Eye BIO 242

Accessory Eye Structures

Include the eyebrows, eyelids or palpebrae, eyelashes, conjunctiva, lacrimal apparatus, and the extrinsic eye muscles.

Conjunctiva

Thin, protective mucous membrane that covers the anterior eye and folds to cover the inner eyelid. The conjunctival fold forms a pocket that keeps contacts from moving toward the posterior part of the eyeball. The palpebral conjunctiva covers the interior of the eyelid, and the bulbar conjunctiva covers the anterior part of the white of the eye, but not the cornea.

Lacrimal Apparatus

Group of structures involved in producing and draining tears. The lacrimal gland produces and secretes tears onto the eye surface, and the lacrimal canals drain the tears from the eyes into the enlarged lacrimal sac. The enlarged nasolacrimal duct receives tears from the lacrimal sac and drains tears into the nasal cavity.

Extrinsic Eye Muscles

Six skeletal muscles that insert on the exterior of the eyeball to move the eyeball in all directions. The superior, inferior, medial, and lateral rectus muscles are parallel to the long axis of the eyeball. The superior and inferior oblique muscles attach to the eyeball at an angle.

Structure of the Eyeball

The wall of the eyeball has 3 layers: the outer fibrous tunic, the middle vascular tunic, and the retina (inner tunic).

Fibrous Tunic

Composed of the cornea and sclera.

Cornea

The transparent layer forming the front of the eye.

Sclera

Tough, white part of the eye that forms the majority of the eyeball.

Scleral Venous Sinus

An opening found at the junction of the cornea and the sclera that drains aqueous humor back into the blood stream.

Middle Vascular Tunic

Composed of the iris, ciliary body, and choroid.

Iris

The iris is the most anterior portion of the vascular tunic and contains pigmented cells. It is made of the sphincter pupillae and dilator pupillae smooth muscles that controls the pupil size.

Pupil

Opening in the middle of the iris that allows light to enter the eyeball and changes size in response to the intensity of light.

Ciliary Body

Begins posterior to the iris at the junction of the cornea and sclera and extends to the ora serrata. Consists of the ciliary muscle that contracts to control the shape of the lens.

Ora Serrata

The jagged anterior margin of the retina.

Ciliary Processes

Folds that protrude from the ciliary body towards the lens. They contain capillaries that secrete aqueous humor, the fluid in the anterior chamber of the eyeball. Suspensory ligaments are thin fibers that attach the lens to these processes.

Choroid

Most posterior part of the vascular tunic that lines most of the interior of the sclera. It contains blood vessels that nourish the retina.

Retina (Inner Tunic)

Begins at the ora serrata and continues posteriorly, lining the interior of the choroid. 2 parts: pigmented layer and neural layer. Retina lines the posterior three quarters of the eyeball. Its primary function is image formation.

Interior of Eyeball

The interior of the eyeball contains the lens, anterior cavity, and vitreous chamber.

Lens

The lens divides the interior of the eyeball into an anterior cavity and vitreous chamber (posterior chamber). Just behind the pupil and iris. The lens is constructed of numerous layers of a protein called crystalline, arranged like the layers of an onion. Normally, the lens is perfectly transparent and is enclosed by a clear capsule held in position by the zonular fibers (suspensory ligaments). Focuses rays toward each other (refracts the light). The lens can accommodate or change shape to bend light rays to focus them on the central fovea.

Cataract

A loss of transparency of the lens.

Anterior Cavity

Space between the cornea and the lens that is filled with watery aqueous humor. This cavity is subdivided into an anterior chamber and a posterior chamber.

Aqueous Humor

Secreted by the ciliary processes in the anterior cavity.

Anterior and Posterior Chamber

Anterior: Between cornea and iris. Posterior: Between the iris and the lens.

Movement of Aqueous Fluid in Anterior Cavity

From the posterior chamber, the fluid permeates the posterior cavity and then passes anteriorly between the iris and the lens, through the pupil into the anterior chamber. From the anterior chamber, the aqueous humor is drained into the scleral venous sinus and passes into the blood.

Vitreous Chamber

Larger, posterior cavity located between the lens and the retina. This cavity is filled with a gel-like substance called the vitreous body.

Vitreous Body

This substance contributes to intraocular pressure, helps to prevent the eyeball from collapsing, and holds the retina flush against the internal portions of the eyeball. Unlike the aqueous humor, the vitreous body does not undergo constant replacement.

Glaucoma

Abnormal elevation of intraocular pressure. Results in degeneration of the retina and blindness.

Outer Pigmented Layer of Retina

The outer pigment layer (nonvisual portion) is a sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina.

Inner Neural Layer of Retina

The inner neural layer (visual portion) is composed of three zones of neurons: Photoreceptor layer (deepest), bipolar cell layer (middle), ganglion cell layer (superficial). Names in the order in which they conduct nerve impulses. The ganglion cell layer is the innermost zone of the neural portion.

