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Skeletal Muscle

Physiology and Pharmacology

QuestionAnswer
Types of muscle Striated - skeletal (tonic, slow twitch and fast twitch) and cardiac Smooth
Structure of skeletal muscle A sarcomere is an individual contractile unit Muscle fibres are the smallest contractile unit and are a single elongated cell. Nerve axon branches contact individual cells to form NMJs Plasma membrane = sarcolemma Cytoplasm = sarcoplasm
The sarcomere A band - Myosin and actin (6 actin per myosin) H-zone - only myosin I band - only actin M-line - connects two myosin filaments Thick filament - myosin Thin filament - actin
What is Titin A large protein with 25000 AAs. Connects Z line to M line Adds stability to the sarcomere Can relax to allow for contraction
Changes to sarcomere on contraction A band does not change I band shortens Z discs move closer together Sarcomere shortens H zone decreases
Excitation-Contraction coupling Processes from depolarisation to contraction Sarcomere depolarises. This passes down t-tubules surrounding each myofibril (at regions where A band meets I band) Sarcoplasmic reticulum secretes Ca2+
What are triads 2 cisternae of the sarcoplasmic reticulum and and a t-tubule
T-tubules Contain voltage sensitive L-type Ca2+ channel (dihydropyridine receptor). When depolarised this opens. This conformational change passed on via mechanical transmission to Ca2+ release channels in the SR (Ryanodine receptors) which flood Ca into sarcoplasm
Evidence for Ryanodine receptors in muscles Muscle in a solution without Ca2+ can still be stimulated to contract Shows no external calcium is needed, intracellular stores are what triggers contraction These are tetramers which span the membrane and can interact with other proteins
Electromechanical coupling Plasmalemma depolarisation opens Ca2+ channel. This is in contact with the ryanodine receptor so mechanical coupling occurs, Ca exit via to RyR activates troponin C and triggers contraction. Ca entering via LTCC is not necessary to open RyR
Process of excitation contraction coupling Action potential in t-tubule alters conformation of DHP receptor. This opens Ca2+ channels in sarcoplasmic reticulum and Ca2+ enters sarcoplasm. Ca2+ binds to troponin allowing actin myosin cross bridges to form
Structure of thin filament Monomers of actin polymerise to form 2 double helices which wind around each other. Tropomyosin sits in the grooves of actin and is bound to a troponin complex TnT binds tropomyosin TnC binds Ca2+ TnI blocks myosin
Structure of thick filament 2 Heavy chains with a rod region(links two myosin), head region(binds to actin) and a hinge region (allows power stroke) 4 regulatory light chains
Role of Ca2+ in contraction Tropomyosin and troponin block myosin binding sites on actin. Ca2+ bins to TnC, causing a conformational change that moves tropomyosin and troponin complex to expose the binding sites
Cross bridge cycle ATP binds to myosin head causing it to dissociate from actin. ATP is hydrolysed causing myosin head to return to a cocked state. A new crossbridge forms (2 actin down from previous) P is released and myosin performs a power stroke. ADP is the released
Termination of contraction Minor mechanism - Ca ATPase and Na-Ca exchanger pumping Ca out sarcoplasm Main mechanism - Reuptake of Ca into sarcoplasmic reticulum. Proteins bind to Ca acting as a buffer to ensure maximal reuptake.
Regulation of active tension Selective recruitment of a greater or smaller number of motor units Changes in the frequency off stimulation Changes in the starting length of relaxed muscle
Motor unit recruitment (spatial summation) Axons of a neuron can connect to multiple cells in a muscle (a motor unit group) Can increase force by recruiting more fibres Innervation ratio = number of muscle fibres innervated by a single neuron. As low as 3 in extraocular muscle but up to 200
Modulation of stimulation frequency (frequency summation) Duration of an AP is 25-100 ms whilst a contraction lasts much longer. At low frequency the contraction finishes before the next depolarisation. At high frequency the 2nd depolarisation summates on the first and no relaxation occurs between
Modulation of fibre length Chemically fix at set length so force measured but no contraction. At small lengths myosin from opposite ends may bind the same actin so oppose each other (reducing force) At higher lengths the chance of myosin binding actin is reduced (less overlap)
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