Special Senses The Eye and Vision Important Concepts Describe the structures of the eye and the role of each structure in vision. Trace the pathway for vis ion from the retina to the visual cortex. Explain how photoreceptors convert light energy into action potentials. Explain signal processing in the retina and in the visual cortex. Anatomy of the Eye Protected in orbits by the bones of the skull Upper and lower eyelids Lacrimal apparatus washes with tears Pupil is an opening that widens and shrinks when pupillary muscles contract Colored ring of pigment is the iris Anatomy of the Eye Two chambers in the eye In front of the lens filled with aqueous humor covered by cornea Behind the lens, larger vitreous chamber filled with vitreous body (humor) Retina layer lines the back of the eye and contains photoreceptors Optic disk (blind spot) is location where neurons join into the optic nerve Optic nerves cross over in the optic chiasm Optic tracts end at the visual cortex in the occipital lobe 1
Lacrimal gland secretes tears. Muscles attached to external surface of eye control eye movement. Sagittal Section of the Eye Upper eyelid Sclera Pupil Zonules: attach lens to ciliary muscle Lens bends light to focus it on the retina. Iris Lower eyelid Canal of Schlemm Aqueous humor Cornea Pupil changes amount of light entering the eye. Iris Optic dis k (blind spot): region whe re optic ne rv e a nd blood vessels leave the eye Ce ntra l retinal artery and vein emerge from center of optic disk. Optic ne rv e Fovea: re gion of sharpest vision Vitreous chamber The orbit is a bony cavity Nasolacrimal duct drains that protects the eye. tears into nasal cavity. Retina: layer that contains photoreceptors Sclera is connective tissue. The Eye Lens shape is adjusted by contraction and relaxation of the ciliary muscle. 2
The Eye and Vision Light enters the eye Focused on retina by the lens Photoreceptors transduce light energy into electrical signal Neural pathways process electrical signals into visual images The Pupil Light enters the eye Size of the pupil modulates the amount of light that reaches photoreceptors Shape of lens focuses the light Pupillary reflex is a consensual reflex Standard part of neurological examination Constrict in response to parasympathetic fibers in bright light Dilate in response to sympathetic nervous system in dim light The Visual Pathway via CNII Figure 10.26c Pathways for vision and the pupillary reflex Optic Optic ner ve chiasm Optic Later al geniculate tr act body (thalamus) Visual cortex (occipital lobe) Collateral pathways leave the thalamus and synapse in the midbrain to control constriction of the pupils. Light Eye Midbr ain Cranial nerve III controls pupillary constriction. 3
The Lens Figure 10.27ab Optics of the Eye Optics describes light behavior and properties Light passing through a curved surface will bend or refract. Light entering eye is refracted, or bent At the cornea and lens Refraction influenced by the angle at which light meets the lens A concave lens scatters light rays. Concave lens Parallel light rays A convex lens causes light rays to converge. Conve x lens Focal point Parallel light rays Focal length The focal length of the lens is the distance from the center of the lens to the focal point. Accommodation Accommodation Process by which the eye adjusts lens shape to keep objects in focus Near point of accommodation is the closest distance at which the lens can focus an object Myopia Focal point falls in front of the retina Hyperopia Focal point falls behind the retina Presbyopia is loss of accommodation Astigmatism Usually caused by a cornea that is not a perfectly shaped dome, resulting in distorted images 4
Accommodation Focusing on objects at different distances requires changing the shape of the lens: flatter lens for distant objects, more rounded (convex) lens for close objects Figure 10.27 Optics of the Eye C ommon vi si on defects can be corrected wi th external l enses. Hyperopia, or far-si ght edness, occur s when the focal point falls behi nd t he r et i na. Hyper opi a ( corr ect ed with a convex lens) Myopia, or near - si ght edness, occur s when the focal point falls in front of the retina. Myopia (corrected with a concave l ens) Phototransduction Converts light energy into electrical signals Photoreceptors Rods and cones Modified ganglion cells contain melanopsin to respond to changing light cues Most acute vision occurs at the fovea and macula Optic disk has no photoreceptors and is called the blind spot Figure 10.29a The Retina Fixation point Dorsal view of a section of the right eye Light Lens Retina Fovea Macula Optic nerve 5
Figure 10.29b The Retina Figure 10.29c The Retina Axons from the retina exit via the optic nerve. The projected image is upside down on the retina. Visual processing in the brain reverses the image. Optic nerve Sclera Fovea The choroid layer contains blood vessels. Pigment epithelium Neural cells of retina Figure 10.29d The Retina Convergence in the retina Light strikes the photoreceptors in the fovea directly because overlying neurons are pushed aside. To optic nerve Bipolar cell Rod Pigment epithelium Pigment epithelium of retina absorbs excess light. Light Ganglion cell Fovea Cone Rod Bipolar neuron Ganglion cell Neural cells of retina FIGURE QUESTION How many rods converge on the ganglion cell in (e)? 6
