Difficulty: Intermediate | Prerequisites: G-protein coupled receptors (GPCRs), second-messenger cascades, ion channel gating, basic eye anatomy.
This section covers how vertebrates detect light and convert it into neural signals. Phototransduction is a metabotropic signalling cascade, meaning it uses G-proteins and second messengers rather than opening channels directly. It stands in contrast to mechanoreception (which is largely ionotropic), so understanding both gives you the full picture of how sensory systems can either gate channels directly or route through an amplifying biochemical pathway. You need to be comfortable with GPCRs, cGMP as a second messenger and the concept of membrane potential to follow this material.
Photoreceptors (rods and cones) are modified epithelial cells that detect light using photopigments made of opsin (a GPCR) plus a chromophore (a vitamin A derivative). In the dark, cGMP holds CNG channels open and the cell is depolarised (the "dark current"); when light hits, a G-protein cascade breaks down cGMP, channels close and the cell hyperpolarises, reducing glutamate release. The vertebrate eye focuses light through the cornea and lens onto the retina, where a layered circuit of photoreceptors, bipolar cells, ganglion cells and lateral interneurons sharpens the signal before it travels down the optic nerve.
Photoreceptor
A light-sensitive cell (rod or cone) that converts photon energy into an electrical signal. Modified epithelial cell, neuron-like but does not fire action potentials; releases glutamate. Think of it as the biological equivalent of a camera sensor pixel.
Photopigment
A light-absorbing protein consisting of two covalently bonded parts: an opsin and a chromophore. Absorbs photon energy and initiates phototransduction. In simple terms, it is the molecule that catches light.
Opsin
A G-protein coupled receptor (GPCR) that forms the protein component of a photopigment. Its structure determines which wavelength of light the photopigment absorbs. Think of it as the colour filter: change the opsin and you change what colour of light the cell responds to.
Chromophore (Retinal)
The photon-binding part of the photopigment, a derivative of vitamin A. Absorbing light converts its double bond from cis to trans configuration. In simple terms, it is the part that physically catches the photon and changes shape.
Rod
A photoreceptor with a rod-shaped outer segment, containing one type of photopigment (rhodopsin). Sensitive to very dim light. Responsible for scotopic (low-light) vision. Think of it as the night-vision cell.
Cone
A photoreceptor with a cone-shaped outer segment. Sensitive to brighter light. Up to three types in mammals (red, green, blue), each with a different opsin. Think of it as the colour-vision cell.
cGMP (Cyclic Guanosine Monophosphate)
A second messenger that keeps CNG channels open in the dark. Light triggers its breakdown by PDE. In simple terms, cGMP is the key that holds the channel door open; light takes the key away.
CNG Channel (Cyclic Nucleotide-Gated Channel)
An ion channel in the photoreceptor outer segment that opens when bound by cGMP, allowing Na+ (and Ca2+) influx. Responsible for the dark current. Think of it as a gate that stays open as long as cGMP is around.
PDE (Phosphodiesterase)
The enzyme activated by the G-protein transducin that hydrolyses cGMP into GMP, reducing cGMP levels and closing CNG channels. In simple terms, PDE is the enzyme that destroys the key.
Transducin (Gi)
The inhibitory G-protein activated by light-activated opsin. It activates PDE. Think of it as the relay: opsin hands the signal to transducin, which hands it to PDE.
Dark Current
The steady inward flow of Na+ through CNG channels in the dark, keeping the photoreceptor depolarised and releasing glutamate continuously. In simple terms, it is the default "on" state of the photoreceptor.
Bleaching
The dissociation of the activated (all-trans) chromophore from opsin after light absorption. Named for the visible colour change it produces in purified rhodopsin. Think of it as the photopigment falling apart after catching a photon; it must be reassembled before it can work again.
Lateral Inhibition
A process by which horizontal and amacrine cells in the retina inhibit neighbouring neurons, sharpening the boundary of a stimulus and increasing contrast. In simple terms, it is the retina's way of making edges look crisper.
Ganglion Cell
The retinal neuron whose axons form the optic nerve. Receives input from bipolar cells (and amacrine cells) and sends action potentials to the brain. Think of it as the output neuron of the retina.
Bipolar Cell
An interneuron in the retina that relays signals from photoreceptors to ganglion cells. In simple terms, it is the middle link in the retinal chain.
Photoreception is the ability to detect visible light, a small portion of the electromagnetic spectrum (ultraviolet to near infrared).
Photoreceptors are neuron-like modified epithelial cells. They do not fire action potentials, but they release glutamate.
Each photopigment has two covalently bonded parts:
Chromophore (retinal): the photon-binding site. A derivative of vitamin A. Absorbing light converts its shape from 11-cis to all-trans.
Opsin: the protein portion, a G-protein coupled receptor (GPCR). The opsin's structure determines which wavelength of light the photopigment absorbs best.
