# What are tail currents?

This is a voltage clamp on an ion channel. When the voltage is stopped there is a still a current at the end (the tail current). I understand that the gates aren't completely closed because of which there is still some current flowing.

What I don't understand is that why does the current suddenly jump to a small value? Shouldn't it gradually go down?

• I'm voting to close this question as off-topic because it appears to be a physics question Commented May 29, 2016 at 7:09
• @rg255 it's Biophysics and on protein ion channel gating. Bio, most definitely
– AliceD
Commented May 29, 2016 at 8:17
• This trace looks a bit funny - I am guessing the OP has abandoned the site, but if OP happens to return and can give a source for this image that will really help with providing an explanation specific to this example. Commented Feb 23, 2017 at 18:15
• @JasonC The artifact would be from a failure to normalize the pipette capacitance. So it wouldn't have anything to do with the biology but rather an incorrect setup of the patch clamp amplifier. In a good electrophysiology experiment you should not see that artifact, the traces should look like Figure 4 of the paper you linked. Commented Feb 23, 2017 at 20:39
• @JasonC And to explain the biology a bit, there are two things going on here: 1) The experimenter is applying a constant voltage using a patch clamp amplifier. On each trial, that constant voltage starts at one value, briefly steps to another value, and then back to baseline. 2) The step in voltage is activating voltage-gated ion channels. The opening of these biological channels changes the amount of current that needs to be applied to maintain the voltage specified - that current reading is what is plotted in the OP's figure. Those channels do not close instantly, which produces tail current Commented Feb 23, 2017 at 20:42

OK, I know this is old, but here goes. The tail current is the current at -55mV after the voltage step, here:

The current shrinks as the voltage changes, this causes a massive shift in the driving force for the ions either side of the cell membrane. Going from -125 to -55 mV for the largest step. As commented, the dense vertical bands either side of the step are due to fast capacitance readings, basically ignore unless you want to go down a whole other avenue.

The reason for the tail currents is some channels remain open and close gradually after the step ends "tailing" off. We can't really say what channels these are as we know nothing about the recording conditions/cells involved. However these currents are large, so most likely to be sodium/potassium.

I think I understand the question a little better now. When voltage increased, OP expected an exponential decay but instead saw a peaking to positive value and then going back down to negative value at which exponential decay happens.

Pretty much think of this as two current sources superimposing on each other. One is the current caused by difference in voltage (Ih). Ih is a mix of potassium and sodium current. While it is voltage dependent, it doesn't follow simple Ohm's law as this has to do with activation of channels rather than membrane. source The other is current caused by capacitive property of the membrane. Since capacitive current is proportional to rate of change in voltage, increase in voltage results in positive capacitive current.

So when voltage increased from -125 to -55 mV, there is a positive capacitive current as well as exponentially decaying Ih. These two superimposing each other almost leads to the picture we see in OP's question. I cannot explain why there is the huge negative spike in current after the small positive peak though.

Source: I learned this in class one year ago.

• This answer doesn't answer the actual question about tail currents and contains several errors. Actually the OP is a better source for what tail currents are from: the channels that are still open and haven't yet returned to the equilibrium at the new voltage. Commented Feb 23, 2017 at 18:13
• @BryanKrause do you mind checking to see if there's still errors? More on those errors would be appreciated. Commented Mar 19, 2017 at 17:14

Tail currents are observed because current $I = g(V-E)$ suddenly changes with $V$, whereas $g$ changes gradually following some kinetics. Thus, at sudden transition of voltage $V$, $I$ changes by a factor, which is observed as tail current with a jump.

• This is correct and might answer the title question, but it doesn't actually answer the OP. Unfortunately this question has sat for quite some time without feedback from OP and we probably need to get rid of it now. Commented Sep 27, 2017 at 17:01
• OP stands for "original poster"/"original post", in this case the question/question asker. Oliver Houston's answer and my comments address the question the OP was actually wondering about, which was the presence of the sharp transients in addition to the tail currents. Commented Sep 27, 2017 at 17:05
• How is jump not explained by this ? Commented Sep 27, 2017 at 17:07
• If you plug some numbers into the equation in your answer, it would be obvious why you don't get what the OP shows. The paper linked up in the comments by JasonC has an example of what tail currents look like when not impacted by artifacts: jn.physiology.org/content/79/5/2345 Commented Sep 27, 2017 at 17:27
• when you close a switch, an arc of lightning jumps in the air because the voltage rises at that particular point in space momentarily towards a theoretical infinity as the current behind the closing switch places pressure on the edge electrons and force them to jump. that change in voltage is similar to a tail current. Commented Sep 27, 2017 at 21:26