Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Cell Biology

Soft shell clams (Mya arenaria) are bivalves native to the east coast of North America. Some individual clams are resistant to a harmful neurotoxin called saxitoxin while others are not. Saxitoxin affects the function of the neurons and can result in paralysis and/or death in clams and in organisms that eat toxic clams. Neurons are affected because the toxin blocks voltage gated sodium channels along nerve axons preventing nerve signal transmission to the axon terminus.

Illustration of the nervous system of a softshell clam.  There are two cords running the length of the clam and another that runs down the foot.
illustration of a nerve cell. at the top are the branching dendrites which connect the cell body.  A long axon runs dwon to the nerve endings that are shaped like tree roots.

Neurons and Action Potential

Neurons (nerve cells) are responsible for receiving, conducting, and sending electrochemical signals throughout the body of an organism. Collectively, neurons comprise an organism's nervous system. A neuron passes current (action potential) down its length due to electrochemical events mediated by voltage gated sodium channels, voltage gated potassium channels and sodium-potassium pumps situated in the membrane along the length of the axon.

The voltage gated sodium channel is an intra-membrane protein that facilitates the movement of sodium ions across the cell membrane on the axon. As these channels open and close down the length of the axon, the action potential travels from the cell body to the neuron axon terminal. More information on action potential

Saxitoxin and Neurons

A 2D and 3D render of Saxitoxin.

The voltage-gated sodium channel has two gates, an extracellular activation gate that opens the channel and an intracellular inactivation gate that closes the channel. The neurotoxin saxitoxin disrupts sodium channel function by (reversibly) binding to the activation gate and preventing it from opening. This means that sodium diffusion into the axon cannot occur. Axons can only transmit nerve signals by virtue of their ability to change voltage differential along their length. If sodium ions cannot diffuse across the axon membrane, the voltage differential between intra- and extra-cellular spaces cannot change and the neuron cannot conduct a nerve signal. When this happens to many neurons, the entire nervous system can be disabled resulting in paralysis and/or death.

Resistance to Saxitoxin

Some clams are resistant to saxitoxin poisoning. A mutation in the gene encoding the voltage gated sodium channel protein prevents the toxin from binding to the channel gate. If saxitoxin cannot bind to the sodium channel it has no effect in the clam. However, the toxin is not neutralized. It resides in the body and can still harm other organisms that might eat the clam.