Mice sense each other's fear
People can usually tell when others are afraid just by the look on their faces. Mice can tell when other mice are afraid too. But instead of using their beady little eyes to detect fear in their fellows, they use their pink little noses.
FEAR-OMONE: Mice smell fear in other mice using a structure called the Grueneberg ganglion. The ganglion has about 500 nerve cells that carry messages between a mouse's nose and brain.
Scientists are beginning to understand how mice sense fear. According to a new study, the animals use a structure which sits inside the tip of their whiskered noses. This Grueneberg ganglion is made up of about 500 specialized cells - neurons - that carry messages between the body and the brain.
Researchers discovered this ganglion in 1973. Since then, they have been trying to figure out what it does.
"It's ... something the field has been waiting for, to know what these cells are doing," says Minghong Ma, a neuroscientist at the University of Pennsylvania School of Medicine in Philadelphia, Pa.
Researchers already knew that this structure sends messages to the part of the brain that figures out how things smell. But there are other structures in a mouse's nose that pick up odors. So, this ganglion's true function remained a mystery.
To investigate further, researchers from Switzerland began testing the ganglion's response to a variety of odors and other things, including urine, temperature, pressure, acidity, breastmilk and message-carrying chemicals called pheromones. The ganglion ignored everything the team threw at it. That only deepened the mystery of what the ganglion was actually doing.
Next, the scientists used highly detailed microscopes (called electron microscopes) to analyze the ganglion in fine detail. Based on what they saw, the Swiss scientists began to suspect that the structure detects a certain kind of pheromone - one that mice release when they're afraid or in danger. These substances are called alarm pheromones.
To test their theory, the researchers collected alarm chemicals from mice that had encountered a poison - carbon dioxide - and were now dying Then, the scientists exposed living mice to these chemical warning signals. The results were revealing.
Cells in the Grueneberg ganglions of the living mice became active, for one thing. At the same time, these mice began acting fearful: They ran away from a tray of water that contained alarm pheromones and froze in the corner.
The researchers conducted the same experiment with mice whose Grueneberg ganglions had been surgically removed. When exposed to alarm pheromones, these mice continued exploring as usual. Without the ganglion, they couldn't smell fear. Their sense of smell wasn't completely ruined, however. Tests showed that they were able to smell a hidden Oreo cookie.
Not all experts are convinced that the Grueneberg ganglion detects alarm pheromones, or that there is even such a thing as an alarm pheromone.
What's clear, however, is that mice do have a much more fine-tuned ability to sense chemicals in the air than do humans. When people are afraid, they usually yell or wave for help. If humans were more like mice, imagine how scary it might be just to inhale the air in an amusement park!
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.
function A relationship between two or more variables in which one variable (the dependent one) is exactly determined by the value of the other variables.
ganglion A sac-like cyst (called a ganglion cyst) formed made of tissue that lines a joint or tendon. Or a collection of nerve cell that cluster together at some site outside the brain.
microscope An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.
nerve A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.
neuron An impulse-conducting cell. Such cells are found in the brain, spinal column and nervous system.
neuroscientist Someone who studies the structure or function of the brain and other parts of the nervous system.
pheromone A molecule or specific mix of molecules that makes other members of the same species change their behavior or development. Pheromones drift through the air and send messages to other animals, saying such things as “danger” or “I'm looking for a mate.”
pressure Force applied uniformly over a surface, measured as force per unit of area.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
variety (in agriculture) The term that plant scientists give to a distinct breed (subspecies) of plant with desirable traits. If the plants were bred intentionally, they are referred to as cultivated varieties, or cultivars.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.