The Eye

Chapter 9: The Eye

* Light is electromagnetic energy that is emitted in form of waves; waves crash into objects and are absorbed, reflected, scattered, and bent * Half of human cerebral cortex is involved with analyzing visual world * MAMMALIAN VISUAL SYSTEM: begins with eye, back of eye is retina (contains photoreceptors specialized to convert light energy to neural activity) * Eyes have features to track moving objects and keep transparent surface clean (i.e. by tears) * Each eye has two overlapping retinas: one specialized for low light levels and one for higher light levels and detection of colour * Retina specialized to detect differences in the intensity of light * Axons of retinal neurons bundled into optic nerves, which distributes visual information (in form of AP) to several brain structures that perform difference functions * First synaptic relay in pathway that serves visual perception occurs in a cell group of dorsal thalamus called the lateral geniculate nucleus (LGN) * Visual information from LGN -> cerebral cortex (where it is interpreted and remembered)

PROPERTIES OF LIGHT: * Light: electromagnetic radiation that is visible to our eyes * Electromagnetic radiation has wavelength (distance between peaks), frequency (# of waves/second), and amplitude (difference between wave peak) * Energy content of electromagnetic radiation is proportional to its frequency (radiation emitted at high frequency – shorter wavelength – has highest energy content, vice versa) * Visible light = wavelength consist of 400 – 700 nm * Optics * Wave of electromagnetic radiation travels in straight line = rays. Rays travel until they interact with atoms and molecules; interactions include reflection, absorption, and refraction. [study of light rays and their interactions is called optics] 1. Reflection: bouncing of light rays off a surface. 2. Absorption: transfer of light energy to a particle or surface. 3. Refraction: forms images in the eye, bending of light rays that can occur when they travel from one transparent medium to another.

THE STRUCTURE OF EYE: * Eye: organ specialized for detection, localization, and analysis of light. * Gross Anatomy of the Eye: * Pupil – opening that allows light to enter the eye and reach the retina; surrounded by light-absorbing pigments in retina (appears black) * Iris – surrounds pupil, pigmentation provides eye’s colour. Contains 2 muscles that can vary size of pupil; one makes it smaller when it contracts, other makes it larger. * Cornea – covers pupil and iris, continuous with sclera (“white of the eye”) * Sclera – 3 pairs of extraocular muscles, which move the eyeball in orbit (muscles behind conjunctiva – membrane that folds back from the inside of the eyelids and attaches to the sclera) * Optic nerve - carrying axons from retina, exits the back of eye, passes through orbit, reaches base of brain near pituitary gland.

* Ophthalmoscopic Appearance of the Eye: * Ophthalmoscopic – device that allows one to peer into the eye through pupil to the retina * Optic disk – where retinal vessels originate and where the optic nerve fibers exit the retina * Sensation of light cannot occur at the optic disk because no photoreceptors present * Macula – middle of each retina – part of retina for central vision, absence of large blood vessels from this region improves the quality of central vision * Fovea – dark spot 2 mm, retina thinner in fovea because it marks center of retina. * Part of retina that lies closer to the nose than the fovea is called nasal, near the temple is called temporal, above fovea is called superior and below it is called inferior. * Cross-Sectional Anatomy of the Eye: shows path taken by light cornea -> retina * Cornea lacks blood vessels and nourished by the fluid behind it, aqueous humor * Transparent lens behind iris is suspended by zonule fibers attached to ciliary muscles, which are attached to sclera * Lens divide inferior of eye into 2 compartments containing 2 different fluids: aqueous fluid (watery) lies between cornea and lens, jellylike vitreous humor lies between lens and retina; it’s pressure keeps eyeball spherical

IMAGE FORMATION BY THE EYE * Bringing objects into focus involves combined refractive powers of the cornea and lens. * Cornea, rather than the lens, is the site of most of the refractive power of the eyes. * Refraction by the Cornea: * Distance from the refractive surface to the point where parallel light rays converge is called focal distance, tighter curve = shorter focal distance. Diopter = unit of measurement * Cornea has refractive power of about 42 diopters * Accommodation by the Lens: * Cornea performs most of eye’s refraction * Flat lens is involved in forming crisp images with little refraction required, and changing the shape of leans (accommodation) for near objects require greater refraction provided by more spherical lens * The Pupillary Light Reflex: * Pupillary light reflex involves connections between retina and neurons in brain stem that control the muscles that constrict the pupils * Constriction of pupil = increases the depth of focus * The Visual Field * Visual field is the total amount of space that can be viewed by the retina when the eye is fixated straight ahead * Visual Acuity – ability of eye to distinguish two nearby points

MICROSCOPIC ANATOMY OF THE RETINA: * Most direct pathway for visual information to exit the eye is from photoreceptors -> bipolar cells -> ganglion cells * Ganglion cells fire AP in response to light -> optic nerve -> rest of brain * Horizontal cells – receive input from the photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors * Amacrine cells – receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells.

* 2 Important Points: 1. The only light-sensitive cells in the retina are the photoreceptors. 2. The ganglion cells are the only source of output from the retina * The Laminar Organization of the Retina: * Light must pass from vitreous humor through ganglion cells and bipolar cells before it reaches photoreceptors * Pigmented epithelium that lies below the photoreceptors absorbs any light that passes entirely throught he retina, minimixing the reflection of light within the eye that would blur the image * Innermost layer = gangion cell layer (contains cell bodies of ganglion cells), inner nuclear layer (contains cell bodies of bipolar cells, horizontal and amacrine cells), outer nuclear layer (contains cells bodies of photoreceptors), and layer of photoreceptor outer segments (contains light-sensitive elements of retina). * Between ganglion cell layer and inner nuclear layer is “inner plexiform layer” which contains the synaptic contacts between bipolar cells, amacrine cells and ganglion cells * Between outer and inner nuclear layers is “outer plexiform layer” where the photoreceptors make synaptic contact with bipolar and horizontal cells * Photoreceptor Structure: conversion of electromagnetic radiation into neural signals occurs in photoreceptors * 4 regions: outer segment, inner segment, cell body, and a synaptic terminal. * 2 types of photoreceptors: rod – long, cylindrical outer segment, containing many disk and cone – have shorter, tapering outer segment with fewer membranous disks. * Rods are 1000 times more sensitive to light than cones because of higher # of disks and higher photopigment concentrations; cones only responsible for our ability to see colour * Regional Differences in Retinal Structure: * Region of retina most highly specialized for high-resolution vision = fovea
PHOTOTRANSDUCTION: rods outnumber cones by ratio of 20:1 in human * Phototransduction in Rods: * Photoreceptor, light stimulation of photopigment activates G-proteins, which in turn activate an effector enzyme that changes the cytoplasmic concentration of a second messenger molecule * Change causes membrane ion channel to close; hence, membrane potential altered * Movement of + change across membrane occurs in dark = dark current. Na+ stimulated to open and are gated by cGMP (second messenger). cGMP produced in photoreceptors continuously by enzyme guanylyl cyclase to keep Na+ channels open. Light reduces cGMP -> Na+ channels close -> MP becomes more negative * Photoreceptors hyperpolarize in response to light * Phototransduction in Cones: * cGMP levels in sunlight fall to the point where the response to light becomes saturated. * Young-Helmholtz trichromacy theory: the three cones; red, green, and blue activate to give rise to other others * Dark and Light Adaptation:
RETINAL PROCESSING: * Only ganglion cells fire AP; all other cells in retina (except some amacrine cells) respond to stimulation with graded changes in membrane potential * Most direct path for information flow in the retina is from a cone to bipolar cell to ganglion cell