In a neuron with a resting membrane potential of -78 mV the intracellular and ex

Question

In a neuron with a resting membrane potential of -78 mV the intracellular and extracellular ion concentrations were found to be respectively: Na+ : 15 and 115 mM K+ : 90 and 3 mM; Cl- : 10 and 120 mM. By ion replacement in the extracellular bathing solution with impermeable salts the following observations were made: halving the external [Na+] caused a very small hyperpolarization, doubling the external [K+] caused a considerable depolarization, halving external [Cl-] had no effect. What can you deduce (with reasons) about the resting membrane conductances?

Explanation / Answer

In general, the negative environment inside the cell wants to pull potassium back in, however the paradox is that the concentration gradient wants to makes potassium escape into the extracellular environment due to the high concentration. Increased extracellular potassium concentration surrounding neurons inactivates Na+channels and opens K+channels. Resulted in decreases potassium gradient accross the cell membrane, and therefore the resting membrane potential will become more positive. More positive inside, the membrane is said to be depolarized. Because less change in external Na+ gradient, sodium leaky channel remain closed but Na+ –K+ ATPase work normally, 3Na+ are exported for every 2K+ brought into the cell, the pump makes a small direct contribution to the resting membrane potential and making it slightly more negative than outside (small hyperpolarization). In neurons with Cl- pump, where VR = ECl, VR (membrane potential at rest VR) therefore, opening chloride channels does not change the membrane potential.

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