# Math Insight

### Applet: Neuron firing rate in response to input and nicotine levels

A fictitious representation of the firing rate $r(i,s)$ of a neuron in response to an input $i$ and nicotine level $s$. The firing rate increases with input $i$, but it increases more slowly as $s$ increases. The firing rate is zero below a threshold input that decreases with $s$. This leads to a fold in the surface that describes the function $r(i,s)$.