Chapter 2: Membrane Excitability


I. Overview

All cells selectively modify the ionic composition of their internal environment to support the biochemistry of life (see 1?II) as shown in Figure 2.1. Moving ions into or out of a cell creates a charge imbalance between the intracellular fluid (ICF) and the extracellular fluid (ECF) and thereby allows a voltage difference to form across the surface membrane (a membrane potential, or V m ). This process creates an electrochemical driving force for diffusion that can be used to move charged solutes across the membrane or that can be modified transiently to create an electrical signal for intercellular communication.

For example, nerve cells use changes in Vm (action potentials) to signal to a muscle that it needs to contract. The muscle cell, in turn, uses a change in Vm to activate Ca2+ release from its internal stores. Ca2+ release then facilitates actin and myosin interactions and initiates muscle contraction. Neuronal and muscle action potentials both involve carefully coordinated sequences of ion channel events that allow selective transmembrane passage of ions (e.g., Na+, Ca2+, and K+) between ICF and ECF.

Figure 2.1 Intracellular fluid (ICF) modification by ion transporters.

ATP = adenosine triphosphate; ECF = extracellular fluid.

II. Membrane Potentials

The term “membrane potential” refers to the voltage difference that exists across the plasma membrane. By convention, the ECF is considered to be at zero volts, or electrical “ground.” Inserting a fine electrode across the surface membrane reveals that the cell interior is negative with respect to the ECF by several tens of millivolts. A typical nerve cell has a resting potential of ?70 mV, for example (Figure 2.2). Vm is established by membrane-permeant ions traveling down their respective concentration gradients and generating diffusion potentials.

Figure 2.2 Membrane potential (Vm).

A. Diffusion potentials

Imagine a model cell in which the plasma membrane is composed of pure lipid, the ICF is rich in potassium chloride (KCl, which dissociates into K+ and Cl?), and ECF is pure water (Figure 2.3). Although there is a strong KCl concentration gradient for diffusion across the membrane, the lipid barrier prevents both K+ and Cl? from leaving the cell and thereby constrains both ions to the ICF.

The charges carried by K+ and Cl? cancel each other out and, thus, there is no voltage difference between ECF and ICF. If a protein that permits passage of K+ alone is inserted into the lipid barrier, K+ is now free to diffuse down its concentration gradient from ICF to ECF, and the membrane is said to be semipermeable (Figure 2.4). Because potassium ions carry charge, their movement causes a diffusion potential to form across the membrane in direct proportion to the magnitude of the concentration gradient. The potential may be significant (tens of millivolts) but involves relatively few ions.

Figure 2.3 Charge distribution in a model cell with an impermeable membrane.

ECF = extracellular fluid; ICF = intracellular fluid.

Figure 2.4Origin of a diffusion potential.

ECF = extracellular fluid; ICF = intracellular fluid.

The principle of bulk electroneutrality notes that the number of positive charges in a given solution is always balanced by an equal number of negative charges. The ICF and ECF are also subject to this rule, even though all cells create a negative Vm by altering charge distribution between the two compartments. In practice, Vm is established by just a few charges moving in the immediate vicinity of the cell membrane and their net effect on overall charge distribution within the bulk of the ICF and ECF is negligible.

B. Equilibrium potentials

When K+ crosses the membrane down its concentration gradient, it leaves a negative charge in the form of Cl? behind. Net charge magnitude builds in direct proportion to the number of ions leaving the ICF (see Figure 2.4), but, because opposite charges attract, K+ movement slows and eventually stops when the attraction of the negative charges inside the cell precisely counters the outward driving force created by the concentration gradient (electrochemical equilibrium). The potential at which equilibrium is established is known as the equilibrium potential for K+.

1. Nernst equation:

Equilibrium potentials can be calculated for any membrane-permeant ion assuming that the ion’s charge and concentrations on either side of the membrane are known:

where Ex is the equilibrium potential for ion X (in mV), T is absolute temperature, z is the valence of the ion, R and F are physical constants (the ideal gas constant and the Faraday constant), and [X]o and [X]i are ECF and ICF concentrations of X (in mmol/L), respectively. Equation 2.1 is known as the Nernst equation. If T is assumed to be normal human body temperature (37°C), Equation 2.1 can be simplified:

Most of the common inorganic ions (Na+, K+, Cl?, HCO3 ?) have an electrical valence of 1 (monovalents). Ca2+ and Mg2+ have a valence of 2 (divalents).

