Eyes Anatomy & Function PDF

**17-3 Internal Eye Structures Contribute to Vision, while Accessory Eye Structures Provide Protection.** \*\*\* We rely more on vision than any other special sense. \*\*\* **Accessory Structures of the Eye** Accessory structures of the eye provide protection, lubrication, and support and include...

**17-3 Internal Eye Structures Contribute to Vision, while Accessory Eye Structures Provide Protection.** \*\*\* We rely more on vision than any other special sense. \*\*\* **Accessory Structures of the Eye** Accessory structures of the eye provide protection, lubrication, and support and include... - Eyelids - Superficial epithelium of the eye - Lacrimal apparatus **The Eyelids** **[Eyelids (palpebrae):]** continuation of skin. - Blinking keeps surface of eye lubricated and clean. - **[Palpebral fissure:]** gap that separates upper and lower eyelids. Eyelids are connected at... - Medial angle, also called, *medial canthus*. - Lateral angle, also called, *lateral canthus*. **[Eyelashes:]** robust hairs. - Help prevent foreign matter from reaching eye. **[Tarsal glands:]** secrete lipid-rich product that helps keep eyelids from sticking together. **[Lacrimal caruncle:]** mass of soft tissue at medial angle of eye. - Contains glands that produce thick secretions. **The Superficial Epithelium of the Eye** **[Conjunctiva:]** mucous membrane covered by an epithelium. **[Conjunctivitis (pinkeye):]** inflammation of conjunctiva. - **[Palpebral conjunctiva:]** covers inner surface of eye. - **[Bulbar conjunctiva:]** covers anterior surface if eye, extends to edges of cornea. **[Cornea:]** transparent part of fibrous outer layer that extends from the bulbar conjunctiva. **The Lacrimal Apparatus** **[Lacrimal apparatus:]** produces, distributes, and removes tears. **[Lacrimal gland:]** tear gland. - Produces tears that cover conjunctiva. - Secretions contain **[lysozyme:]** antibacterial enzyme. **[Fornix:]** pocket where palpebral conjunctiva joins bulbar conjunctiva. - Receives 10-12 ducts from lacrimal gland. **Tears** - Collect in lacrimal lake at medial angle of eye and pass through... - Lacrimal puncta (pores), - Lacrimal canaliculi (canals), - Lacrimal sac, - Nasolacrimal duct, - Nasal cavity. **Anatomy of the Eyeball** Layers of the wall of the eyeball \*\*\* layers are also called tunics. \*\*\* - Outer fibrous layer. - Intermediate vascular layer (uvea). - Deep inner layer (retina). **[Orbital fat:]** cushions and insulates each eye. **[Eyeball ]** - Hollow. - Filled with fluid. - Two interior cavities... - Small **[anterior cavity]** - **[Aqueous humor ]** - Large **[posterior cavity]** - **[Vitreous body ]** **The Fibrous Layer** The fibrous layer is the outermost layer of the eyeball. - **[Sclera:]** white of the eye. - **[Cornea:]** transparent portion. - **[Corneoscleral junction (corneal limbus):]** border between cornea and sclera. **The Vascular Layer** The uvea is the vascular layer of the eyeball. - **[Iris:]** colored part of the eye, contains blood vessels, melanocytes, and two layers of smooth muscle called pupillary muscles. - **[Pupillary muscles:]** change diameter of pupil. - **[Pupil:]** central opening of iris. - **[Ciliary body:]** attaches to iris and extends posteriorly to ora serrata. - Contains ciliary muscle and ciliary processes. - **[Ora serrata:]** serrated anterior edge of neural layer of retina. - **[Ciliary zonule (suspensory ligament):]** attaches lens to ciliary processes. - **[Choroid:]** vascular layer that separates fibrous and inner layers posterior to ora serrata. - Capillaries deliver oxygen and nutrients to retina. **Vascular Layer Functions** - Provides route for blood vessels and lymphatics that supply tissues of eye. - Regulates amount of light entering eye. - Secretes and reabsorbs aqueous humor that circulates within chamber of eye. - Controls shape of lens, which is essential for focusing. **The Inner