1625915643-5d53d156c9525bd62bd0d3434ecdc231 (Netters - Essential Physiology (на английском)), страница 8

3.1), this equation becomes:E K+ = ( 61mV ) log ( 0.1) = −61mVE K+ = ( 61 mV +1) log ( 0.1 M 1.0 M ) , or 61 mVThe electrochemical equilibrium will be greatly changed byaltering the concentration of the permeable ion inside oroutside of the cell. For a system in which only one ion is permeable, the Nernst potential for the permeable ion is equal tothe resting membrane potential. In actual cells, more than oneion is permeable, and thus the resting membrane potential isa result of different permeabilities (conductances) of the ionspresent and the concentration differences of those ions acrossthe cell membrane. The Nernst potential (equilibrium potential) represents the theoretical membrane potential that wouldoccur if the cell membrane were to be selectively permeableto only a specific ion. Given the normal values for concentrations of various ions inside cells and in the interstitial fluid,the approximate Nernst potentials for the major permeableions are listed in Table 3.1.26The Nervous System and MuscleVm 0Vm 61mV100 K10 Na110 Cl10 K100 Na110 Cl–100 K10 Na110 ClExtracellular fluidFigure 3.1 Membrane Potential of a Hypothetical Cell When the cell with the indicated intracellularion concentrations (mEq/L) is placed into a solution with different ion concentrations, there is a gradient fordiffusion of ions.

In this example, the membrane separating inside and outside compartments is selectivelypermeable; it is permeable only to K+. Because the cell is selectively permeable to K+, diffusion of K+ from insideto outside the cell immediately begins, resulting in negative charge on the inside of the membrane and positivecharge outside the membrane. An electrochemical equilibrium is established, in which the gradient for diffusionof K+ is opposed by the electrical gradient, and the membrane is negatively charged.

In a simple system withonly one permeable ion, Vm (the membrane potential) can be calculated by the Nernst equation.Table 3.1Approximate Concentrations andNernst Potentials for Major Ions inCytosol and Interstitial FluidCONCENTRATION (mM)IonCytosolicInterstitialNernst Potential (mV)Na1414061K+1404−94Cl−8108−68+mM, millimolar; mV, millivolts.Goldman-Hodgkin-Katz EquationThe actual resting membrane potential (Vm) for a systeminvolving more than one permeable ion is calculated by theGoldman-Hodgkin-Katz equation (G-H-K equation), whichtakes into account the permeabilities and concentrations ofthe multiple ions:Vm RT In PK[Ko]PNa[Nao]PCl[Cli]ZFPK[Ki]PNa[Nai]PCl[Clo]Where:■■PX is the membrane permeability to ion x[X]i is the concentration of x inside the cell■■■■■[X]o is the concentration of x outside the cellR is the ideal gas constantT is absolute temperatureZ is the charge of the ionF is Faraday’s numberAlthough cells contain many ions, this simplified G-H-Kequation omits ions that are much less permeable to the cellmembrane than K+, Na+, and Cl−, because their contributionto resting membrane potential is usually negligible.

The concentration of Cl− inside appears in the top of the right-handterm and the concentration of Cl− outside appears in thebottom, whereas the situation for [K+] and [Na+] is oppositethat of [Cl−], because of the difference in charge of these ions(negative vs. positive).The resting membrane potential for most cells is approximately −70 mV; in nerve cells it is approximately −90 mV.

K+contributes most to the resting membrane potential, becausecytosolic K+ concentration is high and extracellular K+ concentration is low, and permeability of the plasma membraneto K+ is high relative to other ions. Therefore, while the restingmembrane potential is similar to the Nernst potential for K+,other ions contribute to the resting membrane potential. Specifically, leaking of Na+ through Na+ channels along its electrochemical gradient contributes to the fact that the restingmembrane potential of cells is less negative (more positive)than the Nernst potential for K+. The gradients of Na+ and K+are maintained in living cells by active transport via theNerve and Muscle PhysiologyExtracellular fluidⴙⴙMembraneNaⴙⴙ DiffusioⴙⴙNaⴙArtenspoⴙⴙsffuKⴙDiionⴙⴙⴙⴙⴙⴙAADPⴚⴚⴚResistanceEKⴙⴚ90 mVgK 100MitochondrionEquivalentcircuitdiagramATPaseRMP70 mVⴙEK 61 log [K ⴙ outside][K inside]ⴚⴚⴚEClⴚⴚ70 mVgCl 50 to 150KⴙClⴚDiffusionClⴚgNa 1ENaⴙⴙ50 mVATPctivratⴙnAxoplasmⴚⴚⴚⴚⴚ27Proteinⴚ(anions)ⴙENa 61 log [Na ⴙ outside][Na inside]ⴚBECl 61 log [Cl ⴚ outside][Cl inside]Figure 3.2 The Resting Membrane Potential Ion concentration differences between cytosol (inthis example, axoplasm) and interstitial fluid are produced by the activity of transporters, mainly the Na+/K+ATPase, which pumps Na+ out of the cell in exchange for K+, which is pumped into the cell (A).

