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Figure 7–5 shows myofibrils surrounded by the T tubule–sarcoplasmic reticulum system. The T tubules are very small and run transverse to the myofibrils. They begin at the cell membrane and penetrate all the way from one side of the muscle fiber to the opposite side. Not shown in the figure is the fact that these tubules branch among themselves so that they form entire planes of T tubules interlacing among all the separate myofibrils. Also,where the T tubules originatefrom the cell membrane, they are open to the exterior of the muscle fiber. Therefore, they communicate withthe extracellular fluid surrounding the muscle fiber, and they themselves contain extracellular fluid in their lumens. In other words, the T tubules are actually internal extensions of the cell membrane. Therefore, when an action potential spreads over a muscle fiber
membrane, a potential change also spreads along the T tubules to the deep interior of the muscle fiber. The electrical currents surrounding these T tubules then elicit the muscle contraction.
Figure 7–5 also shows a sarcoplasmic reticulum, in yellow. This is composed of two major parts: (1) large chambers called terminal cisternae that abut the T tubules, and (2) long longitudinal tubules that sur-round all surfaces of the actual contracting myofibrils.
One of the special features of the sarcoplasmic reticu-lum is that within its vesicular tubules is an excess of calcium ions in high concentration, and many of these ions are released from each vesicle when an action potential occurs in the adjacent T tubule.
Figure 7–6 shows that the action potential of the T tubule causes current flow into the sarcoplasmic re-ticular cisternae where they abut the T tubule. This in turn causes rapid opening of large numbers of calcium channels through the membranes of the cisternae as well as their attached longitudinal tubules. These channels remain open for a few milliseconds; during this time, enough calcium ions are released into the sarcoplasm surrounding the myofibrils to cause con-traction.
Calcium Pump for Removing Calcium Ions from the Myofibrillar Fluid After Contraction Occurs. Once the calcium ionshave been released from the sarcoplasmic tubules and have diffused among the myofibrils, muscle contrac-tion continues as long as the calcium ions remain in high concentration. However, a continually active calcium pump located in the walls of the sarcoplasmic reticulum pumps calcium ions away from the myofi-brils back into the sarcoplasmic tubules.This pump can concentrate the calcium ions about 10,000-fold inside the tubules. In addition, inside the reticulum is a protein called calsequestrin that can bind up to 40 times more calcium.
Excitatory “Pulse” of Calcium Ions. The normal restingstate concentration (less than 10-7 molar) of calcium ions in the cytosol that bathes the myofibrils is too little to elicit contraction. Therefore, the troponin-tropomyosin complex keeps the actin filaments inhib-ited and maintains a relaxed state of the muscle.
Conversely, full excitation of the T tubule and sarcoplasmic reticulum system causes enough re-lease of calcium ions to increase the concentration in the myofibrillar fluid to as high as 2 x 10-4 molar concentration, a 500-fold increase, which is about 10 times the level required to cause maximum muscle contraction. Immediately thereafter, the calcium pump depletes the calcium ions again. The total duration of this calcium “pulse” in the usual skeletal muscle fiber lasts about 1/20 of a second, although it may last several times as long in some fibers and several times less in others. (In heart muscle, the calcium pulse lasts about 1/3 of a second because of the long duration of the cardiac action potential.)
During this calcium pulse, muscle contraction occurs. If the contraction is to continue without inter-ruption for long intervals, a series of calcium pulses must be initiated by a continuous series of repetitive action potentials.
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