This is a cardiac muscle cell. Its role is to contract and eject blood from the heart. Sympathetic nervous system stimulation increases its force of contraction. This model examines the physiology behind these events.
Althouth the various elements are symbolized only once they exist as populations. Not all 'individuals' in a population respond exactly the same; there is an 'average' response to stimuli. Thus, when interpreting this model, keep population dynamics in mind.
The overall circular structure represents one cardiac contractile cell. Beneath the cell, at the lower left, is a graph of the action potential caused by ion fluctuations across the cell membrane. Above the graph and to the left of the cell is a portion of an adjacent cell. This cell connects to the contractile cell via gap junctions (cylinder). The cell could be another contractile cell or an autorhythmic cell.
Paired ovals located in the cell membrane (circular outline of the cell) represent ion channels. When the ovals are touching the channel is closed, when apart the channel is open. A thick black arrows indicates a channel changing from one state to the other.
Block arrows represent the diffusion of ions. Twisted block arrows represent the active transport of ions across a membrane; the ATP circle represents the pump.
Cylinders represent passive ion channels that are always open. The block arrow passing through a channel represents the diffusion gradient.
At the lower left third of the cell is a series of numbered paired ovals. These represent the four states assumed by a sodium channel. The small shaded oval between each numbered pair is a fast activation gate. The shaded circle associated with each numbered pair is a slow inactivation gate.
At the lower right is a ACase circle that represents the membrane-bound enzyme adenyl cyclase. The chemical reaction it catalyzes is indicated inside the cell. A second circle (PDase) represents the cytoplasmic enzyme phosphodiesterase. The two triangles just above the ACase circle represent a beta 1 receptor and a stimulatory G-protein. The hexagon represents the alpha subunit that has dissociated from the G-protein and is diffusing through the membrane to activate adenyl cyclase.
The irregular shaped object within the cell represents the sarcoplasmic reticulum. The paired ovals in its membrane represent open calcium channels. The ATP circle and twisted arrow also in the membrane represent a calcium pump.
Last revision: 7/16/2005