Two components of electrical dark noise in toad retinal rod outer segments

D. A. Baylor, G. Matthews, King-Wai Yau

Research output: Contribution to journalArticle

Abstract

Physiological noise in the visual transduction mechanism was studied by recording membrane current from single rod outer segments in pieces of isolated toad retina. The inward current in darkness showed spontaneous fluctuations which disappeared during the response to bright light. The dark noise consisted of two components, a continuous fluctuation of rms amplitude about 0.2 pA and occasional discrete events about 1 pA in size. Intervals between discrete events followed the exponential distribution expected of a Poisson process with a mean rate of about one event per 50 sec (20°C). The amplitude and power spectrum of the discrete events resembled those of single photon effects in the same rod, suggesting that discrete events may arise from spontaneous activation of single rhodopsin molecules. The temperature dependence of the mean frequency of occurrence of discrete events gave an activation energy of 22 kcal mole-1, probably characteristic of thermal isomerization of rhodopsin. The variance of the continuous component of the dark noise rose linearly with the length of the outer segment drawn into the suction electrode, indicating that this component is generated in the outer segment. The power spectrum of a rod's continuous noise was usually fitted by the square of a Lorentzian with the same time constant as that of the four first-order delays in the cell's single photon response. The shot effects composing the continuous component thus appear to be shaped by two of four sequential processes in transduction. The variance and spectrum of the continuous noise are interpreted to reflect shot effects about 1/400 the size of a single photon effect occurring at a frequency of 6 x 103 sec-1. A rod's flash sensitivity was halved by a steady light I0 giving about 8 photoisomerizations sec-1. The much lower mean rate of discrete events indicates that I0 in increment sensitivity experiments on individual receptors is not set by thermal activation of rhodopsin. Values of sensitivity and time-to-peak flash response collected from many cells in darkness were correlated by the same power law relation obtaining in the presence of backgrounds. The correlation observed would be explained if a single variable controlled both the gain and time scale of several stages of the transduction mechanism in background light and in darkness.

Original languageEnglish (US)
Pages (from-to)591-621
Number of pages31
JournalJournal of Physiology
VolumeVOL.309
StatePublished - 1980
Externally publishedYes

Fingerprint

Retinal Photoreceptor Cell Outer Segment
Rod Cell Outer Segment
Retinal Rod Photoreceptor Cells
Anura
Noise
Rhodopsin
Darkness
Photons
Light
Hot Temperature
Suction
Retina
Electrodes
Temperature
Membranes

ASJC Scopus subject areas

  • Physiology

Cite this

Two components of electrical dark noise in toad retinal rod outer segments. / Baylor, D. A.; Matthews, G.; Yau, King-Wai.

In: Journal of Physiology, Vol. VOL.309, 1980, p. 591-621.

Research output: Contribution to journalArticle

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