Layer** Inner layer (retina) consists of a thin layer called the pigmented layer and a thicker covering called the neural layer. **[Pigmented layer:]** absorbs light that passes through neural layer. **[Neural layer:]** contains supporting cells and neurons - Outermost part contains **[photoreceptors]**. - Rods and cones. **Photoreceptors** - **[Rods]** (black/white/gray) - Do not discriminate colors. - Highly sensitive to light. - **[Cones]** (colors) - Provide color vision. - Densely clustered in **[macula]**. - Especially at **[fovea centralis (fovea)]**. - At center of macula, - Site of sharpest color vision. - Visual axis is the line from an object to the fovea. - **[Bipolar cells:]** synapse with rods and cones. - **[Ganglion cells:]** synapse with bipolar cells. **The Neural Layer: The Optic Disc** **[Optic disc:]** origin of optic nerve. - Circular region just medial to fovea. - No photoreceptors (blind spot). **The Chambers of the Eye** Ciliary body and lens divide interior of eye into... - **[Large posterior cavity ]** - **[Small anterior cavity ]** - Divided by iris into anterior and posterior chambers. **[Aqueous humor:]** fluid that circulates within anterior cavity. - Also diffuses through posterior cavity. - Enters scleral venous sinus (canal of Schlemm) at corneoscleral junction. - Reenters circulation at veins in sclera. - **[Intra-ocular pressure:]** fluid pressure in aqueous humor, helps retain eye shape. - **[Glaucoma:]** elevated intra-ocular pressure. **[Vitreous body:]** gelatinous mass in posterior cavity. - Helps stabilize shape of eye. - **[Vitreous humor:]** fluid portion of vitreous body. **Lens** **[Lens:]** held in place by ciliary zonule, also called, *suspensory ligaments*. - **[Cataracts:]** loss of transparency or cloudiness in lens. - **[Senile cataracts:]** most common form; natural consequence of aging. **[Lens fibers:]** enucleate cells in interior of lens. - Filled with **[crystallins]**, which provide clarity and focusing power. **17-4 The Focusing of Light on the Retina Leads to the Formation of a Visual Image.** **An Introduction to Light** Light energy is a form of radiant energy that travels in waves. **Wavelength and Color: visible spectrum acronym: ROY G BIV (red, orange, yellow, green blue, indigo, violet)** R: red (740-625nm) longest visible wavelength V: violet (435-380nm) shortest visible wavelength Visible light is also described as being made up of photons, small energy packets with characteristic wavelengths. Photons of red light carry the least energy and have the longest wavelength, whereas photons of violet carry the most energy and have the shortest wavelength. **Refraction and Focusing of Light** Light is **[refracted (bent)]** as it passes through cornea and lens. - **[Focal point:]** specific point of intersection of light rays on retina. - **[Focal distance:]** distance between center of lens and focal point. **[Astigmatism:]** condition where light passing through cornea and lens is not refracted properly, visual image is distorted. **[Accommodation:]** automatic adjustment of eye to provide clear vision. - Lens becomes rounder to focus on [nearby] objects. - Flatter lens allows focus on [distant] objects. **Image Formation and Reversal** Image arriving at retina is miniaturized, upside down, and reversed from left and right and the brain compensates for this. **Visual Acuity** **[Visual acuity:]** clarity of vision. - Standard rating is 20/20. This means a person can see at 20 feet what should normally be seen at 20 feet. 20/15 is better than normal vision, while 20/30 is slightly worse. A person is legally blind when their vision with assistance is 20/200 or worse (person needs to be at 20 feet to see what can normally be seen at 200 feet). **[Scotoma:]** abnormal, permanent blind spot. - May result from compression of optic nerve, damage to photoreceptors, or central damage. **[Emmetropia:]** normal vision. **[Myopia:]** nearsightedness. **[Hyperopia:]** farsightedness. **[Photorefractive keratectomy (PRK):]** computer-guided laser shapes the cornea to exact specifications. - LASIK is a variation of PRK. **17-5 Photoreceptors Transduce Light into Electrical Signals that are then Processed in the Visual Cortex.