Calcium channels open when the the threshold potential is reached; they are therefore called 'voltage-regulated.' The influx of these ions causes depolarization. Calcium ions diffuse to adjacent cells via gap junctions and cause them to depolarize.
Again, we are dealing with a population of channels. The closed state is shown as a pair of ovals that are touching. The open state is represented as a pair of ovals separated by a block arrow. There is a small block arrow labeled '>-40 mV' pointing from the closed to the open state. This symbolism implies that the calcium channels are voltage-gated rather than chemically-gated. A transmembrane potential greater than (i.e., more positive) -40 mV sets the channels to their open state; one less than (i.e., more negative) -40 mV sets them to their closed state. The -40 mV value is called the threshold potential. Individual channels that have cyclic AMP attached to their interior surfaces are more responsive to this voltage signal.
These channels remain open for 100-150 msec depending on the presence of cAMP. When the channels are open calcium ions diffuse into the cell along their gradient (block arrow). This calcium influx causes the steep rise in the action potential graph (section B) called depolarization. The accumulating free calcium ions:
The rapid increase of free calcium ions (block arrow) is the trigger for the heart beat. Although autorhythmic cells contain few contractile fibers themselves, they do serve as capacitors that temporarily store positive ions (i.e., calcium and potassium). Accumulation of these ions facilitates their diffusion (block arrow) into adjacent contractile cells via gap junctions (cylinder) initiating the contraction of these cells. (See Cardiac Contractile Cell .)
Last updated:7/16/2005