2. Equilibrium potentials:

The ICF and ECF are both strictly regulated, and their ionic composition is well known (see Figure 2.1, also see Table 1.1). Using known values for concentrations of the common ions, we can use the Nernst equation to predict that, for most cells in the body, EK = ?90 mV, ENa = +61 mV, and ECa = +120 mV. Intracellular Cl? concentrations can vary considerably, but ECl usually lies very close to Vm. If any of these ions are provided with a pathway that allows them to diffuse across the plasma membrane, they will drag Vm toward the equilibrium potential for that ion (Figure 2.5).

Figure 2.5 Equilibrium potentials for Na+ (ENa), Ca2+ (ECa), and K+ (EK).

 ECF = extracellular fluid; ICF = intracellular fluid.

Example 2.1. 2nt

A cell has an intracellular free Mg2+ concentration of 0.5 mmol/L and is bathed in a saline solution with a Mg2+ composition that approximates plasma (1.0 mmol/L). The saline is held 37°C. If the cell has a membrane potential (Vm) of ?70 mV, and the membrane contains a gated channel that is Mg2+ permeable, will Mg2+ flow into or out of the cell when the channel opens?

We can use Equation 2.2 to calculate the Mg2+ equilibrium potential for (EMg):

EMg tells us that Mg2+ will flow into the cell, its positive charges tending to drive Vm toward 9 mV.

C. Resting potential

The plasma membranes of living cells are rich in ion channels that are permeable to one or more of the ions mentioned above, and some of these channels are open at rest. Resting Vm (resting potential) thus reflects the sum of the diffusion potentials generated by each of these ions flowing through open channels. Vm can be calculated mathematically as follows:

where gT is total membrane conductance (membrane conductance is the reciprocal of membrane resistance, in Ohms?1); gNa, gK, gCa, and gCl are individual conductances for each of the common ions (Na+, K+, Ca2+, and Cl?, respectively); and ENa, EK, ECa, and ECl are equilibrium potentials for these ions (in mV). Vm can also be calculated using the Goldman-Hodgkin-Katz (GHK) equation, which is similar in form to the Nernst equation above (Equation 2.1). The GHK equation derives Vm using relative membrane permeabilities for each of the ions that contribute to membrane potential.

In practice, most cells at rest have a negligible permeability to either Na+ or Ca2+. Cells do have a significant resting K+ conductance, however. Thus, Vm typically rests close to the equilibrium potential for K+ (Figure 2.6). The approximate value for resting potential in neurons is ?70 mV, ?90 mV in cardiac myocytes, ?55 mV in smooth muscle cells, and ?40 mV in hepatocytes, for example.

Figure 2.6 Resting potential origins.

ECF = extracellular fluid; ICF = intracellular fluid.

D. Extracellular ion effects

The ionic composition of the ECF is regulated within a fairly narrow range, but significant disturbances can occur through inadequate or excessive ingestion of salts or water. Because resting membrane permeability to Na+ and Ca2+ is low, Vm is relatively insensitive to changes in ECF concentration of either ion. Vm is sensitive to changes in extracellular K+ concentration, however, because resting potential is closely tied to the equilibrium potential for K+ (see Figure 2.6). Increasing extracellular K+ concentration (hyperkalemia) reduces the electrochemical gradient that drives K+ efflux, causing the membrane to depolarize (Figure 2.7). Conversely, lowering the extracellular K+ concentration (hypokalemia) steepens the gradient, and Vm becomes more negative.

Figure 2.7 Membrane-potential (Vm) dependence on K+ concentration in the extracellular fluid (ECF).

ICF = intracellular fluid.

E. Transporter contribution

The Na+-K+ ATPase that resides in the plasma membrane of all cells drives three Na+ out of the cell while simultaneously transferring two K+ from ECF to ICF. The three-for-two exchange results in an excess of positive charges being removed from the cell. Because the transporter creates a charge imbalance across the membrane, it is said to be electrogenic. The direct contribution of this exchange to Vm is insignificant, however. The main role of the Na+-K+ ATPase is to maintain a K+ concentration gradient across the membrane, because it is the K+ gradient that ultimately determines Vm via the K+ diffusion potential.