(Size ofrectangles indicates relative concentration of ions.) The cell membrane has differential permeabilities to theions, resulting in leak across the membrane at different rates. The membrane is not permeable to proteins,which are negatively charged and are mainly intracellular. Leakage of K+ out of the cell is the major factorin setting the resting membrane potential, because it is the most permeable ion. This situation can bemodeled as an electrical circuit (B), in which potential differences for the ions are expressed (in mV), basedon the values obtained from the Nernst equation, and the permeability of the ions is expressed in terms ofconductance (g).Na+/K+ ATPase, counteracting the constant leak of Na+ (andthus K+) (Fig.

3.2).Where:■■A cell is said to be hyperpolarized when its membranepotential is more negative than the normal resting membrane potential for the cell; a cell at membrane potential lessnegative than its usual resting membrane potential is said to bedepolarized. The membrane potential is a function of the individual concentration gradients for ions across the cell membrane and the permeabilities of the membrane to those ions.ELECTROPHYSICAL PRINCIPLESSome key electrophysical principles apply to movement ofions in cellular systems, and also to conduction of electricalimpulses in the nervous system.

According to Ohm’s law:V = IR■V is the voltage of potential difference, measured in volts(V) or mVI is the current, measured in amperes (A), nanoamperes(10−9A), or picoamperes (10−12A)R is resistance, measured in ohms or megaohms(106 ohms)In any electrical system, including movement of permeableions across a cell membrane, current will occur (that is,charged particles will flow) driven by the potential differencein the system and impeded by the resistance of the system.Conductance (G) is the inverse of resistance and is measuredin Siemens (S), nanoSiemens, or picoSiemens:G =1 RIn cells, conductance for various ions is a function of “leakiness” of the cell membrane to the ions (related to permeability, or presence of open channels). Ohm’s law can be restated28The Nervous System and Muscleto predict a specific ion current based on membrane potentialand conductance of the ion:I X = G ( Vm − E x )In other words, an ion current across a cell membrane willequal the product of the conductance for the ion and the difference between the membrane potential and the equilibrium(Nernst) potential for the ion.

Thus, the positive ion K+ continues to leak out of the cell (along its concentration gradient)at resting membrane potential, because the Vm (approximately−70 mV in most cells) is less negative than EK+ (−94 mV); Na+will continually leak into a cell, because its equilibrium potential is 61 mV compared with its resting membrane potentialof −70 mV.

By convention, the current associated with thisoutward flux of K+, IK+, is a positive current, whereasthe inward Na+ current, INa+, is a negative current. Again, theconcentration gradients of these ions are maintained inthe face of this constant but slow leak by active transport bythe Na+/K+ ATPase.ACTION POTENTIALSAn action potential is a rapid depolarization that occurs in anexcitable cell (neurons and muscle cells) initiated by an electrical event or chemical stimulation that causes increased ionpermeability in the cell membrane (Fig.

3.3). Some importantcharacteristics of action potentials are as follows:■■■■■A threshold potential, which is the potential at which anaction potential will be initiated by increased conductance of ions (Na+ in many cases). For example, a cellwith a resting membrane potential of −70 mV mighthave a threshold potential of −50 mV; if the cell is depolarized to that threshold, an action potential will occur.“All-or-none” response, with a fixed amplitude (voltagechange) and stereotypic shape of the action potential fora given cell type.Nondecremental propagation, in which an actionpotential at one point on a cell surface will be conductedto adjacent sites along the membrane.A refractory period beginning with the initiation of theaction potential, when it is impossible to elicit a secondaction potential.A relative refractory period following the absoluterefractory period, when a second potential can be evokedbut requires a larger than normal stimulus.The threshold potential results from the balance betweenoutward K+ leakage and inward Na+ current through voltagegated Na+ channels.