Visual System - Brokenshire College PDF

The Visual System Julie Ann Kristy L. Torres, MD, FPCP, FPNA Brokenshire College - School of Medicine  Reference Fundamental Neuroscience for Basic and Clinical Applications, 5th ed. (Haines) Neuroscience, 6th ed. (Purves) OBJECTIVES To present an anatomical overview of...

The Visual System Julie Ann Kristy L. Torres, MD, FPCP, FPNA Brokenshire College - School of Medicine  Reference Fundamental Neuroscience for Basic and Clinical Applications, 5th ed. (Haines) Neuroscience, 6th ed. (Purves) OBJECTIVES To present an anatomical overview of the visual system from periphery to the central tracts To know the circuitry behind visual processing VISION A sensory modality in which the nervous system converts optical images into neural signals and eventually visual experiences VISUAL SYSTEM Creates a location-coded (visuotopic) map of its sensory field (visual world) that is preserved at all levels REVIEW OF THE OPTIC SYSTEM Anatomy of the Eye The cornea is transparent and positioned centrally at the front of the eye. Light entering the eye is refracted by the cornea. Its lateral margin is continuous with the conjunctiva, a specialized epithelium covering the “white” (sclera) of the eye. Cornea Sclera Anterior & Posterior Chamber Ciliary body Canals of Schlemm Aqueous Fluid Vitreous Body Anatomy of the Eye The sclera comprises the majority of the fibrous layer (approximately 85%). It provides attachment to the extraocular muscles. It is visible as the white part of the eye. Sclera and Cornea - Cornea Sclera Anterior & Posterior Main functions to provide shape to the Chamber eye and support the deeper structures. Ciliary body Canals of Schlemm Aqueous Fluid Vitreous Body Anatomy of the Eye The vascular layer of the eye lies underneath the fibrous layer. It consists of the choroid, ciliary body and iris Cornea Sclera Anterior & Posterior Chamber Ciliary body Canals of Schlemm Aqueous Fluid Vitreous Body FUNDUS Inner Layer The inner layer of the eye consists of the retina, the light detecting part of the eye. The retina itself is composed of two cellular layers: neural and pigmented layers. FUNDUS Neural layer – the innermost layer of the retina. It consists of photoreceptors; the light detecting cells of the retina. It is located posteriorly and laterally in the eye. Pigmented layer – the outer layer of the retina. It is attached to the choroid layer and acts to support the neural layer. It continues around the whole inner surface of the eye. FUNDUS Anteriorly, the pigmented layer continues but the neural layer does not – this is part is known as the non-visual retina. Posteriorly and laterally, both layers of the retina are present. This is the optic part of the retina. FUNDUS The optic part of the retina can be viewed during ophthalmoscopy or funduscopy. FUNDUS The center of the retina is marked by an area known as the macula. It is yellowish in colour, and highly pigmented. FUNDUS The macula contains a depression called the fovea, which has a high concentration of light detecting cells. It is the area responsible for high acuity vision. FUNDUS The area that the optic nerve enters the retina is known as the optic disc – it contains no light detecting cells. FUNDUS Normal fundoscopy findings Disc margins are sharp color: yellowish orange to creamy pink shape: round or oval Cup-to-disc ratio: less than 0.5 AV ratio 2:3 AV crossing: no indentation No exudates or hemorrhages Macula is located 2.5 disc distance temporal to disc no vessels are noted around macula Chambers of the Eye Just behind the cornea is the fluid-filled anterior chamber, which is bound posteriorly by the iris and the opening of the pupil Chambers of the Eye A second fluid-filled space, the posterior chamber, is bound anteriorly by the iris and posteriorly by the lens and its encircling suspensory ligament (zonule fibers). Chambers of the Eye The fluid in the posterior chamber is in contact with the vitreous body, the gelatinous mass that fills the main space of the eyeball between the lens and the retina. Chambers of the Eye The ciliary body continuously produces fluid around the rim of the posterior chamber and fluid flows through the pupillary opening into the anterior chamber. Chambers of the Eye It then drains into a set of modified veins, the canals of Schlemm, that are located around the rim of the anterior chamber in the angle where the iris meets the cornea. Iris Circular structure, with an aperture in the centre (the pupil). Directly anterior to the lens The connective tissue, or stroma, of the iris contains melanocytes that reflect or absorb light to give the iris its characteristic color. Iris The diameter of the pupil is altered by smooth muscle fibres within the iris, which are innervated by the autonomic nervous system. It is situated between the lens and the cornea. Iris Sphincter muscles Circular