The Eye Chapter 9 PDF - Fall 2024

The Eye Chapter 9   Discuss the anatomy of the eye & retina  Describe the process of phototransduction Learning  Learn about how visual information is sent from objectives photoreceptors to the ganglion cells  Briefly describe how visual informatio...

The Eye Chapter 9   Discuss the anatomy of the eye & retina  Describe the process of phototransduction Learning  Learn about how visual information is sent from objectives photoreceptors to the ganglion cells  Briefly describe how visual information is processed from ganglion cells to the brain   The special senses, such as vision, all use specialized & distinct receptor cells within the head  Unlike touch which is a general sense that uses modified nerves  70% of body’s sensory receptors are in eye!  Half of cerebral cortex is involved in visual processing Vision   Six straplike extrinsic eye muscles  Originate from bony orbit and insert on eyeball  Enable eye to follow moving objects, maintain shape of eyeball, and hold it in orbit  Four rectus muscles originate from common tendinous ring; names indicate movements  Superior, inferior, lateral, and medial rectus  Two oblique muscles move eye in vertical plane and rotate eyeball Extrinsic eye  Superior and inferior oblique muscles muscles Extrinsic eye muscles   If the eye muscles need to coordinate with each other in order to help your eyes move around properly, what might happen if you have damage to any of the eye muscles?  Diplopia (double vision): occurs when movements of external muscles of two eyes are not perfectly coordinated  Person cannot properly focus images of same area of the visual field from each eye, so sees two images instead of one  Can result from paralysis, extrinsic muscle weakness, or neurological disorders  Strabismus (“cross-eye”): congenital weakness of external eye muscles Clinical  Eye rotates medially or laterally  Eyes may alternate focusing on objects, or only controllable eye is used connection:  Brain begins to disregard inputs from deviant eye, which can become functionally blind if not treated early eye muscles Anatomy of the eyeball   Very important to test for during emergency neurological assessments Pupil constriction & dilation   Retina originates as an outpocketing of brain  Contains:  Millions of photoreceptor cells that transduce light energy  Neurons  Glial cells  Delicate two-layered membrane  Outer pigmented layer (absorbs light, stores vitamin A, helps phagocytize waste)  Inner neural layer Retina anatomy   Neural layer of the retina  Composed of three main types of neurons  Photoreceptors, bipolar cells, ganglion cells  Signals spread from photoreceptors to bipolar cells to ganglion cells  Ganglion cell axons exit eye as optic nerve Retina anatomy   Optic disc  Site where optic nerve leaves eye  Lacks photoreceptors, so referred to as blind spot  Retina has quarter-billion photoreceptors that are one of two types:  Rods  Cones Retina anatomy   Rods  Dim light, peripheral vision receptors  More numerous and more sensitive to light than cones  No color vision or sharp images  Numbers greatest at periphery Photorecepto  Cones  Vision receptors for bright light rs in the  High-resolution color vision  Macula lutea area at posterior pole retina lateral to blind spot  Contains mostly cones  Fovea centralis: tiny pit in center of macula lutea that contains all cones, so is region with best visual acuity  Eye movement allows us to focus in on object so that fovea can pick it up   Rods  Dim light, peripheral vision receptors  More numerous and more sensitive to light than cones  No color vision or sharp images  Numbers greatest at periphery Photorecepto  Cones  Vision receptors for bright light rs in the  High-resolution color vision  Macula lutea area at posterior pole retina lateral to blind spot  Contains mostly cones  Fovea centralis: tiny pit in center of macula lutea that contains all cones, so is region with best visual acuity  Eye movement allows us to focus in on object so that fovea can pick it up Clinical connection: the retina   The lens helps to focus light onto the retina in the eye Focusing light in the eye   Lens is able to adjust Focusing light its curvature to allow for fine focusing in the eye  Can focus for distant vision and for close vision Clinical connection: focusing light in the eye  Photoreceptors (rods and cones) are modified neurons that resemble upside-down epithelial cells  Consists of cell body, synaptic terminal, and two segments: Photoreceptors in  Outer segment: light-receiving region  Contains visual pigments the retina (photopigments) in many layers of discs that change shape as they absorb light  Inner segment of each joins cell body  Inner segment is connected via cilium to outer segment and to cell body via outer fiber  Photoreceptors (rods and cones) are modified neurons that resemble upside-down epithelial cells  Consists of cell body, synaptic terminal, and two segments: Photoreceptors in  Outer segment: light-receiving region  Contains visual pigments the retina (photopigments) in many layers of discs that change shape as they absorb light  Inner segment of each joins cell body  Inner segment is connected via cilium to outer segment and to cell body via outer fiber   Rods are very sensitive to light, making them best suited for night vision and peripheral vision  Contain a single pigment, so vision is perceived in gray tones only  Pathways converge, causing fuzzy, indistinct images  As many as 100 rods may converge into one ganglion Photoreceptors in  Cones have low sensitivity, so require the retina bright light for activation  React more quickly than rods  Have one of three pigments, which allow for vividly colored sight – red, green, blue  Nonconverging pathways result in detailed, high-resolution vision  Some cones have their own ganglion cell, so brain can put together accurate, high- acuity resolution images Photoreceptors in the retina  There are about 100 million rods and 7 million cones in the human eye!   