Vision Lecture Notes Summer 2024 PDF

At the end of the lecture, students should be able to: Understand the basic structure of the eye Understand the function of the pupillary muscles (miosis and mydriasis) Describe photoreceptors and compare their localization, characteristics, and functions Describe the cells...

At the end of the lecture, students should be able to: Understand the basic structure of the eye Understand the function of the pupillary muscles (miosis and mydriasis) Describe photoreceptors and compare their localization, characteristics, and functions Describe the cells of the retina and their neurotransmitters Describe why visual acuity is bigger in the area centralis of the retina Understand the pathway of signals from the retina to the cortex Understand the pupillary light reflex  Vision is an integral part of neural function Eyes are an extension of the brain The spatial area seen by one eye is the visual field Animals have wider peripheral vision than humans Because the visual fields of each eye do not completely overlap THE EXTENT OF OVERLAP OF THE VISUAL FIELDS DEPENDS ON THE ANATOMICAL PLACEMENT OF THE EYES  binoculardisparitybraincangaugethe difference the in ima positionofthe tworetinas disparity closer greater lesserdisparityfarther Dogs have binocular vision only in the area directly in front of them Monocular vision does not provide good depth perception To maintain binocular vision, the eyes move as a functional unit when animals scan their surrounding environment WHEN THE EYES LOOK DIRECTLY AT AN OBJECT, AN IMAGE OF THE OBJECT WILL FALL ON TO THE AREA CENTRALIS – THE RETINAL AREA THAT PROVIDES THE GREATEST ACUITY PATHWAYS INVOLVED IN VISUAL REFLEXES Green = vision and visual processing Violet = interneuronal connections Purple = parasympathetic connections for pupillary light reflex Red = UMN and LMNs for eye, eyelid and head movement  VASCULAR (NUTRITION) PUPIL (OPENING) RECEPTORS (CONES AND RODS) IRIS (DIAPHRAGM - MUSCLE THAT SCLERA (OUTERMOST LAYER OF THE EYE) CAN CONTRACT/RELAX TO MODIFIED ANTERIORLY INTO A CLEAR CONTROL THE RAYS OF LIGHT THAT REGION CALLED CORNEA GOES INSIDE OF THE EYE )  The CHOROID consists of loose connective tissue with numerous vasculature and pigmented cells Normal dog. Choroidal vessels are visible throughout the fundus. Nutritive function Source: Veterian Key Some diurnal animals have melanocytes that absorb light that has passed by the photoreceptors without stimulating them Nocturnal and most domestic mammals have a patch of reflective material called TAPETUM LUCIDUM Light-reflective surface that enhances dark-adapted vision under dim light Eye-shine Normal cat. The tapetal color is typical, and the optic nerve is surrounded by tapetum. Source: Veterian Key Cow eye dissection  The iris controls the ammount of light that passes through the pupil In the dark the pupil will dilate (mydriasis) In the light the pupil will contract (miosis)  Heterochromia in an Australian shepherd The IRIS is a pigmented structure containing muscles that modify the diameter of the PUPIL Contains dilator and sphincter muscles PUPILLARY DILATOR MUSCLE Radially arranged Oposes the action of the sphincter Is part of the pigmented anterior epithelial cells (myoepithelial cells – smooth muscle) Contraction results in pupillary dilation (MYDRIASIS) Dilation reflects the general state of sympathetic tone Pain, fear, anger  PUPILLARY SPHINCTER MUSCLE Circularly arranged near the pupillary margin Contraction results in decreased pupillary size (MIOSIS) Innervated by parasympathetic fibers The size of the pupil regulates the amount of light that enters the eye Behind the iris is the LENS IRIS INNERVATION OF THE CAT  The LENS is suspended by SUSPENSORY LIGAMENTS These fibers are attached to the CILIARY BODY Muscular structure near the base of the iris Help with accommodation of the lens Increase or decrease the tension on the lens Making lens curvature more or less convex Lens can focus on a near or far object  Behind the lens is a chamber filled with VITREOUS HUMOR Gelatinous fluid Give the spherical shape of the eye Contains phagocytic cells Behind the vitreous humor lies the RETINA Zachary, JF. Pathologic Basis of Veterinary Disease. Sixth Edition, 2017, Elsevier.  