If all K + channels are closed or if no K + channels exist in the membrane, p K will be zero. If only a few K + channels are open, p K will be small. Therefore, if many K + channels are open, p K will be high. Permeability refers to the ease with which ions cross the membrane, and is directly proportional to the total number of open channels for a given ion in the membrane. p K, p Na, and p Cl are the relative membrane permeabilities for K +, Na +, and Cl -, respectively. In examining the GHK equation (see below), it is clear that the relative contribution of any given ion is determined not only by its concentration gradient across the plasma membrane, but also by its relative membrane permeability. Moreover, the GHK equation can predict the reversal potential ( V rev) of the current-voltage ( I-V) relationship obtained from a cell in which the predominant ion channels in the plasma membrane are K +, Na +, and Cl - channels. When more than one ion channel is present (and open) in the plasma membrane, the membrane potential can be calculated by using the Goldman-Hodgkin-Katz equation (GHK equation). Therefore, the transmembrane movements of all three ions (K +, Na +, and Cl -) collectively contribute to the membrane potential. The movement of any ion down its own electrochemical gradient will tend to move the membrane potential toward the equilibrium potential for that ion. These selective ion channels allow K +, Na +, and Cl - to each move down its own electrochemical gradient. This is because in neurons at rest, there are K +-, Na +- and Cl -selective channels in the plasma membrane. For example, in a typical mammalian neuron, K +, Na +, and Cl - contribute to a resting membrane potential of around -70 mV, a membrane potential value that is not at the equilibrium potential for K +, Na +, or Cl. In many cells, K +, Na +, and Cl - are the main contributors to the membrane potential. Instead, the membrane potential is generally established as a result of the relative contributions of several ions. If this were the case, the membrane potential could be predicted by the equilibrium potential (V Eq.) for that ion, and could be easily calculated by using the Nernst equation. In living cells, the resting membrane potential ( V m) is seldom governed by only one ion such as K +, Na +, Cl -, etc.
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