Clinical Application 2.1: Hypokalemia and Hyperkalemia

Excitable cell function critically depends on maintaining membrane potential within a narrow range, so plasma levels normally range between 3.5 and 5.0 mmol/L. Hypokalemia and hyperkalemia are both commonly encountered clinically, however. Hypokalemia is generally of less concern than hyperkalemia, although some individuals with a rare inherited disorder (hypokalemic periodic paralysis) can experience muscle weakness when plasma K+ concentrations dip such as following a meal. Hyperkalemia is, potentially, a more serious condition. The slow depolarization caused by rising plasma K+ levels inactivates Na+ channels that are required for muscle excitation, resulting in skeletal muscle weakness or paralysis and cardiac arrhythmias and conduction abnormalities. Hyperkalemia usually results from kidney failure and impaired ability to excrete K+. Treatment typically requires either diuresis or dialysis to remove excess K+ from the body.

III. Excitation

Many cell types use changes in Vm and transmembrane ion fluxes as a means of signaling or initiating intracellular events. Sensory cells (e.g., mechanosensors, olfactory receptors, and photoreceptors) transduce sensory stimuli by generating a Vm change called a receptor potential. Neurons signal to each other and to effector tissues using action potentials. Myocytes and secretory cells also use changes in Vm to increase intracellular Ca2+ concentration, thereby facilitating contraction and secretion, respectively. All such cells are said to have excitable membranes.

A. Terminology

The electrical changes caused by increased membrane permeability to ions do not consider the permeant ion’s species (e.g., Na+ versus K+ or Cl?), only the charge that it carries.

1. Membrane potential changes:

The inside of a cell at rest is always negative with respect to the ECF. When a positively charged ion (cation) flows into a cell, negative charges (anions) are neutralized, and the membrane loses polarization. The influx is said to have depolarized the cell, or caused membrane depolarization. By convention, depolarization is shown as an upward pen deflection on a voltage record (Figure 2.8). Conversely, if a cation leaves the cell, Vm becomes more negative: The efflux hyperpolarizes the cell (membrane hyperpolarization) and yields a downward pen deflection on a recording device.

Figure 2.8 Membrane potential changes and ion currents.

2. Currents:

When positive charges flow into a cell, they generate an inward current. By convention, recording devices, such as oscilloscopes and chart recorders, are configured so that inward currents cause a downward deflection (see Figure 2.8). Positive charges leaving the cell cause an outward current and an upward deflection on a recording device.

Anions and cations are equally effective in changing Vm, but, because anions carry negative charges, their effects are opposite to those of cations. When an anion enters the cell from the ECF, it hyperpolarizes the membrane and yields an outward current. Conversely, anions leaving a cell create an inward current, and the cell depolarizes.

B. Action potentials

Action potential size, shape, and timing may vary widely between the different cell types, but there are several common characteristics, including the existence of a threshold for action potential formation, all-or-nothing behavior, overshoots, and afterpotentials (Figure 2.9). The discussion below focuses on a nerve action potential whose upstroke is mediated by voltage-dependent Na+ channels, but voltage-dependent Ca2+ channels can support action potentials also (e.g., see 17?IV?B?3).

Figure 2.9 An action potential.

Vm = membrane potential; Vth = threshold potential.

1. Threshold potential:

Because action potentials are explosive membrane events that have consequences (e.g., initiating muscle contraction), they must be triggered with care. Vm normally fluctuates over a range of a few millivolts with changes in extracellular K+ concentration and other variables, even at rest, but such changes do not trigger spikes. Neurons only fire action potentials when Vm depolarizes sufficiently to cross the voltage threshold for action potential formation (Vth), which, in a neuron, usually resides at around ?60 mV. Vth corresponds to the voltage needed to open the number of voltage-dependent Na+ channels required to trigger an action potential.

2. All or nothing:

Voltage-dependent Na+ channels that mediate action potentials are typically present in the membrane in high numbers. When Vm crosses threshold, they open to allow a massive inward current, and the membrane depolarizes in a self-perpetuating (regenerative) fashion toward ENa (+61 mV). This “all-or-nothing” behavior can be likened to breaching a dam wall. Once depolarization begins, it does not stop until the ionic flood is complete.

3. Overshoot

The action-potential peak typically does not reach ENa, but it often “overshoots” the zero-potential line, and the inside of the cell becomes positively charged with respect to the ECF.

4. Afterpotentials:

Action potentials are transient events. The downstroke is caused in part by voltage-dependent K+ channels that open to allow K+ efflux, causing Vm to repolarize. In some cells, the action potential may be followed by an afterpotential of varying size and polarity. A hyperpolarizing afterpotential takes the membrane negative to Vm for a period before eventually settling at the normal resting potential.