What is the average difference in electrical charge between the inside and outside of the neuron when it is at its resting potential?

2.1 Resting membrane potential

The resting membrane potential exerts an electrical force on the negatively charged chloride ions. If the membrane potential were the only force acting and provided a sufficient chloride conductance, the steady-state condition would produce a chloride gradient that follows Nernst's law at the resting membrane potential and thereby fix the chloride reversal potential at the resting membrane potential. Countless experiments have shown that the chloride equilibrium potential can be more positive or more negative than the resting membrane potential; thus in addition to the passive equilibrium condition, active transport is shaping the chloride distribution across the cell membrane. Two members of the cation-chloride cotransporter family (Gagnon & Delpire, 2013) represent the most important chloride pumps for neurons. One member of the sodium potassium chloride cotransporter family: Na-K-Cl cotransporter 1 (NKCC1) with a pumping stochiometry of 1Na, 1K, 2Cl, and one member of the potassium chloride cotransporter family: K-Cl cotransporter 2 (KCC2) with a 1K, 1Cl stochiometry (Blaesse et al., 2009; Payne, Rivera, Voipio, & Kaila, 2003). Both pumps use the energy of the gradients established by the Na/K ATPase to pump chloride against a potential electrochemical gradient.

Resting membrane potential

The resting membrane potential is the potential of a cell while at rest. All neural activities start with a change in the resting membrane potential, which is temporary and localized. This effect is a graded potential and decreases over distance from the stimulus. When it is large enough, an action potential is triggered in the membrane of the axon. An action potential is an electrical signal involving nerve cells. A neuron that is not conducting electrical signals is “resting”, usually at about −70 mV, though this varies. The three most important factors about resting membrane potential are as follows:

Large differences in the ionic composition of ECF and intracellular fluid (cytoplasm). The ECF has high concentrations of sodium (Na+) and chloride ions (Cl−). The cytoplasm has significant concentrations of potassium ions (K+) and negatively charged proteins.

Selective cell membrane permeability. Because of selectively permeable membranes, there is no “even” distribution of ions. Lipid areas of the plasma membrane keep ions from crossing easily. Ions only enter or leave a cell through a membrane channel. At resting membrane potential, ions move through leak channels, which are membrane channels that stay open. Some ions are moved in or out of cells by active transport mechanisms, including the sodium-potassium exchange pump.

Ions have different membrane permeabilities. This means that a cell’s passive and active transport mechanisms do not have an equal distribution of charges across the plasma membrane. Negatively charged proteins are unable to cross the membrane due to their large size. The inner membrane surface has excessive negative charges compared to the outer surface.

Passive and active forces determine membrane potential across the plasma membrane.

Sodium-potassium exchange pump

At normal resting membrane potential, sodium is forced outwards and potassium is carry into the cells (see Fig. 2.4). This requires the sodium-potassium exchange pump, which uses ATP in order to operate. Three intracellular sodium ions will be exchanged for every two extracellular potassium ions. At normal resting membrane potential, the sodium ions are ejected as fast as they can enter. The exchange pump then balances the passive forces of diffusion. Since the ionic concentration gradients remain balanced, resting membrane potential remains stable.

Figure 2.4. Sodium-potassium exchange pump.

Publisher Summary

The resting potential of the nerve cell is because of the differences of ionic concentration, which are maintained on the two sides of the membrane by metabolic processes. If undisturbed, the system remains in a steady state, but it is not in thermodynamic equilibrium. When a molecule is dissociated into ions in a solution, and the latter is divided by a semipermeable membrane into two compartments with unequal ionic concentrations, then an electric potential difference exists between the compartments. This chapter discusses how a relationship between potential and concentration differences at equilibrium can be derived. It discusses the exchange of ions across a membrane as a more accurate derivation of the resting potential must take into account that several species are involved in ion exchange across the membranes and as it must include the different permeabilities of the membrane to the various ions.