What would happen if you had a mutation in your cone cell development genes?  Color blindness: lack of one or more cone pigments  Inherited as an X-linked condition, so more common in males  As many as 8–10% of males have some form  The most common type is red-green, in which either red cones or green cones are absent Clinical  Depending on which cone is missing, red can appear green, or vice versa  Rely on different shades to get cues of color connection: rods and cones  1. Photopigments detect & capture light (phototransduction) Light 2. Biochemical signaling cascade is activated information is (phototransduction) to depolarize or hyperpolarize photoreceptor cell transmitted & 3. Synaptic communication occurs along the retina towards processed in the direction of the brain from: 3 steps: photoreceptor cells  bipolar cells  ganglion cells Ganglion cells ultimately send action potentials to the brain   Phototransduction: process by which pigment captures photon of light energy, which is converted into a graded receptor potential  Retinal: key light-absorbing molecule that combines with one of four proteins (opsins) to form visual pigments Retinal  Synthesized from vitamin A  Four opsins are rhodopsin (found in rods pigments only), and three found in cones: green, blue, red (depending on wavelength of light they absorb)  Cone wavelengths do overlap, so same wavelength may trigger more than one cone, enabling us to see variety of hues of colors  Example: yellow light stimulates red and green cones, but if more red are triggered, we see orange   Retinal isomers are different 3-D forms  Retinal is in a bent form in dark, but when pigment absorbs light, it straightens out  Bent form called 11-cis-retinal  Straight form called all-trans-retinal  Conversion of bent to straight initiates reactions that lead to electrical Retinal impulses along optic nerve pigments   Phototransduction is similar in rods and cones, but different types of opsins in cones require more intense light  We will discuss the processing How retinal of light by rod cells, starting pigments with the 3 steps of the pigment cycle: capture light 1. Pigment synthesis  Opsin and 11-cis retinal combine to form rhodopsin in the dark  1. Pigment synthesis  Opsin and 11-cis retinal combine to form rhodopsin in the dark How retinal 2. Pigment bleaching pigments  When rhodopsin absorbs light, 11-cis isomer of retinal capture light changes to all-trans isomer  Retinal and opsin separate (rhodopsin breakdown)  1. Pigment synthesis  Opsin and 11-cis retinal combine to form rhodopsin in the dark 2. Pigment bleaching  When rhodopsin absorbs How retinal light, 11-cis isomer of retinal changes to all-trans isomer pigments  Retinal and opsin separate capture light (rhodopsin breakdown) 3. Pigment regeneration  All-trans retinal converted back to 11-cis isomer  Rhodopsin is regenerated in the outer segments (discs) of rod cells  1. Photopigments detect & capture light (phototransduction) Light 2. Biochemical signaling cascade is activated information is (phototransduction) to depolarize or hyperpolarize photoreceptor cell transmitted & 3. Synaptic communication occurs along the retina towards processed in the direction of the brain from: 3 steps: photoreceptor cells  bipolar cells  ganglion cells Ganglion cells ultimately send action potentials to the brain Phototransduction biochemical pathway  1. Rhodopsin absorbs light; retinal isomer changes shape & activates Phototransduction biochemical pathway  1. Rhodopsin absorbs light; retinal isomer changes shape & activates 2. Transducin (a G protein) is activated by the all-trans retinal configuration Phototransduction biochemical pathway  1. Rhodopsin absorbs light; retinal isomer changes shape & activates 2. Transducin (a G protein) is activated by the all-trans retinal configuration 3. Transducin binds to and activates the enzyme phosphodiesterase (PDE) Phototransduction biochemical pathway  1. Rhodopsin absorbs light; retinal isomer changes shape & activates 2. Transducin (a G protein) is activated by the all-trans retinal configuration 3. Transducin binds to and activates the enzyme phosphodiesterase (PDE) 4. Phosphodiesterase deactivates cyclic GMP (cGMP), a 2nd messenger protein Phototransduction biochemical pathway  1. Rhodopsin absorbs light; retinal isomer changes shape & activates 2. Transducin (a G protein) is activated by the all-trans retinal configuration 3. Transducin binds to and activates the enzyme phosphodiesterase (PDE) 4. Phosphodiesterase deactivates cyclic GMP (cGMP), a 2nd messenger protein 5. cGMP-gated cation channels close, causing hyperpolarization Phototransduction biochemical pathway  1. Photopigments detect & capture light (phototransduction) Light 2. Biochemical signaling cascade is activated information is (phototransduction) to depolarize or hyperpolarize photoreceptor cell transmitted & 3. Synaptic communication occurs along the retina towards processed in the direction of the brain from: 3 steps: photoreceptor cells  bipolar cells  ganglion cells Ganglion cells ultimately send action potentials to the brain photoreceptor cells  bipolar cells  ganglion cells  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light  Darkness Light Synaptic communicatio n in the retina: darkness vs. light   Rhodopsin is so sensitive that bleaching occurs even in starlight  In bright light, bleaching occurs so fast that rods are virtually