 Tonic firing

Tonic firing is a sustained pattern of individual action potentials rather than brief, separated clusters characteristic of burst firing. In a specified preparation it may be relatively regular, but tonic does not require perfectly equal interspike intervals or one universal firing frequency. The term concerns timing, not a permanently fixed identity of the neuron.

For an invented one-second recording, ten action potentials at $0.05,0.15,\dots ,0.95$ seconds give a mean count rate of $10{\mathrm{s}}^{-1}$ and consecutive interspike intervals of $0.1$ seconds. Two clusters containing five spikes each can have the same total count with very different intervals. The average rate therefore does not specify whether a train is tonic or bursting.

Recordings from the lateral habenula in a 2018 study distinguished tonic and bursting modes and examined changes associated with membrane potential and receptor-dependent currents.^[ref] The same cell can change its temporal pattern under changed conditions. This illustrates why a firing-mode label must be attached to the recorded interval and experimental state, rather than inferred solely from the cell's name.