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The Nernst potential for chloride is -70 mV. The neuronal Resting Membrane Potential (RMP) is -70 mV. Under this condition, if the chloride channels are open (as it happens in Inhibitory Post Synaptic Potential - where chloride channels act as postsynaptic ionotropic channels), Chambers† writes that there is a movement of chloride ions along the concentration gradient i.e from outside to inside. However as the RMP and Nernst potential are the same, there is no hyperpolarization, as would happen were the voltage-gated potassium channels to open. However, the result is that there would be a need for much greater excitatory stimuli (influx of sodium) to cause an action potential in this cell. This is called a "Chloride clamp".

I am confused about two things:

  1. Why is there a net movement of chloride ions when the RMP is equal to the Nernst potential of chloride?
  2. As there are more chloride ions inside the cell, why is the cell not hyperpolarized?

Cl‾- mediated IPSP: binding of neurotransmitter opens Cl‾ channels. The resulting intracellular movement of Cl‾ ions usually makes little difference to the postsynaptic membrane potential, as the Nernst potential of Cl‾ (–70 mV) is approximately the same voltage as the RMP. However, to reach threshold, an excitatory signal must trigger sufficient Na+ influx to exceed the combined effects of Cl‾ influx and K+ efflux, making it much more difficult to depolarise the cell membrane; this is known as the ‘chloride clamp’.

† David Chambers Basic Physiology for Anesthetists Second Edition, chapter 53 - Synapses and NMJ Page 233

David
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One Face
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  • Can you quote from your book where it says when the membrane potential is equal to the Nernst potential for chloride there is a net nonzero movement of chloride ions in a particular direction? – Bryan Krause Dec 08 '23 at 14:11
  • For question (2), make sure you understand what causes the membrane potential. This answer I wrote yesterday addresses the same issue: https://biology.stackexchange.com/questions/113638/why-dont-membrane-potentials-violate-the-principle-of-electroneutrality – Bryan Krause Dec 08 '23 at 14:12
  • Also https://biology.stackexchange.com/questions/57064/why-is-it-possible-to-calculate-the-equilibrium-potential-of-an-ion-using-the-ne/57066#57066 https://biology.stackexchange.com/a/79623/27148 https://biology.stackexchange.com/a/60547/27148 – Bryan Krause Dec 08 '23 at 14:13
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    @BryanKrause I have added the verbatim quote from the book. – One Face Dec 09 '23 at 13:57

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I don't think your book is saying that there is net movement of chloride ions at the reversal potential for chloride (Nernst potential): you are correct, that is specifically the voltage where the net flow of an ion is exactly zero; if there is net flow of an ion you are not at the Nernst potential and vice versa. See also https://en.m.wikipedia.org/wiki/Nernst_equation

What your book says is "little difference" and "approximately" rather than "no difference" and "exactly": they're assuming that the resting potential is slightly less negative than the reversal for chloride, so there will be some small but negligible flow of ions and small but negligible change in membrane potential.

The chloride "clamp" or "shunt" will work by opposing any voltage change away from the reversal potential of chloride. The same is really true for any conductance that is open: membrane potential is a weighted sum of the contributions of all permeable ions, so when you open a conductance with a reversal of -70 mV the resulting potential with any other conductances open will be closer to -70 mV than it would be otherwise. See also https://en.m.wikipedia.org/wiki/Goldman_equation

Bryan Krause
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  • Thank you. I understand what you are telling. So in other words, if an excitatory end synaptic potential were to arrive at this point, any movement of sodium ions inside will be accompanied by movement of chloride ions inside - in other words, as the membrane potential goes toward nernst potential for sodium, chloride starts moving in and tries to keep the membrane potential close to RMP (Nernst potential for chloride). This makes it difficult to excite the cell. Thank you. – One Face Dec 10 '23 at 00:51
  • @OneFace Yes, exactly. – Bryan Krause Dec 10 '23 at 02:02