Dear Julieta,

There can be a number of reason why the APs may be small.

What neurons are you recording from? And what is the amplitude of the AP and the peak potential?

What intrapipette and extracellular solution do you use?

A reason why AP become smaller (and broader) is that the series resistance (R between amplifier and the neuron) with your pipette capacitance functions as a low-pass filter. This reduces the peak and broadens your AP. Do you measure these parameters? If yes, what are the values and do you use the bridge balance and capacitance compensation to minimize the effects?

For more information, Barbour’s Electronics for Electrophysiologists is good place to start (https://www.biologie.ens.fr/~barbour/electronics_for_electrophysiologists.pdf).

A second reason is that sodium channels tend to inactivate at depolarized potentials. If your cells are depolarized, the sodium currents as well as your APs will be much smaller. What resting membrane potentials do you get at breaking in and after 5 min?

If your compensation is optimized, have you tried to first hyperpolarize your cells to -80 mV (or go even to more hyperpolarized potentials) and then generate the sodium currents or your APs? This should allow sodium channels to recover from inactivation.

A third reason may be that you damage the axon close to the axon initial segment. As a result, the AIS may be lost or severely damaged. Then, you are left with somatodendritic sodium channels which are for many neurons less abundant and are less likely to open in an all-or-none manner. Did you try to make IV curves for the sodium currents?

A fourth reason, which is probably less relevant to your experiments, is that some neurons are extremely leaky at the soma. The large AP in the axon generates a current from the AIS to the soma, which generates only a small AP at the soma. Examples are auditory brainstem neurons in the medial and lateral superior olivary nuclei.

So for some neurons, small APs are physiological.

Good luck!

Hello Julieta,,

your Q does not have enough info to help you. You need to send your METHODs containing all relevant info (media in and out, what neurons, what animal…etc)

Perhaps, the configuration you made for the recordings is different. “Cell-attached” potential recordings would virtually cause the action potential spikes to be smaller. SN