Vision Interactive: pgs 469-483

Sense of Sight Visual receptors are found in the eye. Accessory organs for sense of sight: Upper and lower eyelids (palpebrae, protection) Eyelashes (protection) Lacrimal apparatus (tear production) Extrinsic eye muscles (eye movement)

Visual Accessory Organs: Eyelids Eyelids = Palpebrae Composed of 4 layers: Skin. Muscle. Connective tissue. Conjunctiva. Orbicularis oculi muscle closes eyelid Levator palpebrae superioris muscle opens eyelid. Tarsal glands secrete oil onto eyelashes. Conjunctiva: mucous membrane lines eyelid and covers portion of eyeball.

Lacrimal Apparatus Lacrimal gland: In orbit, lateral to eye. Secretes tears. Canaliculi: 2 ducts that collect tears. Lacrimal sac: Collects tears from canaliculi. Lies in groove in lacrimal bone. Nasolacrimal duct: Collects from lacrimal sac. Empties tears into nasal cavity. Lysozyme: Antibacterial component of tears.

Extrinsic Eye Muscles Superior rectus: Inferior rectus: Medial rectus: Rotates eye up and media. Inferior rectus: Rotates eye down and medially. Medial rectus: Rotates eye medially. Lateral rectus: Rotates eye laterally. Superior oblique: Rotates eye down and laterally. Inferior oblique: Rotates eye up and laterally.

Structure of the Eye Hollow, spherical organ of sight. Wall has 3 layers: Outer (fibrous) tunic. Middle (vascular) tunic. Inner (nervous/retinal) tunic.

The Outer (Fibrous) Tunic Cornea: Anterior sixth. Transparent. Helps focus light rays. Transmits and refracts light. Sclera: Posterior five sixths. White, opaque. Protects eye, attaches muscles. Pierced by optic nerve and blood vessels.

Middle (Vascular) Tunic Choroid coat: Posterior five-sixths. Provides blood supply. Contains melanocytes. Melanin absorbs extra light. Ciliary body: Anterior portion. Thickest portion, pigmented. Forms ring to hold lens. Changes lens shape for focusing. Iris: Anterior to ciliary body. In front of lens. Pigmented. Controls light entering eye.

Anterior Portion of the Eye Anterior cavity of eye, between cornea and lens, is filled with a watery fluid, aqueous humor. Lens: Transparent, biconvex, lies behind iris, elastic, held in place by suspensory ligaments of ciliary body; helps focus light rays, and changes shape for long-distance or close vision.

Ciliary Body and Lens Ciliary body forms internal ring around the front of the eye. Ciliary processes are the radiating folds. Ciliary muscles contract and relax to move lens. Suspensory ligaments hold lens in position. Lens lies just behind iris and pupil.

Accommodation Accommodation: A change in the shape of the lens, to view close objects. Lens thickens and becomes more convex when focusing on close object. Lens thins and becomes flatter when focusing on distant objects. The ciliary muscle relaxes the suspensory ligaments during accommodation.

Iris Iris controls amount of light entering the eye. Iris consists of connective tissue and smooth muscle (colored portion of eye). Pupil is window or opening in center of iris. Dim light stimulates radial muscles and pupil dilates. Bright light stimulates circular muscles and pupil constricts. Amount and distribution of melanin determines eye color.

Aqueous Humor Fluid in anterior cavity of eye. Secreted by epithelium on inner surface of the ciliary body. Provides nutrients and maintains shape of anterior portion of eye. Leaves cavity through scleral venous sinus.

Inner (Nervous) Tunic Inner tunic consists of retina. Retina contains visual receptors (photoreceptors). Continuous with optic nerve in back of eye. Macula lutea: yellowish spot in retina. Fovea centralis: center of macula lutea; produces sharpest vision. Optic disc: blind spot; contains no photoreceptors; found where nerve fibers from retina leave eye to become optic nerve. Vitreous humor: thick gel that holds retina flat against choroid coat, and helps maintain the eye’s shape.

The Retina

Posterior Cavity Space enclosed by lens, ciliary body, and retina. Contains vitreous humor: thick gel that supports internal structures and maintains shape of eye.

Major Groups of Retinal Neurons Photoreceptor cells, bipolar cells, and ganglion cells: provide pathway for impulses triggered by photoreceptors to reach the optic nerve. Horizontal cells and amacrine cells: modify, integrate impulses.

Light Refraction Bending of light, which occurs when light waves pass at an angle between mediums of different densities. Focusing bends light so the image falls on the fovea centralis.

Types of Lenses Convex lenses cause light waves to converge. Concave lenses cause light waves to diverge.

Focusing on the Retina As light enters eye, it is refracted by: Convex surface of cornea. Convex surface of lens. Image focused on retina is upside down and reversed from left to right Visual cortex corrects the reversals.

Concave lens corrects nearsightedness. Convex lens corrects farsightedness

Photoreceptors Rods: Cones: Photoreceptors are modified neurons of retina that sense light: Rods: Long, thin projections. Contain light sensitive pigment, called rhodopsin. Hundreds of times more sensitive to light than cones. Provide vision in dim light. Produce vision without color in shades of gray. Produce outlines of objects. Cones: Short, blunt projections. Contain light sensitive pigments, called erythrolabe, chlorolabe, and cyanolabe. Provide vision in bright light. Produce sharp images. Produce color vision. Fovea centralis contains only cones.

Rods and Cones Rods and cones are named for shape of receptive ends: rods are cylindrical and cones are conical

Visual Pigments Rods and cones contain light-sensitive pigments that decompose upon absorption of light: Rhodopsin (Visual purple): Light-sensitive pigment in rods. In presence of light, decomposes into Opsin and Retinal. Triggers a complex series of reactions that initiates nerve impulses. Impulses travel along optic nerve. Iodopsins (pigments in cones): Each type of cone contains different light-sensitive pigment. Each type of cone is sensitive to different wavelengths. Color perceived depends on which types of cones are stimulated. Erythrolabe: responds to red light. Chlorolabe: responds to green light. Cyanolabe: responds to blue light.

Rhodopsin in a Rod Rhodopsin is embedded in the many discs of membrane at the end of the rod.

Stereoscopic Vision Provides perception of distance, depth, height and width of objects. Results from formation of two slightly different retinal images from eyes.

Visual Pathways The visual pathway proceeds from the ganglion cells of the retina to the optic nerve, optic chiasma, optic tracts, the thalamus, optic radiations, and visual cortex in occipital lobe of cerebrum. A few fibers branch off before reaching the thalamus, and enter nuclei for visual reflexes.

Life-Span Changes Age-related hearing loss due to: Damage to hair cells in spiral organ. Degeneration of neural pathways to the brain. Tinnitus. Age-related visual problems include: Dry eyes. Floaters (crystals in vitreous humor). Loss of elasticity of lens, decreasing accommodation (presbyopia). Glaucoma. Cataracts. Macular degeneration. Age-related smell and taste problems due to: Loss of olfactory receptors (anosmia).