The brain contains over 100 billion specialized cells called neurons. Neurons can be thought of as the basic processing units of the brain and convey signals by passing electrical impulses called action potentials from one end of themselves to another. Action potentials are generated by the opening and closing of microscopic gates in the cell membrane. These gates are specialized to permit only the passage of certain ions, and the flow of these ions creates electrical current. Action potentials are created at the cell body and are transmitted down the part of the neuron called the axon, which is sheathed in a fatty insulating substance called myelin. At the end of the axon, information is transferred to the next neuron via a specialized structure, called the synapse.
Although most neurons work in basically the same way, they come in many different shapes and sizes, specialized for their particular function. Here are a few examples commonly encountered in the brain:
Pyramidal cells activate other neurons, sometimes making connections with neurons that are quite far away. For this purpose they have long, insulated axons to carry the electrical impulses.
Interneurons (e.g., chandelier cells, double bouquet cells, spiny stellate cells, basket cells) have short dendrites and axons, and create network interactions between neighboring neurons. There are many different types of interneurons, but most are inhibitory, i.e. they suppress the activity of other neurons that are connected to them.