A&P Chp 15: Inner Layer (Retina)

Retina periphery

Contains mostly rods, which continuously decrease in density from there to the macula.

Fovea

Contains only cones

Neural Layer of the Retina

Extends anteriorly to the posterior margin of the ciliary body. This junction is called the ora serrata, the saw-toothed margin; composed of photoreceptors, bipolar cells, and ganglion cells.

Our cone density declines gradually

From the edge of the macula toward the retina periphery.

Foveae (plural of fovea)

Has a sufficient cone density to provide detailed colored vision, so anything we wish to view critically is focused on the foveae.

Retina

Innermost layer of the eyeball; it contains millions of photoreceptors (rods, cones) that convert light energy, other neurons involved in processing light responses, and gilia.

Why is the optic disc called the blind spot?

It lacks photoreceptors, so light focused on it cannot be seen.

Ganglion cells axons

Make a right-angle turn at the inner face of the retina, then leave the posterior aspect of the eye as the thick optic nerve.

Fovea centralis

Minute pit in the macula lutea's center; The retinal structures abutting the vitreous humor are displaced to the sides, which allows light to pass almost directly to the photoreceptors rather than through several retinal layers, greatly enhancing visual acuity.; contains only cones

Because each fovea is only about the size of the head of a pin

Only a thousandth of the entire field is in hard focus (foveal focus) at a given moment.

Rods

Our dim-light and peripheral vision receptors. They are more numerous and far more sensitive to light than cones are, but they don't provide sharp images or color vision. This is why colors disappear and the edges of objects appear fuzzy in dim light and at the edges of our visual field.

For us to visually comprehend a scene that's rapidly changing

Our eyes must flick rapidly back and forth to provide the foveae with images of different parts of the visual field.

Cones

Our vision receptors for bright light and provide high-resolution color vision

Signals

Produced in response to light and spread from the photoreceptors to the bipolar cells and then to the innermost ganglion cells, where action potentials are generated.

Why don't we notice these gaps in our vision?

The brain uses a sophisticated process called filling in to deal with the absence of input.

In the oval region..

The retinal structures abutting the vitreous humor are displaced to the sides which allows light to pass almost directly to the photoreceptors, greatly enhancing visual acuity (ability to resolve detail).

Radiating outward from the optic disc

These vessels give rise to a rich vascular network- Only place where small blood vessels are visible in a living person.

Optic disc (blind spot)

Where the optic nerve exits the eye. It is a weak spot in the fundus (posterior wall) of the eye because it is not reinforced by the sclera

Physicians may observe these tiny vessels

With an ophthalmoscope for signs of hypertension, diabetes, and other vascular diseases.

The retina has 2 layers

-Outer pigmented layer -Inner neural layer **Close together, but not fused. Only the neural layer of the retina plays a direct role in vision.

Neural layer of retina receives its blood supply from 2 sources

-Vessels in the choroid supply the outer third (containing photoreceptors) -The inner two-thirds is served by the central artery and central vein of the retina, which enter and leave the eye through the center of the optic nerve.

The neural layer is composed of 3 main types of neurons:

-photoreceptors -bipolar cells -ganglion cells

The quarter-billion photoreceptors found in the neural layer are of 2 types:

-rods -cones

Pigmented Layer of the Retina

A single-cell-thick lining, abuts the choroid, and extends anteriorly to cover the ciliary body and the posterior face of the iris. These pigment cells, like those of the choroid, absorb light and prevent it from scattering in the eye; They also act as phagocytes participating in photoreceptor cell renewal and store vitamin A needed by the photoreceptor cells.

Retina

Also contains other types of neurons-horizontal and amacrine cells-which play a role in visual processing

Macula lutea (yellow spot)

An oval region that's lateral to the blind spot of each eye and located precisely at the eye's posterior pole with the fovea centralis in the its center; contains mostly cones