Lesson 2.2: Special Senses Part 2 (PDF)

Lesson 2: Special Senses Part 2 - Vision (sight): perception of objects in the environment by means of light they emit or reflect o Light: visible part of the electromagnetic spectrum o Human vision limited to wavelengths of light from 400 to 700 nm o Light mus...

Lesson 2: Special Senses Part 2 - Vision (sight): perception of objects in the environment by means of light they emit or reflect o Light: visible part of the electromagnetic spectrum o Human vision limited to wavelengths of light from 400 to 700 nm o Light must cause a photochemical reaction to produce a nerve signal o Ultraviolet (UV) radiation: has too much enerhghy and destroys macromolecules o Infrared (IR) radiation: too little energy to cause photochemical reaction, but does warm the tissues - Orbital region: area around the orbit (eye socket), contains structures that protect and aid the eye: o Eyebrows: enhance facial expression, protects eyes from glare and perspiration o Eyelids (palpebrae): consist of orbicularis oculi muscle and supportive fibrous tarsal plate ▪ Covered with skin outside and conjunctiva inside ▪ Block foreign objects, help with sleep, blink to moisten ▪ Meet at corners – medial and lateral commissures ▪ Tarsal glands secrete oil that reduces tear evaporation and prevents the eyelids from sticking together ▪ Eyelashes help keep debris from eye o Conjunctiva: transparent mucous membrane lining eyelids and covering anterior eyeball, except cornea ▪ Secretes thin mucous film to prevent drying of eyeball ▪ Richly innervated and vascular (heals quickly) ▪ Conjunctivitis (“pinkeye”): inflammation and associated redness caused by allergic reaction, chemical irritation, infection by pathogen o Lacrimal apparatus: gland and tear ducts for production and drainage of tears ▪ Lacrimal gland in superolateral corner of orbit secretes tears Tears: slightly alkaline watery secretion containing antibodies and bacterial lysozyme. It washes and lubricates eye, deliver O2, and nutrients to conjunctiva Flow through lacrimal punctum (opening on eyelid edge) to lacrimal sac, into nasolacrimal duct, and empty into nasal cavity ▪ Lacrimal caruncle: mass of soft tissue containing glands that produce a thick secretion that forms into a gritty deposit (crust on eyes after waking up from sleep) ▪ Orbital fat: cushions eye, protects vessels and nerves - Extrinsic eye muscles attach to exterior surface of eye o Superior, inferior, medial, and lateral rectus o Superior and inferior oblique o Muscles innervated by cranial nerves: CN IV innervates superior oblique, CN VI innervates lateral rectus, CN III innervates other four extrinsic muscles o Movements: ▪ Superior, inferior, medial, and lateral rectus muscles move the eye up, down, medially, and laterally  ▪ Superior and inferior obliques turn the “12 o’clock pole’ of each eye toward or away from the nose; they also produce slight elevations and depressions of the eye - Three tunics (layers): o Fibrous layer: outer layer derived from two regions ▪ Sclera: touch, fibrous protective layer; white of the eye ▪ Cornea: anterior transparent region; admits light into eye ▪ Corneal limbus: border separating the sclera from the cornea o Vascular layer (uvea): middle with three regions ▪ Choroid: highly vascular, dark pigmented layer behind retina, provides oxygen and nutrients to retina ▪ Ciliary body: extension of the choroid; muscular ring around the lens Supports lens and iris; secretes aqueous humor ▪ Iris: colored diaphragm controlling size of pupil (opening) Dilation of pupil caused by sympathetic stimulation of pupillary dilator muscle Constriction of pupil caused by parasympathetic stimulation of pupillary constrictor muscle Dark eyes: lots of melanin in chromatophores (iris pigmented cells) Blue, green, or gray eyes: reduced melanin ▪ Functions of the uvea: Provides route for blood/lymphatic vessels that supply the eye Regulates amount of light entering the eye Secretes and reabsorbs aqueous humor that circulate within the anterior cavity of the eye Controls the shape of the lens to focus light onto the retina o Inner layer: retina and pigmented epithelium ▪ Pigmented epithelium located next to choroid of uvea Absorbs light passes through the inner neural layer and prevents light from bouncing back into the retina ▪ Inner layer (neural layer of retina) contains the photoreceptors and associated neurons. Photoreceptors include: Rods: do not detect different wavelengths (cannot detect colors); however, very sensitive to light – used in low light surroundings Cones: provides color vision. Cones clustered at fovea centralis at the center of the macula lutea - The optical components o Transparent elements that admit light, refract light rays, and focus images on retina o Cornea: transparent window allowing light into the eye. Initial refraction of light occurs here o Aqueous humor: serous fluid secreted by ciliary body into posterior chamber between chamber and iris ▪ Flows through pupil to anterior chamber between iris and cornea  ▪ Reabsorbed by scleral venous sinus (canal of Schlemm) at same rate it is secreted ▪ Note: the anterior and posterior chambers are both located in the anterior cavity of the eye o Lens: transparent structure composed of flattened, compressed transparent cells called lens fibers ▪ Suspended by suspensory ligament from the ciliary body – the ligaments are connected to the ciliary muscles of the ciliary body ▪ Contraction of ciliary muscle loosens the suspensory ligament and causes lens to become thicker/rounder to