Action potential: Also known as a nerve impulse or spike. A fast, short change in voltage that neurons use to send signals. See "About the Action Potential" in the "Background" section for more information.

Anterior: Front. Opposite of "posterior."

Axon: A long, thin tube extending from a neuron's cell body. Axons transmit information over long distances through action potentials. In this lab, axons carry information from the leech's skin to its ganglia, then toward the front and the back of the leech.

Blood sinus: A major blood vessel of the leech. The ventral blood sinus surrounds the ventral nerve cord and is made of a tough, dark-green material.

Brain: A leech has two brains, one at each end of its body. Each brain is made up of ganglia that have fused together.

Cell body: The main body of a neuron, which contains its nucleus.

Dendrite: A thin tube on a neuron's cell body. Dendrites receive and process information from other neurons. In this lab, you'll see dendrites as the many fine branches on neurons near the center of a ganglion.

Dorsal: Upper side. For example, the dorsal fin of a shark is the fin on its back. Opposite of "ventral."

Electrode: A conductor used to make electrical contact with a nonmetallic substance, such as the cell of a leech. Electrodes are used to record the electrical activity of neurons.

Fluorescent dye: A chemical that emits light when it absorbs electromagnetic radiation (often UV light) from another source. A fluorescent dye can be injected into neurons to view their anatomy under UV light.

Ganglion (plural: ganglia): A mass made up of cell bodies of neurons. The leech has 21 ganglia in its body segments, each containing 350 neurons.

Labeled lines: The concept that each neuron carries a specific type of information. For example, one neuron may carry touch information from the left big toe. When this neuron is active, the brain "knows" that something is touching the left big toe.

Neuroanatomy: The study of the anatomy of neurons or the nervous system.

Neuron: Also known as a nerve cell. A cell that receives, processes, and transmits information through electrical and chemical signals.

Neurophysiology: A study of the physical and chemical processes that underlie the function of neurons. This lab focuses on electrophysiology, the study of the electrical properties of cells and tissues.

Nerve: A bundle of axons that connects a ganglion and/or the brain to other parts of the body.

Nerve cord: A bundle of axons that connects ganglions to each other and the brain, similar to the spinal cord in humans. In the leech, the nerve cord is located ventrally.

Posterior: Rear. Opposite of "anterior."

Resting potential: The voltage of a neuron at rest. Most neurons have a negative resting potential inside with respect to the outside. See "About the Resting Potential" in the "Background" section for more information.

Segment: A repeating unit that makes up the bodies of certain organisms, such as leeches and other worms.

Synapse: A specialized structure that transfers information when two neurons make contact.

Ventral: Under side. Opposite of "dorsal."

Voltage: A difference in the electric potential between two points. The voltage of a neuron is measured as the difference in the electric potential inside the neuron compared to a reference point connected to the ground, which is at 0 volts. This voltage can increase as charged ions accumulate across the cell membrane.