n i n T h a s a f f in ity to tr o p o myo sin a n d a th i r d ( t rop o n i n C ) h a s g r e a t a f f inity f or ca l ci u m i o n s. “Walk-Along” theory of contraction: the sliding during muscular contraction occurs as soon as the actin filaments becomes activated by calcium ions, the myosin heads bind firmly to actin, bends at the junction of the head with the neck dragging the actin filament, then detach, this tilt is called “power stroke”. After tilting, the head automatically breaks away from the active site. Next the head return to the normal perpendicular position combining with a new active sites farther down along actin filament, then the head tilt against to cause a new power stroke, actin filament move another step. Thus, the heads of cross-bridges bend-back and forth and step by step along the actin filament. Neuromusclar junction “Motor-End-plate”: Skeletal muscles are innervated by large myelinated nerve fiber, which branches many times. The nerve ending make junction called neuro -muscular junction near the fiber midpoint. The invagination is called synaptic gutter, and the space between the terminal and the fiber membrane is called synaptic cleft. In the axon terminal are many mitochondria that energize synthesis of acetylcholine transmitter that in turn excites the muscle fiber. Attached to matrix of the basal lamina large quantities of the enzyme choline-estrase which is capable of destroying acetylcholine. Excitation-Contraction Coupling 1-The action potential spreading along the surface of skeletal muscle fiber penetrate to the interior through T tubules which are extension of sarcoplasmic membrane. 2-Then the action potential signal transmitted from T tubule to the cistern. 3-Opening the large number of calcium ion channels through the membrane of cistern and their attached longitudinal tubules. 4-The released Ca ++ from these channels into the sarcoplasm, diffuse to adjacent myofibrils, where strongly bind with troponin C that elicits muscle contraction. 5-Muscle contraction continue as long as calcium ion concentration is high in the sarcoplasmic fluid. 6-Thereafter, the continually active calcium pump in the wall of sarcoplasmic reticulum pumped back calcium ions into the sarcoplasmic reticulum which leads to total depletion of calcium ions in the sarcoplasmic fluid, then release of calcium ions from troponin, when cessation of interaction between actin and myosin (relaxed or rest state). Changes That Occur During and After Contraction I- Excitability Changes The absolute refractory period: In this period no stimulus whatever its strength can excite. Excitability is abolished. The relative refractory period: Stronger stimulus is needed to excite. Excitability is being recovered but still below normal. The total refractory period is about the duration of spike. It coincide to the latent period as skeletal muscle can respond to another stimulus in the phase of mechanical contraction. Super-normal phase: It coincides with the negative after potential. Excitability is higher than normal. In this phase a weak stimulus is needed and give a higher response. Sub-normal phase: It coincide with the positive after potential excitability is lower than normal. Contraction produced in this phase is weaker. II- Mechanical Changes Muscular contraction involves shortening of the contractile elements. Is a weight is lifted by the muscle, work is done. Since the work is the product of force times distance. Accordingly two types of contractions are known: Isotonic Contraction In this type the muscle shorten against a fixed head It do work as it can lift a weight. About 20 to 30% of total energy may be converted into mechanical work and the rest appear in form heat. The tension remain constant (isotonic). Isometric Contraction: Muscle does not shorten during contraction. In this type the muscle contract against force transducer without decreasing the muscle length. No work exerted as the muscle does not lifting any weight. Increased muscle tension. Muscle fatigue: Prolonged and strong contraction of a muscle leads to fatigue, repeated stimulation of the muscle leads to progressive diminishes in the strength of the contraction and prolongs the different phases of twitch. Fatigue is most probably due to accumulation of lactic acid, depletion of energy producing metabolites such as ATP, CP and glycogen. Also depletion of acetyl-choline from the motor nerve ending or muscle receptors may become refractory to the transmitter. Perhaps all these factors together may participate in causing fatigue. III- Metabolic (Chemical) Changes Muscle contraction requires energy, and muscle has been called a machine for converting chemical energy into mechanical work> Sources of Energy for Muscle Contraction: 1-ATP: In is the immediate source of energy in the muscle, it contains two high energy phosphate bond: ATP ⥨ ADP + Ph This energy is used for cross-bridges of myosin in pull the actin filament, pumping Ca +2 from the sarcoplasm back to the reticulum after the contraction is over and pumping of Na + , K + through muscle fiber membrane for propagation of action potential. 2-Creatine phosphate: It contain one high phosphate bond as it used to reconstitute ATP from ADP. 3-Carbohydrate and lipid break down. Changes in pH during Contraction: Process of muscle contraction is accompanied by changes in pH which is contemporary with metabolic events during muscle contraction as follows: 1-During muscle contraction the pH is shifted to acidic side; due to the release of inorganic phosphate from ATP. 2-Thereafter, pH change to the alkaline side, due to release of creatine form CP, this alkalinity occurs after the contraction. 3-The pH shift again towards acid side as a result of lactic acid formation. The Mechanical Efficiency (M.E.) M. e. expresses the ratio of the total energy which is convertible into useful work. 𝑈𝑠𝑒𝑓𝑢𝑙 𝑤𝑜𝑟𝑘 𝑝𝑒𝑟𝑓𝑜𝑟𝑚𝑒𝑑 M. E. = × 100 𝑇𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑒𝑥𝑝𝑒𝑛𝑑𝑖𝑡𝑢𝑟𝑒 M.E. in normal condition about 25 % Factors that influence the M.E.: 1-Type of work: In isometric muscle contraction M.E. equal zero, this is met in to heavy weights to be carried. 2-Training: Improve M.E. where there is better nervous coordination. 3-Fatigue: Whether physical or mental decrease M.E. so period of rest are important in factories to increase the production. 4-Diet: M.E. is more when energy is derived from carbohydrate diet. 5-Rate of performance: There is an optimum rate at which the highest M.E. is obtained Faster or slower rate drop down M.E. 6-Fuel of exercise: Muscles derived the energy from oxidation of carbohydrate and fat, the former is preferred when available. Proteins are not direct energy supplier for muscle contraction. Contraction and Excitation of Smooth Muscle Main characteristics of smooth muscle fibers: 1-Smooth muscle is distinguished from skeletal and cardiac muscle because it lacks visible cross-striations. 2-Instead of Z lines, there are dense bodies. 3-Contains tropomyosin but troponin is absent. 4-Sarcoplasm is poorly developed with few mitochondria and depends on glycolysis for their metabolic needs. 5-Smooth muscle can be generally divided into. A) Visceral smooth muscle. B) Multiunit smooth muscle. 6-Smooth muscle can be stimulated by multiple types of signals: nervous signals, hormonal stimulation 7-Excitation-contraction coupling is a very slow process in plain muscles. The intestinal muscle is innervated both by adrenergic and cholinergic nerves.