The RETINA is where light is transformed into the electrical activity of neurons Normal retina in a black Labrador retriever. Typical tapetum in many canine breeds. Source: VeterianKey The retina is interrupted at a point where axons of the retina ganglion cell layer leave the eye on their way to the brain – OPTIC DISC or BLIND SPOT  The axons leaving the eye at the optic disc give rise to the OPTIC NERVE (CN II) There are more axons in both optic nerves than in all the dorsal roots of the spinal cord The surface of the retina has blood vessels Arteries and veins enter the retina at the optic disc Provides the nutrition of the retina together with vessels of the choroid Normal cat. The tapetal color is typical, and There are about 130 million PHOTORECEPTOR CELLS in the the optic nerve is surrounded by tapetum. retina Cones and Rods  There are 2 types of photoreceptor cells CONE CELLS and ROD CELLS They consist of 2 portions OUTER SEGMENT photosensitive region In cone cells is composed mainly of membranous invaginations In Rod cells contains numerous flattened membranous sacs arranged like a stack of coins The membrane of these invaginations and sacs contains PHOTOPIGMENTS Convert a light stimulus to a receptor potential INNER SEGMENT The metabolic region of the photoreceptor CONE ROD  sourceisverydim scotopicvision light VITAMIN A IS IMPORTANT FOR THE FORMATION OF RHODOPSIN – SEVERE VIT A DEFICIENCY CAN CAUSE NIGHT BLINDNESS Rod cells have RHODOPSIN Photochemical neurotransmitter responsible for the perception of SHADES OF GRAY Rhodopsin has a low threshold of excitability Easy stimulated by low-intensity light Rod cells are 300x more sensitive to light than cone cells Essential for night vision 95% of the photoreceptors present in the retina  oflight vision plenty phototopic Cones have COLOR PIGMENTS or CONE PIGMENTS Photochemical neurotransmitters responsible for the perception of COLOR Less sensitive to light Requires relatively high-intensity light than Rhodopsin There are 3 different types of cones in primates Each one carries a different type of color pigment with a different pick of absorbencies at different wavelengths BLUE SENSITIVE PIGMENT – 445 nanometers THE VISUAL SYSTEM GREEN SENSITIVE PIGMENT – 535 nanometers POLICHROMATIC MIX AND CONTRAST RED SENSITIVE PIGMENT – 570 nanometers VISION THE EFFECT OF EACH CONE CELL  Most mammals have only 2 types of cones (DICHROMATIC) Able to detect the blue and yellow portion of the light spectrum Unable to disinghish reds and oranges Some birds, lizards, turtles and fishes have 4 types 3 types + UV-sensitive cone Richer color perception than humans/primates COLOR IS THE BRAIN’S INTERPRETATION OF DIFFERENCES IN THE WAVELENGTHS OF LIGHT  Light 5 major types of cells make up the retina There are local variations in the density of some cell types 1. PHOTORECEPTOR CELLS Synapse with bipolar and horizontal cells. Glutamate is the neurotransmitter. 2. HORIZONTAL CELLS Inhibitory interneurons that transmit signals horizontally from rods and cones to bipolar cells. GABA is the neurotransmitter. hasasfewasoneconeper note cone ganglioncell whereastherearehundredsof perganglioncells rods 3. BIPOLAR CELLS bicofthisitcanamplifyadim crops source light Retinal interneurons that transmit signals vertically from trade rods forlowaquity s ensitivity the rods, cones and horizontal cells to ganglion and amacrine cells. Glutamate is neurotransmitter.  5 major types of cells make up the retina There are local variations in the density of some cell types 4. AMACRINE CELLS Most are inhibitory interneurons that transmit signals in 2 directions either directly from bipolar to ganglion cells horizontally from bipolar cells to ganglion cells or to other amacrine cells GABA or Glycine are the neurotransmitters 5. GANGLION CELLS Projection neurons that transmit output from the retina through the optic nerve into the brain. Glutamate is the neurotransmitter  neuronreceivessignal mmol Zachary, JF. Pathologic Basis of Veterinary Disease. Sixth Edition, 2017, Elsevier. RETINA IS A SPECIALIZED SENSORY EPITHELIUM THAT CONTAINS PHOTORECEPTORS AND OTHER CELL TYPES ARRANGED IN LAYERS  VEA ALERT The cells in the retinal layer labeled C in the photomicrograph are responsible for which of the following? (A) Absorbing excess photons (B) Acting as interneurons 0 (C) Detecting light, color, and shape (D) Forming the optic (II) nerve with their axons (E) Reflecting light  whatwillbefix ofbipolarcell ex ALL THESE CELLS CAUSE SOME DISTORTION OF LIGHT RAYS THE FOVEA (AREA CENTRALIS) MINIMIZES THIS DISTORTION  rememberrods cones ratio ganglion photoreceptors FOVEA is a minute area in the center of the retina Especially capable to acute and detailed vision In animals is called AREA CENTRALIS Composed almost entirely of cones With special long and slender bodies