An excised, active nerve trunk serves as a bioelectric source located on the axi

Question

An excised, active nerve trunk serves as a bioelectric source located on the axis of a circular cylindrical volume conductor. Field potentials are recorded at various radial distances from the nerve trunk from an appropriate electrode assembly connected to an amplifier. (a) Describe the behavior of the field potential with increasing radial distance from the nerve (angle and axial distance arc fixed). (b) Describe the effect of increasing the specific resistivity rho of the bathing medium on the magnitude of the field potential, and explain how this change in rho might be accomplished experimentally. (c) In what manner would changing the radius of the surrounding volume-conductor affect the magnitude and wave shape of the extracellular field potential? (d) When can a volume conductor of finite dimensions be considered an essentially "infinite" volume conductor?

Explanation / Answer

Answer a:

As the radial distance increases from the axis, where the source is located, the distance frim the charged axis increases and hence the field potential which are given by 1/(4*pi*epsilon not) q/r , also decreases. When angular position is changed without changing the radial position, the distance from the axis is constant and hence field potentials must remain constant.

Answer b:

As the resistivity of the bathing medium changes , i.e. Increase in resistivity means decrease in dielectric constant and hence decrease in dielectric constant means increase in the electric field potential as electric field increases if dielectric constant is small, e.g. Electric field, in metal whose dielectric constant is infinite, is zero. Now increase in resistivity can be accomplished if there is decrease in ions in the biosource ,i.e decrease in the salts from the body.

Answer c:

The change in volume will decrease the charge density if the source gives the same charge into the volume. Hence there will be decrease in the feild potential and there will be change in the waveshape as the field lines will become more diffuse with wider gaps.

Answer d:

When the charge spread occurs negligibly outside a cylinderical volume and there is not significant change in the charge density of the volume, the finite volume can be considered to have reached its infinite limit.

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