For example, when a nerve impulse stimulates a muscle fiber to contract, calcium channels in the cell membrane open to allow calcium ions into the muscle cell. (lpi.oregonstate.edu)
I approached professors, explained that I was willing to work for free, and was soon involved in a molecular biology project concerning N - type calcium channels. (moneysense.ca)
But, for some unknown reason, they come to rely more on calcium channels as they mature. (scientificamerican.com)