This applet simulates the Beeler-Reuter model in a 1D cable or 1D ring of any length between 0.5 cm and 20 cm. Initiation by a stimulus, propagation, and complex dynamics such as discordant alternans can be studied.To learn more about the model in a single cell, use the BR single cell applet.

Features:

• The Start button initiates the applet. An action potential can be produced either by pressing the Apply S1 button or by clicking inside the panel with the mouse, which will produce a stimulus that will generate an action potential if large enough.
• The length can be changed at any time by using the Cable Length slider. The new length is automatically indicated on the horizontal axis (minimum length is 0.5 cm, maximum is 20 cm).
• To observe AP initiation or propagation in more or less detail, the simulation speed can be increased or decreased at any time by using the Simulation Speed slider.
• When initiating a stimulus using the Apply S1 button, attributes of the stimulus can be set by using the Strength, Duration, Length, and Position text boxes.
• The list box in the bottom left corner is used to choose either a cable or a ring simulation. Note: using a long cable before changing from a cable to a ring is recommended to prevent self-termination of the wave.
• The list box in the bottom right corner is used to choose either the original Beeler-Reuter (BR) model or the what is commonly known as the Modified Beeler-Reuter (MBR) model, whose calcium dynamics are twice as fast with consequent action potential duration and wavelength shortening.
• The gna, gnac and gs text-boxes are used to set the sodium and calcium conductances used in the simulation.
• The check boxes on the right indicate which variables are plotted.
• The Reset button brings the membrane potential of all the cells back to the rest state.
• The Stop button is used to stop the simulation at any time. The simulation can be resumed by using the Continue button that replaces the Stop button when the simulation is stopped.

Suggested Experiments:

• Click the Start button and then the Apply S1 button to generate an action potential propagating to the right. As the wave propagates, enlarge the cable to about 16 cm using the Cable Length slider at the top right. When the wave front is close to the right boundary, click the Stop button and use the list box in the lower left corner to change the cable to a ring, then click the Continue button. This allows the wave to reenter, and after a few rotations around the ring it will be a steady wave with a fixed wavelength. Decrease the ring size slowly, and observe the formation of discordant alternans at a length of 13 cm.Repeat as above, but change the sodium and/or calcium conductances and find the lengths at which alternans appear.
• Reset using the Reset button and study the threshold for excitation as a function of the stimulus parameters (strength, duration, and length) as well as the sodium and calcium conductances.