Neuroscience/Objectives/Lecture 27

From PhysioWiki

Vision I: the eye

Discuss what makes vision difficult.

The visual system receives a two-dimensional image of a scene and from it must generate a three-dimensional representation of the world. Common sense and basic geometry tells us that this problem is intractable. Yet the visual system accomplishes this through multiple layers of complex circuitry, heuristics, and other techniques that allow us to make educated guesses about the physical world based on the limited two-dimensional images projected on our retinas.

Discuss the development of the eye.

The eye develops from the optic vesicle, an outpocketing of the embryonic diencephalon. The optic vesicle invaginates to form the optic cup, the inner wall of which becomes the neural retina. Its outer wall becomes the pigmented epithelium.

Describe the basic anatomy of the eye.

Understand the process of accommodation.

Light is refracted by the cornea and (to a lesser extent) the lens. Accommodation is the process by which the lens' refractive power is adjusted to focus an image on the retina. This is primarily achieved by regulating the tension of the ciliary muscles. When contracted, the ciliary muscles pull the lens into its default round shape, which produces greater refraction and is used to focus nearby objects. When relaxed, the ciliary muscles give the lens a flattened shape, which decreases the lens' refractive power to focus on far-away objects.

Discuss refractive errors, their cause, and how they can be corrected.

Myopia

Myopia (nearsightedness) occurs when the refractive power of the eye is too great (possibly due to a cornea that protrudes too much or from a hyperrefractive lens). This results in the image being focused before it reaches the retina. Myopia can be corrected with a diverging (concave) lens.

Hyperopia

Also called farsightedness, hyperopia occurs when the refractive power of the eye is too low (having the opposite causes of myopia). Consequently, light is not refracted soon enough, resulting in images being focused behind the retina. Hyperopia is corrected with converging (convex) lenses, which help the eye converge light earlier.

Presbyopia

Often accompanying age, presbyopia is the eye's impaired ability to accommodate. It may be corrected with bifocal lenses.

Understand the basic organization of the retina, including its cell types and layers.

The retina is an eight-layered sensory structure containing five different cell types. From outside to inside, the layers are:

• Retinal pigmented epithelium: reduces backscattering of light by absorbing photons with the pigment melanin, and helps maintain photoreceptors
• Photoreceptor outer segments: membranous disks studded with photopigments
• Outer nuclear layer: cell bodies of photoreceptors (roughly equivalent to the photoreceptor inner segment)
• Outer plexiform layer: synapses between photoreceptors, bipolar cells, and horizontal cells
• Inner nuclear layer: cell bodies of bipolar and horizontal cells
• Inner plexiform layer: synapses between bipolar cells, amacrine cells, and ganglion cells
• Ganglion cell layer: nuclei of ganglion cells
• Nerve fiber layer: axons of ganglion cells transmitted via the optic nerve

The cell types are:

• Photoreceptor
• Bipolar cell
• Horizontal cell
• Amacrine cell
• Ganglion cell

Understand the basic structure of photoreceptors and how they transduce light.

Photoreceptors have an outer segment containing membranous disks and an inner segment that contains a nucleus and makes connections with bipolar and horizontal cells. Transduction of light is as follows:

In dark

1. ↑cGMP activates Na⁺ channels
2. Depolarization of the cell opens voltage-gated Ca²⁺ channels
3. ↑Ca²⁺ triggers the release of glutamate

In light

1. Photon collides with opsin
2. 11-cis retinal is converted to all-trans retinal
3. All-trans retinal activates transducin
4. Transducin activates a cGMP phosphodiesterase (PDE) which hydroylzes cGMP
5. ↓cGMP closes Na⁺ channels, leading to hyperpolarization
6. Duration of response is limited by arrestin
   • Inhibits rhodopsin's ability to activate transducin
   • Promotes breakdown of activated rhodopsin

Discuss the differences between rods and cones and how their distribution varies across the retina.

Describe the most common diseases of the eye.

Cataract

Any opacity of the lens, possibly resulting from trauma, inflammation, metabolic or nutritional disorder, radiation, and aging. No treatment is available to prevent, delay, or reverse cataract, but surgical replacement with an artificial lens may alleviate some of the symptoms.

Diabetic retinopathy

Occurs when diabetes damages retinal blood vessels. May cause macular edema, causing swelling of the macula and consequent blurred vision. New blood vessels grow to replace the damaged ones, obscuring vision further. If not promptly treated, these vessels may rupture as well, causing bleeding, cloudy vision, and destruction of the retina.

Glaucoma

A specific pattern of damage to the optic nerve, most often caused by increased intraocular pressure. The most common variety is open-angle glaucoma in which the trabecular meshwork fails to allow the aqueous humor to drain, causing intraocular pressure to rise.

Age-related macular degeneration

Most common cause of vision loss after 55 years of age. Caused by degeneration of photoreceptors. Up to 90% of cases are "dry", characterized by gradual dissapearance of retinal pigmented epithelium. Vision is lost over a period of years and no treatment is available. The remaining cases are "wet", associated with abnormal blood vessel growth under the macula; these vessels leak blood and fluid onto the retina. Vision is lost over a period of months and can be treated with laser surgery. Can be tested with Amsler grid.

Retinitis pigmentosa

This degeneration of photoreceptors begins with a decline in the number of rods. It causes night blindness and loss of peripheral vision, with complete vision loss over a number of years. Legal blindness occurs once the visual field contracts to 20° and/or central vision is 20/200 or worse.