dark adaptation curve physiology guyton fig 51.9
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Here is Figure 51.9 from Guyton & Hall, along with a full explanation of the physiology:
Dark Adaptation Curve — Guyton & Hall Fig. 51.9
Figure 51.9 — Dark adaptation: cone adaptation vs. rod adaptation (retinal sensitivity on a log scale vs. minutes in the dark)
Physiology Explained
What is Dark Adaptation?
When a person remains in bright light for hours, large amounts of photopigment (rhodopsin in rods; iodopsin in cones) are broken down into retinal + opsins, and much of the retinal is further converted to vitamin A. This depletes photosensitive chemicals → reduced retinal sensitivity = light adaptation.
On entering darkness, the reverse occurs:
- Retinal + opsin → rhodopsin (rods) / iodopsins (cones) are regenerated
- Vitamin A → retinal, replenishing the substrate
- The final ceiling is set by the amount of available opsin
This regeneration process = dark adaptation.
The Biphasic (Two-Segment) Curve
The curve has a characteristic inflection point at ~7–10 minutes, creating two distinct segments:
| Phase | Cells | Time course | Sensitivity gain |
|---|---|---|---|
| Early (fast) phase | Cones | 0–10 min | ~30–40× |
| Late (slow) phase | Rods | 10–40+ min | up to ~25,000× total |
Why cones adapt first: All photochemical events in cones occur ~4× faster than in rods. Cone adaptation is complete within ~7 minutes.
Why rods dominate the late phase: Rods continue adapting slowly for 20–40 minutes or more, achieving a far greater total sensitivity increase than cones can. Their sensitivity is further amplified by neural convergence — 100+ rods synapse onto a single ganglion cell, allowing spatial summation.
Key Quantitative Milestones (from the text)
| Time in dark | Sensitivity increase |
|---|---|
| 1 minute | ~10-fold |
| 20 minutes | ~6,000-fold |
| 40 minutes | ~25,000-fold |
| Full range (dark→light) | 500,000–1,000,000-fold |
Other Mechanisms Contributing to Adaptation
- Pupillary dilation — rapid (~seconds), accounts for ~30-fold sensitivity change
- Neural adaptation — occurs at successive retinal and brain processing stages (bipolar cells, ganglion cells, lateral geniculate, visual cortex); works rapidly but contributes a smaller fraction of the total range
Clinical Correlate
Impaired dark adaptation (prolonged or absent rod phase) is a hallmark of:
- Vitamin A deficiency (retinal substrate unavailable → rhodopsin cannot regenerate → night blindness)
- Retinitis pigmentosa (rod degeneration → absent rod adaptation phase)
- Oguchi disease (stationary night blindness — rod function absent after 30 min dark adaptation but recovers with prolonged dark adaptation: Mizuo-Nakamura phenomenon)
Guyton and Hall Textbook of Medical Physiology, Chapter 51 (p. 637)
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