A nerve cell that transmits electrochemical impulses. Neurons were thought to work somewhat like electric switches: when triggered at the dendrite end by an incoming nerve pulse, they generate a pulse down the axon (with help from the Nodes of Ranvier, which work like signal boosters) to the terminal button, where the pulse triggers the release of neurotransmitter chemicals into the synpase, the space between one neuron and another. These chemicals then trigger a pulse (or inhibit one) in other neurons. As it turns out, however, each neuron is more like a microchip than a simple switch. At any moment only abut 5-10% of the human brain's neurons are sparking, but eventually they all do, a fact that contradicts the common but inaccurate idea that people use only a small portion of their brains. (How much of the brain a person uses at once, or regularly, is another matter. ) The adult brain contains about 10 billion neurons, with the brain as a whole drawing 20-40 watts of power, and with an ultimate storage capacity of (very roughly) 100 terrabytes: about the same as every book ever written, digitized. It's hard to estimate because each dendrite contacts about 10,000 other neurons in extraordinarily complicated neural nets.