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I am reading about dopamine, and I have realized that dopaminergic neurons can fire phasically or tonically. What is the difference between these two modes of firing?

egFace
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1 Answers1

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Tonic firing typically occurs without presynaptic input and can be viewed at as background activity. Tonic activity is often characterized by a steady action potential firing at a constant frequency. Note that not all neurons may have tonic activity at rest. It may serve as keeping a steady background level of a certain neurotransmitter or it can serve as a mechanism where both an inhibition or increase in presynaptic input can be transmitted. When a neuron is silent at rest, only an increase in presynaptic activity can be transmitted postsynaptically.

In contrast, phasic firing occurs after a neuron is activated due to presynaptic activity and it incurs activity on top of any background activity a neuron may have. It is typically restricted to one, a few, or a short burst of action potentials, whereafter the activity quickly returns to the resting state.

Reference
Tsai et al., Science (2017); 324: 1080-4

AliceD
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  • Could you please explain what causes tonic firing? I.e. how can a neuron depolarize itself? – Vivek Subramanian Jul 01 '17 at 21:35
  • @VivekSubramanian: cyclic changes in restig membrane potential or inputs from other neurons – AliceD Apr 11 '18 at 09:57
  • @AliceD , from the paper you reference, both tonic and phasic activity seem evoked, instead of tonic activity being passive background activity. The definition I would extract from that paper is instead that phasic activity consists of faster firing, maybe only bursting, and tonic firing is lower-frequency firing: Caption figure 2: "(D) Whole-cell recording of DA neurons showing spontaneous activity and tonic and phasic firing evoked by 1-Hz and 50-Hz light flash trains, respectively". Or: "low-frequency tonic activity and phasic bursts of action potentials" (first page) – dasWesen Feb 16 '23 at 13:15
  • Maybe tonic = constant for a longer duration (like muscle tone), whereas phasic = in phases ? – dasWesen Feb 16 '23 at 13:23