Andrews University BIOL 221 L_ch16_Sen 3_Eye PDF

Andrews University BIOL 221-001 Chapter 16 Sensory Organs Part 3 Professor: Brian Y.Y. Wong, Ph.D.  Anatomy & Physiology of Human Eye Contains lymph and its purpose is to facilitate changes in the volume of the lens. As the lens expands in positive accom...

Andrews University BIOL 221-001 Chapter 16 Sensory Organs Part 3 Professor: Brian Y.Y. Wong, Ph.D.  Anatomy & Physiology of Human Eye Contains lymph and its purpose is to facilitate changes in the volume of the lens. As the lens expands in positive accommodation, its volume increases. This results in compression of the hyaloid canal, so that the volume of the eye remains constant Tunica fibrosa Tunica vasculosa (uvea) Tunica interna 130 million rods and 6.5 million cones Most astrocytes and other glial cells 4,000 tiny cone cells (no rods) No neuronal convergence Each foveal cone cell has “private line to brain”  Light and Vision Vision (sight)— perception of objects in the environment by means of light they emit or reflect Light— visible electromagnetic radiation – Human vision: limited to wavelengths of light from 400 to 700 nm – Light must cause a photochemical reaction to produce a nerve signal – Ultraviolet radiation: < 400 nm; has too much energy and destroys macromolecules – Infrared radiation: > 700 nm; too little energy to cause photochemical reaction, but does warm the tissues Three types of cones are named for absorption peaks of their photopsins – Short-wavelength (S) cones peak sensitivity at 420 nm – Medium-wavelength (M) cones peak at 531 nm – Long-wavelength (L) cones peak at 558 nm 16-3 Accessory Structures of the Orbit  Accessory Structures of the Orbit Orbital region of face is the area around the eye socket (orbit) Eyebrows enhance facial expression – Protect eyes from glare and perspiration Eyelids (palpebrae) – Block foreign objects, help with sleep, blink to moisten – Meet at corners (commissures) – Consist of orbicularis oculi muscle and tarsal plate covered with skin outside and conjunctiva inside – Tarsal glands secrete oil that reduces tear evaporation – Eyelashes help keep debris from eye 16-5 Accessory Structures of the Orbit (3) Conjunctiva—a transparent mucous membrane that lines eyelids and covers anterior surface of eyeball, except cornea – Richly innervated and vascular (heals quickly) – Secretes a thin mucous film that prevents the eyeball from drying Orbital fat— cushions eye, protects vessels and nerves of orbit Accessory Structures of the Orbit (4) Lacrimal apparatus makes, distributes and drains tears. Tears from lacrimal gland wash and lubricate eye, deliver O2 and nutrients, and prevent infection with a bactericidal lysozyme Tears flow through lacrimal punctum (opening on eyelid edge) to lacrimal sac, then into nasolacrimal duct emptying into nasal cavity  Accessory Structures of the Orbit Six extrinsic muscles attach to exterior surface of eyeball – Superior, inferior, lateral, and medial rectus muscles, superior and inferior oblique muscles Innervated by cranial nerves – CN IV innervates superior oblique – CN VI innervates lateral rectus – CN III innervates other four extrinsic muscles  Trochlea Optic nerve Superior oblique Muscles: tendon Superior oblique Superior rectus Medial rectus Muscles: Superior rectus Lateral rectus Inferior rectus Inferior oblique Inferior rectus Levator palpebrae superioris (cut) (a) Lateral view (b) Superior view TSO Levator palpebrae OLSMII Superior Trochlear oblique superioris muscle nerve (IV) muscle Superior rectus muscle Lateral Medial rectus Oculomotor Abducens rectus muscle muscle nerve (III) nerve (VI) Inferior rectus ALR muscle Inferior oblique muscle (c) Frontal view  Innervation of the 6 Intrinsic (3 pairs) Eye Muscles TSO OSMII ALR Superior, inferior, medial, and lateral rectus muscles move the eye up, down, medially, and laterally (respectively) Superior and inferior obliques turn the “twelve o’clock pole” of each eye toward or away from the nose; they also produce slight elevations and depressions of the eye  Anatomy of the Eye Tunica fibrosa Tunica vasculosa (uvea) Tunica interna Three principal components of the eyeball – Three layers (tunics) that form the wall of the eyeball – Optical components admit and focus light – Neural component: retina and optic nerve  The Tunics Tunica fibrosa— outer fibrous layer – Sclera: dense, collagenous white of the eye – Cornea: transparent region of modified sclera in front of eye that admits light Tunica vasculosa (uvea)— middle vascular layer – Choroid: highly vascular, deeply pigmented layer behind retina – Ciliary body: extension of choroid; a muscular ring around lens Supports lens and iris Secretes aqueous humor – Iris: colored diaphragm controlling size of pupil (opening) If there is a lot of melanin in chromatophores (cells) of iris—brown or black eye color If there is reduced melanin—blue, green, or gray eye color Tunica interna— retina and beginning of optic nerve 16-12  The Optical Components Optical components— transparent elements that admit light, refract light rays, and focus images