Glossary

• Action Potential ... A type of neuron output signal which does not lose strength over long distances. Commonly called "nerve impulse", and when an action potential occurs, the neuron is often said to "fire an impulse".
• Axon ... A long "branch" of a neuron which carries the output signal (action potential) from the cell. A single axon may be as long as a meter. Contrast dendrite.
• Central Nervous System (CNS) ... The set of neurons, fibers, and support structures within the skull and spine. Contrast peripheral nervous system.
• Dendrite ... A "branch" of a neuron which conducts voltages to transfer information from one part of a cell to another. Dendrites typically serve an input function for the cell, but many dendrites also have output functions. Contrast axon.
• Electron Microscopy ... In general, any form of high-resolution imaging which uses a beam of electrons to probe the sample. Practical resolution for biological samples is about 2 nm, though the theoretical limit is about an order of magnitude higher.
• Gap Junction ... A place where the cell membranes of two cells come in direct contact, allowing electrical potentials to be conducted directly from one to another. Contrast synapse.
• Glia ... Cells in nervous system which are not neurons, but serve various support functions (e.g., provide myelin for axons, clean up after cell damage or death, etc.). Some evidence indicates that certain types of glia (esp. astrocytes) may serve information-processing roles as well.
• Neuron ... A cell specialized for processing information. A typical neuron has many input branches (dendrites) and one output branch (axon), though there are many exceptions. The human brain contains roughly 10^12 neurons.
• Myelin ... A fatty substance which surrounds many axons, enabling them to conduct action potentials more quickly.
• Peripheral Nervous System (PNS) ... The set of neurons, nerve fibers and support structures outside the brain and spine. Contrast central nervous system.
• Synapse ... A chemically mediated connection between two neurons, so that the state of the one cell affects the state of the other. (In some cases, a cell makes a synapse onto itself.) Synapses typically occur between an axon and a dendrite, though many other arrangements also occur.