|MadSci Network: General Biology|
Thank you for your question! First I'd like to give some background information in case other readers don't understand your question. Then I'll go into a more detailed answer.
Daphnia is the name of a group of small, aquatic crustaceans commonly called water fleas. Because their exoskeletons are clear, it is possible to watch Daphnia hearts without cutting them open. This allows the changes in Daphnia heart rate to be studied quite easily. To learn more about the uses of Daphnia in experimentation, see this site at Cornell University which includes pictures and Daphnia culturing methods.
One common experiment done with Daphnia tests chemicals for an effect on heart rate. The chemical to be tested is added in various concentrations to the water in which the crustaceans are swimming. The heart rate in these experimental treatments can be compared with the heart rates of Daphnia in control groups in which no chemical has been added. By comparing the heart rates of Daphnia in the two groups, you can tell whether the tested chemical is a heart stimulant, depressant, or has no effect on the heart rate. Sample results for this type of experiment using nicotine and alcohol can be seen at this site.
From your question, I assume that you have conducted an experiment demonstrating that adding ethanol to the water in which Daphnia swims causes their heart rate to slow down. Perhaps you are now writing up your results, and wish to include relevant information in your discussion to explain the physiologic mechanism by which the change in heart rate is brought about.
Unfortunately, there is not a lot of documentation of the mechanisms by which alcohol slows down Daphnia heart rate. But you don't need all the answers to write a good discussion of your results. You only need to explain what might be causing the phenomenon you've proven experimentally. I can help you with that.
First, it's important to understand how crustacean hearts work. Ethanol must be interfering somehow with the normal process of heartbeat regulation. So what regulates crustacean heartbeats? As is the case in most animals, crustacean heartbeats are regulated primarily by nerve pulses. In the case of crustaceans, these impulses are generated by pacemaker neurons located in a group of nerve cells called the cardiac ganglion. The impulses are then transferred to larger follower neurons which carry the signal to the cardiac muscle, causing it to beat. The University of Calgary has a site describing crustacean hearts, with a page specifically about the nerve fibers involved.
Somehow ethanol is interfering with the heart rate. The method of action is probably by interfering with these nerves. (If the quality of the heart beat was affected, we might consider the cardiac muscle to be the site of action. In this case, however, only the rate is involved, so it's most likely the nerves that are affected.)
So how does alcohol interfere with nerves? All cells have a type of protein embedded in their cell membranes called receptor proteins. These receptors allow communication between cells to occur. Nerves can receive communication from cells far away or from nerve cells near by. Cells far away secrete hormones, which find their way to the target nerve's hormone-receptors. Once these hormones attach to the receptor, changes in the nerve take place. The hormone-receptors, when activated, can change how quickly a nerve generates an impulse, or alter how fast the nerve passes impulses along. Some hormones stimulate an increase in heart rate (such as during exercise) and other hormones decrease heart rate (such as during sleep).
Other receptors allow communication between adjacent nerve cells (such as between the pacemaker neurons and the follower neurons). Instead of hormones, this type of receptor binds to neurotransmitters.
Receptors of any type, however, are not perfect. Sometimes they accidently bind molecules that they shouldn't. This can cause an inappropriate increase in nerve activity, or it may cause an inappropriate decrease in nerve activity. (The change depends on the type of receptor involved, and the type of binding the molecule uses.) The nerve fibers conducting pulses to the hearts of Daphnia may contain receptors that inappropriately bind to ethanol (or a product of ethanol after it is broken down) causing an inappropriate decrease in nerve activity.
There are ways to prove more conclusively that receptors are involved, and also ways to prove what kind of binding is taking place. But that seems like it is beyond the scope of the present discussion. If you're interested in learning more about determining characteristics of cell membrane receptor-binding, please ask us!
These other MAD Scientist answers to similar questions may also be of help to you:
Re: How can I test for physiological processes of the Artemia & Daphnia?
Re: Why does the heart rate of daphnia increase with temperature?
Re: Why do Ethanol, Acetyl choline, Adrenalin affect the heart rate of Daphnia?
Good luck with your project!
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