The dependence of motoneurone membrane potential on extracellular ion concentrations studied in isolated rat spinal cord.

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Abstract
1. Intracellular recordings from ninety‐nine motoneurones have been made in an in vitro hemisected spinal cord preparation. Their mean resting membrane potential in normal artificial cerebrospinal fluid (CSF) was ‐71 +/‐ 0.5 mV (+/‐ S.E.M.). The mean amplitude of the action potential was 84.0 +/‐ 1.4 mV (n = 50), and the mean input conductance was 101 +/‐ 7 nS (n = 49). 2. Both membrane potential and input conductance were sensitive to changes in [K+]o, [Na+]o, [Cl‐]o and [Ca2+]o. 3. Replacement of extracellular Ca2+ by Mn2+ resulted in less than 1 mV hyperpolarization and a decrease in input conductance from 102 +/‐ 7 to 93 +/‐ 6 nS (n = 15). 4. At high [K+]o (greater than 10 mM) the membrane potential followed the potential predicted by the Nernst equation for K+ ions with a slope of 58 mV per 10‐fold change in [K+]o. At low [K+]o (less than 10 mM) there was significant deviation from K+ equilibrium potential (EK). 5. [K+]i was found to be 106 mM when estimated from the reversal potential of the after‐hyperpolarization of the antidromic action potential. 6. The reversal potential of the recurrent inhibitory postsynaptic potential (IPSP) in normal CSF was used to calculate [Cl‐]i. This was 6.6 mM, which is less than would be expected if Cl‐ was passively distributed, indicating the presence of an outwardly directed Cl‐ pump. 7. Decreasing [Cl‐]o from control (134 mM) to 4 mM resulted in a depolarization of 6.9 +/‐ 0.9 mV and a decrease in input conductance from 102 +/‐ 5 to 90 +/‐ 5 nS (n = 14) in 3 mM [K+]o. 8. Decreasing [Na+]o from 156 to 26 mM by substitution with choline resulted in a 6.2 +/‐ 0.5 mV hyperpolarization and a decrease in input conductance from from 102 +/‐ 4 to 76 +/‐ 4 nS (n = 5) in 3 mM [K+]o. 9. The input conductances for Na+, Cl‐ and K+ at the resting potential were calculated. After allowing for a microelectrode leak conductance, the relative input conductances were gNa/gK = 0.13 and gCl/gK = 0.25.