Special Senses: The Eye

Collin County Community College BIOL 2401: Week 9 Special Senses: The Eye 1 VISION As humans, we rely on Vision more than any other special sense. The eye itself is surrounded by accessory structures Eyelids (palpebrae) that separate the palpebral fissue Eyelashes Tarsal glands or Meibomian glands in the eyelids Lacrimal apparatus Lacrimal gland Lacrimal canuncle with lcrimal canaliculi Lacrimal sac and naso lacrimal duct 2 1

VISION Conjuctiva : Epithelium covering the inner eyelid and outer surface of the eye Extends to the edges of the cornea. The cornea itself is transparent and contains no blood-vessels. Lacrimal apparatus : Produces, distributes and removes tears. 3 VISION: The EYE The eye is a more or less spherical structure, almost the size of a ping pong ball and about 8 grams in weight. It is located within the eye socket and cushioned by orbital fat. The eye-ball is hollow and it can be divided into two chambers according to the location with respect to the lens. Posterior cavity or vitreous chamber : filled with gelatinous liquid called vitreous humor or vitreous body Anterior cavity located in front of the lens and divided into two chambers ( anterior and posterior chamber). It is filled with aqueous humor. 4 2

VISION: The EYE The eye is a more or less spherical structure, almost the size of a ping pong ball and about 8 grams in weight. It is located within the eye socket and cushioned by orbital fat. Excessive fat behind the eye causes the eyes the bulge forward ( such as may occur due to hyperthyroidism). Exophthalmos 5 VISION: The EYE The eye-ball has 6 extra-occular (external) muscles that allows us to move the eye in quick fashion in almost all directions. 6 3

VISION: The EYE The medial rectus ( not visible) Cranial nerves that activate these muscles : A : Abducens nerve (VI) E : Trochlear nerve (IV) B,C,D and the medial rectus : Occulomotor nerve (III) 7 VISION: The EYE 8 4

VISION: The EYE There are basically three main layers (tunics) to the wall of eye organ Outer fibrous tunic Sclera, cornea, limbus Middle vascular tunic Iris, ciliary body, choroid Inner nervous tunic Retina 9 VISION: The EYE 10 5

The EYE: Internal Structures Fibrous Tunic The outer layer Composed out of the SCLERA and CORNEA SCLERA is white part of the eye (the majority ) while the cornea is transparent anterior portion. The limbus is the border between these two 11 The EYE: Internal Structures Vascular Tunic or UVEA The middle layer Composed out of the CHOROID, CILIARY BODY and IRIS CHOROID is follows the majority of the sclera and is very vascular. It also contains many melanocytes near the border with the sclera. 12 6

The EYE: Internal Structures Near the anterior portion of the eye, the choroid develops into the CILIARY BODY It is composed out of the ciliary smooth muscles that extend inwards towards the lens. The lens attaches to the ciliary body via suspensory ligaments. It keeps the lens in front 13 of the iris and centered. The EYE: Internal Structures Vascular Tunic or UVEA The IRIS extend as a flap of tissue beyond the Cilairy body and provide a central opening for light to enter the eyeball ( the pupil) It is composed out of two layers of smooth muscles : radial dilator muscles and circular constrictor muscles The iris also contains pigments and blood vessels. 14 7

The EYE: Internal Structures Vascular Tunic or UVEA 1 : 2 : 3 : 4 : 5 : Sclera Cornea Iris Pupil Lens 6 : Limbus 7 : Ciliary Body 15 The EYE: Internal Structures Vascular Tunic or UVEA Contraction of the two layers of muscles results in pupil dilation or pupil constriction. It thus functions just like a camera diaphragm, letting more or less light inside. Circular constrictor contraction is under para-sympathetic influence (part of pupillary reflex; makes pupil smaller) Radial dilator contraction is under sympathetic influence (makes pupil larger). 16 8

The EYE: Internal Structures 17 The EYE: Internal Structures Retina or Neural Tunic It is the innermost layer of the eye wall Consist out of a pigmented part and a neural part Pigmented part is also called the pigmented epithelial layer It has an important function in preventing light from bouncing back It also is important in providing biochemical feedback to the light receptors in the retina 18 9

The EYE: Internal Structures Retina or Neural Tunic The neural part is the actual retina with the light receptors Rods : do not discriminate color - good for gray shades - highly sensitive to light - good for dim light vision Cones : discriminate color - require higher light intensities It extends anteriorly only as far as what is called the ora serrata. (light does not hit the inside of the eye anterior to the ora serrata. Thus no need for a retina there). The retina contains several layers of cells that are important in relaying captured light energy to the brain. 19 The EYE: Internal Structures Retina or Neural Tunic 8 : Ora Serrata 20 10

