The human brain contains billions of neurons and supporting cells. What is a neuron? Neurons are cells in the central nervous system that communicate with each other at certain points on their surfaces called synapses. Because neurons are so small, they are difficult to study; we will learn about the methods that researchers use in the section on methodology. Neurons are specialized to conduct electrical signals, called action potentials, along arm-like appendages called axons. In order to communicate, neurons send each other messages via molecules called neurotransmitters. We will learn more about this communicative process in the sections on the structure of neurons, the action potential, and neurotransmitters. The brain processes information through circuits of neurons that are modified over time according to the organism's biological development and the experiences it has. Glia, which we will study in the section on supporting cells, are incapable of signaling, but carry out other functions necessary to keep these neural circuits running smoothly.