** **Physiology of Vision** The rods and cones of the retina are called photoreceptors because they detect photons, the basic units of visible light. **Anatomy of Photoreceptors: Rods and Cones** **[Rods:]** detect pressure or absence of photons. **[Cones:]** provide information about wavelengths of photons. Both rods and cones have... - Inner segment containing major organelles. - Outer segment with membranous discs that contain visual pigments. **Physiology of Photoreceptors** **[Visual pigments:]** absorb pigments. - First step in photoreception. - Derivatives of rhodopsin. - Opsin (protein) plus retinal (pigment). - Retinal is synthesized from vitamin A. **[Color vision:]** provided by blue cones, green cones, and red cones. - Each type has a different form of opsin. **[Color blindness:]** inability to distinguish certain colors. **Steps in Photoreception** - Absorption of a photon changes retinal from 11-cis to 11-trans form. - This activates opsin. - Opsin activates transducin (G-protein). - Which activates phosphodiesterase (PDE). - PDE reduces levels of cyclic GMP - Chemically gated sodium ion channels close. - Dark current is reduced. - Rate of neurotransmitter release declines. **Bleaching and Regeneration of Visual Pigments** **[Bleaching ]** - After absorbing a photon, rhodopsin splits into retinal and opsin. - 11-trans retinal is converted back to 11-cis retinal. - Requires ATP. - Retinal then recombines with opsin. **Synthesis and Recycling of Visual Pigments** **[Night blindness (nyctalopia):]** results from deficiency of vitamin A. **Light and Dark Adaption of Visual Pigments** **[Dark-adapted state:]** visual pigments are fully receptive to stimulation. **[Light-adapted state:]** rates of bleaching and reassembly of visual pigments are balanced. **[Retinitis pigmentosa (RP):]** inherited disease characterized by progressive retinal degeneration. **The Visual Pathways** - Begin at photoreceptors, - End at visual cortex of cerebral hemispheres. - Messages must cross two synapses before moving toward brain. - Photoreceptor to bipolar cell, - Bipolar cell to ganglion cell. **Processing by the Retina** **Ganglion Cells** **[Ganglion cells:]** monitor specific portions of field of vision. **[M cells:]** ganglion cells that monitor rods. - Relatively large. - Provide information about... - General form of an object. - Motion. - Shadows in dim lighting. **[P cells:]** ganglion cells that monitor cones. - Smaller and more numerous than M cells. - In fovea, ratio of cones to ganglion cells is 1:1. - Provide information about... - Edges. - Fine detail. - Color. **[On-center neurons:]** excited by light arriving in center of receptive field. - Inhibited when light strikes edges. **[Off-center neurons:]** excited by light at edges. - Inhibited by light in central zone. **Central Processing of Visual Information** - Axons from ganglion cells converge on optic disc. - Penetrate wall of eye. - Proceed toward diencephalon as optic nerve (II). - Two optic nerves reach diencephalon after partial crossover at **[optic chiasm]**. - Information travels to visual cortex in occipital lobe. - **[Optic radiation:]** bundle of projection fibers linking lateral geniculates with visual cortex. **The Field of Vision** **[Field of vision:]** combined visual images from left and right eyes. - **[Depth perception:]** obtained by comparing relative positions of objects between images received from both eyes. **Brainstem and Visual Processing** **[Circadian rhythm:]** daily pattern of activity tied to day-night cycle. - Established from visual information. - Affects metabolic rate, blood pressure, etc.