Basis Of Brain Electrical Activity 21 Membrane Polarity

All neurons and glia have lipid bilayer membranes separating the delicate internal machinery of the cell from the external environment. The neuronal membrane is an excellent insulator and separates different concentration of ions inside the cell from those outside the cell. The activity of ion channels is fundamental to signaling in the nervous system. The movement of ions that carry electrical charge through ion channels results in voltage changes across the membrane.

Electrical potentials are generated across the membranes of neurons because there are differences in the concentration of specific ions across the membrane, and the membrane is selectively permeable to ion flow. Movement of ions across membranes occurs through ion channels, which are proteins that transverse the neuronal membrane and allow certain ions to cross in the direction of their concentration gradient. Whereas Na+ and Cl_ are more concentrated outside the cell, K+ and organic anions, consisting of amino acids and proteins, are more concentrated inside the cell. Na+ and Cl_ ions, therefore, tend to flow into the cell along this concentration gradient, whereas K+ ions tend to flow outward. Because of their size, large organic anions are unable to move out of the intracellular compartment.

Ion flow is not only determined by ionic concentration gradients, but also is dependent on the selective permeability of ion channels (often referred to as the channel's conductance), as well as electrical forces that arise from the membrane potential. Ultimately, in the resting state, ion flow ceases when concentration dependent forces are offset by opposing electrical forces based on the voltage across the membrane, the resting membrane potential. Because of the selective permeability of ion channels and the unequal distribution of anions and cations inside and outside of the neuron, there arises a potential difference across the neuronal membrane. The charge separation gives rise to a difference of electrical potential, or voltage, across the membrane, called the membrane potential. The membrane potential, Vm, is defined as

m in out where Vin is the potential on the inside of the cell and Vout the potential on the outside. At rest, the membrane potential is called the resting membrane potential. The outside of the cell is by convention defined as zero and the resting membrane potential is, therefore, equal to the voltage inside the cell. In neurons, the usual range is -60 mV to -70 mV.

Ions are, thus, subject to two forces driving them across the membrane: a chemical driving force that depends on the concentration gradient across the membrane and an electrical driving force that depends on the electrical potential across the membrane. Ions flow from high-concentration areas to low-concentration areas (chemical driving force); ions flow to areas of opposite charge, like charges repel, unlike charges attract (electrical driving force). The net electrochemical driving force is determined by the electrical potential difference across the membrane and the concentration gradient of the ions selective for the channel.

To illustrate these physiological features, the flow of K+ ions will be considered (Fig. 1). Because K+ ions are highly concentrated inside the cell, K+ ions tend to diffuse from inside to outside the cell down their chemical concentration gradient. Because of this ionic flow, the

Diffusion Electrical Gradient

Fig. 1. K+ channel. In neurons, K+ has a higher concentration inside neurons than outside (A). Because of the concentration differences, K+ diffuses from inside the cell to the outside. With K+ outflow, the inside of the cell becomes even more negative, because the K+ ion is carrying a positive charge. At some point, a steady state is reached, in which the electrical and chemical driving forces are equal and opposite, and there is a balance between K+ entering and leaving the cell (B). Modified from Kandel et al., 2000, with permission.

Fig. 1. K+ channel. In neurons, K+ has a higher concentration inside neurons than outside (A). Because of the concentration differences, K+ diffuses from inside the cell to the outside. With K+ outflow, the inside of the cell becomes even more negative, because the K+ ion is carrying a positive charge. At some point, a steady state is reached, in which the electrical and chemical driving forces are equal and opposite, and there is a balance between K+ entering and leaving the cell (B). Modified from Kandel et al., 2000, with permission.

outside of the membrane accumulates a slight positive charge and the inside a slight negative charge. As K+ ions accumulate outside the cell, there grows a counteracting electrostatic force opposing further K+ efflux, because the positive charges repel each other. Once K+ diffusion has proceeded to a certain point, a potential develops across the membrane at which the electrical force driving K+ ions into the cell exactly balances the chemical force driving K+ ions out of the cell, that is, the outward movement of K+ (driven by its concentration gradient) is equal to the inward movement of K+ (driven by the electrical potential difference across the membrane). This potential is called the potassium equilibrium potential, EK.

The equilibrium potential for any ion X can be calculated from the Nernst equation:

where R is the gas constant, T the temperature, z the valence of the ion, F the Faraday constant, and [X] the concentrations of the ion inside (i) and outside (o) of the cell. Because RT/F equals 25 mV at 25°C, z is +1 for K+, and the concentrations of K+ outside and inside the axon are 20 mM and 400 mM, respectively, the Nernst equation for K+ in the squid axon can be rewritten as:

Table 1

Extracellular and Intracellular Ion Concentrations in Squid Axon

Intracellular (mM)

Extracellular (mM)

Potassium Sodium Chloride Calcium

400 50 50 0.00001

20 440 560 2

Na+ is more common outside the cell than inside, therefore, it tends to flow into the cell down its chemical concentration gradient. There is also an electrical driving force that drives Na+ into the cell by virtue of the negative electrical potential difference across the membrane. The equilibrium potential of +55 for Na+ is not reached at resting conditions because there are so many more open K+ channels than Na+ channels. The relatively larger conductance of K+ compared with that of Na+ at resting conditions dictates the resting membrane potential of the cell. Thus, despite the large chemical and electrical forces driving Na+ into the cell, the influx of Na+ is small compared with K+ if there are many open channels.

The resting membrane potential, Vm, is not equal to either EK or ENa, but lies between them. As a rule, if Vm is determined by two or more ions, the influence of each ion is determined not only by the concentration of the ion inside and outside the cell but also by the relative permeability of the membrane to each ion. The Goldman equation describes voltage when more than one ion is active:

P K ]o + PJ Na]o + Pa[Cl]i where V is the voltage and P is the permeability of the membrane to each ion. Often, conductance, G, is used in lieu of permeability, P, in biological systems.

The Goldman equation is an extension of the Nernst equation that considers the relative permeabilities of the ions involved. Table 1 provides the extracellular and intracellular ion concentrations that apply in common physiological circumstances.

Because ion leaks could eventually result in a run down of Na+ and K+ gradients, the resting membrane potential would eventually be altered. The Na+-K+ pump, which moves Na+ and K+ ions against their net electrochemical gradients, extrudes Na+ from the cell while bringing K+ into the cell. The energy to run this pump comes from the hydrolysis of ATP. At the resting membrane potential, the cell is not in equilibrium but, rather, in a steady state. The continuous passive influx of Na+ and efflux of K+ ions is counterbalanced by the Na+-K+ pump.

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Responses

  • paula
    What forces determine the direction of Na movement?
    7 years ago

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