when a neuron fires
Excitatory vs. Inhibitory inputs. Some inputs to a neuron are excitatory which depolarize a neuron while others are inhibitory and polarize the neuron. As can be seen in the graph below the neuron will not fire until the summation of the inhibitory and excitatory inputs cross a depolarized threshold. One the neuron fires as a result of releasing the signal it will hyper-polarize so that it cannot firing again for a certain time period. http://scienceblogs.com/clock/20... Also not all neurons have the same amount of inputs. Lets look at an example of a type neuron that does not fire all the time. Most neurons in the barrel cortex (http://en.wikipedia.org/wiki/Bar...) do not fire unless there is a somatosensory event that activates them (in this case whiskers moving in some way.)