Boo!

In my last column (“when sleep goes awry”), I wrote about a sleep disorder called narcolepsy. I mentioned that there are several well-known videos of narcoleptic dogs collapsing without warning in the middle of playtime. Watching those videos made my mind think back to another kind of animal that I’ve heard sometimes collapses without warning, and I wanted to investigate it further. The animal? The fainting goat.

Fainting goats have gone by many names over the years, including Tennessee goats, nervous goats, stiff-leg goats, wooden-leg goats, and Tennessee scare goats. All of these names are reflective of the fact that these guys (a) originate from Tennessee and (b) have the unfortunate tendency to fall over when startled. When spooked or surprised, the muscles in their legs become as stiff as a board, resulting in the goat falling on its side, as if in a dead faint. (In animals less severely affected, though, they don’t actually fall over. They just stand there on unbending legs, looking really awkward.) Mind you, the goat has not really fainted, since it is fully conscious - it just can’t move its muscles! After 10-15 seconds, the muscles will relax enough to let the goat walk, albeit stiffly at first. Once the stiffness has worn off, though, they walk and run just like any other goat. (To see a picture of what they look like when they’ve fallen over, check out: http://agonline.com/runningbird/goats.asp. It’s a good illustration of a goat that has fainted in the midst of ones that haven’t.)

This may sound similar to what I described for a narcoleptic/cataplectic attack, but it is actually something different. Fainting goats suffer from a muscle condition called myotonia congenita. Myotonia congenita occurs in many animal species, including humans. It is a disease of the cells in skeletal muscles, preventing normal muscle contraction and expansion. Let’s take a look at how muscle cells work to understand what’s going wrong in these poor goats.

When muscles expand and contract, they do so in a coordinated fashion, regulated by the flow of ions in and out of the muscle cells. Ions are electrically charged atoms due to the fact that they have unequal numbers of protons (conferring positive charge) and electrons (conferring negative charge). Ions are essential for life – there are literally thousands of cellular processes regulated by them. Ions used by your cells include sodium, potassium, calcium, magnesium, zinc and chloride; because they are so important, their movement and levels are carefully controlled. Muscle contraction and expansion requires a tightly regulated flow of chloride and calcium ions through dedicated ion channels that sit in the membranes of the muscle cells. These channels act like tunnels through the barriers of the cell, letting only their designated ions through. There are many such barriers in a cell, including the outer membrane (separating cell from non-cell) and organelle membranes (enclosing numerous compartments within the cell). One important compartment in muscle cells is called the sarcoplasmic reticulum. The sarcoplasmic reticulum is a major storage compartment of intracellular calcium (a positively charged ion).

When your skeletal muscles receive a signal to contract (the signal coming from the nervous system), calcium channels in the membrane of the sarcoplasmic reticulum allow a rush of calcium to flow from the sarcoplasmic reticulum to the interior of the cell (the cytoplasm). The calcium allows the proteins that control muscle contraction to work. When the contraction is finished, the cells must have some way of balancing out the positive charge in the cytoplasm – if they don’t, the contraction would not end! There are 2 ways to achieve this balance. First, a different set of channels pump the calcium back into the sarcoplasmic reticulum. And second, there is a counterbalancing inrush of chloride ions. Chloride, being negatively charged, cancels out the remaining positive charge conferred by the calcium. This balance of electrical charge in the cell keeps the muscles from contracting abnormally.

(It’s important to emphasize that what I’m talking about here is skeletal muscle. There are actually 3 different kinds of muscles in your body – skeletal, cardiac and smooth. Both cardiac and smooth muscles contract and relax involuntarily, while skeletal muscle function can be both voluntary and involuntary. The muscles differ in their organization and mechanism of action, and what I’ve written above is specific for skeletal muscle.)

Okay, back to the goats. Fainting goats have a genetic defect in the chloride channel in their skeletal muscles. This mutation - in a gene called CLCN1 - results in chloride channels with an abnormal shape – as a result, they cannot properly control the flow of chloride in or out of the muscle cells. If the goat is startled, it will contract its muscles to run or jump away. Once the muscles are contracted, though, they cannot be relaxed as normal because the chloride ions are not flowing properly. The result - their muscles remain contracted abnormally long. And tightly contracted leg muscles equates to falling over. Eventually, the calcium ions causing the contraction get pumped back into the sarcoplasmic reticulum in the goat’s muscle cells, which is why they are able to move again after a while.

I do want to emphasize that this condition is painless and harmless to the goats. While it makes them easier prey for goat-eating predators like coyotes, if properly protected, they suffer no ill effects from their condition. The only muscles affected are the skeletal muscles, not their hearts or digestive muscles. In fact, if anything, these goats tend to be slightly more muscular than their unaffected counterparts.

Of course, only a goat could tell you whether being buff would make up for the embarrassment of falling over every time you were startled.