📌 Professor’s Notes: Quick Revision
High-yield revision notes for rapid review before university examinations.
- Vision begins when light activates retinal photoreceptors, which convert light energy into electrical signals. The two photoreceptors are rods and cones. Each has an outer segment containing visual pigments, an inner segment, a cell body and a synaptic terminal.
- Rods are mainly present in the peripheral retina and are absent from the centre of the fovea. They are highly sensitive to dim light and are responsible for night vision. Rods contain rhodopsin and show high sensitivity but relatively poor visual acuity.
- Cones are densely packed in the fovea and provide high visual acuity and colour vision. There are three cone pigments: long-wavelength-sensitive pigment (about 560 nanometres, red), medium-wavelength-sensitive pigment (about 530 nanometres, green), and short-wavelength-sensitive pigment (about 420 nanometres, blue).
- Photopigments contain 11-cis-retinal, derived from vitamin A, and an opsin protein. Retinal is common to the visual pigments, while differences in opsin determine their wavelength sensitivity.
- Rhodopsin is the rod visual pigment. It consists of 11-cis-retinal and scotopsin and has peak sensitivity around 500 nanometres. Light converts 11-cis-retinal into all-trans-retinal, producing a conformational change that activates rhodopsin.
- In darkness, photoreceptors are relatively depolarized at about −40 millivolts. High cyclic guanosine monophosphate keeps cyclic guanosine monophosphate-gated sodium channels open, producing a continuous dark current. Calcium enters at the synaptic terminal, causing continuous release of glutamate.
- The sequence of phototransduction in rods is: light → 11-cis-retinal to all-trans-retinal → metarhodopsin II → transducin activation → phosphodiesterase activation → decreased cyclic guanosine monophosphate → closure of sodium channels → hyperpolarization → decreased calcium entry → decreased glutamate release → bipolar cell response.
- Light therefore causes hyperpolarization, not depolarization. The membrane potential may reach about −65 to −70 millivolts. This decreases glutamate release and allows the visual signal to be transmitted through bipolar and ganglion cells to the optic nerve.
- Cone phototransduction follows essentially the same mechanism as rods but uses a different G protein, Gt2.
- After light exposure, rhodopsin undergoes bleaching because all-trans-retinal separates from opsin. All-trans-retinal is converted back to 11-cis-retinal in the retinal pigment epithelium and recombines with opsin to regenerate rhodopsin.
- The cyclic guanosine monophosphate-gated channel allows sodium and calcium entry. During illumination, reduced intracellular calcium promotes restoration of cyclic guanosine monophosphate and helps return the photoreceptor towards its resting state.
- Melanopsin is present in intrinsically photosensitive retinal ganglion cells and contributes to circadian rhythm regulation and the pupillary light reflex.
📝 Practice MCQs
🎙️ Frequently Asked Viva Questions
High-yield viva questions with concise answers for rapid university examination preparation.
Q1. Differentiate the functions of rods and cones.
Answer: Rods mediate dim-light (night) vision with high light sensitivity but low visual acuity. Cones mediate daylight vision, color vision, and high visual acuity.
Q2. Why are rods more sensitive to dim light than cones?
Answer: Many rods converge onto a single bipolar cell, and rhodopsin amplifies the light signal, allowing detection of very low levels of light.
Q3. What is rhodopsin and what are its components?
Answer: Rhodopsin is the visual pigment of rods. It consists of 11-cis-retinal (chromophore) and scotopsin (opsin protein).
Q4. Why is vitamin A essential for normal vision?
Answer: Vitamin A is the precursor of 11-cis-retinal, the light-absorbing component of all visual pigments. Deficiency impairs rhodopsin formation and vision.
Q5. What is phototransduction?
Answer: Phototransduction is the process by which light energy is converted into electrical signals in rods and cones.
Q6. Why do photoreceptors hyperpolarize in response to light?
Answer: Light reduces intracellular cGMP, causing closure of cGMP-gated sodium channels. Sodium influx decreases while potassium efflux continues, leading to hyperpolarization.
Q7. Why are photoreceptors depolarized in darkness?
Answer: In darkness, cGMP levels remain high, keeping sodium channels open. Continuous sodium entry (dark current) maintains partial depolarization.
Q8. What is the dark current?
Answer: The dark current is the continuous inward flow of sodium ions through cGMP-gated channels in photoreceptors during darkness.
Q9. What is the role of transducin in phototransduction?
Answer: Activated transducin stimulates phosphodiesterase, which lowers cGMP concentration and initiates photoreceptor hyperpolarization.
Q10. What is the function of phosphodiesterase (PDE) in phototransduction?
Answer: Phosphodiesterase converts cGMP into 5′-GMP, lowering cGMP levels and closing cGMP-gated sodium channels.
Q11. Why does glutamate release decrease in light?
Answer: Hyperpolarization closes voltage-gated calcium channels, reducing calcium entry and thereby decreasing glutamate release from photoreceptors.
Q12. What is bleaching of rhodopsin?
Answer: Bleaching is the separation of all-trans-retinal from opsin after light absorption, making the visual pigment temporarily inactive.
Q13. How is rhodopsin regenerated after exposure to light?
Answer: In the retinal pigment epithelium, all-trans-retinal is converted back to 11-cis-retinal, which recombines with opsin to regenerate rhodopsin.
Q14. What is the function of melanopsin?
Answer: Melanopsin regulates circadian rhythm, synchronizes the biological clock with light, and contributes to the pupillary light reflex.
Q15. Why is phototransduction considered a highly amplified process?
Answer: A single activated rhodopsin molecule activates many transducin molecules, leading to hydrolysis of thousands of cGMP molecules and a large electrical response.
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