||The English used in this article may not be easy for everybody to understand. (November 2011)|
A cell may have hundreds or thousands of dendrites, but may have only one axon. The dendrites carry signals from other neurons into the soma, whereas the axon carries a single signal from the soma to the next neuron or to a muscle fiber.
A dendrite from one neuron and an axon from another neuron meet at a synapse, which is a very narrow gap between the two cells. When electrical impulses (action potentials) reach the end of an axon, the signal must pass over a synapse again. There are two types of synapse: chemical or electrical. If it's a chemical synapse, the active potential triggers the release of special chemicals called neurotransmitters from the axon. These chemicals cross the synapse to the dendrite, where they trigger the flow of ions into or out of the cell.
The movement of the charged ions in the dendrite causes an electrical current, which spreads to the soma briefly before being restored to normal. If the current is positively charged, it will make the soma (which is normally negatively charged compared to its surroundings) become more positive. If the current is negatively charged, then it will make the soma even more negative. If a strong enough positive charge builds up in the soma quickly enough (caused by the dendrites receiving lots of signals from axons), then it will set off a chain reaction in a part of the soma called the axon hillock. The axon hillock is the place where the soma connects to the axon. The chain reaction that begins in the axon hillock is a strong electrical current called an action potential that flows down the axon to the next synapse.