|
OK, the t-tubule provides the stimulus (depolarisation) and the SR contains calcium that is released in response to the stimulus.
How is the stimulus transmitted?????
This is a good question.
There is evidence of a physical connection between the t-tubule membrane and that of the SR - The so-called 'end feet' projections. Available evidence suggests that these end-feet projections allow the
t-tubule to cause the SR to release calcium. THE MECHANISM IS UNKNOWN.
It may be:
The end-feet appear to be composed of 2 proteins
-
Dihydropyridine (DHP) receptors (an integral protein in the t-tubular membrane)
-
Ryanodine receptors (sometimes called calium release channels) (an integral protein in the SR membrane)
-
The current theory would have you believe that the DHP receptor senses the voltage change across the t-tubular membrane and communicates this to the SR membrane via the ryanodine receptor. At the present time the jury os out over what the actual stimulus is - electrical or mechanical.
The ryanodine receptor protein structure is less well understood but there is good evidence that it contains a channel through which calcium can exit the SR and flood into the sarcomere - thus activating the myofibrils.
|