GPCRs as a family also include olfactory receptors, pheromone receptors and some taste receptors. Phototransduction is metabotropic (works through a second-messenger cascade), in contrast to mechanoreception in hearing which is largely ionotropic.
Vertebrates have ciliary photoreceptors of two types:
Rods
Outer segment is rod-shaped.
Contain one type of photopigment (rhodopsin).
Extremely sensitive to dim light (scotopic vision).
Cannot distinguish colour.
Cones
Outer segment is cone-shaped.
Contain up to three types of photopigment in mammals, each tuned to a different wavelength (red, green, blue).
Sensitive to brighter light (photopic vision).
Responsible for colour vision.
Segment structure (both rods and cones)
Outer segment: contains the photopigments (stacked membrane discs in rods, folded membrane in cones). This is where phototransduction occurs.
Inner segment: contains the nucleus and synaptic terminal. Forms synapses with bipolar and horizontal cells.
The two segments are connected by a cilium.
The dark current (no light)
In the dark, cGMP levels are high.
cGMP binds to CNG (cyclic nucleotide-gated) channels in the outer segment, keeping them open.
Na+ (and some Ca2+) flows in through open CNG channels, keeping the photoreceptor depolarised.
The depolarised photoreceptor continuously releases glutamate at its synapse.
This steady inward current is called the dark current.
Phototransduction steps (light present)
The chromophore (11-cis retinal) absorbs a photon.
The chromophore isomerises from cis to all-trans configuration (changes shape).
The activated chromophore dissociates from opsin (this is called bleaching).
The now-active opsin activates the G-protein transducin (Gi), an inhibitory G-protein.
Transducin activates PDE (phosphodiesterase).
PDE hydrolyses cGMP into GMP, reducing cGMP concentration.
With less cGMP available, CNG channels close.
Na+ influx decreases, so the membrane hyperpolarises.
Glutamate release decreases.
The key logic to remember: light closes channels and hyperpolarises the cell. This is the opposite of what many students expect.
Summary of the cascade
Photon hits chromophore, chromophore changes shape, opsin activates transducin, transducin activates PDE, PDE destroys cGMP, CNG channels close, Na+ entry drops, cell hyperpolarises, glutamate release decreases.
Sclera: the white outer surface of the eye.
Cornea: transparent layer at the front; allows light to enter and begins refracting it.
Iris: two layers of pigmented smooth muscle that control the size of the pupil.
Pupil: the opening in the iris through which light enters.
Lens: focuses the image onto the retina. Shape is adjusted by the ciliary body.
Ciliary body: muscles attached to the lens that change its shape for near/far focus (accommodation).
Retina: the layer of photoreceptors and interneurons at the back of the eye.
Choroid: a pigmented layer behind the retina that absorbs stray light (prevents internal reflection).
Tapetum: a reflective layer in the choroid of nocturnal animals (this is why cat eyes glow in the dark).
Aqueous humour: fluid filling the anterior chamber (between cornea and lens).
Vitreous humour: gelatinous mass filling the posterior chamber (between lens and retina).
Fovea: the region of highest visual acuity, densely packed with cones.
Optic disc: the point where the optic nerve exits the retina. Contains no photoreceptors (the blind spot).
Light path: light enters through the cornea, passes through the pupil, is focused by the lens and refracted onto the retina, where photoreceptors absorb it.
Retinal cell layers (back to front)
The retina is arranged in a seemingly counterintuitive way: light must pass through several cell layers before reaching the photoreceptors at the back.
Photoreceptors (rods and cones): located at the back of the retina, outer segments facing away from the incoming light.
Bipolar cells (interneurons): receive input from photoreceptors and relay to ganglion cells.
Ganglion cells: receive input from bipolar cells, send axons through the optic nerve to the brain. The only retinal cells that fire conventional action potentials.
Three layers exist because multiple processing steps improve the clarity and precision of the visual signal before it leaves the eye.
Lateral interneurons
Horizontal cells: sit between photoreceptors and bipolar cells. Mediate lateral inhibition at the first synapse.
Amacrine cells: sit between bipolar cells and ganglion cells. Mediate lateral inhibition at the second synapse.
Lateral inhibition
When a cluster of photoreceptors is stimulated, the cell at the centre of the stimulus is activated most strongly. Through horizontal and amacrine cells, it sends inhibitory signals (axoaxonic synapses) to its neighbours. The neighbours are suppressed, so only the central cell fires strongly. The result is increased contrast at the edges of a stimulus, making boundaries sharper.
The optic nerve
Axons of ganglion cells converge at the optic disc and exit the retina as the optic nerve. Because the optic disc has no photoreceptors, it creates the blind spot.
Night blindness (nyctalopia) results from vitamin A deficiency, because without vitamin A the body cannot synthesise enough retinal chromophore for the rods. Colour blindness arises from mutations in one or more cone opsins, eliminating sensitivity to that wavelength range. Retinitis pigmentosa, a family of genetic disorders that degrade photoreceptors, is a target for gene therapy and retinal prostheses that aim to replace the lost phototransduction step with an electronic one.
