what are T tubules? describe their function and location
the T tubules surround myofibrils. The T tubules come in close apposition to the SR. Where the T tubules come into close apposition to the SR, there is a specialized coupling of the proteins in the two membranes. A voltage-sensitive cluster of four calcium channels, called a tetrad, in the T tubule membrane is linked to a calcium release channel in the SR.
The arrival of an action potetnial in the T tubule membrane causes a conformational change in the voltage-sensitive tetrad of channels, which opens the calcium release channel in the SR membrane. Some Ca ions flow through the tetrad, and even more Ca ions flow through the caclium release channel, and the resulting increase in free Ca ions in the cytosol causes the myofibril to contract