Scared Stiff: The Story Behind "Fainting Goats"
Have you ever seen a goat (or any animal, for that matter) do this?
Neither had I. But these are the sorts of things that come up at family parties and pique my curiosity. Perhaps the nickname and title of the YouTube video “fainting goats” is a misnomer (as National Geographic pointed out) as the goats are not actually losing consciousness when they go rigid and topple over. So why the wipe-outs?
Myotonia congenita is a condition studied more frequently in humans than in animals, but myotonic goats provide a good model organism for research. A 1996 article by Carol Beck et al. describes the origin of the condition as resulting from a shift in the activation threshold for a chloride channel. This shift can be due to a single amino acid substitution. The channel, it turns out, is highly conserved among species: the paper cites homologous channels in rats and in humans, both of which can undergo similar myotonia-producing mutations. The subsequent modification decreases the channel’s probability of being open, and the decrease in intracellular chloride reduces the stimulation necessary for neurons to reach action potential threshold. Additionally, the normal post-action potential rise in extracellular potassium has a greater effect on the membrane potential of neurons with these channel defects, causing a kind of “after-depolarization” that can bring the neurons back to threshold and result in additional, unintended action potentials. This after-depolarization seems to be the reason for the prolonged muscle contraction that sends the goat to the ground.
Researchers at Vanderbilt University are studying potential gene therapy for humans who suffer from myotonia congenita. Their methods include the use of genetically-engineered ribozymes to repair the defective messenger RNA that codes for the mutant chloride channels and it has been working in at least some of the cells the researchers have tested it on. As for the goats, myotonia congenita is a characteristic feature of their breed: it looks like they are just doomed to keep “fainting”.
And just for fun…
Molecular basis for decreased muscle chloride conductance in the myotonic goat – Proceedings of the National Academy of Sciences