A neuron can be defined as a nerve cell. The neuron is often thought of as the "building block" of the nervous system, and for good reason. The neuron is the fundamental unit which makes up a nerve pathway, neural firing (neurotransmitter release) takes place at the level of the neuron, and many aspects of the physiology-behavior relationship can be explained in terms of activity at the neuronal level.
Figure 1 is a graphic representation of the "typical" neuron. In a general sense, you can think of the neuron as a miniature self-contained information processor. It receives inputs, processes information, and generates outputs. The structure most associated with receiving is the dendrite, the structure most associated with processing is the cell body (also called soma), and the process most associated with the output is the axon, more specifically the terminal buttons (Figure 2 - Requires Shockwave plug in to run). If we move to a slightly more detailed level we will find that neural signals most often are received by specialized areas on the dendrites called dendridic spines. As for the cell body, the "processing" actually occurs in the nucleus, within the cell body. Continuing our analogy of the nerve cell as an independent information processor, we can think of the nucleus as the "brain" of the cell, where many important cellular activities are initiated, as we'll discuss in the internal structure section below. Also, it's important to note that a general characteristic of the terminal buttons is that they contain the chemical messengers, neurotransmitters, which we will talk about in more detail in the class lecture. These chemical messengers are responsible for communication among neurons.
Two other important neuronal structures are the synapse, and the myelin sheath. The synapse is the very small space or gap between adjacent neurons, and, as we will see, many important processes occur in this tiny gap. Finally, the myelin sheath consists of a fatty sheath that is wrapped around the axons of many neurons. The "wrapping" is interrupted by small spaces called nodes of Ranvier. The main function of the myelin sheath is to speed the neural signal.