Students often think light depolarises photoreceptors. It does the opposite: light causes hyperpolarisation by closing CNG channels. The depolarised state is the dark (resting) state.
A common mistake is assuming photoreceptors fire action potentials. They do not. Photoreceptors modulate glutamate release; ganglion cells are the first retinal neurons to fire APs.
Students sometimes think the choroid reflects light. It absorbs light to prevent internal scattering. The tapetum (found only in nocturnal animals) is the reflective layer.
Many students forget that rods and cones face backward in the retina. Light must pass through ganglion cells, bipolar cells and other layers before reaching the photoreceptors. This arrangement seems odd but allows the pigment epithelium to support and recycle photopigments.
⚠️ The phototransduction cascade is almost certain to appear on the exam. Know every step from photon absorption to hyperpolarisation, and be able to explain what each molecule does (chromophore, opsin, transducin, PDE, cGMP, CNG channel).
⚠️ Understand the dark current: in the dark, CNG channels are open, Na+ flows in, the cell is depolarised, glutamate is released. Light reverses all of this.
⚠️ Be ready to compare rods and cones: sensitivity, number of photopigment types, shape, and role in dim vs bright light.
⚠️ Know the retinal cell layers in order (photoreceptors, bipolar cells, ganglion cells) and the role of horizontal and amacrine cells in lateral inhibition.
⚠️ Lateral inhibition is a favourite exam topic. Be able to explain how it increases contrast and why three retinal layers improve visual precision.
True or False: In the dark, CNG channels are closed and the photoreceptor is hyperpolarised.
False. In the dark, CNG channels are open and the photoreceptor is depolarised (the dark current).
Fill in the blank: The enzyme that breaks down cGMP into GMP during phototransduction is called ______.
PDE (phosphodiesterase).
True or False: Cones are more sensitive to dim light than rods.
False. Rods are the dim-light specialists; cones require brighter light.
Fill in the blank: The two lateral interneurons in the retina responsible for lateral inhibition are ______ cells and ______ cells.
Horizontal; amacrine.
True or False: Ganglion cell axons form the optic nerve.
True.
Q: Walk through the phototransduction cascade, starting from a photon striking rhodopsin and ending with a change in glutamate release.
A: A photon is absorbed by the chromophore (11-cis retinal), which isomerises to all-trans retinal. The activated chromophore dissociates from opsin (bleaching). The active opsin activates the G-protein transducin (Gi). Transducin activates PDE, which hydrolyses cGMP to GMP. The drop in cGMP causes CNG channels to close. With fewer Na+ ions entering, the membrane hyperpolarises. The hyperpolarised photoreceptor releases less glutamate at its synapse with bipolar cells.
Q: Explain the dark current. Why is it called a "current" and why does it matter?
A: In the dark, high cGMP levels keep CNG channels open, allowing a steady influx of Na+ (and some Ca2+) into the outer segment. This inward ion flow is the dark current. It keeps the photoreceptor depolarised and causes continuous glutamate release. It matters because light must reduce this current to signal, meaning phototransduction works by suppression rather than activation.
Q: Why are horizontal and amacrine cells important for vision?
A: They mediate lateral inhibition. When a photoreceptor at the centre of a light stimulus is strongly activated, horizontal cells (at the photoreceptor-bipolar synapse) and amacrine cells (at the bipolar-ganglion synapse) inhibit the surrounding, less-stimulated cells. This enhances edge contrast and makes the visual image sharper.
Q: Compare the ionic composition of endolymph and perilymph, and explain why this distinction matters for hair-cell transduction but the analogous question (aqueous vs vitreous humour) does not arise in phototransduction.
A: Endolymph is high K+/low Na+; perilymph is high Na+/low K+. This matters for hearing because hair-cell depolarisation depends on K+ influx from endolymph through mechanically gated channels. In vision, phototransduction depends on cGMP-gated Na+ channels and a standard extracellular ionic environment; there is no specialised high-K+ compartment involved.
Q: A patient has a mutation that prevents PDE from being activated. What happens to their vision in the light?
A: Without functional PDE, cGMP cannot be broken down even when light activates opsin and transducin. CNG channels remain open, the dark current persists and the photoreceptor stays depolarised. Glutamate release does not decrease in response to light, so the photoreceptor cannot signal "light" to the bipolar cells. The patient would be functionally blind despite having intact photoreceptors and optic nerve.
Phototransduction links directly to mechanoreception (Part 1 of these notes): both systems use modified epithelial cells that release glutamate rather than firing action potentials, but mechanoreception is largely ionotropic while phototransduction is metabotropic. The G-protein signalling cascade here (opsin, transducin, PDE, cGMP) is a specific case of the GPCR signalling you will encounter throughout pharmacology and endocrinology. Lateral inhibition in the retina is a model for contrast enhancement that recurs in somatosensory cortex and other neural processing contexts.
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