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USC BISC 307L - Muscle Figs
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BISC 307L 2nd Edition Lecture 12 Current Lecture Excitation Contraction Coupling In the top right of the figure below in blue is the smooth ER of the muscle fiber known as the sarcoplasmic reticulum SR Organelles in a skeletal muscle fiber have the sarco prefix in a skeletal muscle fiber the sarcoplasm is the cytoplasm and the sarcolemma is the plasma membrane Also shown are the yellow transverse tubule rings that circle where the Z disks are And the SR wraps the myofibril in between the t tubules This structure repeats down the length of the muscle fiber An action potential going down the muscle fiber works and spreads the same way as a typical unmyelinated axon but with one difference it spreads down both directions but it also spreads INTO the fiber through the t tubule system The t tubules which branch through the whole fiber and form ring like membranous structure tubules are continuous with the ECF They are transversely oriented regularly spaced one per sarcomere and they extend in a ring around the myofibril If you magnified it you would see the picture on the bottom left Flanking the t tubule on either side are the membranous sacks of the SR An action potential that is going down the muscle fiber is able to spread into the ttubular system because around the opening of each t tubule are high concentrations of voltage gated Na channels which act as current amplifiers that allow Na into the tubule depolarizing it If we zoom in even further on the connection between the t tubule and the SR we get the box on the right hand side Recognize that a membrane potential exists across the membrane of the t tubule because the lumen is continuous with the outside the membrane is polarized with the membrane potential so the inside is negative with respect to the outside In the t tubular membrane there is a protein called the dihydropyridine receptor called this because dihydropyridine is the class of drugs that is used to block this system This receptor is a voltage sensing protein but not a channel This receptor is connected to a Ca releasing channel called the ryanodine receptor through the dotted line which are proteins that serve as mechanical linkages between the voltage sensing dihydropyridine receptor on the left and the Ca channel on the right It undergoes a conformational change when the t tubular membrane is depolarized and the links pull and push and cause the Ca channel on the right to open Because this channel is gated by mechanical forces transmitted by links from the voltage sensitive dihydropyridine receptor it is said to be mechanically not voltage gated and voltage sensitive Now muscle fiber contractions are triggered by internal Ca concentration And at rest Ca concentration is low enough that there are no contractions This low internal Ca concentration is maintained by a Ca ATPase in the SR membrane a uniport that takes Ca out of the cytoplasm and pumps I into the lumen of the SR The Ca held inside the lumen of the SR is not free but is bound so that the pump isn t working across a gigantic Ca gradient These Ca ATPases in the smooth ER membrane of all cells are largely responsible for maintaining the low cytoplasmic Ca concentrations that all cells have by hiding the Ca in the lumen of the SR But now when the SR ryanodine Ca channels open there is an electrochemical gradient for Ca to go out into the cytoplasm raising intracellular calcium concentration allowing crossbridge cycling and consequently contractions to happen This continues until Ca is sequestered again Ca levels need to rise about 100x to activate contraction Random tangent by Herrera The total capacitance of a muscle fiber is way higher than that of an axon Total amount of membrane between inside and outside of a cell is way higher in a muscle fiber because of all the internal membranes How Calcium triggers contraction Tropomyosin is the long grey protein surrounding the actin and troponin is the brown kidney shaped protein Troponin has a Ca binding protein and at a Ca conc of 10 8 M or below Ca is NOT bound to TN and tropomoyosin is in the position shown where it is blocking the binding site on G actin for myosin In this state very few of the cross bridges are attached and the muscle is free to relax But when Ca rises 10 100x there is sufficient Ca to bind to troponin and the resulting conformational change causes tropomyosin to shift and uncover the binding site on actin to which cross bridges can bind And this cycle can continue as long as binding is permitted Muscles are characterized by how fast force builds up and decays due to one AP The rate at which force builds up during a twitch is a function of the ATPase activity of myosin Muscle cells carry several different genes for myosin that code for different isoforms of final proteins each different in a number of properties including ATPase activity Which gene gets expressed and what mixture of different myosin is made is different in different organs and individuals and in different developmental stages The rate at which force falls off is determined by how fast Ca is resequestered and that is a function of the number and activity of Ca pumps in the SR So these two processes ATPase activity and Ca pumping determines the duration of the twitch Force Length Relationship Experimentation on muscle follows the same principle pass a current through a whole organ enough for one AP and see how it behaves However muscles not only generate force but they also shorten It is better to look at these variables separately because they can be exclusive For example it is possible to try to pick something up but be too weak in this case force would be generated but length wouldn t change This would be called an isometric contraction and it is shown in the experiment pictured above Simply fix a muscle at a certain length stimulate it and see what force it generates Then stretch it a little but keep it fixed stimulate it and record again What you find as you stretch it more and more is that the force generated gets weaker If you fix it at a shorter length force falls off as well Recording the data creates a U shaped curve between force and length This can be understood to be a function of the extent of overlap between thick and thin filaments L 0 the length at which you get maximum force occurs when the muscle has optimal overlap between filaments As you stretch the muscles more you get the situation on the right where there is very little overlap between filaments


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