9 Chapter Properties of Light 1. Light a. Wavelength distance between successive peaks or troughs b. Frequency - number of waves per second c. Amplitude difference between wave trough and peak d. Radiation is proportional to its frequency e. Radiation emitted at high frequency (short wavelengths) has highest energy content f. Radiation at emitted at lower frequencies (longer wavelengths) has less energy 2. Optics a. Study of light rays and interactions is called optics b. Reflection bouncing of light rays off a surface c. Absorption transfer of light energy to a particle or surface i. Black absorbs energy of all visible wavelengths d. Refraction bending of light rays that can occur when they travel from one transparent medium to another Structure of the Eye 1. Gross anatomy a. Pupil - opening that allows light to enter the eye and reach retina b. Iris pigmentation provides what we call the eye's color i. surrounds pupil ii. contains two muscles that can vary the size of the pupil c. cornea glossy transparent external surface that covers iris and pupil d. sclera the white of the eye, forms the tough wall of the eyeball e. Extraocular muscles move the eyeball in the orbit f. Conjuctiva membrane that folds back from the inside of the eyelids and attached to the sclera g. Optic nerve carrying axons from the retina, exit the back of the eye, pass through the orbit and reaches the base of the brain near the pituitary gland 2. Ophthalmoscopic Appearance of the Eye a. Optic disk pale circular region where optic nerve fibers exit retina i. Sensation of light cannot occur because no photoreceptors here 1. Neither near large blood vessels because they cause shadows on the retina b. Macula part of the retina for central vision i. no large blood vessels in this region of the retina is one of the specializations that improves quality of central vision c. Fovea dark spot about 2mm in diameter and retina thinner here than anywhere else i. convenient anatomical reference point 3. Cross-sectional anatomy of the eye a. Aqueous humor fluid the nourishes the cornea
b. Lens- suspended by ligaments (Called zonule fibers) attached to the ciliary muscles, which are attached to the sclera and form a ring inside the eye c. Vitreous humor (viscous and jellylike) lies between the lens and the retina; its pressure serves to keep the eyeball spherical Image formation by the eye 1. refraction by the cornea a. Light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye b. Those that enter the center of the eye pass straight to the retina c. Focal distance distance from the refractive surface to the point where parallel light rays converge i. Depends on curvature of the cornea- the tighter the curve, the shorter the focal distance 2. Accommodation by the lens a. Lens involved more importantly in forming crisp images of objects located closer than about 0m from the eye b. greater refractive power is required to bring rays into focus on the retina i. additional focusing power is provided by changing the shape of the lens, a process called accommodation ii. during accommodation, ciliary muscle contracts and swells in size, thereby making the area inside the muscle smaller and decreasing the tension in the suspensory ligaments 1. lens become rounder and thicker which increase the curvature of the lens surfaces, thereby increasing their refractive power 2. relaxation of the ciliary muscle increases the tension in the suspensory ligaments and the lens is stretched into a flatter shape 3. Pupillary light reflex a. Involves connections between the retina and neurons in the brain stem that control the muscle that constrict the pupils b. It is consensual, shining a light into only one eye causes the constriction of the pupils in both eyes i. Lack of consensual papillary light reflex is often taken as a sign of a serious neurological disorder involving the brain stem c. Constriction of the pupil has the effect of increasing the depth of focus 4. Visual Field a. Image is inverted; left visual field is imaged on the right side of the retina, and the right visual field is imaged on the left side of the retina Microscopic Anatomy of the Retina 1. Most direct pathway is from photoreceptors to bipolar cells to ganglion a. Ganglion cells fire action potentials in response to light and these impulses propagate down the optic nerve to the rest of the brain
b. Horizontal cells receive input from the photoreceptors and project neuritis laterally to influence surrounding bipolar cells and photoreceptors c. Amacrine cells receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells d. Only light sensitive cells in the retina are the photoreceptors e. Galglion cells are the only source of output from the retina 2. Laminar organization of the retina a. Cells are organized in layers b. Light must pass through vitreous humor, through ganglion cells and bipolar cells before it reaches the photoreceptors i. Inside-out arrangement is advantageous because the pigmented epithelium that lies below the photoreceptors plays a critical role in the maintenance of the photoreceptors and photopigments ii. Pigmented epithelium also absorbs any light that passes entirely through the retina, thus minimizing the reflection of light within the eye that would blur the image c. Innermost layer is ganglion cell layer, then inner nuclear layer, which contains cell bodies of the bipolar cells, the horizontal and amacrine cells, then outer nuclear layer which contains the cell bodies of the photoreceptors, then layer of photoreceptor outer segments contains the light-sensitive elements of the retina i. Outer segments are embedded in the pigmented epithelium ii. Inner plexiform layer lies between the ganglion cell layer and inner nuclear layer and contains the synaptic contacts between bipolar cells, amacrine cells and ganglion cells iii. Outer plexiform layer lies between outer and inner nuclear layers and is where photoreceptors make synaptic contact with the bipolar and horizontal cells 3. Photoreceptor Structure a. Every photoreceptor has four regions: an outer segment, an inner segment, a cell body and a synaptic terminal b. Photopigments in the disk membranes absorb light thereby triggering changes in the photoreceptor membrane potential c. Rod photoreceptors have a long cylindrical outer segment containing many disks d. Cone photoreceptors have a shorter, tapering outer segment with fewer membranous disks e. Rods are 1000X more sensitive to light than cones are because of greater number of disks i. In nighttime (scotopic) conditions, rods contribute to vision ii. Under daytime lighting (photopic) conditions, cones do the bulk of the work
