Question: Does the human brain use a specific activation function?

I've tried doing some research, and as it's a threshold for whether the signal is sent through a neuron or not, it sounds a lot like ReLU. However, I can't find a single article confirming this. Or is it more like a step function (it sends 1 if it's above the threshold, instead of the input value)?

Answer: 
The answer is **We do not know**. Odds are, we will not know for quite a while. The reason for this is we cannot understand the "code" of the human brain, nor can we simply feed it values and get results. This limits us to measuring currents of the input and output on test subjects, and we have had few such test subjects that are *human*. Thus, we know almost nothing about the human brain, including the activation function.
My interpretation of the question was 'what activation function in an artificial neural network (ANN) is closest to that found in the brain?'

Whilst I agree with the selected answer above, that a single neuron outputs a dirac, if you think of a neuron in an ANN as modelling the output firing rate, rather than the current output, then I believe ReLU might be closest?

<http://jackterwilliger.com/biological-neural-networks-part-i-spiking-neurons/>