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Special Senses Vision Anatomy of the Eye ● 70% of all the sensory receptors in the body are in the eyes ● Vision is the sense that requires the most “learning”, and the eye is often “fooled” ● Most of the eye is enclosed and protected by a cushion of fat and the walls of the bony orbit ● the accessory structures include: ○ ○ ○ ○ extrinsic eye muscles eyelids conjunctiva lacrimal apparatus ● Eyelids protect the anterior of the eye. The eyelids meet at the medial and lateral corners of the eye (medial and lateral commissure (canthus)) ● palpebral fissure - the space between the eyelids in an open eye ● Tarsal glands - modified sebaceous glands associated with the eyelid edges ○ produce an oily secretion that lubricates the eye ○ ciliary glands - modified sweat glands that lie between the eyelashes ● Conjunctiva - a delicate membrane lining the eyelids and covering part of the outer surface of the eyeball ● The conjunctiva fuses with the corneal epithelium ● The conjunctiva helps to lubricate the eyeball and keep it moist Lacrimal Apparatus ● The lacrimal Apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity. ● The lacrimal glands are located above the lateral end of each eye and release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. ● tear drainage from the eye follows the following “tract” ○ ○ ○ ○ lacrimal canaliculi lacrimal sac nasolacrimal duct nasal cavity ● Lacrimal secretion contains ○ antibodies ○ the enzyme lysozyme - enzyme that destroys bacteria ● The lacrimal apparatus ○ cleanses the eye ○ protects the eye surface ○ moistens and lubricates ● The six extrinsic (external) eye muscles produce gross eye movements ○ possible to follow a moving object Name Action Cranial Nerve Lateral rectus Moves eye laterally abducens (VI) Medial rectus moves eye medially Oculomotor (III) Superior rectus elevates eye and turns it medially Oculomotor (III) Inferior rectus depresses eye and turns it medially Oculomotor (III) Inferior oblique elevates eye and turns it laterally Oculomotor (III) superior oblique depresses ey and turns it laterally Trochlear (IV) Internal Structures: The Eyeball ● The eyeball is a hollow sphere whose wall is composed of three layers and its interior is filled with fluids (humors) ● The lens,the main focusing apparatus of the eye, is supported upright within the eye cavity Layers of the Wall of the Eyeball ● Fibrous layer - outer layer ○ sclera - thick, glistening white connective tissue -” white of the eye” ○ cornea - transparent “window” through which light enters the eye ■ abundant nerve endings (pain) ■ great ability to repair itself when damaged ■ contains no vascular tissue, therefore, is beyond immune system’s reach - may be transplanted without fear of rejection ● Vascular layer - Middle layer has three distinguishable regions ○ choroid - blood-rich nutritive tunic that contains a dark pigment that prevents light from scattering within the eye ○ Muscle structures - ciliary body - attaches to lens and ciliary zonule - suspensory ligament of lens, ○ iris - pigmented, circularly and radially arranged smooth muscle fibers that regulates the amount of light entering the eye ■ pupil - opening in the center of the iris through ● Sensory layer - Innermost layer containing the two-layered retina ○ pigmented layer ■ absorbs light and prevents light from scatering inside the eye. ■ acts as phagocytes to remove dead or damaged receptor cells ■ Stores vitamin A ○ neural layer - transparent inner layer containing photoreceptors - rods and cones ● Electrical signals from the photoreceptors pass through a two-neuron chain - bipolar cells and the ganglion cells - before leaving the retina via the optic nerve ● blind spot (optic disc) - area around the optic nerve that does not contain photoreceptors ● Rods are most dense at the edge of the retina and decrease in number as the center is approached ● Rods allows us to see in gray tones in dim light, and they provide for peripheral vision ● cones allow us to see the details of our world in color under bright light conditions ● Cones are more dense in the center of the retina and decrease in density toward the edge ● fovea centralis - tiny pit containing only cones lateral to the blind spot. The area of greatest visual acuity (sharpest vision) ● Varieties of cones respond to different wavelengths of light (figure 8.6) ○ blue cones ○ green cones ○ green to red (red cones) ● Signals from more than one type of cone is interpreted as intermediate colors. ● The lack of cones leads to colorblindness Lens ● A flexible biconvex crystal-lke structure held upright in the eye by the ciliary zonule attached to the ciliary body. ● cataracts - result from the lens becoming more opaque and hardened as one ages ● the lens divides the eye into two segments ● Anterior (aqueous) segment contains a clear watery fluid, the aqueous humor ○ similar to blood plasma and is secreted continuously by the choroid ○ maintains pressure inside the eye (intraocular pressure) ○ provides nutrients for the lens and cornea which lacks vascular tissue ● Posterior (vitreous) segment contains a gellike substance, the vitreous humor ○ reinforces the eyeball internally (avoiding collapsing Refraction of light through the eye ● Refraction - the bending of light when the speed of light changes as it passes from one substance to another substance that has a different density ● Light rays are bent in the eye as they encounter the cornea, aqueous humor, lens, and vitreous humor ● The refractive power of the cornea and humors is constant ● The refractive power of lens can be changed by changing its shape ○ the greater the lens convexity, or bulge, the more it bends the light ○ the flatter the lens, the less it bends the light ● The lens does not need to change shape to focus properly on the retina for light from a distant source (over 20 ft) ● Light from a close object tends to diverge (spread out) and the lens must bulge more to make close vision possible ○ the ciliary body contracts, allowing the lens to become more convex ● Accommodation - the ability of the eye to focus specifically for close objects ● The image formed on the retina because of accommodation is a real image ○ left-right reversed, inverted, and smaller than the object ● Vision problems occur when: ○ the lens is too weak (underconverging) ○ the lens is too strong (overconverging) ○ structural problems of the eyeball ■ myopia (nearsightedness) - eyeball too long ■ hyperopic (farsightedness) - eyeball is too short Visual Fields and Pathways ● Axons from the retina are bundled as the optic nerve. ● Fiber tracts from the optic nerve split ○ optic tracts ■ fibers from medial side of each eye cross over to the opposite side of the brain at the optic chiasma ■ Fibers from the lateral side of the eye stay on the same side of the brain ● The optic tract fibers synapse with neurons in the thalamus ● Thalamus axons form the optic radiation which runs to the occipital lobe of the brain (visual center) ● Optic radiation axon synapse with the cortical cells, and visual interpretation (seeing) occurs ● Each side of the brain receives visual input from both eyes - lateral field of vision of the eye on it own side and from the medial field of the other eye. ● the visual fields of both eyes overlap ● biocular vision - (two-eyed vision) provides for depth perception (3-D vision) Eye Reflexes ● Internal eye muscles (controlled by autonomic nervous system) ○ Ciliary body muscles - alters lens curvature ○ radial and circular muscles of the iris - controls pupil size ● External eye muscles (rectus and oblique muscles)- control eye movement and make it possible to follow moving objects ○ convergence - reflexive movement of the eyes medially when we view close objects. ■ both eyes are aimed toward the near object being viewed ○ Extrinsic muscles are controlled by somatic fibers of cranial nerves III, IV, and VI ● Photopupillary reflex - the constriction of the pupils immediately after sudden exposure to bright light. ● accommodation pupillary reflex - the reflexive constriction of the pupils when we view close objects ○ provides for more acute vision