The pacemaker of the heart consists of this type of cell. It is responsible for adjusting the heart rate in response to various stimuli such as parasympathetic stimulation which decreases heart rate and sympathetic stimulation which increases it. This mind map examines the physiology behind these activities.
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 mind map, keep population dynamics in mind.
The overall circular structure represents one cardiac autorhythmic cell. Beneath the cell is a graph of the action potential that is the result of a series of ion fluctuations across the cell membrane.
At the 12 o'clock position on the map a pair of ovals represents a closed potassium channel. To its left is a circle with a curved block arrow pointing to a calcium ion outside the cell; this represents a calcium pump that removes these ions from the cell. To the left of the pump, at the 10 o'clock position, is a pair of ovals with a curved block arrow passing between them. This represents an open potassium channel allowing the efflux of potassium.
At the 1 o'clock position is a circle with two curved block arrows. This is the sodium- potassium pump moving potassium ions into the cell in exchange for sodium ions which are pumped out. To the right sodium ions are shown re-entering the cell though a passive sodium channel (cylinder).
At the 2 o'clock position is a cylinder representing a gap junction through which calcium ions are diffusing into an adjacent cell.
At 3 o'clock a pair of ovals represents a closed calcium channel and below it an open calcium channel is represented as a pair of ovals with a block arrow passing between them; calcium ions are shown diffusing into the cell from the interstitial fluid.
At 6 o'clock a triangle represents a beta 1 receptor at the outer membrane surface of the cell. The sympathetic nervous system is releasing norepinephrine which will bind to this receptor. The triangle at the inner membrane surface represents a stimulatory G-protein. This protein is composed of three subunits (alpha, beta and gamma) shown as two small hexagons; the alpha subunit is shown moving away through the membrane.
At 8'oclock is another pair of triangles; the outer one is a muscarinic 2 receptor shown receiving acetylcholine from the parasympathetic nervous system, the inner one is an inhibitory G-protein. The subunits of this G-protein are both shown traveling through the membrane.
At 7'oclock is a circle representing the membrane-bound enzyme adenyl cyclase. It is catalyzing the intercellular conversion of ATP to cyclic AMP. The other circle inside the cell represents the enzyme phosphodiesterase that converts cAMP to its inactive form, AMP.
This tutorial is divided into four mini-tutorials listed below. An on-line image of the section of the map under discussion will be visible but it is handy to have a printed image of the entire map for an overall perspective.
Last updated:7/16/2005