The EYE: Internal Structures 21 The EYE: Internal Structures Rods and Cones Synapse with Bipolar Cells Synapse with Ganglion Cells Their Axons form the Optic Nerve Axons of the ganglion cells leave the eye at the optic disc ( contains no light receptors ; reason for the blind spot) 22 11

The EYE: Internal Structures Macula lutea is a depression in the retina where no rods occur. Fovea centralis is the center and has highest levels of cones ; provides the sharpest vision. 23 The EYE Chambers Ciliary body and lens divide the anterior cavity of the eye into posterior (vitreous) cavity and anterior cavity Anterior cavity further divided anterior chamber in front of eye posterior chamber between the iris and the lens Aqueous humor circulates within the anterior eye cavity Made by ciliary body and diffuses through the walls of the anterior chamber passes through canal of Schlemm and re-enters circulation Vitreous humor fills the posterior cavity. Not recycled permanent fluid 24 12

The EYE: Internal Structures Blockage of this drainage pathway may result in an increase in ocular pressure, resulting in glaucoma! 25 The EYE: The Lens The lens is located Posterior to the cornea and forms the anterior boundary of the posterior cavity The Lens helps to focus light on the retina by refracting (bending) it as it passes through lens. The lens is made from slender, elongated cells filled with transparant proteins called crystallins. Loss of transparency = cataract ( cloudy lens) 26 13

The EYE: The Lens Hyper-mature cataract 27 The EYE: The Lens Refraction Light changes direction when it passes from one medium to another medium with different density. Most of the refraction occurs when light passes through the cornea. The lens provides the extra refraction adjustments needed to focus the light onto the retina. If light is not centered on the retina, we end up with blurry vision. Why do things look blurry under water when we open our eyes? 28 14

The EYE: The Lens Refraction Accommodation is the process where the shape (thickness) of the lens is adjusted to keep the focal distance constant During Accommodation the lens becomes fatter when we try to focus on a near-by object and thinner when the object is distant. 29 The EYE: The Lens Accommodation Accommodation is executed via CN III by action on the ciliary body Contraction of the cilary muscles causes relaxation of the ligaments and bulging of the lens Relaxation of the ciliary muscles results in a pull on the suspensory ligaments, which in turn flattens the lens 30 15

The EYE: The Lens Accommodation Problems 31 VISUAL PHYSIOLOGY VISUAL physiology relates to understanding how we actually see images. First of all, the image created on the retina is almost the same as when we look through a microscope : it is upside down and backwards The brain compensates for this image reversal and we are never aware of this. 32 16

VISUAL PHYSIOLOGY The rods and the cones are responsible for picking up the information in visible light ( the photons) Our rods and comes are sensitive to visible light only, a portion of the electromagnetic spectrum with wavelenghts between 400 nm to 700 nm. 33 VISUAL PHYSIOLOGY Anatomy of Rods and Cones Rods and cones are elongated specialized nerve receptor cells There is an Outer segment embedded into the pigmented epithelial layer contains membranous discs with visual pigments Narrow stalk connecting outer segment to inner segment Inner segments synapse with bipolar cells 34 17

VISUAL PHYSIOLOGY RODS Functional characteristics Very sensitive to dim light Best suited for night vision and peripheral vision Perceived input is in gray tones only Pathways converge, resulting in fuzzy and indistinct images CONES Functional characteristics Need bright light for activation (have low sensitivity) Have one of three pigments that furnish a vividly colored view Nonconverging pathways result in detailed, high-resolution vision 35 VISUAL PHYSIOLOGY Visual Pigments Visual pigments are located in the membranes of the membraneous discs pigments The visual pigment is called Rhodpsin Rhodopsin is a molecule made from Opsin ( a larger protein) Retinal ( a smaller visual pigment) Retinal is a derivative of Vitamin A In cones, the Opsin protein is slightly different, accounting for the color sensitivity of the cones 36 18

VISUAL PHYSIOLOGY PhotoReception Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment. Rod discs It is made from retinal, the Vitamin A derivative, and a larger protein part, called Opsin Visual pigment consists of Retinal Opsin 37 VISUAL PHYSIOLOGY PhotoReception Retinal is derivative from Vitamin A (Retinol) Retinal has two different configurations Trans form : molecule has a straight tail Cis form : molecule tail has a bend in it Light energy converts Retinal from the Cis to Trans form. The conversion from the Cis to Trans state is at the basis of photoreception 38 19

