Anatomy of a Neuron
A neuron consists of the following parts:
- Cell Body (soma) - Anatomically there aren't anything too special about the neuronal cell bodies themselves. Some are large, and in invertebrates, cell diameters can be as large as hundreds of microns. More typical size for a large vertebrate neuron would be tens of microns in diameter, smaller neurons are well under ten microns. Most somata are round, but there are other shapes also such as fusiform (elongated double-pointed cylinder)and pyramidal (somewhat conical).
- Axon - A neuron usually has one long thin tube extending from the cell body called an axon. Some neurons have none, some have more than one. The axons are specialized to transmit information from input regions (usually dendrites and the soma) to the output regions at the end of the tube called axon terminals. At the terminal, the axon makes a contact to another cell (typically a dendrite or a soma) and makes a specialized structure called a synapse.
- Dendrite - The soma have one to many tubelike processes called dendrites. Dendrites are specialized for receiving information. The information is transmitted from another cell to the dendrite by means of a synapse. The cell that transmits the information is called a pre-synaptic neuron, and the cell that receives information is called a post-synaptic neuron. A dendrite receives many "synaptic inputs" from many cells.
Of course, like in most things in biology, there are exceptions. That's why there are words like "usually" or "most" or "normally" all over the place in this document. For example, the leech neuron you are going to encounter do not work like the textbook neuron described above. In our touch sensitive neurons, the information starts at the periphery at the end of the axons and travel towards the cell body, but in fact the cell body isn't really involved. On the other hand, this anatomical arrangement is very similar to the human touch cells.
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