Retina

Definition

The light-sensitive neural tissue layer lining the interior surface of an eye that receives focused optical images and converts photic stimuli into electrochemical signals transmitted to the visual processing centers. In arthropods, the retina comprises the rhabdomeric photoreceptor cells (retinula cells) arranged in ommatidia of compound eyes or beneath the lens of simple eyes (ocelli); in vertebrates and some molluscs, it consists of rods, cones, and associated interneurons. The retina initiates visual transduction through photopigment activation and provides the substrate for spatial resolution, spectral sensitivity, and light adaptation.

Etymology

From Medieval Latin retina, probably from Latin rete (net), referring to the network of blood vessels and neurons; the term was introduced into anatomical usage in the 14th century.

Example

In nocturnal moths, the retina contains a reflective tapetum behind the rhabdoms that enhances photon capture, whereas diurnal butterflies possess retinas with tiered rhabdoms adapted for color vision and polarization sensitivity.

Related Terms

• Rhabdom
• Ommatidium
• retinula cell
• compound eye
• ocellus
• tapetum
• photoreceptor
• visual acuity
• rhabdomere
• Cornea

Usage Notes

Arthropod retinas differ fundamentally from vertebrate retinas in cellular organization: insect and arachnid photoreceptors are typically rhabdomeric (with microvillar light-gathering structures) rather than ciliary, and their axons project to optic ganglia rather than a central brain structure homologous to the vertebrate visual cortex. The term is sometimes used loosely for any photoreceptive layer, but specialists reserve it for organized neural tissue with image-forming capability; simple photoreceptive patches in larvae or primitive eyes may be termed eyespots rather than retinas. In histological descriptions of arthropod eyes, 'retina' often refers specifically to the layer of retinula cells and their rhabdoms, distinct from the overlying dioptric apparatus (cornea, crystalline cone) and underlying basement membrane.