group Iris Dilator muscles Radial group Lens Lens focuses light on the retina; simple convex lens that inverts and reverses the image on the retina. Uvea Uvea - Underneath the fibrous layer - Vascular tunic comprised of Iris, Ciliary Body, Choroid. Uvea Choroid lies between the pigmented layer called retinal pigment epithelium and sclera. Provides nourishment to the outer layers of the retina. Uvea Ciliary body comprised of two parts 1. ciliary muscles Smooth muscles fibers 2. ciliary processes Attach ciliary muscles to the lens of the eye The ciliary body controls the shape of the lens, and contributes to the formation of aqueous humor. Retina Retina Composed of neural retina and retinal pigment epithelium Retinal Layers Retinal Pigment Epithelium (RPE) Neural Retina  Why the photoreceptors are located at the outer (rather than at the inner) layer? 1. RPE functions to phagocytose the shed outer disc of the photoreceptors 2. RPE regenerates photopigment molecules after they have been exposed to light. 3. Choroid capillaries supply nourishment to the photoreceptors I. Retinal Pigment Epithelium (RPE) Continuous sheet of pigmented cuboidal cells Bound together by tight junctions that block the flow of plasma or ions I. Retinal Pigment Epithelium (RPE) Functions: it supplies the neural retina with nutrition in the form of glucose and essential ions; it protects retinal photoreceptors from potentially damaging levels of light; and it plays a key role in the maintenance of photoreceptor anatomy via phagocytosis. II. Neural Retina contains the photoreceptors and associated neurons of the eye specialized for sensing light and processing the resultant information II. Neural Retina Photoreceptors Absorbs quanta of light or photons Convert this input into electrical signal Retinal neurons or ganglion cells Send processed signal to the brain via axons that collectively form the Optic Nerve II. Neural Retina 7 layers (outer to inner) 1. Photoreceptor cell outer and inner segments 2. Outer nuclear layer 3. Outer plexiform layer 4. Inner nuclear layer 5. Inner plexiform layer 6. Ganglion cell layer 7. Nerve fiber layer or optic fiber layer II. Neural Retina Limiting membranes consist of glial cell processes joined by tight junctions Inner – between nerve fiber layer and vitreous Outer – between layers 1 and 2 Retinal Layers (Inner to Outer) In Inner Limiting Membrane New Nerve Fiber Layer Generation Ganglion Cell Layer It Inner Plexiform Layer Is Inner Nuclear Layer Only Outer Plexiform Layer Ophthalmologists Outer Nuclear layer Examining External Limiting Membrane Sick Segments Layer Retina Retinal Pigment Epithelium Retinal cells (Outer to Inner) 1. Photoreceptors – rods and cones 2. Bipolar cells 3. Horizontal cells 4. Amacrine cells 5. Ganglion cells - third order neurons - resemble nervous ganglia Photoreceptors Rods Cones Scotopic vision Photopic vision Mesopic vision Most most of what we think of as normal “seeing” is mediated by the cone system Macular degeneration Night blindness Segments Layer Photoreceptor outer and inner segments Outer segment site of light detection and transduction; interdigitate with the melanin-filled processes of pigment epithelial cells Inner segment contains mitochondria Cilia connects Outer to Inner Segment Segments Layer Segments Layer Disc shedding - Outer segment (OS) is constantly renewed - Distal one tenth of the outer segment is broken off and phagocytosed by the pigment epithelium - New discs are formed at the base of the outer segment and move outward so that the shed discs are replaced - Rods every morning; Cones every evening Segments Layer cGMP- mediates standing Na and Ca current into the outer segment resulting in high resting potential (-40 mV) Photons  Opsin pigment protein Transducin, Phosphodiesterase reduce cGMP  resting potential becomes -60mV (rod cell becomes hyperpolarized) Photoreceptors are the only sensory neurons that hyperpolarize in response to the relevant stimulus. Segments Layer 3 types of Cones: 1. L - cones (red cones) Sensitive to long wavelengths 2. M - cones (green cones) Sensitive to medium wavelengths 3. S - cones (blue cones) Sensitive to short wavelengths Correspond to 3 different types of Cone Opsin (“Photopsin”) pigments Segments Layer Only 1 type for rods Pigment is Rod Opsin or “Rhodopsin” Outer Nuclear Layer Nuclei of the photoreceptor cells Outer Plexiform Layer Synaptic terminal Constantly release the transmitter glutamate unless when light- hyperpolarized “Spherule” in rods; “Pedicle” in cones Synaptic ribbon characteristic dark sheet of protein in the synaptic terminal act as a “conveyor belt,” organizing vesicular release of transmitter Larger in cones Outer Plexiform Layer Triad Contacts between a single cone pedicle or rod spherule + A centrally placed postsynaptic Bipolar Cell process + Two laterally placed Horizontal Cell processes