Chapter 19 Special Senses: Vision

Fig Structures that prevent foreign objects from entering eye: eyebrows eyelashes (length ideally 1/3 width of eye opening)length ideally 1/3 width of eye opening eyelids (AKA palpebrae) include thin layer of skin, muscle, fibrous core tarsal glands produce secretion that prevents tear overflow and keeps eyelids from sticking together Accessory Structures of the Eye

Fig Accessory Structures of the Eye Lacrimal caruncle Medial palpebral commissure Palpebral fissure Pupil Sclera Lateral palpebral commissure Iris opening between eyelids is palpebral fissure eyelids join at medial and lateral palpebral commissures lacrimal caruncle is at medial palpebral commissure houses ciliary glands, modified sweat glands that produce thick secretion (grit in your eye when you wake up)

Fig Nostril Nasal cavity Nasolacrimal duct Lacrimal sac Lacrimal canaliculi Lacrimal caruncle Lacrimal puncta Lacrimal gland (orbital part) Lacrimal gland (palpebral part) Lacrimal Apparatus Produces, collects, and drains lacrimal fluid (tears) lubricates anterior surface of eye cleanses and moistens eye surface contains antibacterial enzyme to help prevent infection

Fig Lacrimal puncta are visible on surface of eyelid as small hole Lacrimal canaliculi are canals within eyelid Puncta and canaliculi are part of lacrimal caruncle Nostril Nasal cavity Nasolacrimal duct Lacrimal sac Lacrimal canaliculi Lacrimal caruncle Lacrimal puncta Lacrimal gland (orbital part) Lacrimal gland (palpebral part)

Fig Nostril Nasal cavity Nasolacrimal duct Lacrimal sac Lacrimal canaliculi Lacrimal caruncle Lacrimal puncta Lacrimal gland 1. Lacrimal fluid (tears) is produced in the lacrimal gland. 2. Lacrimal fluid is dispersed across eye surface when we blink. 3. Lacrimal fluid enters the lacrimal puncta, drains into the lacrimal canaliculi, and collects in the lacrimal sac. 4. Lacrimal fluid from the lacrimal sac drains through the nasolacrimal duct. 5. Lacrimal fluid enters the nasal cavity.

Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Human eye is about 2.5 cm in diameter Orbital fat cushions eye against bone provides blood vessels and supports nerves Fibrous tunic is tough external layer Sclera is “white” of eye, made of dense, irregular connective tissue Cornea is clear surface of anterior eye convex shape bends light coming into eye 3 layers: inner simple squamous epithelium, collagen fibers, outer stratified squamous epithelium sclera and cornea meet at limbus (AKA corneal scleral junction) Fig a Anatomy of the Internal Eye

Fig. 19.9b Eyebrow Conjunctival fornix Ocular conjunctiva Palpebral conjunctiva Superior tarsal plate Superior eyelid Eyelashes Inferior eyelid Inferior tarsal plate Cornea Conjunctiva produces mucus and (some) tears Palpebral conjunctiva covers inner surface of eyelid Ocular conjunctiva covers sclera connects to palpebral conjunctiva at conjunctival fornix Accessory Structures of the Eye

Fig. 19.9b Eyebrow Conjunctival fornix Ocular conjunctiva Palpebral conjunctiva Superior tarsal plate Superior eyelid Eyelashes Inferior eyelid Inferior tarsal plate Cornea conjunctiva contains blood vessels does not cover surface of cornea so blood vessels and nerves don’t block vision Accessory Structures of the Eye

Fig Optic disc Lens (b) Central artery of retina Central vein of retina CN II (optic) Fovea centralis Posterior cavity Retina Choroid Sclera Anterior cavity Anterior chamber Posterior chamber Dilator pupillae Sphincter pupillae Pupil Cornea Iris Limbus Suspensory ligaments Ciliary muscle Ciliary process Ciliary body

Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Vasclar tunic (AKA uvea) 3 regions Iris is colored portion of eye black hole at center is pupil two layers of pigment-forming cells give eye color contains two groups of smooth muscle fibers, controls size of pupil Fig a Anatomy of the Internal Eye

Fig Lens Dilator pupillae Sphincter pupillae Pupil Cornea Iris Within iris, two layers of muscles sphincter pupillae is in concentric circles, constricts pupil dilator pupillae extends in radial pattern, dilates pupil

Fig a Anatomy of the Internal Eye Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Vascular tunic (AKA uvea) 3 regions Ciliary body is continuous with iris composed of ciliary muscles and ciliary processes that cover muscles suspensory ligaments extend to lens, focus eye by contracting or relaxing

Fig Lens Ciliary muscles Suspensory ligaments

Vascular tunic (AKA uvea) 3 regions Choroid is most posterior region, black color prevents reflection of excess light back into retina in cats, cows, etc., choroid covered with tapetum lucidum that reflects light Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Fig a Anatomy of the Internal Eye

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Fibrous tunic Sclera Cornea Vascular tunic Iris Ciliary body Choroid Retina Pigmented layer Neural layer Fig a Anatomy of the Internal Eye Retina is most internal layer composed of 2 layers pigmented layer is attached to choroid provides Vitamin A for photoreceptor cells transports nutrients and oxygen to photoreceptor cells, removes waste neural layer is internal to pigmented layer houses photoreceptors and other neurons involved in receiving and processing light signals not attached to other layers, except at optic nerve

