Paramecium behavior in various testing solutions is indicative of the activity of specific subsets of the ion channels that control the underlying action potential. The action potential essentially controls the duration of an increase in internal calcium concentration by keeping the ion channel that lets in calcium, the voltage dependent Ca++ channel, open for as long as the action potential. The internal calcium concentration is what controls the direction of ciliary beating. At concentrations below 10-6 M the cilia tend to beat forward and at concentrations above 10-6 M they tend to reverse.

The ion channels that regulate the shape of the action potential are found in the cell membrane and their opening and closing is coordinated by voltage, calcium, calmodulin, and other unknown regulatory proteins (presumably protein kinases and protein phosphatases are involved). In response to soem stimuli, a localized deploarization occurs that is sufficient to activate the volatge dependent Ca++ channel. Calcium enters through the voltage-dependent Ca++ channels to initiate the action potential. It is followed by the opening of the voltage-dependent K+ channel which allows K+ to leave the cell and repolarize the cell. After the calcium enters via the voltage-dependent Ca++ channel, a series of calcium-dependent channels are activated; a calcium-dependent Mg++ channel, a calcium-dependent Na+ channel, and a calcium-dependent K+ channel. These channels act together to regulate the backward swimming response in Paramecium. Briefly, the individual roles of the various channels are as follows;

1) the voltage-dependent Ca++ channels allow calcium to come into the cell and depolarize (make the inside of the cell less negative) the cell. The incoming calcium initiates the backward swimming response.

2) the calcium-dependent Na+ channels open in response to the incoming calcium and act to lengthen the backward swimming response by lengthening the amount of time that the cell is depolarized (thus keeping the voltage dependent Ca++ channel open.

3) the voltage-dependent K+ channel and the calcium-dependent K+ channel act to repolarize the cell and therefore return the cell to a forward swimming behavior.
 

Work is now directed at understanding how the opening and closing frequencies of these channels are regulated by other proteins. Some progress has already been made, in that we now know from genetic experiments that the calcium-dependent Na+ and K+ currents are regulated by the calcium-binding protein, calmodulin. Thus, in a nicely efficient manner, the calcium that comes in from the voltage-dependent Ca++ channels acts (through calmodulin) to initially extend the time of backward swimming and then (after a period of time) to allow forward swimming to resume.

Paramecium also respond to stimulants like lysozyme or GTP. These are called chemorepellants and they provoke a sustained backward swimming response (up to 10-15 minutes). After that time the cells will 'adapt' and will begin to swim forward slowly. If you were to remove them and then test them again in GTP or lysozyme, you would find that they are still adapted. We know very little about the molecular mechanisms at play here.

To learn more about how to conduct a behavioral test using Paramecium click here.
Last updated Monday July 2, 2001 Webmaster Dean Fraga.