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Mechanism behind extracellular [Ca2+]'s effect on threshold potential?

Discussion in 'Medical Students - MD' started by CuriousGeorge2, Mar 8, 2011.

  1. CuriousGeorge2


    Dec 5, 2010
    Hypocalcemia causes a decrease in threshold potential, explaining why you get all those hyperexcitability signs with a low serum Ca2+. Hypercalcemia does the opposite.

    My understanding was that the threshold potential is the point where Na+ conductance (and influx) just exceed K+ conductance (and efflux). Based on this, these serum calcium levels must be having some sort of effect on the sodium or potassium channels.

    I looked through a bunch of texts and online and couldn't find any explanation as for how changes in serum calcium alter the threshold potential. Is this just because nobody has any idea?
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  3. PiBond

    PiBond Call me Bond...PiBond 7+ Year Member

    Aug 12, 2009
    My understanding is that since I have less positive charge (Ca2+) in the ECF, the interior is relatively more positive to the exterior since the membrane potential is just the difference in charge across a membrane. Therefore, the cell becomes hyperexcitable since it's relatively more positive and thus can more easily achieve threshold.
  4. CuriousGeorge2


    Dec 5, 2010
    That's what I thought, too, but when I read further, it seems that the the membrane potential itself does not change; only the threshold does. Several texts make this point to distinguish it from the effects of potassium: K+ affects the membrane potential but does not affect the threshold potential...
  5. Everglide

    Everglide EM PGY3 10+ Year Member

    Aug 6, 2006
    Also there is a Na/Ca antiport exchanger, low serum calcium would increase sodium entry

  6. mannyowens

    mannyowens 7+ Year Member

    Mar 16, 2009
    Yes, I believe its a combination of the aforementioned points. The thought is that hypocalcemia increases membrane permeability to Na+ ions, increasing the frequency and duration of action potentials. As previously stated, hypocalcemia increases the voltage difference between the intra & extra-cellular components of the cell>> as a result increases the resting membrane potential and reduces the threshold for reaching an action potential. I also believe the Na/Ca exchanger is some how involved but I cant exactly reason how. The Na/Ca normal expels Ca while working off the electrochemical gradient of Na, it does not have a high affinity for Ca and only works under conditions of high Ca concentrations, and its actions are reversible once the cell is depolarized to a certain critical point (imports Ca rather than expels it), anyone have more insight as to how Na/Ca exchanger might be involved?
  7. startoverat40

    startoverat40 R1 Family Medicine 5+ Year Member

    Feb 11, 2011
    calcium entering the cell activates a potassium current (outward) which leads to hyperpolarization, and harder to get to threshold. so low extracellular calcium means reduced ca-dependent K current, so easier to get to threshold.

    calcium and magnesium neutralize the negative charge on the cell membrane, so hypercalcemia means less negative charge on outside of membrane, which means greater negative potential across the membrane, and harder to get to threshold.

    another reason is calcium ions block sodium channels. so more Ca outside means harder for Na to enter the cell.

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