Optic disc Optic nerve Retina Sclera Choroid Fovea centralis Organization of the Retina Blood vessels leave eye through optic disc, travel through center of optic nerve out back of eye

Fig a & b Bipolar cells Choroid Photoreceptor cells Horizontal cell Rod Cone Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve Pigmented layer Neural layer Incoming light Nerve signal Optic disc Optic nerve Retina Sclera Choroid Organization of the Retina Light travels through neural layer and is received by photoreceptor cells on deepest layer of retina Retina

Fig a & b Bipolar cells Choroid Photoreceptor cells Horizontal cell Rod Cone Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve Organization of the Retina Photoreceptor cells –rods are most sensitive in dim light, process black and white, most numerous outside fovea centralis –cones process color, are most sensitive in high- intensity light, most numerous within fovea centralis

Fig a & b Bipolar cells Choroid Photoreceptor cells Horizontal cell Rod Cone Amacrine cell Ganglion cells Axons of ganglion cells to optic nerve Organization of the Neural Layer Rods and cones receive signals, send signal to bipolar cells Horizontal cells synapse between bipolar cells, creating convergent signals Bipolar cells send signals to amacrine cells, which process and integrate signals between bipolar and ganglion cells Ganglion cells have axons that extend to optic disc, converge into optic nerve

Fig Bipolar cells Ganglion cells LM 250x Pigmented layer Choroid Rods and cones Axons of ganglion cells Posterior cavity Neural layer Retina

Fig a, Organization of the Retina Area around fovea centralis is macula lutea Contains numerous rods and cones, no bipolar or ganglion cells Light doesn’t have to pass through other cells to get to rods and cones –produces crispest vision Optic disc Optic nerve Fovea centralis Macula lutea

Medial Lateral Optic disc Blood vessels Fovea centralis Macula lutea

(c) Amsler grid, seen with normal vision (a) Normal vision (b) As viewed by a person with macular degeneration (d) Amsler grid, as viewed by a person with macular degeneration Macular Degeneration Loss of photoreceptors and thinning of pigmented layer in macula May also involve bleeding, capillary proliferation, scar tissue formation Major cause of blindness –caused by diabetes, infection, smoking, hypertension, trauma to the eye Laser surgery can slow degeneration, but not restore lost sight

27 Space between lens and cornea is anterior cavity divided into anterior chamber between iris and cornea, and posterior chamber between iris and lens filled with aqueous humor (clear liquid)

28 Space behind lens is posterior cavity filled with vitreous humor clear, jelly-like substance helps maintain eye shape

Fig Optic disc Lens (b) Central artery of retina Central vein of retina CN II (optic) Fovea centralis Posterior cavity Retina Choroid Sclera Anterior cavity Anterior chamber Posterior chamber Dilator pupillae Sphincter pupillae Pupil Cornea Iris Limbus Suspensory ligaments Ciliary muscle Ciliary process Ciliary body

How do these structures work together to produce vision? Cornea bends light as it enters eye Light passes through aqueous humor without bending Iris controls amount of light entering eye Lens further bends light Light is focused on fovea centralis Fovea centralis

Fig Ciliary muscles relaxed Lens flattened Suspensory ligaments taut When ciliary muscles are relaxed, suspensory ligaments are tight, lens is pulled flat, eye focuses on distant objects Lens thickened, more spherical Suspensory ligaments relaxed Ciliary muscles contract, moving ciliary body closer to the lens. When ciliary muscles contract, suspensory ligaments relax, eye becomes more rounded eye focuses on close objects

Page 579 Normal eyeEye with a cataract Normal vision Cataract Image seen through cataract What happens when a lens stops being transparent? cataracts are opacities within the lens major cause of blindness caused by diabetes, UV exposure, infection surgery to correct cataracts often replaces lens eye drops to dissolve cataracts being developed

Fig Binocular vision Left eye Right eye Uncrossed axon Crossed axon Projection fibers (optic radiation) Inferior view Optic nerve: Axons of retinal ganglion cells form optic nerves and exit the eye. Optic chiasm: Most optic nerve axons cross at the optic chiasm. Optic tract: Contains axons from both eyes that will project to either the superior colliculus or the lateral geniculate nucleus. Lateral geniculate nucleus of thalamus: The majority of the optic tract axons project to the lateral geniculate nucleus in the thalamus. Superior colliculus: Some optic tract axons project to the superior colliculus. Primary visual cortex: Receives processed information from the thalamus of the occipital lobe. Right eye only (monocular vision) Left eye only (monocular vision)

Page 584 Focal plane Emmetropia (normal vision) Hyperopia (farsightedness): Eyeball is too short so near objects are blurry. Myopia (nearsightedness): Eyeball is too long so far objects are blurry. Focal plane Hyperopia (uncorrected)Myopia (uncorrected) Concave corrective lens Corrected focal plane Convex corrective lens Corrected focal plane Vision correction using (center) convex and (right) concave lenses.

1 2 3 Cornea is sliced with a sharp knife. Flap of cornea is reflected, exposing deeper corneal layers. A laser removes microscopic portions of the deeper corneal layers, thereby changing the shape of the cornea. Corneal flap is put back in place, and the edges of the flap start to fuse within 72 hours. LASIK laser vision correction procedure.