f. Retina is duplex a scotopic retina using only rods and a photopic retina using mainly cones g. Only cones are responsible for color 4. Regional differences in the retinal structure a. Peripheral retina has a higher ratio of rods to cones and higher ratio of photoreceptors to ganglion cells i. More sensitive to light because rods are specialized for low light and there are more photoreceptors feeding information to each ganglion cell 1. Good visual acuity requires a low ratio of photoreceptors to ganglion cells b. Region of retina most highly specialized for high-resolution vision is the fovea i. Pitlike appearance is due to the lateral displacement of the cells above the photoreceptors, allowing light to strike the photoreceptors without passing through the other retinal cell layers ii. Central fovea contains no rods Phototransduction 1. Phototransduction in rods a. Light stimulation of the photopigment activates G-proteins, which in turn activate an effector enzyme that changes the cytoplasmic concentration of a second messenger molecule i. Causes a membrane ion channel to close and the membrane potential is thereby altered b. In complete darkness, the membrane potential of the rod outer segment is about -30mV i. Depolarization is caused by the steady influx of Na+ through special channels in the outer segment membrane ii. Movement of positive charge across the membrane, which occurs in the dark, is called dark current iii. Sodium channels are stimulated to open by an intracellular second messenger called cyclic guanosine monophosphate (cGMP) 1. cGMP is continually produced in the photoreceptor by the enzyme guanylyl cyclase, keeping the Na+ channels open 2. light reduces cGMP causing the Na+ channels to close, and the membrane potential becomes more negative 3. therefore, photoreceptors hyperpolarize in response to light a. Initiated by the absorption of the electromagnetic radiation by the photopigment in the membrane of the stacked disks in the rod's outer segments i. In rods, this pigment is rhodopsin 1. receptor with protein a prebound chemical agonist
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a. prebound agonist called retinal, a derivative of vitamin A i. absorption of ligh causes change in the conformation of retinal so that it activates opsin ii. called bleaching because it changes the wavelengths absorbed by the rhodopsin receptor protein is opsin and has seven transmembrane alpha helices typical of G-protein-coupled receptors throughout the body bleaching of rhodopsin stimulates Gprotein called transducin in the disk membrane which in turn activates the effector enzyme phosphodiesterase (PDE) which breaks down the cGMP that is normally present in the cytoplasm of the rod (in the dark) reduction of cGMP causes the Na+ channels to close and the membrane to hyperpolarize amplification leads to many gproteins being activated from one photopigment molecule and each PDE enzyme breaks down more than one cGMP molecule gives visual system ability to detect a single photon
2. Phototransduction in cones a. in sunlight, cGMP levels in rods fall to the point of saturation where additional light causes no more hyperpolarization b. cones require more energy to become bleached, so vision during day is dependent on cones c. process of phototransduction is nearly same as rods except major difference is in type of opsins in the membranous disks of the cone outer segments i. three opsins that give photopigments different spectral sensitivities 1. blue cones maximally activated at 430nm 2. green cones at 530nm 3. red cones at 560nm
d. color detection i. color perceived is largely determined by relative contributions of the blue, green, and red cones to the retinal signal ii. when all three cones are equally active, white is perceived iii. peak sensitivity of the rods is to a wavelength of about 500nm, about blue-green under photopic conditions e. dark and light adaptation i. transition from all-cone daytime vision to all-rod nighttime vision takes about 20-25minutes 1. this is called dark adaptation 2. dilation of the pupils allows more light to enter the eye a. changes in its size can increase pupil area by a factor of only 16 3. larger component of dark adaptation involves the regeneration of unbleached rhodopsin and adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell ii. Light adaptation 1. reversing the changes in the retina that accompanied dark adaptation 2. light-dark adaptation in the duplex retina gives our visual system the ability to operate in light intensities ranging from moonless midnight to bright high noon f. Calcium's role in light adaptation i. Light level adaptation by eye relies on calcium concentrations within the cones ii. When from dark room to bright light, cones are hyperpolarized as much as possible 1. constriction of pupil helps a bit in reducing light entering the eye 2. most important change however, is gradual depolarization of the membrane back to about -35mV a. cGMP-gated sodium channels also admit calcium b. in dark, calcium enters the cones and has an inhibitory effect on the enzyme that synthesizes cGMP c. when cGMP-gated channels close, flow of calcium into the photoreceptor is curtailed; as a result, more cGMP is synthesized (because the synthetic enzyme is less inhibited), thereby allowing the cGMP-gated channels to open again i. when channels close, a process is initiated that gradually reopens them even if the high level does not change
