Rods and cones (photoreceptors)

Neuroepithelium retinae: bacilli et coni

Also known as: photoreceptors, rods, cones, rhodopsin, iodopsin, opsins, S cones, M cones, L cones, colour vision, color blindness

The light-sensitive neurons of the retina: ≈92 million rods for dim-light (scotopic), achromatic vision and ≈4.6 million cones for bright-light (photopic), high-acuity colour vision.

Show on body map

Overview

Rods≈92 million (78–107 million)

Cones≈4.6 million (4.1–5.3 million)

Peak foveal cone density≈200 000/mm²

Peak absorbance (rod; S; M; L)≈498; 420; 530; 560 nm

Red–green colour deficiency≈8% of males, ≈0.5% of females

Each photoreceptor has an outer segment (stacks of membranous discs packed with visual pigment — free-floating discs in rods, continuous infoldings in cones), a thin connecting cilium, an inner segment (mitochondria-rich ellipsoid and myoid; protein synthesis and the Na⁺/K⁺-ATPase), a cell body in the outer nuclear layer, and a synaptic terminal (rod spherule, cone pedicle) with ribbon synapses that release glutamate tonically. Visual pigments consist of an opsin (a G-protein-coupled receptor) bound to the chromophore 11-cis-retinal (vitamin A aldehyde).

Rods contain rhodopsin (peak sensitivity ≈498 nm). They can respond to a single photon, saturate in daylight, are absent from the foveola and most dense ≈15–20° from fixation. Many rods converge on each ganglion cell, so rod vision is sensitive but blurry and colourless — this is why faint stars are seen better slightly off-centre.

Cones contain one of three photopsins: S (short-wavelength, "blue", peak ≈420 nm), M (medium, "green", ≈530 nm) and L (long, "red", ≈560 nm). Colour is encoded by the relative activity of the three types (trichromacy) and then by opponent channels (red–green, blue–yellow, light–dark) in ganglion cells. Cones are concentrated in the fovea (peak ≈200 000/mm²), have low convergence and fast kinetics, and adapt to bright light. S-cones make up only ≈5–10% of cones and are absent from the central foveola. The genes for M and L opsins lie on the X chromosome; S-opsin is on chromosome 7 and rhodopsin on chromosome 3.

Location: Outer retina — outer segments interdigitating with the RPE, nuclei in the outer nuclear layer, terminals in the outer plexiform layer.

Parts & subdivisions

Rods
≈92 million; rhodopsin (λmax ≈498 nm); scotopic vision; high sensitivity, high convergence, low acuity, no colour; slow responses; absent from the foveola.
S cones ("blue")
λmax ≈420 nm; ≈5–10% of cones; absent from the central foveola; OPN1SW gene on chromosome 7.
M cones ("green")
λmax ≈530 nm; OPN1MW genes on Xq28.
L cones ("red")
λmax ≈560 nm; OPN1LW gene on Xq28; L:M ratio varies widely (≈2:1 average).
Outer segment
Disc membranes containing ≈10⁸ visual-pigment molecules; renewed every ≈10 days (rods) by new discs at the base and shedding at the tip.
Inner segment and terminal
Metabolic machinery and the ribbon synapse onto bipolar and horizontal cells.

Anatomy

Histology: Rod outer segments are long and cylindrical; cone outer segments are conical (except the slender foveal cones, which look rod-like). Connecting cilium has a 9+0 axoneme.

Function

Absorb photons and convert them into graded changes in membrane potential (phototransduction)
Scotopic (night) vision — rods
Photopic, high-acuity and colour vision — cones

Physiology: Photoreceptors are depolarised in the dark (≈−40 mV) and hyperpolarise to light (towards ≈−70 mV), decreasing glutamate release — see phototransduction. Dark adaptation: cones recover in ≈7–10 min, rods over ≈20–30 min (rod–cone break on the dark-adaptation curve). Purkinje shift: in dim light peak sensitivity moves from ≈555 nm (photopic) to ≈507 nm (scotopic), so reds look darker at dusk. Mesopic vision uses both.

Clinical significance

Colour vision deficiency

X-linked red–green deficiency (protan — L-cone; deutan — M-cone; most commonly deuteranomaly) affects ≈8% of males and ≈0.5% of females of Northern European ancestry. Tritan (S-cone) defects are rare and autosomal; acquired blue–yellow defects occur in glaucoma, diabetic retinopathy and optic nerve disease. Screening uses Ishihara pseudo-isochromatic plates.

Night blindness (nyctalopia)

Vitamin A deficiency impairs rhodopsin synthesis: night blindness, conjunctival xerosis, Bitot spots and keratomalacia — the leading preventable cause of childhood blindness worldwide. Also an early symptom of retinitis pigmentosa (rod-first degeneration).

Achromatopsia

Rare autosomal recessive absence of functional cones: total colour blindness, photophobia, nystagmus and poor acuity.

Test yourself

Why does the fovea provide the highest visual acuity?

It contains the highest density of rods
It is supplied directly by the central retinal artery
It lies at the optic disc where ganglion axons converge
Inner retinal layers are displaced and densely packed cones connect almost 1:1 to ganglion cells

Answer: Inner retinal layers are displaced and densely packed cones connect almost 1:1 to ganglion cells

The foveola is rod-free, packed with slender cones (≈200 000/mm²), the overlying layers and vessels are pushed aside, and each cone has a "private line" via a midget bipolar to a midget ganglion cell. It is avascular and nourished by the choroid.