U2 L4 Eye and Vision Reading Notes

Eye and Associated Structures

70% of all sensory receptors are in the eye.
Most of the eye is protected by a cushion of fat and the bony orbit.
Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

Structure of the Eyeball

A slightly irregular hollow sphere with anterior and posterior poles.
The wall is composed of three tunics: fibrous, vascular, and sensory.
The internal cavity is filled with fluids called humors.
The lens separates the internal cavity into anterior and posterior segments.

Fibrous Tunic

Forms the outermost coat of the eye and is composed of:
- Opaque sclera (posteriorly).
- Clear cornea (anteriorly).
The sclera protects the eye and anchors extrinsic muscles.
The cornea lets light enter the eye.

Vascular Tunic (Uvea)

Has three regions: choroid, ciliary body, and iris.
- Choroid region:
  - A dark brown membrane that forms the posterior portion of the uvea.
  - Supplies blood to all eye tunics.
- Ciliary Body:
  - A thickened ring of tissue surrounding the lens.
  - Composed of smooth muscle bundles (ciliary muscles).
  - Anchors the suspensory ligament that holds the lens in place.
- Iris:
  - The colored part of the eye.
    - Pupil – central opening of the iris.
  - Regulates the amount of light entering the eye during:
    - Close vision and bright light – pupils constrict.
    - Distant vision and dim light – pupils dilate.
    - Changes in emotional state – pupils dilate when the subject matter is appealing or requires problem-solving skills.

Sensory Tunic: Retina

A delicate two-layered membrane.
- Pigmented layer – the outer layer that absorbs light and prevents its scattering.
- Neural layer, which contains:
  - Photoreceptors that transduce light energy.
  - Bipolar cells and ganglion cells.
  - Amacrine and horizontal cells.

The Retina: Ganglion Cells and the Optic Disc

Ganglion cell axons:
- Run along the inner surface of the retina.
- Leave the eye as the optic nerve.
The optic disc:
- Is the site where the optic nerve leaves the eye.
- Lacks photoreceptors (the blind spot).
Rods:
- Respond to dim light.
- Are used for peripheral vision.
Cones:
- Respond to bright light.
- Have high-acuity color vision.
- Are found in the macula lutea.
- Are concentrated in the fovea centralis.

Inner Chambers and Fluids

The lens separates the internal eye into anterior and posterior segments.
The posterior segment is filled with a clear gel called vitreous humor that:
- Transmits light.
- Supports the posterior surface of the lens.
- Holds the neural retina firmly against the pigmented layer.
- Contributes to intraocular pressure.
The anterior segment is composed of two chambers:
- Anterior – between the cornea and the iris.
- Posterior – between the iris and the lens.
Aqueous humor:
- A plasma like fluid that fills the anterior segment.
- Filters from the capillaries of the ciliary processes.
- Drains via the canal of Schlemm into the scleral venous sinus.
- Supports, nourishes, and removes wastes.

Lens

A biconvex, transparent, flexible, avascular structure that:
- Allows precise focusing of light onto the retina.
- Is composed of epithelium and lens fibers.
  - Lens epithelium – anterior cells that differentiate into lens fibers.
  - Lens fibers – cells filled with the transparent protein crystallin.
- With age, the lens becomes more compact and dense and loses its elasticity.

Light

Electromagnetic radiation – all energy waves from short gamma rays to long radio waves.
Our eyes respond to a small portion of this spectrum called the visible spectrum.
Different cones in the retina respond to different wavelengths of the visible spectrum.

Refraction and Lenses

When light passes from one transparent medium to another its speed changes and it refracts (bends).
Light passing through a convex lens (as in the eye) is bent so that the rays converge to a focal point.
When a convex lens forms an image, the image is upside down and reversed right to left.

Problems with Refraction

Emmetropic eye – normal eye with light focused properly.
Myopic eye (nearsighted) – the focal point is in front of the retina.
- Corrected with a concave lens.
Hyperopic eye (farsighted) – the focal point is behind the retina.
- Corrected with a convex lens.

Focusing Light on the Retina

Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors.
Light is refracted:
- At the cornea.
- Entering the lens.
- Leaving the lens.
The lens curvature and shape allow for fine focusing of an image.

Focusing for Distant Vision

Light from a distance needs little adjustment for proper focusing.
Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.).

Focusing for Close Vision

Close vision requires:
- Accommodation – changing the lens shape by ciliary muscles to increase refractory power.
- Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye.
- Convergence – medial rotation of the eyeballs toward the object being viewed.
- Corrected with a convex lens.

Functional Anatomy of Photoreceptors

Photoreception – process by which the eye detects light energy.
Rods and cones contain visual pigments (photopigments).

Rods

Functional characteristics:
- Sensitive to dim light and best suited for night vision.
- Absorb all wavelengths of visible light.
- Perceived input is in gray tones only.
- Sum of visual input from many rods feeds into a single ganglion cell.
- Results in fuzzy and indistinct images.

Cones

Functional characteristics:
- Need bright light for activation (have low sensitivity).
- Have pigments that furnish a vividly colored view.
- Each cone synapses with a single ganglion cell.
- Vision is detailed and has high resolution.

Chemistry of Visual Pigments

Retinal is a light-absorbing molecule. Combines with opsins to form visual pigments.
Similar to and is synthesized from vitamin A.
Two isomers: 11-cis and all-trans.
Isomerization of retinal initiates electrical impulses in the optic nerve.

Excitation of Rods

The visual pigment of rods is rhodopsin (opsin + 11-cis retinal).
Light phase:
- Rhodopsin breaks down into all-trans retinal + opsin (bleaching of the pigment).
Dark phase:
- All-trans retinal converts to 11-cis form.
- 11-cis retinal is also formed from vitamin A.
- 11-cis retinal + opsin regenerate rhodopsin.

Excitation of Cones

Visual pigments in cones are similar to rods (retinal + opsins).
There are three types of cones: blue, green, and red.
Intermediate colors are perceived by activation of more than one type of cone.
Method of excitation is similar to rods.

Adaptation

Adaptation to bright light (going from dark to light) involves:
- Dramatic decreases in retinal sensitivity – rod function is lost
- Switching from the rod to the cone system – visual acuity is gained
Adaptation to dark is the reverse
- Cones stop functioning in low light
- Rhodopsin accumulates in the dark and retinal sensitivity is restored

Visual Pathways

Axons of retinal ganglion cells form the optic nerve
Medial fibers of the optic nerve decussate at the optic chiasm
Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus
Other optic tract fibers end in superior colliculi (initiating visual reflexes) and pretectal nuclei (involved with pupillary reflexes)
Optic radiations travel from the thalamus to the visual cortex
Some nerve fibers send tracts to the midbrain ending in the superior colliculi
A small subset of visual fibers contain melanopsin (circadian pigment) which:
- Mediates pupillary light reflexes
- Sets daily biorhythms

Depth Perception

Achieved by both eyes viewing the same image from slightly different angles
Three-dimensional vision results from cortical fusion of the slightly different images
If only one eye is used, depth perception is lost and the observer must rely on learned clues to determine depth