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For the Public: What does it mean to do research on "excitable membranes"?

Certain cells in the body must generate and transmit electrical signals to function properly. For example, the cells of the nervous system generate and transmit these signals (known as "action potentials") along the nerves in the body and in the brain. These action potentials are necessary for communication between cells in the nervous system and throughout the body. Muscle cells also generate action potentials in order to contract, this includes the muscle cells that contract to help us move and the cardiac cells that keep our hearts beating. The cell membranes that generate and transmit action potentials are known as "electrically excitable membranes," or just "excitable membranes."

Life depends on the finely-tuned functioning of many types of excitable membranes. In order to hear certain sounds, some cells in the brain make action potentials that are shorter than one-thousandth of a second, while healthy heart cells make action potentials that are two-hundred times longer in duration. Different types of excitable cells make action potentials with different shapes to perform particular functions for the body. In the healthy body, thousands of different types of excitable membranes must remain finely tuned to do their jobs well. When something goes wrong and excitable membranes don't function properly, a wide range health problems like epilepsy and cardiac arrhythmias can result.

Plasticity

The term "Plasticity" is common in neuroscience but might sound a little strange to a nonscientist. In general, plasticity refers to changes in the functioning of a biological system. Excitable cells display plasticity - they can remodel their excitable membranes so that the action potentials are bigger or longer or have a different shape. These changes allow the body to adapt to different conditions. Plasticity of excitable cells in the nervous system is one of the mechanisms behind learning and memory. Plasticity of excitable cells in heart muscle enables the heart to adapt to regular exercise, and to recover after a heart attack.

The Bottom Line

All of this means that excitable cells have to meet two competing demands. They must be very reliable under some circumstances, yet they must be able to change under other circumstances. That is to say, they sometimes must be precisely regulated, but at other times they must be precisely modulated. In technical terms, excitable cells display intrinsic plasticity - the ability of a particular cell to maintain and change its pattern of electrical excitability. Our health and survival depends on the correct functioning and plasticity of excitable cells. On the other hand, problems with intrinsic plasticity of excitable cells leads to a wide range of disease states.

My research focuses on trying to understand the intrinsic plasticity of excitable cells. I am interested in two broad questions: How are excitable membranes created during development? And - what are the mechanisms behind the intrinsic plasticity of excitable cells?

So why do you work with fish?

I don't just work with any old fish! I work with a unique type of fish - the electric fish. At the mention of electric fish most people instantly think of the electric eel -- the true monster of the electric fish that can deliver a massive electric shock. The eel, however, is just one of thousands of species of electric fish, and the majority of these fish generate much smaller electric fields. These fish generate electric fields in the water, and they also can sense these electric fields with special sensors on their skin. They use these electric fields to "see" in the dark. Objects in the water distort the electric fields and, by sensing these distortions, the fish can image its world in the dark. Interestingly, these fish also use their electric sense to communicate with each other.

What does an electric fish have to do with intrinsic plasticity of excitable membranes? The answer lies in the fact that these fish have taken the same mechanisms that nerve cells and muscle cells use to generate tiny electrical impulses, and created big excitable cells where they use these same mechanisms to generate big electrical impulses. These big electricity generating cells are known as electrocytes, and these electrocytes are packed into an electric organ in the body that the fish uses to generate electric fields in the water. The electric organ discharge is the origin of the electric fields emitted by these fish, and we refer to it as the EOD for short.

In order to communicate with their electric fields, some species quickly change the shape of their EODs, and they do this by quickly changing the excitable membranes of the electrocytes. These fish specialize in making and using electrocytes that display a remarkable degree of intrinsic plasticity! Not only do these fish specialize in regulating and modulating excitable membranes, they also provide a unique window into understanding how excitable membranes develop. The electrocytes in the electric organ actually start off as muscle cells. In very young fish, certain muscle cells fuse together and lose the mechanisms that make them contract while retaining their electrical excitability. This creates a big cell that makes a big action potential - the basis for the EOD.

What this all means is that electric fish give us a truly unique way to study how excitable membranes develop, and to study the mechanisms behind intrinsic plasticity of excitable cells.

 
  Copyright (c) 2008 Michael R. Markham -- Updated May 30, 2013