19.3.1 Passive properties

The resting membrane potential of a neuron is the electrical potential inside the cell relative to the adjacent extracellular space. In conjunction with the input resistance, which is measured by the changes of membrane potential to small somatic current injections, and the action potential threshold, the resting membrane potential determines how likely a neuron is to fire an action potential in response to synaptic input. In both PV-expressing (Goldberg et al., 2011; Lazarus and Huang, 2011; Miyamae et al., 2017; Okaty et al., 2009; Oswald and Reyes, 2011; Pangratz-Fuehrer and Hestrin, 2011; Takesian et al., 2013; Yang et al., 2014) and SST-expressing (Kinnischtzke et al., 2012; Lazarus and Huang, 2011; Pan et al., 2016; Takesian et al., 2013) interneurons, the resting membrane potential at birth is significantly depolarized relative to adult levels. The resting membrane potential then hyperpolarizes to adult levels over the first postnatal week, concurrent with changes to other physiological properties. Another passive membrane property that changes significantly over this time period is the input resistance. A neuron's input resistance determines how much its membrane potential will change in response to a given synaptic current. Interneuron input resistances are markedly higher at birth than those seen in mature interneurons and decrease over the first 1–2 postnatal weeks. The depolarized resting potential and increased input resistances of immature interneurons lead to a higher likelihood of action potential generation given any synaptic input. As these neurons mature and decrease their input resistance, they become more integrating than rapid-following neurons. These changes are similar to those reported in the maturation of cortical pyramidal neurons (Franceschetti et al., 1998; Kasper et al., 1994; Kinnischtzke et al., 2012; McCormick and Prince, 1987; Metherate and Aramakis, 1999; Oswald and Reyes, 2008; Zhang, 2004) which demonstrate hyperpolarization of the resting membrane potential, decreasing input resistance, and decreasing membrane time constants over the first 1–2 postnatal weeks. Genetic studies of the maturation of fast-spiking interneurons demonstrate that both the hyperpolarization of the membrane potential and the decrease in input resistance correlate with an increase in inward rectifier potassium channels Kir2.2 and Kir2.3 (Goldberg et al., 2011) and the potassium leak currents TWIK1 and TASK1 (Goldberg et al., 2011; Okaty et al., 2009). Pharmacological blockade of Kir2.2 channels directly depolarizes the resting membrane potential, identifying the maturational increase in expression of these channels as likely causative of the hyperpolarization of the resting membrane potential that occurs during this same period.

A final passive membrane property of fast-spiking interneurons that is of note is the gamma frequency oscillation of the membrane at potentials just hyperpolarized to the action potential threshold (Llinás et al., 1991). These oscillations enhance cortical sensory processing (Cardin et al., 2009; Sohal et al., 2009) and are disrupted in schizophrenia (Sun et al., 2011). In the first postnatal week, fast-spiking interneurons have a primary oscillation frequency of ∼10 Hz, but this increases to >50 Hz by the end of the third postnatal week (Goldberg et al., 2011), correlated with increased expression of TASK 1/3. Direct knockout of TASK 1/3 leads to a marked impairment of the maturational increase in membrane potential oscillations, suggesting that increased expression of this gene is likely causative of this important physiological change.

The Effects of Elevated Extracellular Calcium on Voltage Measurements

The resting potentials we obtained were more negative than previously reported for rat fast7,21 and slow8,21 twitch muscle fibers. In vivo studies suggest that the physiologic mean resting potential of rat fast twitch muscle is about –95 mV.21 Several factors contribute to the large negative values of the resting potentials reported here. The membrane potentials were adjusted for the depolarization of about 5 mV produced by impaling the cell with a microelectrode, and the high extracellular calcium produced by a 6-mV hyperpolarization. These two factors resulted in the membrane potentials reported here being about 11 mV more negative than most of the previously reported values. In addition, we used a relatively low potassium concentration (3.5 mM KCl) in the Tyrode solution in order to further hyperpolarize cells. In this way, it would not be necessary to subject a fiber to a very large hyperpolarization to completely remove SI. To compare our values for Vh1/2 and Vs1/2 with those previously obtained, it is necessary to adjust for the 11-mV shift described above, and for the 2-mV depolarizing shift produced by 6 mM Ca+2 Tyrode solution on inactivation gating. Consequently, the adjusted values are as follows. Vh1/2 is about –68 mV for EDL and –61 mV for soleus, and Vs1/2 is about –99 mV for EDL and –77 mV for soleus. The adjusted values of Vh1/2 agree well with the values previously reported for intact rat muscle of –70 mV to –73 mV for fast twitch fibers7,21,22 and –63 mV for soleus fibers.8

The corrected Vs1/2 suggests that only about one-third of the total available sodium channels in fast twitch fibers are excitable at the in vivo resting potential (Figure 13.5). Prior studies using voltage clamp techniques other than the loose-patch voltage clamp reported sodium current densities of 2.4 to 3.8 mA/cm2 for rat fast twitch fibers maintained at about –90 mV,4,7,22 which agrees well with the value of 3.1 mA/cm2 predicted by our data with Vs1/2 = –99 mV and a maximal sodium current density of 17.6 mA/cm2. In human muscle fibers that were not hyperpolarized sufficiently to remove SI, DeCoursey et al.5 reported a current density of 2.4 mA/cm2. The differences between the resting potentials and Vh1/2 of slow and fast twitch fibers were qualitatively similar to the findings of Duval and Leoty,8 who reported that slow fibers had mean resting potentials that were 8 mV more positive and Vh1/2 that was approximately 10 mV more positive than in fast twitch fibers.

