# Tens of Picoamperes in the Dark: Why Eyes Burn Energy in Darkness > **Canonical Edition:** https://mariuscomper.uk/patruzeci-de-picoamperi/en/ > **Author:** Marius Comper > **Domain:** Quantitative Sensory Biophysics · Retina --- ## 1. The Visual Energy Paradox Unlike digital image sensors, retinal photoreceptors remain electrically and metabolically active in absolute darkness. When you turn off the lights, an unceasing inward stream of charge known as the **dark current** makes the cells consume oxygen and ATP. In tissue- and method-specific comparisons, retinal energy demand can exceed that of the cortex. Daylight acts as a metabolic brake that reduces this expenditure. The dark current is on the order of tens of picoamperes, but its value depends on species and protocol. In 1979, Baylor, Lamb, and Yau measured saturating responses up to 27 pA in toad (*Bufo marinus*) rods. In human rods, Kraft, Schneeweis, and Schnapf recorded flash-evoked photocurrents up to about 20 pA in 1993. This page uses 20 pA as an illustrative upper-bound reference, not as a universal human dark-current constant. The current is carried by sodium and calcium ions flowing through cyclic nucleotide-gated (CNG) cation channels in the outer segment membrane. Due to this inward positive current, the resting membrane potential of the rod in darkness remains depolarized at approximately **−40 millivolts**, substantially higher than the typical −70 millivolt resting baseline of cortical neurons. At this depolarized voltage, photoreceptor ribbon synapses tonically release glutamate at maximum rates. --- ## 2. The ATP Price of Molecular Pumps In an upper-bound model based on 20 picoamperes, each rod receives approximately **1.25 × 10⁸ equivalent ionic charges per second**. This converts current into elementary-charge units; it is not a count of sodium ions because the actual current also includes calcium. To preserve ionic balance, the rod inner segment is packed with Na⁺/K⁺-ATPase molecular pumps. If the equivalent charge is treated as a didactic proxy and the 3-to-1 sodium-to-ATP stoichiometry is applied, the model estimates about **4.2 × 10⁷ ATP molecules per second** for one rod. This is an illustrative calculation, not a measurement of every human rod. Across the approximately 120 million rods in one retina, the same model reaches about **5.0 × 10¹⁵ ATP molecules per second**. For both eyes, the value is about **1.0 × 10¹⁶ molecules per second**. Studies of rabbit and cat retina show high oxygen demand, but the 2–3× comparison with cortex depends on tissue, species, and method. --- ## 3. The Photonic Cascade: Why Light Acts as a Brake When a single photon is absorbed by a rhodopsin molecule in the outer segment discs, its 11-*cis*-retinal chromophore isomerizes into *all-trans*-retinal in less than 200 femtoseconds. This conformational switch sets off an immense enzymatic cascade: - In intact mouse rods, Yue and colleagues estimated approximately **12–14 effective transducin–PDE complexes** for each activated rhodopsin; other preparations and models can yield different values. - The active complexes stimulate PDE6, which hydrolyzes cGMP and closes CNG channels; this page presents the cascade as an amplification model, not a universal constant. - The localized plunge in cGMP forces CNG channels to close. - The dark current drops, the rod hyperpolarizes to **−70 millivolts**, glutamate release halts, and the metabolic burden on the sodium pumps plummets. --- ## 4. Guided Body Experiment: The Scotopic Foveal Scotoma 1. **Fixate on a faint star:** Look straight at a faint night sky light source. Notice that the point of light vanishes from perception. 2. **Avert your gaze by 10–15 degrees:** Shift your gaze slightly to the side. The faint light immediately reappears substantially brighter. 3. **The reason:** The central foveola contains exclusively cones and 0 rods. At 10°–15° eccentricity, rod density reaches its peak (~160,000 rods/mm²), powering dim-light scotopic vision. --- ## 5. Primary Scientific Bibliography 1. Baylor, D. A., Lamb, T. D., & Yau, K. W. (1979). *The membrane current of single rod outer segments.* J Physiol, 288, 589–611. [PubMed](https://pubmed.ncbi.nlm.nih.gov/112242/) 2. Kraft, T. W., Schneeweis, D. M., & Schnapf, J. L. (1993). *Visual transduction in human rod photoreceptors.* J Physiol, 464, 747–765. [PubMed](https://pubmed.ncbi.nlm.nih.gov/8229828/) 3. Ames, A., Li, Y. Y., et al. (1992). *Energy metabolism of rabbit retina as related to function.* J Neurosci, 12(3), 840–853. [PubMed](https://pubmed.ncbi.nlm.nih.gov/1312136/) 4. Okawa, H., Sampath, A. P., et al. (2008). *ATP consumption by mammalian photoreceptors in darkness and in light.* Curr Biol, 18(24), 1917–1921. [PubMed](https://pubmed.ncbi.nlm.nih.gov/19084410/) 5. Linsenmeier, R. A. (1986). *Effects of light and darkness on oxygen distribution and consumption in the cat retina.* J Gen Physiol, 88(4), 521–542. 6. Curcio, C. A., et al. (1990). *Human photoreceptor topography.* J Comp Neurol, 292(4), 497–523. 7. Rieke, F., & Baylor, D. A. (1998). *Single-photon detection by rod cells of the retina.* Rev Mod Phys, 70(3), 1027–1036. 8. Yue, W. W. S., et al. (2019). *Elementary response triggered by transducin in retinal rods.* Proc Natl Acad Sci, 116(11), 5144–5153. [PubMed](https://pubmed.ncbi.nlm.nih.gov/30796193/)