Neuro: Vision

• The eyeball has a diameter of 1 inch
• Approximately 5/6 of the eyeball is inside the orbit and protected
• Eyelashes and eyebrows protect the eye from foreign objects, perspiration, and sunlight
• Sebaceous glands at the base of the eyelashes can become clogged and inflamed, resulting in a sty
• The palpebral fissure is the gap between the eyelids
• Six muscles insert on the exterior surface of the eyeball
• Cranial Nerves III, IV, or VI innervate the eye
• There are four rectus muscles: superior, inferior, lateral, and medial
• There are two oblique muscles: inferior and superior
• About 1 ml of tears are produced daily and spread over the eye by blinking; tears contain a bactericidal enzyme called lysozyme
• The eye includes three layers: the fibrous tunic is the outer layer, the vascular tunic (uvea) is the middle layer, and the nervous tunic is the inner layer

Cornea

• The cornea is transparent and colorless
• Plays a role in focusing light, known as refraction
• The cornea is made of 5 layers of epithelial tissue, connective tissue, and endothelial tissue
• Transplants of the cornea are common and successful
• The cornea has no blood vessels; as a result, no antibodies cause rejection
• Tears and aqueous humor nourish the cornea

Sclera

• Known as the "white of the eye
• Contains a dense irregular connective tissue layer made of collagen and elastic fiber
• Provides shape and support to the eye
• The junction of the sclera and cornea features an opening, called the scleral venous sinus

Choroid Layer

• Contains pigmented epithelial cells (melanocytes) & blood vessels
• Provides nutrients to the retina
• Melanocytes use black pigments to absorb scattered light

Ciliary Body

• Includes ciliary muscle and ciliary processes
• Ciliary muscle can alter the shape of the lens

Iris

• Constitutes the colored portion of the eye
• The iris is shaped like a flat donut
• It is suspended between the cornea and lens
• The iris contains muscle fibers, pigment, connective tissue, and epithelial tissue
• Regulates the amount of light entering the eye
• Autonomic reflexes regulate the amount of light entering the eye
• Circular muscle fibers contract in bright light to constrict the pupil
• Constrictor pupillae (circular) are innervated by parasympathetic fibers
• Dilator pupillae (radial) are innervated by sympathetic fibers
• The response of the pupil varies based on light levels

Lens

• The lens is avascular
• It is made of crystallin proteins in layers like an onion
• The clear capsule is transparent
• Held in place by suspensory ligaments
• The lens focuses light on the retina
• There is an age-related loss of elasticity in the lens
• Suspensory ligaments attach the lens to the ciliary process
• The ciliary muscle controls tension on the ligaments

Retina

• The retina makes up the posterior 3/4 of the eyeball
• The optic disc has an optic nerve exiting the back of the eyeball
• The central retinal’s blood vessels fan out to provide nourishment to the retina
• Those blood vessels can be inspected or hypertension and diabetes
• A detached retina can result from trauma, diabetes, or AIDS and cause fluid retention between layers
• Vision relies on rods, which specialize in dim light and allow one to see shapes and movement
• Vision relies on cons, which specialize in bright light, most densely concentrated in the central fovea

Macula Lutea

• Refers to the posterior portion of the retina near the center, corresponding to the visual axis of the eye
• The central fovea in the macula lutea is the area of sharpest vision
• Rods are absent from the fovea and macula but are more dense toward the periphery of the retina
• Consists of several structures and layers
• Including: Pigmented Epithelium, Photoreceptor layer, Outer synaptic layer, Bipolar cell layer, Inner synaptic layer, Ganglion cell layer, and the Optic nerve fibers

Rods and Cones

• Rods are shaped liked rods
• Shades of gray in dim light
• Approximate 120 million cells are rods
• Able to see shapes and movements
• Distributed along periphery
• Cones are shaped like cones
• Able to see sharp, color vision
• Approximate 6 million cells are cones
• Associated with the fovea of macula lutea
• Densely packed at the exact visual axis of the eye
• Sharpest resolution in vision

Light and Image Processing

• Light penetrates the retina, allowing rods & cones to transduce that light into action potentials
• Rods & cones excite bipolar cells
• Bipolar cells excite ganglion cells
• Axons of ganglion cells form the optic nerve, which the eyeball leaves at the optic disc (blind spot)
• Information is transferred to thalamus (LGN)
• Third-order neurons end at the primary visual cortex
• The eye has an anterior cavity and a posterior cavity

Anterior Cavity

• The segment that is anterior to the lens
• Filled with aqueous humor
• Humor created by the ciliary body: continually drained replaced every 90 minutes
• Includes 2 chambers: anterior chamber between cornea and iris, posterior chamber between iris and lens

Posterior Cavity

• Posterior to the lens
• The space filled with vitreous humor (jellylike)
• Formed once during embryonic life
• Floaters refer to debris in vitreous body of older individuals
• Pressure in the eye, is named intraocular pressure & produced mainly by the aqueous humor
• This pressure maintains the shape of the eyeball
• Keeps the retina smoothly applied to the choroid for clear images
• Glaucoma causes increased intraocular pressure due to drainage of aqueous humor
• Glaucoma may produce degeneration of the retina and eventual blindness
• Aqueous humor is continuously produced by the ciliary body and flows from posterior chamber into anterior chamber through the pupil
• The scleral venous sinus includes the canal of Schlemm, it provides lymphatic-like drainage at the junction of cornea & sclera
• Aqueous humor from eye is drained to the bloodstream

Accommodation

• Accommodation of the lens is defined as changing the shape of lens so that light is focused
• Can also include Constriction of the pupil, which constitutes less light entering the eye
• Refraction of light occurs by cornea & lens
• Light rays must fall upon the retina
• Refraction is described as the bending of light
• Refraction occurs when light moves through substances with different refractive indices
• The refractive index is the degree to which a substance can bend light
• The refractive index is the ratio of the velocity of light in air to the velocity of light in the substance
• Speed of light, air travels close to 300,000 km/s
• Speed of light, glass travels close to 200,000km/s
• The refractive index is calculated as 300,000/200,000, which totals approximately 1.5
• Four refractive surfaces comprise the eye
• Although the cornea bends light the most, it cannot change shape
• As such, its refractive power is constant
• The lens is critical for the focus of objects as the distance between objects
• Lens power can change, with a range from 20-34 diopters
• The Diopter can be defined as a measure of the refractive power of a lens
• Diopter is the reciprocal of the focal length of the lens (measured in meters)
• The focal point of a len’s convergence of parallel light rays after a lens
• Incoming images are focused inverted upon the retina, but reversed from left to right
• 75% of refraction with vision is accomplished by the cornea
• 2+ processes need to occur for focusing on the retina
• Light rays that are from distances of around 20ft need little refraction
• Light rays that are at less than 6ft are very divergent