Ridecamp: Re: [RC] Lisa's comments on thumps

> So what you are saying is that any muscle, the large hind end
> ones in cows, and probably horses too, can lose Ca+ but for some
> reason, certain horses lose Ca+ in the diaphram causing thumps.
> Why the diaphram remains a question.  Perhaps there is a genetic
> predisposition.  Perhaps that is why certain horses, getting what
> would be considered adequate electrolytes, still thump.

I agree that there is a genetic predisposition to thumping.  And to back up
a bit, no, thumps is not about calcium and muscle contraction--it is about
multiple e-lytes and leakiness of the membrane of the phrenic nerve.  Where
the calcium depletion comes in (per Lisa's post) is that calcium has been
consumed during work by repeated muscle contraction, so that the ionized
calcium may be low in circulation.  Hence, the whole e-lyte balance is out
of whack.  (Along with e-lytes lost in sweat, etc.)  What happens in thumps
is that the phrenic nerve anatomically crosses the heart.  The heart has an
electrical discharge with each contraction, which is accomplished in a
rather complex manner by electrolytes being rapidly transported across cell
membranes to produce an electric potential, and then being scavanged back
across to end the "shock" so to speak.  The myelin sheath on the phrenic
nerve is the "insulation" that protects the nerve from "firing" every time
this discharge takes place in the heart--just like the coating around an
electrical wire.  With an imbalance in e-lytes, this myelin sheath ceases to
do its job properly, so the "firing" of the heart in turn causes e-lytes to
"leak" into the phrenic nerve, causing it to "fire" as well.  Hence, since
it services the diaphragm, the diaphragm in turn contracts every time the
heart beats.  What I think is genetic is the intolerance of the phrenic
nerve's myelin sheath to variations in the e-lyte "milieu" to which it is
subjected--some horses seem to have a much greater sensitivity to even the
slightest variation, while others would be long dead of outrageous imbalance
s long before their phrenic nerve's myelin sheath becomes "leaky."

Can these horses be managed?  Sure, in all but the most serious ones, by
carefully micromanaging their e-lytes.  Is this something that can be
avoided by horse selection?  To a reasonable extent, yes.

Heidi


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