•Fig. 2.4 Current-Voltage Relationship of a Hypothetical Cell Containing Na+-, K+-, and Cl−-Selective Channels. Membranecurrentsareplottedoverarangeofmembranevoltages(i.e.,current-voltagerelationships).EachioncurrentiscalculatedwiththeuseofOhm’slaw,theNernstequilibriumpotentialfortheion(ECl,EK,andENa),andthemembraneconductancefortheion.Thecurrent-voltagerelationshipforthewholecellisalsoshown.Totalcellcurrent(Icell)wascalculatedwiththechordconductanceequation(see
Eq.2.7).Because80%of cellconductanceisduetoK+ ,therestingmembranevoltage(Vm)of−64.4mVisneartothatoftheNernstequilibriumpotentialforK+ .
Equation 2.7 where ˜G = G ++ G ++ G −.
Inspection of
Eq. 2.7 , which is often called the chord conductance equation, reveals that the Vm will be near to the Nernst equilibrium potential of the ion to which the membrane has the highest conductance. In
Fig. 2.4 , 80% of the membrane conductance is attributable to K+; as a result, Vm is near to the Nernst equilibrium potential for K+ (EK+). For most cells at rest, the membrane has a high conductance to K+ , and thus the Vm approximates EK+. Moreover, the Vm is greatly influenced by the magnitude of EK+ , which in turn is greatly influenced by changes in the [K+] of the ECF. For example, if the intracellular [K+] is 120 mEq/L and the extracellular [K+] is 4 mEq/L, EK+ has a value of −90.8 mV. If the extracellular [K+] is increased to 7 mEq/L, EK+ would be −79.9 mV. This change in EK+ depolarizes the Vm (i.e., Vm is less negative). Conversely, if the extracellular [K+] is decreased to 2 mEq/L, EK+ becomes −109.4 mV, and the Vm hyperpolarizes (i.e., Vm is more negative).
•Fig. 2.5 Nerve Action Potential Showing the Changes in Na+ and K+ Conductances ( G and GK+ , Respectively) and the Membrane
Potential (Vm). Atrest,themembranehasahighK+ conductance,andVmisneartheNernstequilibriumpotentialforK+ (EK+ ).Withtheinitiationoftheactionpotential,thereisalargeincreaseintheNa+ conductanceofthemembrane,andtheVmapproachestheNernstequilibriumpotentialforNa+ (ENa+ ).TheincreaseinNa+ conductanceistransient,andtheK+ conductancethenincreasesaboveitsvaluebeforetheactionpotential.ThishyperpolarizesthecellasVmapproachesEK+.AstheK+ conductancereturnstoitsbaselinevalue,Vmreturnstoitsrestingvalueof−70mV.(ModifiedfromLevyMN,KoeppenBM,StantonBA.Berne & Levy’s Principles of Physiology. 4thed.St.Louis:Mosby;2006.)