Muscle Figs

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Muscle Figs

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Lecture Note
University of Southern California
Bisc 307l - General Physiology
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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 t-tubular 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. ...

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