The contribution by glial cells to surface recordings from the optic nerve of an amphibian.

1. The contribution by glial cells to surface recordings has been examined in the optic nerve of the amphibian Necturus maculosus. The method of current injection was employed selectively to alter the membrane potential of glial cells without affecting that of the axons. The resulting changes in potential were recorded simultaneously from the surface of the nerve using the sucrose gap method and intracellularly from a glial cell near the gap.2. The sucrose gap method recorded 40% of the changes in glial membrane potential. This percentage was not affected when the current electrode was inserted into different glial cells while maintaining the recording conditions constant.3. Following axonal degeneration, produced by removing the eye 2-3 months earlier, the percentage contribution by glia increased to 84%.4. By measuring sucrose gap responses to changes in K(o) it was possible to estimate that the sucrose gap method recorded 31-60% of changes in axonal membrane potential. It was also determined that the axons, unlike glial cells, are relatively insensitive to reductions in K(o). Surface responses to decreases in external potassium thus reflect the magnitude of the glial contribution.5. It is concluded that changes in glial membrane potential contribute about as much to surface recordings from the optic nerve of Necturus as do equivalent changes in axonal membrane potential. The contributions by the glial cells and axons are related to the relative volumes of tissue they respectively occupy. The significance of these findings to the analysis of surface recordings from the mammalian brain is discussed. Since mammalian glial cells, like those in Amphibia and the leech, become depolarized during neuronal activity and on the basis of electron microscopic evidence appear to be electrically coupled, it is likely that they contribute to the electroencephalogram.