"Excitability is a natural, physical property of nerve cells. In every neuron, the membrane potential undergoes ceaseless fluctuations in voltage. Those fluctuations are due in large part to the random release of vesicles of neurotransmitters at some of the neuron synapses. In the final analysis, this randomness arises fron thermal noise, whitch constantly rocks and rolls our mollecules around. One would think that evolution would minimize the impact of this noize, as engineers do in digital chips, when they set very distinct voltages for 0s and 1s that thermal noise cannot offset them. Not so in the brain: neurons not only tolerate noise but even amplify it – probably because some degree of randomness is helpful in many situations where we search for an optimal solution to a complex problem.
Whenever a neuron's membrane fluctuations exceed a threshold level, a spike is emitted. Our simulations show that these random spikes can be shaped by the vast sets of connections that link neurons into columns, assemblies, and circuits, until a global activity pattern emerges. What starts out as local noise ends up as a structured avalanche of spontaneous activity that corresponds to our covert thoughts and goals. It is humbling to think that the 'stream of consciousness,' the words and images that constantly pop up in our mind and make up the texture of our mental life, finds its ultimate origin in random spikes sculpted by the trillions of synapses laid down during our lifelong maturation and education."
from Consciousness and the Brain by Stanislas Dehaene