Calcium channel

Calcium channels are very effective in signalling processes such as triggering muscle contraction or initiating embryonic development as the calcium ions (Ca²⁺) have a very low concentration in the cytosol, (~10^-7M) where as its concentration in the extra cellular fluid and in the lumen of the endoplasmic reticulum (ER) (and sarcoplasmic reticulum (SR)) is high (~10^-3M).

This therefore creates a gradient for Ca²⁺ ions to rush into the cytosol and activating Ca²⁺ responsive proteins in the cell. Calcium channels in the plasma membrane open in response to a ligand binding or membrane depolarisation such as the depolarisation of the pre-synaptic membrane in nerve cells. This causes the calcium channels to open releasing Ca²⁺ ions. The release of Ca²⁺ ions causes the vesicles to fuse with the pre-synaptic membrane and release their neurotransmitters across the synaptic cleft to the post synaptic membrane. This process allows the depolarisation of the presynaptic membrane to be passed along to the postsynaptic membrane, carrying the action potential to the target cell(s).