Neurons in the human brain vary widely in their size and shape. However, all neurons have some features in common. A neuron consists of a cell body, called the soma, with arms that reach out to connect to the network of other neurons in the brain. One arm, the dendrite, receives signals, while the other arm, the axon, sends signals. All electrical signals are unidirectional; they flow from the receiving point on the dendrite or cell body out through the axon. A typical neuron is pictured below.

The cell body of a neuron is called the soma. It contains the same organelles as other cells in the body, including the nucleus, mitochondria, endoplasmic reticulum, ribosomes, and Golgi apparatus. The basic functions that keep the cell alive, such as replication, respiration, and protein production, are carried out in the cell body.

The axon is a long, thin structure which sends out signals from the cell. The end of the axon is called the terminal bouton. Each signal travels along the neuron's axon to the terminal bouton, where it is then transmitted to the next neuron. The axon is covered in myelin, a thick substance that insulates the nerve to help transmit the electrical signal along the length of the axon. Periodic gaps in the myelin sheath, called nodes of Ranvier, allow the signal to refresh itself as it propagates along the axon. The small area, like a neck, that lies between the soma and the axon is called the axon hillock. Electrical charge builds up at the axon hillock until it is sufficient to generate an action potential. When there is enough charge to make an action potential capable of propelling itself all the way down the axon, the action potential initiates at the axon hillock and travels down the axon to the terminal bouton.

There are many types of dendrites, but, in general, a dendrite looks like a tree whose trunk ends in the soma. Its branches, called dendritic spines, are stretched out to receive signals from the axons of other neurons. Dendrites contain many receptors which can bind to signaling molecules called neurotransmitters. These receptors are sometimes also found on the soma. When these receptors bind enough neurotransmitter molecules, the neuron undergoes a change, becoming more or less likely to send a signal to the next neuron. The type of change depends on the type of neurotransmitter that is bound. Some neurotransmitters excite the cell while others depress it. Still others have cause various biochemical reactions within the postsynaptic neuron. We will see more on neurotransmitters later (see neurotransmitters).