focus near objects ▪ Relaxation of ciliary muscle tightens the suspensory ligament and causes lens to flatten to focus on far objects o Vitreous body: gelatinous substance that fills vitreous chamber (posterior cavity) located behind lens ▪ Maintains intraocular pressure ▪ Helps hold retina against wall of eye and blood supply ▪ Lens fibers do not have a nucleus nor organelles. They are filled with the protein crystallins which provides transparency to the lens - Cataracts: clouding of the lenses o Cataracts consists of: lens fibers darken with age, fluid-filled bubbles and clefts filled with debris appear between the fibers o Cataracts can be induced by diabetes, smoking, drugs, UV radiation, and certain viruses o Treatment include replacing natural lens with plastic one - Glaucoma: pressure within the eye due to obstruction of scleral venous sinus causing improper drainage of aqueous humor and increased intraocular pressure. This causes compression of blood vessels and lack of oxygen and death of retinal cells (peripheral vision first affected) - Macular degeneration: death of photoreceptor cells in macula lutea (central part of retina) o Causes loss of vision in center of visual field o Cannot be cured but progression can be slowed - Diabetic retinopathy: retinal degeneration caused by effects of diabetes mellitus. It causes blockage of normal retinal blood vessels and growth of abnormal vessels and blood leakage into the posterior cavity o Causes most adult blindness in the US o Can often be prevented with early detection and control of diabetes - Optic disc: medial to macula lutea where ganglion cell axons converge and leave the eye as the optic nerve o Optic disc is blind spot – no photoreceptor cells located here o Brain fills in blind spot area (visual filling) o Scotomas: abnormal blind spots (blind spots in areas other than the optic disc). Causes include: compression of optic disc, damage to photoreceptors, or damage to the visual pathway - Visual process: light enters eye, focus on retina, and produces a tiny inverted image o Iris diameter controlled by two sets of contractile elements:  ▪ Pupillary constrictor: smooth muscle encircling pupil; parasympathetic stimulation narrows pupil ▪ Pupillary dilator: spoke-like myoepithelial cells located distal to pupil compared to pupil constrictor muscle; sympathetic stimulation widens pupil o Pupillary constriction and dilation occur in response to emotions, changes in light intensity, and when gaze shifts between distant and nearby objects o Photo pupillary reflex: pupillary constriction in response to light - Refraction: bending of light o Refractive index of a medium: how much a substance refracts light rays o Cornea refracts light more than lens o Lens fine-tunes the image and focuses it on the fovea centralis of the macula lutea o Astigmatism: condition where light passing through the cornea and lens is not refracted properly causing visual distortions - The near response o Emmetropia: normal vision where the eye is relaxes and focused on object more than 20 ft away ▪ Light rays coming from that object are essentially parallel ▪ Rays focused retina without effort o Near response: adjustment to close-range vision ▪ Involves three processes to focus image on retina: Convergence of eyes Constriction of pupil Accommodation of lens ▪ Convergence of the eyes: eyes orient their visual axis toward object ▪ Constriction (miosis) of the pupil: blocks peripheral light rays and reduces spherical aberration (blurry edges) ▪ Accommodation of the lens: change in the curvature of the lens Ciliary muscle contracts, suspensory ligaments slacken, and lens takes more convex (thicker) shape Light refracted more strongly and focused onto retina - Hyperopia (farsightedness): far objects appear clear, but nearby objects are blurry o Presbyopia: age-related hyperopia in adults 40+ - Myopia (nearsightedness): nearby objects appear clear, but far objects are blurry - Cellular layout of the retina: o Pigmented layer (pigmented epithelium): most posterior part of retina. It absorbs stray light so visual image is not degraded - Neural components of retina, from rear (near pigmented layer) forward to posterior cavity: o Photoreceptor cells: absorb light and generate a chemical or electrical signal (but they are not neurons) ▪ Rods, cones, and certain ganglion cells ▪ Only rods and cones produce visual images o Bipolar cells: synapse with rods and cones, serve as first-order neurons, serve. As first-order neurons that carry signal to ganglion cells  o Ganglion cells: second-order neurons of visual pathway. Their axons form the optic nerve - Each rode or cone has an outer segment and an inner segment, separated by constriction of microtubules o Outer segment: points toward wall of eye (towards pigmented layer); contains modified cilium specialized to absorb light o Inner segment: facing interior of eye; contains organelles and gives rise to cell body and process that form synapse - Rods: responsible for night (scotopic) vision o Images produced are in shades of gray (monochromatic vision) o Outer segment contains stack of 1,000 membranous discs studded with globular protein, the visual pigment rhodopsin - Cone: responsible for day (photopic) vision and color (trichromatic) vision. Function in brighter light o Outer segment tapers to a point(cone-shaped) o Outer segment contain discs that are plasma membrane infoldings (not detached as in rods) that contain pigments for light absorption - Rods and cones continually renew their discs o Add new ones a proximal (basal) end of outer segment o Old discs shed from distal tips of cells, phagocytized by cells in pigmented epithelium - Horizontal and amacrine cells: don’t form separate layers in the retina o Horizontal cell modulates information from photoreceptors to bipolar cell; amacrine cell modulates information from bipolar cell to ganglion cell. This enhances perception of contrast, edges of objects, moving objects, and changes in light intensity - Retina contains extensive neural convergence o 92 million rods and 5 million cones in each retina, but only 1 million nerve fibers in optic nerve o Multiple rods or cone cells synapse on one bipolar cell o Multiple bipolar cells synapse on one ganglion cell - Retinitis pigmentosa: most common inherited visual problem. It is caused by a mutation in a visual pigment that results in photoreceptor cell death, and ultimately blindness - visual pigments: o rods contain visual pigment rhodopsin (visual purple) ▪ two parts of rhodopsin: opsin protein and vitamin A derivate called retinene (retinal) dietary vitamin A deficiency can lead to night blindness (nyctalopia) ▪ opsin is embedded in disc membranes of rod outer segment ▪ all rods contain a single kind of rhodopsin – cannot distinguish colors o cones contain pigment called photopsin ▪ retinal moiety same as in rods ▪ opsin moiety contains different amino acid sequences that determine wavelengths of light absorbed  ▪ three kinds of cones (red, green, blue): each type expresses a different type of opsin, therefore absorbs slightly different wavelengths of light ▪ brain compares the response levels across all three types to determine color of object - light adaptation: adjustment in vision when moving from dark or dim area into brighter light o pupil constriction reduces light intensity (and any discomfort that may accompany sudden brightness) o color vision and acuity below normal for 5 to 10 minutes ▪ time needed for pigment bleaching to adjust retinal sensitivity to high light intensity o rods quickly bleach and become nonfunctional; cones take over - dark adaptation: adjustment in vision when moving from bright area to dark or dim area o dilation of pupils occurs o in the dark, rhodopsin od rods is regenerated o in 1 to 2 minutes, night (scotopic) vision begins to function o after 20 to 30 minutes, amount of regenerated rhodopsin is sufficient for eyes to reach maximum sensitivity - duplicity theory of vision: a single receptor system cannot produce both high sensitivity and high resolution o it takes one type of cell and neural circuit for sensitive night vision o it takes a different cell type and neuronal circuit for high-resolution daytime vision o explains why we need both rods and cones o rods are sensitive and respond even in very dim light ▪ extensive neuronal convergence ▪ 600 rods converge on one bipolar cell ▪ Many bipolar cells converge on each ganglion cell ▪ Results in high degrees of spatial summation – one ganglion cell receives information from 1mm of retina producing only a coarse image o Edges of retina have widely spaced rod cells that act as motion detectors ▪ Low-resolution system only ▪ Cannot resolve finely detailed images o In light bright enough to activate cones (for you to see clearly in color), rods are photobleached and not contributing to vision o Fovea contains only 4000 tiny cone cells (no rods) ▪ No neuronal convergence ▪ Each foveal cone cell has “private line to brain” o High resolution color vision ▪ Little spatial summation: less sensitivity to dim light - Color blindness: hereditary alteration or lack o fone photopsin or another o Most common red-green color blindness ▪ Results from lack of either red or green cones since individual has difficulty distinguishing shades of red and green ▪ Sex-linked recessive trait: occurs in 8% of males, 0.5% of females - Stereoscopic vision (stereopsis): depth perception, the ability to judge distance to objects  o Requires two eyes with overlapping visual fields which allows each eye to look at the same object from different angles o Unlike panoramic vision – having eyes on sides of head (horse or rodents are alert to predators but have no depth perception) o Fixation point: point in space on which the eyes are focused ▪ Looking at object within 100 feet, each eye views from slightly different angle ▪ Provides brain with information used to judge position of objects relative to fixation point - Bipolar cells of retina are first-order neurons - Retinal ganglion cells are second-order neurons o Axons exiting the back of each eye at optic disc leave the eye as the optic nerve (CN II) o Half the fibers from each optic nerve cross over (hemi decussation) at the optic chiasm) ▪ Right cerebral hemispheres sees objects in left visual field because their images fall on the right half of each retina ▪ Each side of brain sees what is on side where it has motor control over limbs - Optic tracts pass laterally around the hypothalamus with most of their axons ending in the lateral geniculate nucleus of the thalamus - Third-order neurons arise here and form the optic radiation of fibers in the white matter of cerebrum o Project to primary visual cortex of occipital lobe where conscious visual sensation occurs o Some fibers project to hypothalamus to affect circadian rhythm (day/night) cycle o A few optic nerve fibers project to midbrain and terminate in the superior colliculi and pretectal nuclei o Superior colliculi control visual reflexes of extrinsic eye muscles o Pretectal nuclei are involved in photo pupillary and accommodation reflexes