on retina: cornea, aqueous humor, lens, vitreous body – Cornea: transparent anterior cover – Aqueous humor Serous fluid secreted by ciliary body into posterior chamber—posterior to cornea, anterior to lens Reabsorbed by scleral venous sinus at same rate it is secreted 16-13  The Optical Components Aqueous humor is released by ciliary body into posterior chamber, passes through pupil into anterior chamber, then reabsorbed into scleral venous sinus  The Optical Components – Lens Lens fibers— flattened, tightly compressed, transparent cells that form lens Suspended by suspensory ligaments from ciliary body Changes shape to help focus light – Rounded with no tension or flattened with pull of suspensory ligaments – Vitreous body (humor) fills vitreous chamber Jelly fills space between lens and retina 16-15  The Neural Components Include retina and optic nerve Retina – Formed from optic vesicle—outgrowth of diencephalon – Attached to eye only at optic disc (posterior exit of optic nerve) and ora serrata (anterior edge of retina) – Pressed against rear of eyeball by vitreous humor – Detached retina causes blurry areas of vision and can lead to blindness Examine retina with opthalmoscope – Macula lutea: patch of cells on visual axis of eye – Fovea centralis: pit in center of macula lutea – Blood vessels of the retina 16-16 The Neural Components  The Neural Components Macula lutea— patch of retina on visual axis of eye (3 mm diameter) – Fovea centralis: center of macula; finely detailed images due to packed receptor cells Opthalmoscope—tool used to examine retina and blood vessels  The Neural Components Optic disc— blind spot – Optic nerve exits retina and there are no receptors there Blind spot— use test illustration above – Close right eye, stare at X and red dot disappears Visual filling— brain fills in green bar across blind spot area – Brain ignores unavailable information until saccades (fast eye movements) redirect gaze  Cataracts and Glaucoma Cataract— clouding of lens – Lens fibers darken with age, fluid-filled bubbles and clefts filled with debris appear between the fibers – Induced by diabetes, smoking, drugs, ultraviolet radiation, and certain viruses – Treat by replacing natural lens with plastic one 16-20  Cataracts and Glaucoma Glaucoma— elevated pressure within the eye due to obstruction of scleral venous sinus and improper drainage of aqueous humor – Death of retinal cells due to compression of blood vessels and lack of oxygen Illusory flashes of light are an early symptom Colored halos around lights are late symptom Lost vision cannot be restored – Intraocular pressure measured with tonometer 16-21  Formation of an Image Light passes through lens to form tiny inverted image on retina 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 Sympathetic stimulation widens pupil 16-22  Formation of an Image Pupillary constriction and dilation occurs: – When light intensity changes – When gaze shifts between distant and nearby objects Photopupillary reflex— pupillary constriction in response to light – Mediated by autonomic reflex arc Brighter light signaled to pretectal region of midbrain Excites parasympathetic fibers in oculomotor nerve that travels to ciliary ganglion in orbit Postganglionic parasympathetic fibers stimulate pupillary constrictor 16-23  Refraction Refraction— the bending of light rays Speed of light is 300,000 km/s in a vacuum, but slower in air, water, glass, or other media Refractive index of a medium is a measure of how much it retards light rays relative to air Angle of incidence at 90° light slows but does not change course – Any other angle, light rays change direction (are refracted) The greater the refractive index and the greater the angle of incidence, the more refraction  Refraction Light passing through center of the cornea is not bent Light striking off-center is bent toward the center Aqueous humor and lens do not greatly alter the path of light Cornea refracts light more than lens does – Lens merely fine-tunes image – Lens becomes rounder to increase refraction for near vision  The Near Response Emmetropia— state in which eye is relaxed and focused on an object more than 6 m (20 ft) away – Light rays coming from that object are essentially parallel – Rays focused on retina without effort Light rays coming from a closer object are too divergent to be focused without effort 16-26  The Near Response Near response— adjustment to close-range vision requires three processes – Convergence of eyes Eyes orient their visual axis toward object – Constriction of pupil Blocks peripheral light rays and reduces spherical aberration (blurry edges) – Accommodation of lens: change in the curvature of the lens that enables you to focus on nearby objects Ciliary muscle contracts, suspensory ligaments slacken, and lens takes more convex (thicker) shape Light refracted more strongly and focused onto retina Near point of vision— closest an object can be and still come into focus (lengthens with age) 16-27 The Near Response The Near Response The Near Response Common Defects of Image Formation  Sensory