11-cis-retinal VISUAL PHYSIOLOGY Vitamin A 2H + Oxidation Reduction 2H + 2 Regeneration of the pigment: Enzymes slowly convert all-trans retinal to its 11-cis form in the pigmented epithelium; requires ATP. 11-cis-retinal Rhodopsin Dark Light Opsin and All-trans-retinal 1 Bleaching of the pigment: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all-trans) and eventually releases from opsin. All-trans-retinal 39 VISUAL PHYSIOLOGY In vision, cgmp is important in that it opens up a chemically regulated Na + channel. This is similar like camp opening up Na + channels in olfaction. In photoreceptors, under resting conditions (dark conditions), cgmp is present in high concentrations. GTP Guanyl Cyclase cgmp 40 20

VISUAL PHYSIOLOGY THUS, when not stimulated by light, the photoreceptors cells are always depolarized (around - 40 mv) due to a Na + ion current ( called the dark current)! The cells continuously release neurotransmitters at the bipolar synapse! Steps in PhotoReception STEP 1 : Light energy converts Retinal from the CIS form to the TRANS form. This now activates the OPSIN part of Rhodopsin STEP 2 : OPSIN activates the enzyme TRANSDUCIN (G-protein complex) TRANSDUCIN in turn activates a phosphodiesterase. Phosphodiesterase breaks down cyclic GMP (cgmp) levels 41 VISUAL PHYSIOLOGY PhotoReception STEP 3 : Phosphodiesterase thus reduces cgmp levels ; this reduces the numbers of Na+ channels that are open. It results in a hyper-polarization! STEP 4 : The membrane potential drifts back to around - 70 mv. This reduces the release of neurotransmitters at the synapse with the bipolar cells. Thus, in contrast with what we seen so far, stimulation of the receptor results in a decrease in released number of N.T. This is a graded response; the higher the intensity of light, the greater the hyperpolarization and the less the amount of N.T. released! 42 21

VISUAL PHYSIOLOGY 43 VISUAL PHYSIOLOGY 44 22

VISUAL PHYSIOLOGY 1 Light (photons) activates visual pigment. Visual pigment Light All-trans-retinal Phosphodiesterase (PDE) 11-cis-retinal Transducin (a G protein) Open cgmp-gated cation channel Closed cgmp-gated cation channel 2 Visual pigment activates transducin (G protein). 3 Transducin activates phosphodiester ase (PDE). 4 PDE converts cgmp into GMP, causing cgmp levels to fall. 5 As cgmp levels fall, cgmp-gated cation channels close, resulting in hyperpolarization. 45 VISUAL PHYSIOLOGY So, how does the brain receive the information of the light stimuli? The N.T. released by the photoreceptors are inhibitory neurotransmitters (glutamate). They thus cause IPSP s in the bipolar cells ( resulting in nyperpolarixzation) The bipolar cells in turn reduce their frequency of stimulation to the ganglion cells. The reduction in N.T. release induced by the light stimuli thus reduces the amount of IPSP s. The removal of inhibition equates with stimulation. It is similar like having your foot on the brake and slowly releasing the action of your foot! 46 23

VISUAL PHYSIOLOGY In the dark 1 cgmp-gated channels open, allowing cation influx; the photoreceptor depolarizes. 2 Voltage-gated Ca 2+ channels open in synaptic terminals. 3 Neurotransmitter is released continuously. 4 Neurotransmitter causes IPSPs in bipolar cell; hyperpolarization results. 5 Hyperpolarization closes voltage-gated Ca 2+ channels, inhibiting neurotransmitter release. 6 No EPSPs occur in ganglion cell. 7 No action potentials occur along the optic nerve. Ca 2+ Na + Ca 2+ Photoreceptor cell (rod) Bipolar cell Ganglion cell mv - 40 mv - 70 mv Dark Due to the open Na+ channel, the rod experiences a dark current that keeps the cell depolarized 47 In the light VISUAL PHYSIOLOGY Photoreceptor cell (rod) Bipolar cell Ganglion cell Light Ca 2+ 1 cgmp-gated channels are closed, so cation influx stops; the photoreceptor hyperpolarizes. 2 Voltage-gated Ca 2+ channels close in synaptic terminals. 3 No neurotransmitter is released. 4 Lack of IPSPs in bipolar cell results in depolarization. 5 Depolarization opens voltage-gated Ca 2+ channels; neurotransmitter is released. 6 EPSPs occur in ganglion cell. 7 Action potentials propagate along the optic nerve. mv - 40 mv - 70 mv Dark Light Light response closes Na+ channel due to the cgmp breakdown and the rod experiences a hyperpolarization. 48 24