that aids their detection in the visual image Other cells are all displaced allowing light to pass unimpeded to the cones  Photoreceptors, bipolar cells, horizontal cells, and amacrine cells use graded potentials rather than action potentials All the retinal neurons conduct their visual signals along the cell by direct flow of eletric current – ELECTROTONIC CONDUCTION The same degree of hyperpolarization in the rod and cone is conducted by direct electric current flow in the cytoplasm all the way to the synaptic body Where the neurotransmitter will be released and transmitted to the next cell Allow graded conduction of signals strengh Strenght of the hyperpolarizing output signals is directly related to the intensity of the illumination The signal is not all or none!  1 in I yia outerperi i retina 4 Rodsturnon 5 heteIfaih ftp wave of length IN THE DARK cGMP PHOSPHODIESERASE IS 6 INACTIVE (PDE6) cGMP ACCUMULATES AND BIND TO LIGAND GATED Na/Ca ION CHANNELS Depolarization FLOW OF CATIONS INTO THE Glutamate RODS KEEPS THE MEMBRANE release DEPOLARIZED CALCIUM IS TRANSPORTED USES ENERGY FROM ALLOWING TO FORCE CALCIUM IONS TO BACK OUT AGAIN BY AN Na/K TO RUN DOWN THEIR BE TRANSPORTED AGAINST ANTIPORTER ELETROCHEMICAL GRADIENTS THEIR GRADIENT ACTIVATION OF RHODOPSIN BY LIGHT LEADS TO ACTIVATION OF A G PROTEIN TRANSDUCIN THE ACTIVATED TRANSDUCIN SPLIT AWAY AND ACTIVATES PDE6 ACTIVATED PDE6 Glutamate release hyperpolarization HYDROLYSES cGMP TO 5’- GMP (inactive form) reduced LEADS TO HYPERPOLARIZATION DECREASE IN GLUTAMATE CAUSING Na/Ca CHANNELS (MEMBRANE POTENTIAL RELEASED WILL EXCITE TO CLOSE MORE NEGATIVE) BIPOLAR CELLS DECREASE IN GLUTAMATE RELEASED WILL EXCITE BIPOLAR CELLS Glutamate release hyperpolarization reduced i GANGLION CELLS WILL GENERATE ACTION POTENTIALS  Acuity of visual images reflects several factors Population of retinal cells The ratio of rods to cone photoreceptor cells The ratio of photoreceptors cells to ganglion cells Cone cells provide better acuity than rod cells Hundreds of rods feed signals via bipolar cells to a single ganglion cell Only few cone cells feed signals to a single ganglion cell There are more ganglion cells in the area centralis than there And are in the peripheral portion of the retina Area centralis provides the highest visual acuity in the retina  The eye is optically equivalent to the usual photographic camera Lens system Variable aperture system – Pupil Film - Retina  The eye catches the light reflected by the objects and guide its passage until the image is formed A sequence of transparent media causes the light to be refracted (direction change) 1. The interface between the air and the anterior surface of the cornea 2. The interface between the posterior surface of the cornea and the aqueous humor 3. The interface between the aqueous humor and the anterior surface of the lens of the eye 4. The interface between the posterior surface of the lens and the vitreous humor  The lens works by converging the light rays to a certain focal point on the retina Light rays have almost unobstructed access to the photoreceptors Central retina is the area of highest visual acuity in most mammals No distinct FOVEA is formed in most mammals  The images formed on the retina are real, inverted and smaller than the object THE DISTANCE BETWEEN THE LIGHT RAYS EMANATING FROM THE TOP AND THE BOTTOM OF THE TREE GET CLOSER TOGETHER AS THEY APPROACH THE EYE. AT SOME POINT, THEY CROSS AND REVERSE BY REVERSING AND DECREASING THE IMAGE, THE EYE IS ABLE TO VISUALIZE OBJECTS MUCH LARGER THAN IT  The images formed on the retina are transformed by cones and rods into nerve impulses Visual signals are carried by optic nerve fibers that form the optic nerve, optic chiasm, and optic tract Fibers terminate at: thalamus (lateral geniculate nucleus)  optic radiation  occipital lobe (visual area) - for visual perception midbrain (pretectal nucleus and rostral colliculus)  tectospinal tract and cranial nerve nuclei – for visual reflexes i.e. body and ocular reflex, pupillary constriction THE BRAIN PROCESS AND REORIENTS THE IMAGE  Is a reflex arc composed by: RECEPTORS FOR LIGHT – in the retina Photoreceptors AFFERENT NEURONS Optic nerve (CN 2) BRAINSTEM CENTERS (INTEGRATION) Pretectal nucleus Parasympathetic nucleus of oculomotor nerve (CN 3) EFFERENT NEURONS Oculomotor nerve (CN 3) EFFECTOR ORGAN Pupillary sphincter muscle contracts (miosis) under bright light DIRECT PLR  miosis in illuminated eye CONSENSUAL/INDIRECT PLR  miosis in the eye not illuminated (weaker than direct PLR)