Transduction in the Retina Retina converts light energy into action potentials Structure of retina – Pigment epithelium: most posterior part of retina Absorbs stray light so visual image is not degraded – Neural components of retina (from rear forward) Photoreceptor cells— absorb light and generate a chemical or electrical signal – Rods, cones, and certain ganglion cells – Only rods and cones produce visual images Bipolar cells— (first-order neurons) have dendrites that synapse with rods and cones and axons that synapse with ganglion cells Ganglion cells— (second-order neurons) are largest neurons in the retina and are arranged in a single layer next to the vitreous body 16-32  Sensory Transduction in the Retina Light-absorbing cells – Rods and cones derive from same stem cells as ependymal cells of brain Rod cells (night, or scotopic, vision or monochromatic vision) – Outer segment: modified cilium specialized to absorb light Stack of 1,000 membranous discs studded with globular protein, the visual pigment rhodopsin – Inner segment: contains organelles (mitochondria) sitting atop cell body with nucleus  Sensory Transduction in the Retina (3) Cone cell (color, photopic, or day Back of eye vision) – Similar to rod except: Outer segment tapers to a point Discs with pigment are plasma membrane in-foldings (not detached) Sensory Transduction in the Retina Histology of the retina – Pigment epithelium – Rod and cone cells – Bipolar cells Rods and cones synapse on bipolar cells Bipolar cells synapse on ganglion cells Sensory Transduction in the Retina – Ganglion cells Single layer of large neurons near vitreous Axons form optic nerve Some absorb light with pigment melanopsin and transmit signals to first-order neurons brainstem second-order neurons – Detect light intensity for pupil control and circadian rhythms; do not contribute to visual image Sensory Transduction in the Retina 130 million rods and 6.5 million cones in retina Only 1.2 million nerve fibers in optic nerve Neuronal convergence and information processing in retina before signals reach brain – Multiple rod or cone cells synapse on one bipolar cell – Multiple bipolar cells synapse on one ganglion cell H&A cells- Enhance perception of contrast, edges of objects, moving objects, and changes in light intensity  Sensory Transduction in the Retina Horizontal cells and amacrine cells are present, but do not form separate layers within retina Horizontal and amacrine cells form horizontal connections between cone, rod, and bipolar cells – Enhance perception of contrast, edges of objects, moving objects, and changes in light intensity Much of the mass of the retina is astrocytes and other glial cells 16-38  Visual Pigments Rods contain visual pigment rhodopsin (visual purple) – Two major parts of molecule Opsin— protein portion embedded in disc membrane of rod’s outer segment Retinal (retinene)— a vitamin A derivative – Has absorption peak at wavelength of 500 nm Rods cannot distinguish one color from another 16-39  Visual Pigments Cones contain photopsin (iodopsin) protein – Retinal moiety same as in rods – Opsin moiety contains different amino acid sequences that determine wavelengths of light absorbed – Three kinds of cones, identical in appearance, but absorb different wavelengths of light to produce color vision 16-40 Visual Pigments Generating the Optic Nerve Signal (in Rods) Light changes rhodopsin: – In dark, retinal is bent (cis-retinal) and retinal and opsin are together – In light, retinal molecule straightens (trans-retinal), and retinal dissociates from opsin (bleaching) – To reset, it takes 5 minutes to regenerate 50% of bleached rhodopsin Cones function similarly – But are faster to regenerate their photopsin— 90 seconds for 50% Generating Visual Signals (in Cones) Three types of cones are named for absorption peaks of their photopsins Short-wavelength (S) cones peak sensitivity at 420 nm Medium-wavelength (M) cones peak at 531 nm Long-wavelength (L) cones peak at 558 nm Ganglion cells are the only retinal cells that produce action potentials  Generating the Optic Nerve Signal In dark, rods steadily release the neurotransmitter glutamate from basal end of the bipolar cell When rods absorb light, glutamate (inhibitory NT) secretion ceases Bipolar cells are sensitive to these on-and-off pulses of glutamate secretion – Some bipolar cells are inhibited by glutamate and excited when secretion stops That is: these cells are excited by rising light intensities – Other bipolar cells are excited by glutamate and respond when light intensity drops 16-44  Generating the Optic Nerve Signal When bipolar cells detect fluctuations in light intensity, they stimulate ganglion cells directly or indirectly Ganglion cells are the only retinal cells that produce action potentials Ganglion cells respond to the bipolar cells with rising and falling firing frequencies Via optic nerve, these changes provide visual signals to the brain 16-45  Light and Dark Adaptation Light adaptation (walk out into sunlight) – Pupil constriction reduces light intensity (and any discomfort that may