The Eye

Vision More than half the sensory receptors in the human body are located in the eyes, and a large part of the cerebral cortex is devoted to processing visual information.

Human eye

Accessory Structures of the Eye Accessory structures are the eyebrows, eyelashes, eyelids, extrinsic muscles that move the eyeballs and lacrimal (tear-producing) apparatus. Eyebrows and eyelashes help protect the eyeballs from foreign objects, perspiration, and the sun. Eyelids shade the eye during sleep, protect from light and foreign objects and lubricate (by blinking)

Accessory Structures of the Eye 6 extrinsic eye muscles cooperate to move each eyeball right, left, up, down and diagonally. Lacrimal glands are about the size and shape of an almond. They secrete tears through the lacrimal ducts onto the eyelid. Tears flow over the eyeball toward the nose into 2 lacrimal canals that allow the tears to drain into the nasal cavity.

Components of the human eye

Layers of the eyeball About 2.5 cm in diameter Our eyes stay the same size. They were the size they are now when you were born! Cornea – transparent coat that covers the iris, helps focus light Sclera – the white of the eye, gives the eye shape Conjunctiva – layer that covers the sclera (conjunctivitis – pink eye)

Vascular Tunic The middle layer of the eyeball. Made up of choroid, ciliary body and the iris. Choroid – lines most of the sclera, contains many blood vessels that help nourish the retina. It contains melanocytes and appears dark brown in color. Ciliary muscle – alters the shape of the lens for viewing objects up close or at a distance. The lens, a transparent structure that focuses light rays onto the retina.

Vascular Tunic Iris – colored part of the eyeball Pupil – the hole in the center of the iris, through which light enters the eyeball. The smooth muscle of the iris regulates the amount of light passing through the lens.

Retina The 3rd and inner coat of the eyeball The Retina itself has 3 layers – photoreceptor layer, bipolar cell layer and the ganglion cell layer. Photoreceptors are specialized cells that begin the process by which light rays are converted to nerve impulses. There are 2 types of photoreceptors – rods and cones

Retina Rods – allow us to see shades of gray in dim light Cones – allow us to see highly acute color vision Blue cones – sensitive to blue light Green cones – sensitive to green light Red cones - sensitive to red light These colors are blended so we see in many different colors There are about 6 million cones and 120 million rods

Retina Cones are most densely concentrated in the central fovea – a small depression in the center of the macula lutea (yellow spot) in the exact center of the retina. This is the area of highest visual acuity or resolution (sharpness) because of the high concentration of cones. This is why you move your head and eyes while looking at something to place it on your fovea. They function in bright light and provide color vision. Blind spot – area with no rods or cones, this is the exit to the optic nerve Pg 297

Retina Photopigment – substance that absorbs light, this is called rhodopsin in the rods. Night blindness – inability to see well at low light levels. This person has lost rods and reduced Vitamin A resulting in low amount of rhodopsin. Color blindness – the absence of deficiency of one of the three types of cones from the retina cannot distinguish some of the colors. They DO NOT see in black and white!

Interior of eye Behind the lens is a large cavity of the eyeball, the vitreous chamber. It contains a clear, jellylike substance called the vitreous body (humor). The substance keep the eyeball from collapsing and holds the retina flush against the choroid. Intraocular pressure – pressure inside the eye. Glaucoma – most common cause of blindness in the United States. It’s a buildup of pressure that causes irreversible destruction of the retina, damage to the optic nerve and blindness.

Refraction of light rays The bending of light is refraction. About 75% of the total refraction of light occurs at the cornea. Then, the lens of the eye further refracts the light rays so that they come into exact focus on the retina. Images focused on the retina are inverted (upside down). The reason the world does not look inverted is that the brain learns early in life to coordinate visual images with the orientations of objects.

Accomodation A surface that curves outward, like the surface of a ball is convex. When the eye is focusing on a close object, the lens becomes more convex and refracts the light rays more. This increase in the curvature of the lens for near vision is called accommodation.

Refraction The normal eye – emmetropic eye, refracts light rays from an object so a clear image is focused on the retina. Myopia – nearsightedness, when the eyeball is too long relative to the focusing power of the cornea and lens. Can see near objects but not distant objects. Hyperopia (hypermetropia) – farsightedness, the eyeball is short relative to the focusing power of the cornea and lens. Can see distant objects clearly but not near objects.

Refraction

Refraction Astigmatism – where either the cornea or the lens has an irregular curvature.

Visual pathway After stimulation by light the rods and cones trigger electrical signals in bipolar cells. These then transmit to the ganglion cells. The ganglion cells become depolarized and generate nerve impulses. The axons of the ganglion cells exit the eyeball as the optic nerve.

Disorders Cataracts – the lens becomes cloudy due to changes in the structure of the lens proteins. Trachoma – a serous form of conjunctivitis. It’s the greatest single cause of blindness in the world. The disease produces an excessive growth of subconjunctival tissue and invasion of the blood vessels into the cornea, which progresses until the entire cornea is opaque, causing blindness.