Optic Refraction & Light Detection at the Retina

optic refraction - Overview

The bending of light when a wave travels from a medium with one refractive index to a medium with another.

light reception - Overview

Occurs within the photoreceptors of the retina, of which there are two main categories: cones and rods.

Cones detect color vision and require bright light.
Rods detect black/white ("night") vision, so they only require low levels of illumination.

Optic Refraction

Anatomy

The cornea has a pronounced curvature and is transparent to allow for the passage of light.
Where the cornea ends, the outer layer becomes the sclera, which is opaque, so it blocks the transmission of light. The portion of the sclera we can see is the "white of the eye"; conjunctiva covers it.
The biconvex lens is also transparent and serves to focus a target on the retina, specifically on the area of maximal visual acuity: the fovea centralis of the macula.
The anterior cavity, which lies in front of the lens, contains aqueous humor.
The posterior cavity, which lies behind the lens, contains vitreous humor – it's referred to as the vitreous chamber, or vitreous body.

Like aqueous humor, vitreous humor is primarily water, but the presence of glycosaminoglycans and collagen gives it a gel-like composition, which helps maintain the eye's shape.

Physiology

There are 4 key anatomical components to optic refraction:

Primary mediator is:

The Cornea

It comprises 70% of the power of optic refraction through its pronounced curvature and through its corneal refractive index, which is substantially higher than that of the environmental air. The refractive index is the degree to which a medium bends light.

Secondary mediators are:

Aqueous humor
The Lens
Vitreous humor

Rays of light enter the cornea and bend to ultimately converge at the macula.*

Light Detection within the eye

Anatomy

The uvea.

In front of the lens, lies the pigmented iris, which forms an adjustable diaphragm to funnel light through the pupil – the pigmented epithelium of the iris blocks light transmission and funnels light through the pupil.
Posterior to the iris, lies the ciliary body.
The choroid, which is a thin, brown, highly vascular layer is sandwiched between the sclera and retina; it nourishes the retina and removes heat produced during phototransduction, which is the process wherein the photoreceptors transform light into neural signal, and its brown pigment helps absorb light.
The ciliary body anchors suspensory ligaments, collectively called zonule, which stretch the lens and alter its refractive power.

The retina lies internal to the choroid.

It transitions into optic nerve when it exits the eye, posteriorly, at the lamina cribrosa.
Key aspects of the retina are: the optic nerve head and the macula, the area of highest visual acuity (in the center of it, lies the fovea centralis).

At the macula, the various retinal layers are displaced to the sides to allow light the best passage directly to the photoreceptors.

Brief notes on the Physiology of Light Detection and Phototransduction

Rays of light enter the cornea and bend to ultimately converge at the macula.
Electrical impulse, then, passes along the retina to the optic nerve.

pathophysiology of optic refraction

Emmetropia refers to normal refraction.
Ametropia refers to abnormal refraction.
In myopia, objects focus in front of the retina (the eyeball is too long).
In hyperopia, objects focus behind the retina (the eyeball is too short).
In astigmatism, there is image blurring irrespective of object distance from unequal curvatures in the various parts of the cornea.

Histology of the Retina & Physiology of Phototransduction - Overview

Light reception occurs via photoreceptors within the retina.

The pigmented layer is involved in photoreceptor metabolism; it comprises retinal pigemented epithelium (commonly abbreviated RPE), which captures light not picked up by the photoreceptors.
The photoreceptor cells divide into cones and rods.
Cones provide high-resolution color vision via photoreceptors that are large, conical, active in bright light, and are located predominantly centrally within the retina, meaning in the fovea of the macula,
Rods provide low-resolution black/white (or "night") vision via photoreceptors that are small, narrow and cylindrical, and require only dim light (low-level illumination), and are predominantly located peripherally within the retina – meaning outside of the macula.
Differences in the number of receptor subtypes and their opsins – the photoreceptor proteins the determine the color waves they capture.

For the cones, there are generally three types of the photoreceptors which contain opsins that ultimately handle either red, green, or blue light.
Rods possess one type of photoreceptor, which contains rhodopsin.
The photoreceptor cell segments, themselves, are metabolically dependent upon the pigmented epithelium for photoreceptor regeneration and waste disposal.

Interneuronal cell bodies comprise multiple cell body types and perform multiple functions, including passing forward electrical signal from the photoreceptor cells to the ganglion cells.
The ganglion cells send axons, which form the nerve fibers, which are unmyelinated so as to NOT impeded the light from passing through the retina to the photoreceptor cell layer.

Summary

Light passes through the retina and is captured by the photoreceptor cell segments where the phototransduction cascade occurs, which converts light to neural signal.
Electrical signal is passed back through the retina.

Optic Refraction & Light Detection at the Retina

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