 Dear students, in this module we shall discuss the voltage-gated calcium channels in detail. The voltage-gated calcium channels occur virtually in all cell types. They have selective permeability for calcium ions. Their activation allows the calcium ions to move into the cell. The structure of these channels is very similar to the molecular structure of sodium channels. However, the calcium channels act slowly as compared to the sodium channels. They are activated with depolarization about 10-20 times more slowly than the sodium channels. That is why they are called slow channels as compared to the fast sodium channels. Dear students, we shall discuss the role of calcium channels in neurons and skeletal muscles. In neurons and skeletal muscles, both the sodium voltage-gated sodium channels and voltage-gated calcium channels are present. When depolarization occurs, that is a stimulus acts, both types of channels open. Sodium channels open first, but calcium channels also follow them slowly. As a result, the inward depolarizing current contains both the sodium and calcium ions. However, the calcium current is comparatively weaker in these types of muscles and cannot initiate an action potential on its own. However, in many of the smooth muscles, sodium voltage-gated sodium channels are almost absent. These cells have large number of voltage-gated calcium channels. So, action potentials are caused entirely by the activation of voltage-gated calcium channels. Dear students, the action potentials produced due to the calcium channels and calcium current have similar properties as the action potentials produced due to the flow of sodium current. However, as the calcium channels act slowly, they open comparatively late due to threshold, but they also remain open for longer duration. That is why they produce action potentials of longer duration. Dear students, the calcium ions that enter the cell due to the opening of voltage-gated calcium channels perform two types of functions. First function is the production of action potential, which are propagative, but their second role is to act as intracellular second messengers. These second messengers subsequently produce many types of intracellular responses. For example, the release of neurotransmitter molecules and contraction of muscles.