neuron

A neuron is one nerve cell, and it is built as a one-way street: three working parts plus the output they send. Dendrites receive signals from other cells and carry them toward the cell body, which holds the nucleus and adds the arriving signals up; if the total is strong enough, a new impulse starts and travels down the single long axon, away from the cell body, to the axon terminals, where the signal is passed to the next cell — another neuron, a muscle or a gland. Some axons wear a fatty myelin sheath that speeds the impulse along by letting it jump between the gaps in the wrapping. The direction is written into the parts, which is why an item that runs the impulse backwards — terminals first, dendrites last — is a mistake about the neuron's anatomy rather than a detail of timing.