C Excitability of Neuromuscular Tissues

The resting membrane potential is the voltage across a given cell membrane during the resting stage. In neuromuscular tissues (e.g., nerves, cardiac and skeletal muscle), it is determined primarily by the potassium concentration gradient across the cell membrane or the ratio of ICF to ECF potassium ([Ki]/[Ke]). The threshold potential is the potential at which an action potential is generated during depolarization. The excitability of the membrane is related to the difference between resting potential and threshold potential. Because the variations in plasma potassium will cause relatively large changes in the [Ki]/[Ke] ratio as a result of the smaller range of [Ke] as compared with [Ki], the abnormal plasma potassium levels are more likely to alter the excitability of the neuromuscular tissues, which leads to cellular dysfunction, changes in cardiac conductivity, and the weakness and paralysis of skeletal muscle. Therefore, the regulation of potassium homeostasis requires the maintenance of the proper amount of total body potassium and maintaining an optimal range for the [Ki]/[Ke] ratio to protect the integrity of the neuromuscular function.

The Resting Membrane Potential, Phase 4, Is Set by EK and Large gK

The resting membrane potential of ventricular contractile cells is determined by the conductance-weighted average of the equilibrium potentials for all of the diffusable ions, as described in Eqn [5.5.1]. The equilibrium potential for Na+ is given by the Nernst Equation, whose derivation was described in Chapter 3.1:

[5.5.3]ENa=RTzℑln[Na+]o[Na+]l

where ENa is the equilibrium potential for Na+, R is the gas constant, which in electrical units is 8.314 J mol−1 K−1, T is the temperature in K, z is the valence on the Na+ ion, which is 1.0, and ℑ is the faraday=9.649×104 C mol−1. The logarithm is the natural log. Expressions such as that in Eqn [5.5.3] can be used for each ion, and the resulting equilibrium potentials are listed in Table 5.5.1 for the ionic conditions in the cardiomyocytes.

Table 5.5.1. Concentrations of Selected Ions in the Extracellular Fluid and Intracellular Fluid and Their Calculated Equilibrium Potentials for Cardiomyocytes

The resting membrane potential in the ventricular cells is −0.080 to −0.090 V. This is because at rest in these cells, gK>>gNa, gCa, and gCl, so by Eqn [5.5.1] the resting membrane potential is closer to EK than to ENa, ECa, or ECl. Two specific channels account for the large resting gK. These channels carry the delayed rectifying K+ current (IK) and the inwardly rectifying K+ current (IK1). The resting Em is never as negative as EK, however, because there is residual conductance to Na+ that carries the background current, Ib. This is the stable situation that pertains to the resting cell and accounts for phase 4 of the action potential for ventricular cells shown in Figure 5.5.1.

IVB Inward-Rectifier K+ Channels

The RP of maturing myotubes gets closer to EK because the PNa/PK ratio is gradually decreased during development (see Fig. 25.8B). These characteristics are also observed in chick embryonic cardiomyocytes during development (Sperelakis and Shigenobu, 1972) and are produced by marked expression of the inward-rectifier K+ channels in the surface membrane of the myocytes (Shin et al., 1997). Therefore, it seems likely that the hyperpolarization of the RP of skeletal myocytes in culture is produced by a similar developmental change of inward-rectifier K+ channels.

The IK1 is present in skeletal muscle cells from early embryonic amphibian (Linsdell and Moody, 1995). Although there is a brief period (approximately 4 h) during which its density decreases, the overall trend is an increase during development.

In ascidian myocytes, IK1 exhibited dramatic changes during development (Fig. 25.10C) (Greaves et al., 1996). In the ascidian, the inward-rectifier K+ current is gained after fertilization of the egg. (The same change also occurs in other species.) When gastrulation ends (at 16 h after fertilization), the current density suddenly decreases from 4 to 0.5 pA/pF. After the tailbud stage (22 h after fertilization), the current density progressively increases again and reaches a value of 5 pA/pF before hatching. Because IK1 is one of the most important resting conductances (which stabilizes and helps to set the RP), this transient decrease and subsequent increase in the current density parallels the generation of spontaneous APs.

Evaluating the human myoblast fusion revealed that the IK1 increases during differentiation (Liu et al., 1998). In addition, inhibition of Kir 2.1 expression with an antisense-Kir2.1-RNA reduced the endogenous IK1 and blocked fusion (Fischer-Lougheed et al., 2001). Therefore, it seems likely that the increase in IK1 is required for human muscle differentiation.

Resting Membrane Potential

Increases in the resting membrane potential during development have been reported in several species (Table 51-3). Increases in activity of the sodium-potassium ATPase pump have been reported with maturation. The increase in sodium-potassium ATPase activity noted during development may in part result from expression of different isoforms of the sodium-potassium ATPase pump.16 Increased activity of the sodium-potassium ATPase pump increases the concentration of potassium intracellularly and decreases the concentration of sodium. This results in a more strongly negative resting membrane potential.

An increase in membrane permeability to potassium with development also contributes to the more negative resting potential observed with maturation. Specific increases in the current density of the inward rectifier current, IK1, the main outward potassium current responsible for maintenance of the resting membrane potential, have been reported in chicks and in mammalian species.17-22

In the human fetus, resting membrane potentials at midgestation (about 20 weeks) are similar to values reported in adult mammalian tissue.23 In studies performed in atrial myocytes isolated from young infants undergoing heart surgery, the resting membrane potential is similar to that of the adult.23,24