accompany sudden brightness) – Color vision and acuity below normal for 5 to 10 minutes – Time needed for light intensity pigment bleaching to adjust retinal sensitivity to high – Rods quickly bleach and become nonfunctional; cones take over Dark adaptation (turn lights off) – Dilation of pupils occurs – In the dark, rhodopsin of rods is regenerated – In 1 to 2 minutes, night (scotopic) vision begins to function – After 20 to 30 minutes, amount of regenerated rhodopsin is sufficient for eyes to reach maximum sensitivity 16-46  The Dual Visual System Duplicity theory of vision explains why we have both rods and cones – A single type of receptor cannot produce both high sensitivity and high resolution It takes one type of cell and neural circuit for sensitive night vision It takes a different cell type and neuronal circuit for high-resolution daytime vision 16-47  The Dual Vision System Results in high degree of spatial Each foveal cone cell has “private line to brain” summation High-resolution color vision One ganglion cell receives information Little spatial summation: less sensitivity to dim from 1 mm2 of retina producing only light a coarse image  The Dual Visual System Rods sensitive— react even in dim light – Extensive neuronal convergence – 600 rods converge on one bipolar cell – Many bipolar cells converge on each ganglion cell – Results in high degree of spatial summation One ganglion cell receives information from 1 mm2 of retina producing only a coarse image Edges of retina have widely spaced rod cells that act as motion detectors – Low-resolution system only – Cannot resolve finely detailed images 16-49  The Dual Visual System Fovea contains only 4,000 tiny cone cells (no rods) – No neuronal convergence – Each foveal cone cell has “private line to brain” High-resolution color vision – Little spatial summation: less sensitivity to dim light 16-50  Color Vision Primates have well-developed BS GM RL color vision – Nocturnal vertebrates have only rods Three types of cones are named for absorption peaks of their photopsins – Short-wavelength (S) cones peak sensitivity at 420 nm – Medium-wavelength (M) cones peak at 531 nm – Long-wavelength (L) cones peak at 558 nm Color perception based on mixture of nerve signals representing cones of different absorption peaks  Color Vision (Isahihara color vision test) Color blindness—have a hereditary alteration or lack of one photopsin or another Most common is red–green color blindness – Results from lack of either L or M cones – Causes difficulty distinguishing these related shades from each other – Occurs in 8% of males, 0.5% of females (sex linkage)  Stereoscopic Vision Stereoscopic vision is depth perception— ability to judge distance to objects – Requires two eyes with overlapping visual fields which allows each eye to look at the same object from different angles – Unlike panoramic vision— having eyes on sides of head (horse or rodents are alert to predators but have no depth perception) 16-53  Distant object Stereoscopic Vision D Fixation point— point in space on which the eyes are Fixation focused point F – Looking at object within 100 feet, each eye views Near object from slightly different N angle – Provides brain with information used to judge position of objects relative to fixation point N N F D D F  The Visual Projection Pathway Bipolar cells of retina are first-order neurons Retinal ganglion cells are second-order neurons whose axons form optic nerve – Two optic nerves combine to form optic chiasm – Half the fibers cross over to the opposite side of the brain (hemidecussation) and chiasm splits to form optic tracts Right cerebral hemisphere 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 the side where it has motor control over limbs 16-55  The Visual Projection Pathway 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 the cerebrum – Project to primary visual cortex of occipital lobe where conscious visual sensation occurs – A few optic nerve fibers project to midbrain and terminate in the superior colliculi and pretectal nuclei Superior colliculi controls visual reflexes of extrinsic eye muscles Pretectal nuclei are involved in photopupillary and accommodation reflexes 16-56  The Visual Projection Pathway Uncrossed Crossed Optic radiation (ipsilateral) (contralateral) fiber fiber Object location, motion, color, Right eye shape, boundaries Store visual memories (recognize printed words) Fixation Hemidecussation Occipital lobe point (visual cortex) Left eye Controls visual reflexes of extrinsic eye muscles Optic Optic Pretectal Lateral Superior nerve chiasm nucleus geniculate colliculus Optic tract nucleus of thalamus Photopupillary and accommodation reflexes  The Visual Projection Pathway (4) Some processing begins in retina – Adjustments for contrast, brightness, motion, and stereopsis Primary visual cortex is connected by association tracts to visual association areas in parietal and temporal lobes which process retinal data from occipital lobes – Object location, motion, color, shape, boundaries – Store visual memories (recognize printed words) 16-58