Thus a receptor may be a protein, enzyme, membrane channel, or a region of a foreign pathogen.

Calcium is stored in the endoplasmic reticulum (er), where it is buffered by calcium binding chaperones such as calnexin and calreticulin, and is released via the IP3 Receptor channel (1).

But antipain specificity is ensured by allowing it to enter via the TRPV1 channel, a capsaicin receptor found only in pain-sensing neurons.