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E THE EXCITABLE TISSUES: NERVE AND MUSCLE Chapter 20. ELECTROPHYSIOLOGY A. Introduction to Electrochemistry Some aspects of selective ion transport across membranes will be described in this section. It was noted in Chapter 17 that the intracellular and extracellular fluids are quite different in composition. This discussion will be mainly concerned with the two cations of sodium (Na + ) and potassium (K + ). Part of the movement of ions across membranes is due to the "leakiness" of the lipoprotein barrier, which allows ions to flow down concentration gradients. It was noted earlier that the hydrated K + ion (K + . (H 2 O) x ) is smaller than the hydrated Na + ion and that the concentration gradient for K + is larger than that of Na + . As a result of the hydration differences, K + is some 50 to 75 times more mobile than is Na + . Hereafter, any discussion of these ions will simply assume the hydrated condition. In all cells, intrinsic leakiness of the plasmalemma and the sizeable concentration gradients (K + i >> K + o and Na + o > Na + i ), results in greater loss (via efflux) of K + than gain (influx) of Na + . This passive movement of cations renders the inside of cells slightly less positive (or more negative) relative to the outside, so that there is a resting potential or voltage of about 60 millivolts across the membrane. Because the voltage is expressed inside relative to outside, the membrane in its resting state has a negative potential, in this case, -60 mV. The membrane is said to be polarized . An important "housekeeping" function of cells that offsets the redistribution of ions arising from the aforementioned leakiness is the Na + /K + - adenosine triphosphatase (ATPase) located in the membrane itself. This was described in Chapter 12. The normal concentrations of Na + and K + found inside cells and outside thus reflect a dynamic balance between redistribution due 2007 version – page 148
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to concentration gradients and membrane leakiness and ongoing operation of the energy consuming Na + /K + - ATPase that moves ions against their concentration gradients. The membrane behaves as a dielectric, because it serves to separate charges on either side. A simplified description of the meaning of the electrochemical terms is provided in Box 20-1. The reader should be comfortable with the use of these terms before proceeding with the remainder of this section. The cell membrane is an imperfect dielectric because, as noted above, some leakiness of ions is quite normal and metabolic energy is expended by the pump to sustain the ion distribution. A sizable proportion of the total energy cost of keeping an animal alive (approx. 20 to 25%) is expended in driving the Na + /K + -ATPase pump. An added complexity arises from the presence of ion channels embedded in the hydrophobic lipid matrix, as described in Chapter 11. The channels provide a means for rapid transit of ions across the membrane under certain controlled conditions, depending on whether the channel is open or
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