Electrical Properties of the External Limiting Membrane in the Receptor Layer of the Frog Retina

Abstract

Retinal photoreceptors generate currents which can be recorded extracellularly. When these currents spread along the receptor cells they flow through the external limiting membrane (e.l.m.), a transversal structure formed by neuroglia cells (Müller cells). The present study describes the electrical properties of this membrane-like structure and the resulting interference with the spatial distribution of the receptor currents. Measurements of currents were made with an array of two or three microelectrodes which were moved through the receptor layer. Four groups of results were obtained:

• i) Large voltage drops were frequently recorded when the electrodes were advanced into the e.l.m. of the dark-adapted retina.

• ii) Voltage drops between a pair of microelectrodes caused by transretinal current pulses increased 3 to 8 times at the depth of the e.l.m. indicating an increase in resistance in this region. Locally, up to a 20 fold increase in resistance was observed by means of current injection through the recording electrode. Recordings of current spread in the lateral direction suggested that laterally the resistance increased by a factor of 3 to 4 at the depth of the e.l.m.

• iii) Large photoresponses which accompanied large voltage drops were sometimes observed in regions of high local resistance. The lateral spread of the responses was less than 10 μm.

• iiii) The resistance of the e.l.m. did not change during light responses.