Excitation:AP conducted down the T-Tubule Excitation Ca2+ 2 Ca2+ Sarcotubule Ca2+ Cisterna troponin C 1-L-type Ca2+channel 2-Ca2+-gated Ca2+channel(ryanodine receptor) Notable exceptions to this scheme: Atrial myocytes:Do not have T-tubules;Calcium events have to move into the cell interior by diffusion and are thus slow.As a result they are relatively slow contracting. Purkinje fibers.Do not have T-tubules.These are cells that are specialized for conduction,not contraction.They are more like an axon than a muscle cell.They are minimally contractile