d. calcium affects photopigments and phophodiesterase in way that decrease their response to light e. calcium based mechanisms ensure that the photoreceptors are always able to register relative changes in light level, though info about the absolute level is lost Retinal Processing 1. transformations in the outer plexiform layer a. photoreceptors release NT when depolarized b. photoreceptors are depolarized in the dark and hyperpolarized by light c. bipolar cells create the direct pathway from photoreceptors to ganglion cells i. horizontal cells feed information laterally in the outer plexiform layer to influence the activity of neighboring bipolar cells and photoreceptors 2. bipolar cell receptive fields a. OFF bipolar cells i. Glutamate-gated cation channels mediate a classical depolarizing EPSP from the influx of Na+ b. ON biopolar cells have G-protein-coupled receptors and respond to glutamate by hyperpolarizing c. Each bipolar cell receives direct synaptic input from a cluster of photoreceptors i. # of photoreceptors in this cluster ranges from one at the center of the fovea to thousands in the peripheral retina d. receptive field of a bipolar cell (or any other cell in the visual system) is the area of retina that, when stimulated with light, changes the cell's membrane potential i. two parts 1. circular area of retina providing direct photoreceptor input called the receptive field center 2. surrounding area of retina providing input via horizontal cells, called the receptive field surround e. response of a bipolar cell's membrane potential to light in the receptive field center is opposite to that of light in the surround i. if illumination of the center causes depolarization of the bipolar cell (an ON response) then illumination of the surround will cause an antagonistic hyperpolarization of the bipolar cell ii. if cell is depolarized by a spot turning from light to dark in the center of its receptive field (an OFF response), it will be hyperpolarized by the same dark stimulus applied to the surround iii. these cells are said to have center-surround receptive fields 1. antagonistic surround appears to come from a complex interaction of horizontal cell, photoreceptors and bipolar cells at their synapses
Retinal output 1. ganglion cell receptive fields a. ON-center and OFF-center ganglion cells receive input from the corresponding type of bipolar cell b. An ON-center ganglion cell will be depolarized and respond with a barrage of action potentials when a small spot of light is projected onto the middle of its receptive field c. An OFF-center cell will respond to a small dark spot presented to the middle of it receptive field d. Response to stimulation of the center is cancelled by the response to stimulation of the surround e. Most retinal ganglion cells are not particularly responsive to changes in illumination that include both the receptive field center and the receptive field surround i. It appears that the ganglion cells are mainly responsive to difference in illumination that occur within their receptive fields 2. types of ganglion cells a. large m-type ganglion cells i. 5% of the ganglion cell population ii. Respond to stimulatin of their receptive field centers with a transient burst of action potentials more rapidly in the optic nerve iii. Move sensitive to low-contrast stimuli b. small p-type ganglion cells i. 90% of the ganglion cell population ii. Respond with a sustained discharge as long as the stimulus is on c. nonM-nonP ganglion cell i. 5% of the ganglion cell population d. Color-opponent ganglion cells i. Some P cells and some nonM-nonP cells are sensitive to differences in the wavelength of light 1. majority of these are color-opponent cells 2. response to one wavelength in the receptive field center is canceled by showing another wavelength in the receptive field surround 3. two types of opponency area found, red versus green and blue versus yellow a. cell with red ON center and a green OFF surround i. center of the receptive field is fed mainly by red cones; therefore, the cell responds to red light by firing action potentials ii. excited by red in the receptive field center and inhibited by green in the surround iii. because white light contains all visible wavelengths, both center and surround would be equally activated, thereby canceling the response of the cell
b. blue ON center and yellow OFF surround i. blue light would be an effective stimulus but yellow on the surround would cancel the response ii. perceived colors is based on the relative activity of ganglion cells whose receptive field enters receive input from red, green, and blue cones 3. parallel processing a. in central visual system, these streams are compared to give information about depth, the distance of an object from the observer b. there appear to be independent streams of info about light and dark that arise from the ONcenter and OFF-center ganglion cells in each retina c. ganglion cells of both ON and OFF varieties have different type of receptive fields and response properties d. M cells can detect subtle contrasts over their large receptive fields and are likely to contribute to low-resolution vision e. P cells have small receptive fields that are well suited for the discrimination of fine detail f. P cells and nonM-nonP cells are specialized for the separate processing of red-green and blue-yellow information