Vision and Structure of the Eye

General Info

• Our eyes give us our most important sense —sight or vision • Estimated that 90% of the data processed by our brains about the environment is related to vision • Processed in the visual cortex at the posterior of the brain • Eyes contain photoreceptors that are sensitive to various qualities of light

Accessory Structures

• Several structures assist with the proper functioning of the eye, though not directly related to perceiving light • These include the eyelids, extrinsic muscles, and the lacrimal apparatus which is responsible for producing tears

Eyelids

• Protect anterior surfaces of the eyes • Eyelashes help protect from debris and shade the eyes slightly • The conjunctiva is a mucous membrane which lines the inside of the eyelid and folds back to cover the anterior portion of the eye; secretes mucus to help moisten and lubricate the eye

Lacrimal Apparatus

• Consists of a lacrimal gland, ducts, and the nasolacrimal duct which channels excess fluid into the nasal cavity • Tears produced by the lacrimal gland are slightly salty and are released continuously • Help to moisten, lubricate, and clear eye of debris • Also help to create a smooth surface for refraction of light

Structure of the Eye

• Approx. 1 inch in diameter • Made of three distinct layers, or tunics, on its outer surface • These are the fibrous tunic, vascular tunic, and nervous tunic listed from superficial to deep • Inside is filled with fluid which is divided into chambers

Fibrous Tunic

• Made up of two parts; the posterior sclera and the anterior cornea • Sclera is also known as the “white of the eye” • Dense with blood vessel; penetrated on the posterior surface by the optic nerve • Cornea is the clear “window” that bulges out slightly

Vascular Tunic

• Very dense with blood vessels that supply nourishment to various structures of the eye • Components include the choroid, the ciliary body, and the iris. Associated with the lens • Choroid is a thin, dark brown pigment that absorbs light bouncing around within the eye; reduces distortion of images

Vascular Tunic cont’d

• Anterior of the choroid are the ciliary body and the iris which are made of smooth muscle tissue • Ciliary body connects to the lens by way of a suspensory ligament • Contraction of muscles in the ciliary body changes the shape of the lens to adjust focus, called accommodation

• The lens provides separation of the interior of the eye into two chambers • The anterior chamber, located between the lens and the iris, is filled with a fluid called the aqueous humor, which circulates and is continuously recycled • The posterior chamber, located between the lens and the retina, is filled with a fluid called the vitreous humor which is not recycled

Vascular Tunic cont’d

• The iris is the beautifully colored, donut-shaped portion of the eye. • Contraction of it’s muscle changes the size of the pupil (its blackened middle) which controls how much light enters into the eye

The Nervous Tunic

• Made up of the retina, which is imbedded with numerous photoreceptors and nerves which transmit the data detected to the brain; rely on photo-reactive pigments • Rod cells – contain an elongated, cylindrical dendrite, sensitive to small levels of light Photoisomerization • Cone cells – contain a dendrite that tapers to a point, color sensitive and produce a sharper image

Rods and Cones

The Nervous Tunic cont’d

• The axons from the rods and cones converge at a spot near the center of the retina called the optic disc • There are no photoreceptors at the optic disc which forms a blind spot • The macula lutea and fovea centralis are areas of the retina which have the highest concentration of cones which provide optimal visual acuity

Extrinsic Muscles

• Humans have six • Move and rotate the eyeball within its socket • Originate from the walls of the orbit and insert upon the outer surface of the eye

Muscles that Move the Eye

Superior rectus Inferior rectus Lateral rectus Medial rectus Inferior oblique Superior oblique Turns eye upward Turns eye downward Turns eye laterally Turns eye medially Rotating movements of eye Rotating movements of eye

MR =

Medial Rectus

SR =

Superior Rectus

SO =

Superior Oblique

LR =

Lateral Rectus

IR =

Inferior Rectus

IO =

Inferior Oblique

Common Vision Conditions

Emmetropia Normal vision Hypermetropia Astigmatism Nyctalopia Cannot see close objects clearly; farsightedness Inability to focus an image clearly on the retina due to unequal curvature of the lens, cornea, or retina Inability to adjust vision to darkness; nightblindness

Common Vision Conditions Cont’d

Strabismus Myopia Inadequate muscle action or an increase in the strength of one muscle over another; cross-eyed Cannot see distant objects clearly; nearsightedness Cataract Glaucoma Reduced vision due to the presence of opaque spots on the lens Increase in intraocular pressure that interferes with vision