VISUAL PHYSIOLOGY Recovery after stimulation After light stimulation, retinal is in the TRANS form and does not spontaneously convert back to the CIS form Shortly after light stimulation, rhodopsin actually breaks down into retinal and opsin. (called bleaching effect). Photoreceptors cannot function with damaged rhodopsin. Thus, before it can become an active molecule again, it needs to be glued back together. This can only occur if retinal is in the CIS position. The occurs in the dark and requires enzymes and ATP. 49 VISUAL PHYSIOLOGY Recovery after stimulation 50 25

VISUAL PHYSIOLOGY Dark Adapted State When exposed to the dark long enough, all photoreceptors are loaded and ready. Our visual system is then in a highly sensitive state and receptive to small amounts of light. Light Adapted State When moving from a dark area to a bright area, all photoreceptors become bleached and thus reduce the immediate sensitivity to a series of additional light stimuli. 51 VISUAL PHYSIOLOGY Color Vision White light is a spectrum of all different colors. If an object absorbs all color, it appears black. The color of an object is determined by the wavelength it reflects ( and thus does not absorb )! A red apple looks red because it absorbs all wavelengths of the visible spectrum except the wavelengths between 620 and 700 nm (red colors). 52 26

VISUAL PHYSIOLOGY The cones are sensitive to colored light. There are three different kinds of cones : Blue cones have highest sensitivity in blue region Green cones are most sensitive in green region Red cones are most sensitive in red region In a person with normal vision 16 % Blue cones 10 % Green cones 74 % Red cones Perception of color is due to the relative integration of information arriving from each cone type. 53 VISUAL PHYSIOLOGY ColorBlindness is the inability to perceive certain colors. It occurs when one or more of the classes of cones become non functional The most common type of color blindness ids red-green color blindness; the red cones are missing and the person cannot differentiate between red or green. The genes for the cones are located on the X chromosome. Thus, color blindness is more common in males ( 10% of all males) than females (0.67% of al females). Total colorblindness is rare! 54 27

VISUAL PHYSIOLOGY Visual Pathway The retina has about 130 million rods/cones 6 million bipolar cells 1 million ganglion cells Thus considerable convergence occurs with respect to signal processing Most convergence occurs with the rods! About thousand rods converge on a single ganglion cell. This ganglion cell ( and thus the rods) monitor a certain portion of the visual field. Such a cell is called an M cell! Since rods are more effective in dim light, M cells provide information about the fact that light has arrived in a certain area. 55 VISUAL PHYSIOLOGY Visual Pathway The cones shows little convergence The ratio of cones to bipolar cells is almost 1 : 1 in the fovea Ganglion cells that monitor cones are called P cells They are active in bright light and provide information about detail, color from a very specific area About thousand rods converge on a single ganglion cell. Difference between M and P cells can be explained in terms of a computer screen or photography M cells (rods) provide grainy, fuzzy pictures with low resolution, blurry details P cells (cones) provide high resolution, fine grained, sharp and clear detail 56 28

VISUAL PHYSIOLOGY Visual Pathway in CNS Axons from the ganglion cells meet up at the optic disc and exit the eyeball They proceed as the OPTIC nerve towards the diencephalon The optic nerves cross over at the optic chiasm and become the optic tracts The optic tract proceeds to the Laterate Geniculate Nucleus in the Thalamus Here information is passed on via the projection fibers, called the optic radiation, to the visual cortex of the occipital lobe. Collateral fibers at the Geniculate Nucleus connect with the Superior collicluli, pineal gland, RAS, and other nuclei 57 VISUAL PHYSIOLOGY The Binocular zone is the overlapping zone seen by both eyes, which is important for depth perception. Not all axons cross over at the optic chiasm. The axons that come from the left side of each eye proceed to the left hemisphere, while those from the right proceed to the right hemisphere. This partial cross over thus brings together that information that comes from the same area of the visual field 58 29

VISUAL PHYSIOLOGY This partial cross over has some interesting effects. 59 30