TOILETS AND NEURONS
Many students have a hard time grasping the concept of an action potential. Most have a difficult time understanding what is meant by an all-or-nothing reaction. This short activity uses a toilet, an object that all students can relate to, to help them grasp the concept of an action potential and its effects on voltage-gated channels.
This activity can be as short as 10 minutes or as long as 30 minutes, depending on the teachers discretion.
1. Review action potential and voltage-gated channels.
2. Write the leading question on the board or overhead.
3. The teacher will initiate a class discussion or break students into teams to answer the question below.
"How is a flushing toilet like the effect of an action potential on voltage-gated channels?"
Compare and contrast a flushing toilet to an action potential and its effects on population of
Just as the toilet handle triggers the flushing if pressed hard enough, so too does a population of voltage-gated channels trigger an action potential if the membrane is stimulated with enough voltage (threshold).
If not fully pressed, a partial flush can happen, and so too can a few channels open and generate a nearly threshold voltage change (This is called a local, subthreshold response.)
If pressed hard enough a complete flush will occur in its entirety but pushing even harder on the handle will not make a bigger flush. This is true for the action potential, hence it too is all-or-none.
The flush involves movement of "materials" through the plumbing; the action potential involves movement of Na ions through specially designed "plumbing" (voltage-gated sodium channels in the membrane.)
The driving force for fluid movement in the toilet is the water pressure from the water supply in your town (as from a water tower).The pressure source for Na ion movement is the electrochemical gradient for Na. Both concentration gradient forces (diffusion) and electrical gradient forces action on sodium ions tend to move sodium into the cell, but these forces will move sodium in only when their channels are open.
After flushing there is a period of time when another flush cannot occur even if the handle is pressed; so too in the action potential. There is a period, called the refractory period, when the channels, as a whole are unresponsive to a second stimulus that follows too soon after the first.
A number of materials can clog a toilet and prevent it from properly flushing; similarly, a number of natural and human-made toxins interfere with action potentials.
The purpose of the toilet is to get rid of undesirable wastes and flushing, therefore, has a negative connotation. The purpose of the action potential is to generate a beneficial electrical signal.
The action potential can move from one part of the cell to another as a moving wave of permeability change (called conduction or propagation); toilets and flushing are stationary and local .
One action potential involves a tiny amount of Na ions moving across the membrane in comparison to the huge number of sodium ions that are available. Therefore, Na concentration changes inside and outside the membrane during one action potential are negligible. The flushing toilet is a large scale gushing of water and other materials through the plumbing. Flushing a toilet is not nearly as subtle as the action potential.
An action potential involves a positive feedback effect wherein opening of voltage-gated channels leads to an initial depolarization that in turn leads to more opening of channels. this cycle is disrupted by the action of the Na channel h-gate which flips shut spontaneously in a large population of channels at about the same time. The toilet doesn't have a comparable feedback mechanism for its interruption.
About the Authors:
Liza Kobayashi teaches at Waiakea High School on the Big Island of Hawaii. Her school's
address is 155 West Kawili Street, Hilo Hawaii 96720. She can be reached at
Kay Widmer is a biology and ecology teacher at Hopewell Valley Central High School in
Pennington, New Jersey. Kay can be contacted at Hopewell Valley High school,
Pennington Titusville Road, Pennington, NJ 08534 or by e-mail at