Inner Nuclear Layer Bipolar Cell form a straight-through pathway for visual input Comparators or “edge detectors” of the retina First retinal cells to exhibit the Center- Surround Receptive Field organization Cone bipolar cells and Rod bipolar cells Inner Nuclear Layer Bipolar cell type based on response: 1. On/ Depolarizing BC respond to a light stimulus in the receptive field center by depolarizing Has a Sign- inverting synapse with PC (Glutamate from PC hyperpolarizes the BC Inner Nuclear Layer Bipolar cell type based on response: 2. Off/ Hyperpolarizing BC respond to a light stimulus by hyperpolarizing Has a Sign- conserving synapse with PC (Glutamate from PC hyperpolarizes the BC Bipolar Cells Inner Nuclear Layer Horizontal cells courses parallel to the plane of the retina to nearby and distant photoreceptors receive glutaminergic input from photoreceptors and in turn form γ- aminobutyric acid (GABA)ergic synaptic contacts on adjacent rods and cones. Like BC, establish the concentric center-surround receptive field organization of retinal ganglion cells Lateral inhibition responsible for sharpening the edges of images perceived in the visual system. Inner Nuclear Layer Amacrine cells have a small soma, no obvious axon, and dendrites that are few but highly branched may be displaced into the ganglion cell layer Like HC, also have dendrites that extend over long distances, sampling and modifying bipolar cell output modify the input of groups of bipolar cells onto Y class ganglion cells making the GC sensitive to moving visual stimuli. Ganglion Cell Layer formed by the somata of Ganglion Cells Output cells of the retina Like BC, have center- surround types of receptive fields Ganglion Cell Layer - Types based on presence of Melanopsin: 1. Non- Melanopsin- Containing GC (99%) Function: Image formation Depend on rods and cones for their sensitivity to light Ganglion Cell Layer Melanopsin- Containing Ganglion Cell (1%) Characteristic large- sized cell bodies and expansive overlapping dendritic fields Some are found in the Inner Nuclear Layer Sensitive to blue light Has connections to the suprachiasmatic and pretectal nuclei Function: 1. involved in Circadian rhythms 2. Involved in Pupillary light reflex may also be considered a third type of photoreceptor Intrinsically sensitive to light (even w/o input from Rods & Cones)--- “3rd type of Photoreceptor”; ipRGCs Ganglion Cell Layer Type according to morphology and physiologic role: 1.Alpha cells large cell bodies and widely branching dendritic fields predominantly in the peripheral retina, and receive input mainly from rods more extensive dendritic trees and thicker axons than those of other ganglion cell types aka “M cells”--- connect to other large cells in the magnocellular layers in the lateral geniculate nucleus Physiologic classification: aka “Y cells”---- participate mainly in motion sensing Ganglion Cell Layer Type according to morphology and physiologic role: 2.Beta cells medium- sized cell bodies and more restricted dendritic fields predominantly in the central retina, and receive input mainly from cones aka “P cells”--- connect to small cells in the parvocellular layers in the lateral geniculate nucleus Physiologic classification: aka “X cells”---- participate mainly in color perception Ganglion Cell Layer Type according to morphology and physiologic role: 3. Gamma, Delta, Epsilon cells Smaller cell bodies and axons, varied receptive field sizes and physiologic response Physiologic classification: aka “W cells” Nerve Fiber Layer Axons of the Ganglion Cells that later converge on the optic disc and form the optic nerve Retinal surface Fundus- retinal surface, seen through the pupil Optic disk/ papilla- whitish circular area without photoreceptors; found nasally Physiologic “blind spot”/scotoma Useful gauge of ICP Passing through are: 1. Ophthalmic artery and vein enter/ leave 2. Retinal axons leave to become optic nerve Retinal surface Macula/ Macula lutea Circular region near the center of the region Contains yellow pigment Xanthophyll which is UV- protective Region of high VA  Fovea centralis Retinal surface Most of the visual input that reaches the brain comes from the fovea. Only the Cones along with its segments are present; Rods and other retinal layers do not continue Green and red- sensitive Cones predominate Blue- sensitive Cones are mostly outside the Fovea Processing of Visual Input in the Retina All retinal cells utilize graded potentials to carry information except: Ganglion cells - utilize action potentials via voltage- gated sodium channels on their axonal membranes to carry information Amacrine cells - utilize calcium spikes to carry information Processing of Visual Input in the Retina Receptive field properties of each retinal cell depend on the processing of information passing through the neurons between the photoreceptor and the