Photoreceptor Layer

The photoreceptor layer contains rods and cones. Called this because of their shape. Structurally, the photoreceptor layer is just internal to the pigment layer, which lies adjacent to the choroid.

Rods

Used in night vision and respond to low levels of light, allowing us to perceive shades of gray, black, and white. Visual acuity with rods is low. Permit discernment of shapes and movement. Increase in density toward the periphery of the retina.

Cones

Require brighter light for stimulation, but allow us to see colour and provide high visual acuity. Sharpness of vision. Most densely concentrated in the central fovea.

Synapses of Layers in the Retina

The rods and cones synapse with the bipolar neurons in the bipolar cell layer, which synapses on the ganglion cells in the ganglion layer.

What Makes up the Optic Nerve

Axons from the ganglion cells extend through the optic disc and leave the eyeball as the optic nerve.

Order of Light Going to the Brain

Pigmented epithelial layer, photoreceptors, bipolar cells, ganglion cells whose axons gather together to form the optic nerve, to the optic chiasm to the optic tract to the thalamus to the occipital lobe of the cerebral cortex (primary visual area). Pigment epithelium absorbs light. Macrophages clean up burnt out tips of photoreceptors. Blood vessels are concentrated 'behind the scenes'.

Optic Disc

Does not contain photoreceptors and forms the blind spot. It is also the site where the central retinal artery and vein enter and leave the retina, and the only place where the retina is secured to the other layers of the eyeball.

Macula Lutea

The site of macular degeneration. Is in the center of the neural portion of the retina (spot of highest resolution). In the middle of the macula is the central fovea. Exact center of the posterior portion of the retina and corresponds to the visual axis of the eye.

Central Fovea

This area of the retina has the highest density of cones and is not covered by ganglion and bipolar cell layers. No rods, only cones. Therefore, this area has the highest visual acuity (sharpness of vision) of any area of the retina. When we look at an object, the light rays reflected from the object are focused onto the central fovea.

Viewing the Retina

The retina can be viewed with an ophthalmoscope. It illuminates the interior of the eye, and the retina appears red from the many blood vessels. Blood vessels can be seen branching from the optic disc while the circular macula lutea appears dark because of the absence of blood vessels.

Vitamin A Deficiency

Will create a longer after image.

Accommodation

The ability of the lens to change shape in order to focus images on the retina.

What is Light Refracted By?

Cornea, lens, vitreous humor.

Visual Acuity Tests

Visual acuity tests measure the ability of the lens to focus light reflected from an object on the central fovea of the retina.

Visual Acuity Test- Blind Spot

Use a card with a dot and an X and move it closer until they can't see one object anymore, that's your blind spot.

Visual Acuity Test- Distance

Uses the Snellen eye chart that's 20ft away (you see from 20 feet what most people see at 20 ft, example of 20/20 vision).

Visual Acuity Test- Near

14 inches from their face and same thing as distance.

Observing Objects Farther from 20 feet

At 20 feet, light rays from an object are nearly parallel and do not have to bend as much to focus on the central fovea. At this distance, the lens is flattened and the refractive power (ability to bend light rays) of the lens is lowest.

Observing Objects Closer than 20 feet

To observe objects closer than 20 feet, the lens must change shape or accommodate to focus the light rays on the central fovea. The lens bulges to increase the refractive power.

Emmetropic

Individuals who have normal distance vision and near vision.

Hyperopic

Individuals who have normal distance vision but blurry near vision. Farsighted. Image falls behind retina, corrected with convex lens.

Myopic

Individuals who have blurry distance vision but normal near vision. Nearsighted. Image falls in front of the retina, corrected with concave lens.

Presbyopia

As we age, the ability of the lens to accommodate diminishes and the ability to focus on very close objects decreases, a condition called presbyopia.

Near Point of Vision

The closest distance that a person can focus on an object. The average near point of vision is 10cm for young adults and 20cm for adults in their 40s and 80cm for people over 60.

Astigmatism

Caused by irregularities in the curvature of the cornea or lens (this causes parts of an image to be blurry). To find this you use the chart that looks like a sun. If some lines are lighter or less distinct you have this.

Red-Green Colour Blindness

Red-green colour blindness is an inherited disorder and is the most common form of colour blindness. Due to absence of either red or green cones. There are 3 types of cones in the human retina, and these cones differ in the type of photopigment present. The approximate distribution of cones are 64% red, 34% green, and 2% blue. Perception of different colours occurs when different wave lengths of colour selectively activate different photopigments. In order to differentiate between red and green, both red and green cones must be present. Boys are more prone because it is a gender-linked trait. Males have only one gene for color blindness (x chromosome) while females have two, and it is recessive.