Properties
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These are extensions of the sarcolemma.
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They contain extracellular fluid (high in Ca and Na ions)
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They are continuous tubes of sarcolemmal membrane that run through (transversely) the muscle fiber.
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In mammals the T-tubules lie at the boundary of the A and I bands (so there are 2 tubules per sarcomere).
Function
To some extent this explanation suffers from 'chicken and egg' syndrome. For it to make sense you have
to understand and accept certain things about the SR membrane and how its activity is regulated.
However, let's not let that put us off.
You need to be reminded of some things first, however.
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A muscle fiber contains many 10's of microfibrils (see practical support material).
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Each microfibril runs parallel to the fiber
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Each microfibril is a functional unit of the muscle, it shows sarcomere striations and each sarcomere is assocated with an SR membrane (see below)
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For synchronous contraction, each SR surrounding each sarcomere in every microfibril (even though deep within the fiber) has to be simulated simultaneoulsly.
So, the t-tubule serves to propagate the sarcolemmal action potential deep into the fiber, bringing the excitation
close to the SR membrane that surrounds each sarcomere.
