Potential induced in a spherical cell by an intracellular point source and an extracellular point sink

Oocytes from Xenopus laevis are commonly used as an expression system for ion channel proteins. The most common method for their electrophysiological investigation is the two-microelectrode voltage clamp technique. The quality of voltage clamp recordings obtained with this technique is poor when membrane currents are large and when rapid charging of the membrane is desired. Detailed mathematical modeling of the experimental setup shows that the reasons for this weak performance are the electrical properties of the oocytes and the geometry of the setup. We measured the cytosolic conductivity to be approximately 5 times lower than that of the typical bath solution, and the specific membrane capacitance to be approximately 6 times higher than that of a simple lipid bilayer. The diameter of oocytes is typically approximately 1 mm, whereas the penetration depth of the microelectrodes is limited to approximately 100 microm. This eccentric current injection, in combination with the large time constants caused by the low conductivity and the high capacitance, yields large deviations from isopotentiality that decay slowly with time constants of up to 150 micros. The inhomogeneity of the membrane potential can be greatly reduced by introducing an additional, extracellular current-passing electrode. The geometrical and electrical parameters of the setup are optimized and initial experiments show that this method should allow for faster and more uniform control of membrane potential.

Section: Theorymentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

Section: General Electrostatic Descriptionmentioning

confidence: 99%

See 1 more Smart Citation

Two-Microelectrode Voltage Clamp of Xenopus Oocytes: Voltage Errors and Compensation for Local Current Flow

Baumgärtner

Islas

Sigworth

1999

“…Solutions for the interior potential and current flow resulting from current injection into spherical cells have been obtained (Eisenberg and Johnson, 1970;Engel et al, 1972;Pickard, 1971a,b), but in all these cases the exterior is assumed to be at Earth potential. Two papers (Peskoff and Eisenberg, 1975;Peskoff and Ramirez, 1975) do consider the external field, but only for the purpose of showing that the position of an external sink and the conduction of the external fluid have little effect on the membrane potential. In the case of motor-nerve terminals, for which most experimental evidence is available, the problem that requires solution relates to charge spread when applied at a point on a cable of non-negligible diameter in a volume conductor, so that current flows both transversely and along the length of the cylindrical cable.…”

Section: Introductionmentioning

confidence: 99%

Quantal Potential Fields around Individual Active Zones of Amphibian Motor-Nerve Terminals

Bennett

Farnell

Gibson

et al. 2000

The release of a quantum from a nerve terminal is accompanied by the flow of extracellular current, which creates a field around the site of transmitter action. We provide a solution for the extent of this field for the case of a quantum released from a site on an amphibian motor-nerve terminal branch onto the receptor patch of a muscle fiber and compare this with measurements of the field using three extracellular electrodes. Numerical solution of the equations for the quantal potential field in cylindrical coordinates show that the density of the field at the peak of the quantal current gives rise to a peak extracellular potential, which declines approximately as the inverse of the distance from the source at distances greater than about 4 microm from the source along the length of the fiber. The peak extracellular potential declines to 20% of its initial value in a distance of about 6 microm, both along the length of the fiber and in the circumferential direction around the fiber. Simultaneous recordings of quantal potential fields, made with three electrodes placed in a line at right angles to an FM1-43 visualized branch, gave determinations of the field strengths in accord with the numerical solutions. In addition, the three electrodes were placed so as to straddle the visualized release sites of a branch. The positions of these sites were correctly predicted on the basis of the theory and independently ascertained by FM1-43 staining of the sites. It is concluded that quantal potential fields at the neuromuscular junction that can be measured with available recording techniques are restricted to regions within about 10 microm of the release site.

“…Classical analysis had a cavalier treatment of the spatial spread of potential. The actual spatial spread of potential is enforced by a combination of the cable (i.e., telegrapher's or transmission line) equation in one dimension (20)(21)(22)(23) that describes the properties of the axon ''cable'', and Poisson's equation of electrodynamics in three dimensions (24)(25)(26). Rattay et al (19) have computed strength duration curves produced by extracellular stimulation, including the spatial dependence of potential, and found dramatic deviations from classical theory.…”

mentioning

confidence: 99%

Brilliant Stimulation, One Cell at a Time

Eisenberg

2018