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Lecture 14 – Muscle Physiology II

Ach gates channels open both na and k cross the

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Ach gates channels open both Na and K cross the membrane Na influx exceeds K influx because the electrochemical driing force is greater for Na 2. Ach initiates Muscle AP Addition of net positive charge to the muscle fiber depolarizes the membrane and  creates end-plate potential (EPP) The EPP almost always exceeds threshold and a muscle AP is initiated This AP travels across the surface of the muscle fiber and down into the T-tubule 3. Muscle AP causes Ca to be released from the SR the action potential that travels down the T-tubules cuases Ca release from the lateral  sacs of the SR Free Ca levels in the cytosol of resting musle are normally quite low After the Ca release, they increase about 100 times 4. Ca combines with troponin to initate contraction the TROP complex changes configuration and contraction occurs Relaxation Contraction nedns when Ca is removed from the cytosol and TROP complex covers  binding sites Ca removal is achieved by the SR by using a Ca-ATPase When Ca is pumped into the SR, the EQ is disturbed and Ca dissociates from troponin  and the tropomyosin slides back to block the mtosin binding sites Cross bridges release and the muscle fiber relaxes undr the influence of elastic fibers  (titin) in the sarcomere and in the connective tissues of the muscle T-tubule and SR  Mechanism Ca stored in the lateral sacs is released following membrane excitation T-tubule les directly between, and is intimatelay associated with, the lateral sacs of the  adjact segemnts of the SR The T-tubules and later sacs surrounf the myofibrils in the region of the sarcomere  where the A-band meets the I-band
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The lumen of the T-tubule is continuous with the extracellular fluid surrounding the  myofibril The membrane of the T-tubule is capable of propagating an AP Physialc Realtionship Specialized mechanism coulples T-tubule AP with the release of CA sotred in the SR  lateral sscs T-tuble are intimate contct with the lateral sacs and are connected by junctional feet  (foot processes) This junction involves 2 membrane proteins – one of the T-tubule membrane and one of  the lateral sac membrane The t-tubule protein is a voltage sensitive Ca ion cnnel called a DHP receptor DHP acts as a voltage sensor Portin embedded in the SR called ryanodine receptor (RYR) Binds the foot processes  Forms Ca channel
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