So membrane. So SR basically stands for sarcoplasm reticulum. It's the specialized type of er which is basically used to store the calcium ions found within that muscle cell. So let's remember how the muscle contraction actually takes place. So our nervous system basically generates this action potential and electrical signal that moves into that muscle cell. And once it moves into the muscle cell, what happens is within the membrane of the sarcoplas reticulum, we have these protein channels, not protein pumps, but protein channels.