retinal cell in question Bipolar and Ganglion cells respond to incremental/decremental changes in light Receptive Field of a Neuron region of the visual world in which a stimulus of the proper characteristics will influence excitatory the activity of the neuron Concentric center-surround organization during the early stages of visual information processing RETINA Blood Supply 1. Choriocapillaries From the Choroid layer 2. Central Retinal Artery Branch of the Ophthalmic artery THE OPTIC SYSTEM Afferents to the brain CN II - Optic: VISION and PUPILLOCONSTRICTION CN V - Trigeminal: GENERAL SENSATION ✴ Ocular pain ✴ Tearing reflex ✴ Corneal reflex ✴ Proprioception from the extraocular muscles Two motor systems Peripheral - CN III, IV, VI, carotid sympathetic nerve Central - find, fixate, focus/align on, follow visual targets VISUAL PATHWAY Non- visual axons Visual axons (Tectal area) Pupillary constriction (miosis) Parasympathetic innervation Neurotransmitter: Acetylcholine (muscarinic) Pathway: 1. Preganglionic neurons in Edinger Westphal nucleus 2. Ciliary ganglion 3. Neuromuscular synapse on sphincter muscles Pupillary dilatation (mydriasis) Sympathetic innervation Neurotransmitter: Norepinephrine Pathway: 1. Preganglionic neurons in intermediolateral cell column of spinal cord 2. Superior cervical ganglion 3. Neuromuscular synapse on the dilator muscles Pupillary Reaction to Light The pupillary light reflex, a contraction of the pupil in response to light, is used to assess the function of the nervous system, particularly at midbrain levels. Pupillary Reaction to Light Contraction of pupil in response to light Circumference of pupillary margin changes by a factor of six Acetylcholine is released onto both the sphincter and dilator muscles. Effects: Activate muscarinic receptors that depolarize sphincter muscle cells and cause contraction Acetylcholine released by As the sphincter contracts, the collaterals onto the dilator muscle dilator relaxes, strengthening mediates presynaptic inhibition of norepinephrine release and blocks the pupillary response to light. dilator contraction VISUAL PATHWAY Visual axons (Retinogeniculate tract) Cortical Magnification Optic Radiation Parietal radiation fibers More medial parts of the optic radiation pass under the Parietal lobe cortex, carry information from the inferior portion of the contralateral visual field. Damage results in Inferior homonymous quadrantanopsia Optic Radiation Temporal radiation fibers (Meyer’s Loop) optic radiation axons run out into the temporal lobe on their route to the striate cortex carries information from the superior portion of the contralateral visual field Damage results in Superior homonymous quadrantanopsia VISUAL FIELD Temporal hemiretina receives image input from the nasal visual field. Nasal hemiretina receives image input from the temporal visual field. VISUAL FIELD Upper retinal quadrants receive image input from the lower visual fields. Lower retinal quadrants receive image input from the upper visual fields. VISUAL FIELD Within each eye, the axons projecting from the medial side of the retina decussate at the optic chiasm. For example, the axons from the medial retina of the left eye cross over to the right side of the brain at the optic chiasm. However, within each eye, the axons projecting from the lateral side of the retina do not decussate. VISUAL FIELD Temporal visual field = nasal retina = right brain Nasal visual field = temporal retina = left brain Superior field = inferior retina = inferior bank Inferior field = superior retina = superior bank VISUAL PATHWAY Right monocular blindness Bitemporal hemianopsia Right homonymous hemianopsia Right superior quadrantanopsia Right inferior quadrantanopsia Right homonymous hemianopsia Right homonymous hemianopsia with macular sparing  PRIMARY VISUAL CORTEX The primary visual cortex or striate cortex or V1 (Brodmann area 17) receives most of the axons from the lateral geniculate nuclei of the thalamus. It lies on either bank of the calcarine sulcus in the occipital lobe.  CALCARINE SULCUS Superior bank – On the cuneus – receives input from the inferior part of the contralateral hemifields Inferior bank – On the lingual gyrus – receives input from the superior part of the contralateral hemifields PRIMARY VISUAL CORTEX  PRIMARY VISUAL CORTEX The central part of the visual field (i.e., the macula and fovea) is represented in the poles of the primary visual cortex [central 10 degrees of visual field occupies half of the visual cortex]  PRIMARY VISUAL CORTEX The six-layered neocortex of area 17 is characterized by a wide layer IV which contains the stria of Gennari that indicates the large geniculocalcarine input to this layer.  PRIMARY VISUAL CORTEX Layer VI is also prominent in striate cortex which is the source of a cortical feedback projection to the lateral geniculate nucleus. THANK YOU.