Photoreceptors Rods function well in low light and are used in night vision Cones are responsible for high-acuity vision and color vision during the daytime Outer, inner, and basal segments Photoreceptors Visual pigments convert light energy into a change in membrane potential Rods contain rhodopsin PIGMENT EPITHELIUM Me l a n i n g ran u l e s OUTER SEGMENT Light transduction takes place in the outer segment of the photoreceptor using visual pigments in membrane disks. Disk s Connec ting stalks The dark pigment epithelium absorbs extra light and prevents that light from reflecting back and distorting vision. Old dis k s a t tip a re phagocytized by pigment epithelial cells. Disk s Cones contain three pigments primary excited by red, green, and blue light. Color-blindness INNER SEGMENT Location of major organelles and metabolic operations, such as photopigment synthesis and ATP production Mi to c h o n d ri a Cone Rods Rhodopsin molecule Re tinal Ops in SYNAPTIC TERMINAL Synapses with bipolar cells. Bipolar ce ll LIGHT 7
Figure 10.31 Light absorption by visual pigments Light absor ption (percent of maximum ) 100 75 50 25 0 Blue cones Rods Gr een cones Red cones Disk Transducin (G protein) Inactive rhodopsin (opsin and retinal) In darkness, rhodopsinis inactive, cgmp is high, and CNG and K + channels are open. Pigment epithelium cell cgmp levels high CNG c hannel open K + Ca 2+ Na + Light bleaches rhodopsin. Ops in decreases cgmp, closes CNG channels, and hyperpolarizes the cell. Ac tiva te d re tinal Ops in (bleachedac tiva te s pigment) transducin Ca s c a de Decreased cgmp CNG c hannel closes K + Ca 2+ Na + In the recovery phase, retinal recombines with opsin. Re tinal conve rte dto inactive form Retinal recombines with opsin to form rhodopsin. Violet Blue Gr een Yellow Or ange Red Me m b ran e p o te n ti a l in dark = 40mV Me m b ran e hyperpolarizes to 70 mv GRAPH 400 450 500 550 600 650 700 Wavelength (nm) QUESTIONS 1. Which pigm ent absor bs light over the br oadest spectr um of wavelengths? 2. Over the nar r owest? 3. Which cone pigm ent absor bs the m ost light at 500 nm? Rod Tonic release of neurotransmitter onto bipolar neurons Light Neurotransmitter release decreases in proportion to amount of light. FIGURE QUESTION One rod c ontains about 1 0,00 0 CNG channels open in the dark. One photon of light ac tiva tes one rhodopsin. Ea c h rhodopsin activates 800 transducin. Each transducin cascade removes 6 cgmp. A decrease of 24 cgmp closes one CNG channel. How many photons are needed to close all the CNG channels inone rod? Signal Processing Photoreceptor cells converge in bipolar neurons Multiple bipolar neurons converge onto one ganglion cell Horizontal cells synapse with photoreceptors and bipolar cells Signal Processing Amacrine cells modulate information between bipolar and ganglion cells The central portion of the visual field is the binocular zone Processed to give 3D vision 8
Figure 10.33c Visual fields The retina uses contrast rather than absolute light intensity for better detection of weak stimuli. Visual Field Type Field Is On-Center/Off-Surround Field Is Off-Center/On-Surround On-center, off-surround Figure 10.34 Binocular vision Binocular zone is whe re le ft a nd right visual fields overlap. Monocular zone is the portion of the visual field associated with only one eye. Left visual field Visual field Binocular zone Right visual field Bright light onto center Ganglion cell is excited by light in the center of the visual field. Ganglion cell is inhibited by light in the center of the visual field. Off-center, on-surround Bright light onto surround Ganglion cell is inhibited by light on the surround of the visual field. Ganglion cell is excited by light on the surround of the visual field. Optic c hia sm Optic ne rv e Bright light onto surround Both field types Optic tra c t Diffuse light on both center and surround Ganglion cell responds we a k ly. Ganglion cell responds we a k ly. Lateral geniculate body (thalamus) 2016 Pearson Visual Education, cortex Inc. Test Your Knowledge An area of the retina that contains only cones and is the site of sharpest vision is the A) outer segment. B) inner segment. C) fovea. D) optic disc. E) tapetum lucidum. 9
An area of the retina that contains only cones and is the site of sharpest vision is the A) outer segment. B) inner segment. ØC) fovea. D) optic disc. Accommodation describes the focusing of light on the retina by changing A) the shape of the lens. B) pupillary diameter. C) the shape of the cornea. D) distance of the retina from the lens. E) tapetum lucidum. Accommodation describes the focusing of light on the retina by changing ØA) the shape of the lens. B) pupillary diameter. C) the shape of the cornea. D) distance of the retina from the lens. Which of the following cells involved in processing visual information in the retina synapse with the rods? A) horizontal cells only B) ganglion cells only C) bipolar cells only D) horizontal and bipolar cells only E) horizontal, ganglion, and bipolar 10
Which of the following cells involved in processing visual information in the retina synapse with the rods? A) horizontal cells only B) ganglion cells only C) bipolar cells only D) horizontal and bipolar cells only ØE) horizontal, ganglion, and bipolar Photoreceptors secrete the neurotransmitter A) acetylcholine. B) norepinephrine. C) glutamate. D) dopamine. Photoreceptors secrete the neurotransmitter A) acetylcholine. B) norepinephrine. ØC) glutamate. D) dopamine. 11