Glossary of Terms

• Action Potential: Also known as nerve impulse or spike. One generally talks about a cell "firing" or "generating" an action potential, or simply "spiking." An action potential is a brief change in membrane potential caused by the rapid opening and closing of transmembrane channels that pass specific ions through. Action potentials travel along axons and transfer information over distance. In this virtual lab, action potentials are generated in many of the cells you can find and appear as an almost vertical line superimposed on a horizontal oscilloscope trace. The total number and the rate of firing of action potentials can encode information, as well as the actual shape of the action potential (some are longer lasting than others, as you can see in the lab). See Background on Nervous System for more details.
  
  
• Amplifier: The amplifier stage is omitted in the virtual lab to simplify the controls, but in reality, between the electrode and the oscilloscope, there will be a special amplifier that costs between $500 to $8,000. This amplifier is designed to specifically deal with an electrode that is very high in electrical resistance, but that's another story.
  
  
• Anterior: A general anatomical orientation term that means towards the front, which is usually defined as the head of the animal. It can be used as "anterior sucker," meaning the forward sucker, or "The brain is anterior to the gut," meaning the brain is toward the front when compared to the gut. Opposite to posterior.
  
  
• Axon: A type of a thin tube extending from the cell body of a neuron. Axons are distinguished by their specialization to conduct nerve impulses (action potentials) for long distances, normally in a direction away from the cell body. We say "normally" because it's not always the case. In the touch sensitive cells encountered in this virtual lab, the cell bodies are located in the ganglion, but the action potentials are initiated in the body wall and travel towards the cell body. Also, a neuron can have more than one axon. Again, in our touch sensitive cells, there are axons that disappear from the field of view towards the left and right, in the direction of neighboring ganglia. These axons conduct the touch information to different parts of the nervous system, away from the cell body.
  
  
• Brain: The brain of the leech is a rather simple affair. It is essentially a set of specialized ganglia that have fused to become bigger.
  
  
• Dendrite: A type of thin tube extending from the cell body of a neuron. Dendrites are specialized to receive information from other neurons, and they do not usually conduct action potentials. In the cells featured in this lab, examples of dendrites can be seen as many finer branches near the center of the ganglion. Dendrites do not transmit information for very long distances, but are instead involved in processing the information they receive from other neurons.
  
  
• Dorsal: Another anatomical orientation term that means towards the top, e.g. "dorsal fin of sharks." In most animals, the top will be the animal's back. It gets a bit complicated in humans and other bipeds because we move perpendicularly to the axis defined by our backs. Opposite to ventral.
  
  
• Electrode: Electrodes are what you use here to record the activity of the neurons. The generic term "electrode" is defined as a conductor that is used to establish electrical contact with a nonmetallic substance. So an electrode is what one uses to deliver electric shock to the patient's head in electroshock therapy or to the patient's heart in the emergency room (that pair of iron-like things doctors hold when they are shouting, "Clear!") A glass microelectrode is used in our experiment.
  
  
• Electrolyte: Ionic solution that conducts electricity. Typically, it's created by dissolving some salt. It is used in the real version of our experiment to fill up the glass microelectrode. The choice of salt and the techniques developed to fill up the electrode are themselves interesting topics, but a little too esoteric perhaps for this lab.
  
  
• Electrophysiology: The study of life processes and of the physical and chemical processes involved, particularly the electrical aspects. These days, most scientists use the term "neurophysiology" instead of electrophysiology.
  
  
• Fluorescent Dye: A chemical that emits light when it absorbs electromagnetic radiation from another source. It is often used in neurobiology to view one or a selected number of neurons. The problem is that neurons are usually transparent (unless you are studying the gorgeously colored brain of some sea-slugs). This makes them hard to see. The neurons are also usually packed tightly together, which makes them almost impossible to see individually. Scientists have developed a bag of tricks to overcome this problem, and one of these tricks is to inject (or somehow stain) the neurons with fluorescent dyes. By illuminating the neurons with certain frequencies of light or near-light, the dye "lights up," making the study of their shapes and identities easier. Lucifer Yellow is an example of a fluorescent dye.
  
  
• Ganglion (pl ganglia): A ganglion is a collection of neuronal cell bodies. In the leech, the central nervous system consists of the brain and 21 segmental ganglia connected by the ventral nerve cord. Each ganglion contains 350 neurons, of which 14 respond to touch and pressure.
  
  
• Lucifer Yellow: A fluorescent dye that was introduced in 1978. It is based on the chemical luciferin discovered in 1888 that is present in luminescent organisms such as fireflies. It has proven to be a fiendishly useful product with many applications. In this experiment, it is used in a dissolved form and injected into a neuron, but it has also been used with good effect for immunohistochemistry. Lucifer Yellow fluoresces with bright yellow-green color when stimulated with ultraviolet (wavelength 280 nm) or blue (wavelength 430 nm) light. Because of this, to view the fluorescence, you need an expensive halogen or mercury high-pressure lamp and appropriate filters to block out UV, which can damage your eye. This set-up will cost $3,000 or more, in addition to the cost of the microscope.
  
  
• Microelectrode: An electrode that is very very small. It can be made from a coated tungsten metal or other metal under special fabrication techniques, but in this lab, we use a glass microelectrode. A glass microelectrode is made with a specialized "electrode puller" that heats up the middle of a fine glass tube (usually about 1 mm diameter and about 10 cm in length) and pulls two ends of it apart at high velocity with or without rapid cooling. The result is two electrodes, each with a very fine tip, down to smaller than 0.1 µm depending on the settings on the machine. The electrodes are then filled with electrolytes with or without fluorescent dyes. The electrode puller can cost anywhere from $1,000 to $20,000 (these use laser to pull quartz tubing for extremely fine tips).
  
  
• Morphology: The form and structure of an organism or any of its parts. Here it is the shape of the neuron. It also refers to the branch of biology that deals in such matters.
  
  
• Nerve Cord: This is basically a bundle of nerve fibers that connects the ganglia located in each segment to one another and to the brain. In the leech, the nerve cord is located ventrally. It is the leech equivalent of the human spinal cord.
  
  
• Neuroanatomy: The study of the anatomy of neurons or the nervous system.
  
  
• Neuron: A nerve cell. See background on nervous system for details.
  
  
• Neurophysiology: A study of the physical and chemical processes of neurons. Scientists generally use this term to mean "neuronal electrophysiology."
  
  
• Posterior: Opposite to anterior. Toward the rear.
  
  
• Resting Potential: The voltage of a cell at rest. The intracellular space of most cells is at a negative potential or voltage compared to the extracellular space. The actual voltage varies from cell to cell, but in neurons, it is typically around -60 mV to -80 mV. In our lab, you can observe the resting potential when you first encounter a neuron; when you enter the cell, the voltage seen by the electrode that is displayed on the oscilloscope jumps from near the top of the screen (representing 0 mV) to near the bottom of the screen (representing -60 mV). See Background on Nervous System for more details.
  
  
• Segment: The leech is a segmental animal like a caterpillar or an earthworm. Its body is made up of repeating similar units.
  
  
• Sinus: The circulation system of the leech is a little bizarre. I mean who's ever heard of an animal that sticks its nervous system inside itse blood vessels?? Yet that's exactly how it is. The ventral sinus of the leech is made of a tough fibrous material that appears dark green.
  
  
• Skin: For convenience, we sometimes use the term "skin" in this lab. But the proper term is "body wall," which is a heavily muscled structure that allows the leech to extend or contract at will.
  
  
• Ventral: Opposite to dorsal. Toward the bottom.