nonfunctional  Cones respond to bright light  So, activation of rods and cones depends on:  Light adaptation Light & dark  Dark adaptation adaptation of rods and cones   Light adaptation  When moving from darkness into bright light we see glare because:  Both rods and cones are strongly stimulated  Large amounts of pigments are broken down instantaneously, producing glare  Pupils constrict  Visual acuity improves over 5–10 minutes as:  Rod system turns off Light & dark  Retinal sensitivity decreases  Cones and neurons rapidly adapt adaptation of Me leaving the movie theater in the middle of the day: rods and cones   Dark adaptation  When moving from bright light into darkness, we see blackness because:  Cones stop functioning in low-intensity light  Bright light bleached rod pigments, so they are still turned off  Pupils dilate  Rhodopsin accumulates in dark, so retinal sensitivity starts to increase Light & dark  Transducin returns to outer segments  Sensitivity increases within 20–30 minutes adaptation of rods and cones   Area of retina where light changes neuron’s firing rate  Fields change in shape and Receptive stimulus specificity. fields  Receptive fields often form the basis of a neural “map” of sensory information   Receptive field: ON and OFF bipolar cells Bipolar cell  Receptive field: Stimulation in a small part of the visual field receptive changes a cell’s membrane potential. fields  Antagonistic center-surround receptive fields   ON-center bipolar cell  Depolarized by light in receptive field center  Hyperpolarized by light in receptive field surround  OFF-center bipolar cells are the inverse Bipolar cell receptive fields   Ganglion cells action potentials differ between ON and OFF- center fields  ON-center and OFF-center ganglion cells associated with the preceding bipolar cells  Responsive to differences in illumination – and edge detection! Ganglion cell receptive fields   Ganglion cells action potentials differ between ON and OFF- center fields  ON-center and OFF-center ganglion cells associated with the preceding bipolar cells  Responsive to differences in illumination – and edge detection!  Responses to light–dark edge crossing an OFF-center ganglion cell receptive field: Ganglion cell  Influence of contrast on the perception of light and dark receptive fields   Ganglion cells action potentials differ between ON and OFF- center fields  ON-center and OFF-center ganglion cells associated with the preceding bipolar cells  Responsive to differences in illumination – and edge detection!  Responses to light–dark edge crossing an OFF-center ganglion cell receptive field: Ganglion cell  Influence of contrast on the perception of light and dark receptive fields   Types of ganglion cells  Categorized by appearance, connectivity, and electrophysiological properties  M-type and P-type ganglion cells  Adaptation speed differences Diversity of ganglion cells gives us even more visual acuity   Types of ganglion cells  Categorized by appearance, connectivity, and electrophysiological properties  Color-specific ON/OFF-center ganglion cells Diversity of ganglion cells gives us even more visual acuity   Simultaneous input from two eyes  Input from eyes compared in cortex  Determines depth and distance of object  Information about light and dark: ON-center and OFF-center ganglion cells Parallel  Different receptive fields and response properties of retinal ganglion cells: M and P cells, and nonM–nonP cells processing of visual information   Axons of retinal ganglion cells form optic nerve  Medial fibers from each eye cross Relaying of over at the optic chiasm then continue on as optic tracts, which visual means each optic tract:  Contains fibers from lateral information to (temporal) aspect of eye on same side and medial (nasal) aspect of the brain opposite eye, and  Each carries information from same half of visual field   Most fibers of optic tracts continue on to lateral geniculate nuclei of thalamus  From there, thalamic neurons form optic radiation, which projects to Relaying of primary visual cortex in occipital lobes visual  Conscious perception of visual images occurs here information to  Depth perception requires input the brain from both eyes  Both eyes view same image from slightly different angles  Visual cortex fuses these slightly different images, resulting in a three-dimensional image, which leads to depth perception   Other optic tract fibers send branches to midbrain  One set ends in superior colliculi, area controlling extrinsic Relaying of eye muscles visual  A small subset of ganglion cells in retina contains melanopsin information to (circadian pigment), which projects to: the brain  Pretectal nuclei: involved with pupillary reflexes  Suprachiasmatic nucleus of hypothalamus: timer for daily biorhythms   Retinal cells split input into channels that include information about:  Color and brightness, but also complex info such as angle, direction, and speed of movement of edges (sudden changes in brightness or color)  Lateral inhibition decodes “edge” information Processing of  Job of amacrine and horizontal cells visual  Ganglions pass information to lateral geniculate nuclei of thalamus to be processed for depth perception, with cone input emphasized information in  Primary visual cortex contains topographical map of retina the brain  Neurons here respond to dark and bright edges and to object orientation  Provide form, color, motion inputs to visual association areas  Info is also passed on to temporal, parietal, and frontal lobes, where objects are identified and location in space determined   Pathway of visual information on a cellular level (who sends Quiz hint! info to whom) Latino Medical Student Association Questions?