Ohm's Law

I = g * V

The voltage present across the cell's membrane can be measured between a reference electrode and a conducting glass capillary tube drawn out to a very fine point, and inserted into the cell. At rest the neuronal interior is negative relative to the outside at between -60 to -80mV, while most non-neuronal cells feature a potential of about -30 mV. The current is carried by ions moving across the membrane at ion conductances when specific channels allow them to pass. Ignoring its active properties, the axon can in electrical terms be viewed as an insulated cable. An electric potential is able to spread passively along any stretch of membrane. In the process its strength decays and the slope of on- and offsets becomes less steep with distance due to the passive cable properties of the membrane. The latter include resistance along its length and both a resistance and a capacitance component across it: Signals will spread fastest when longitudinal resistance is low (e.g., via increased axonal diameter) and they will spread furthest when resistance across the membrane is high (e.g., with layers of myelin for added electrical insulation).		a. b. c. d. The electric properties of the neuronal membrane can be expressed as a circuit diagram. © 2004 lobsterman. a. Golgi-stained neuron with long axonal process. b. Circuit diagram of neuronal membrane combining membrane resistance (Rm), membrane capacitance (Cm) and internal or axial resistance (Ri). c. Decay in signal size over distance with Space Constant. d. Change along the axon in Voltage response to the injection of a square current pulse due to membrane capacitance

The Resting Membrane Potential

The resting membrane potential arises from an unequal distribution of ions across the cell's membrane. An excess of K⁺ and of various Anions^- exists on the inside and of Na⁺, Cl^-on the outside. The membrane is dotted with proteins (i.e., ion channels) that control the flow of specific ions across it. In a cell at rest only potassium ions (K⁺) are able to flow freely between the two compartments. Initially we observe a net flow of positively charged potassium ions from the inside where they are more numerous to the outside where their concentration is much lower. With a net flow of positively charged ions from the inside across the membrane, the inside becomes increasingly negative relative to the outside. The resting potential settles into an equilibrium when as many potassium ions are pushed out of the cell along their concentration gradient, as will enter the cell along the accompanying electrical gradient. This equilibrium potential for potassium can be calculated using the Nernst potential for known inside and outside concentrations of any given ion

The Nernst potential for potassium is thus:

The cell's resting membrane potential combines all relevant ion currents with K⁺ ions figuring most prominently due to their high resting conductance. In addition a small number of Na⁺ ions leak into the cell. The resulting resting membrane potential is thus slightly lower compared to the E_K+ at around -65mV. The concentration gradient across the membrane is maintained by the Na⁺/K⁺ ATPase (ion pump).

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last modified: 12/26/03

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