Textbook Notes (363,084)
Canada (158,183)
Neuroscience (289)
NROC64H3 (81)
Chapter 9

NROC64 - Chapter #9 Notes.docx

8 Pages
Unlock Document

University of Toronto Scarborough
Matthias Neimier

NROC64 - Chapter #9: The Eye:  Sensitivity to light allows us to detect prey, predators and mates  Light is electromagnetic energy that is emitted in the form of waves  About half the human cerebral cortex is involved with analyzing the visual world  At the back of the eye is the retina – contains photoreceptors specialized to convert light energy into neural activity  Eye has the ability to track moving objects and the ability to keep its transparent surfaces clean (by blinking)  The retina is actually a part of the brain  Each eye has 2 overlapping retinas: one specialized for low light levels that we encounter from dusk to dawn and another for higher light levels and for the detection of colour from sunrise to sunset  Axons of retinal neurons are bundled into optic nerves which distribute visual information to several brain structures that perform different functions  The first synaptic relay that serves perception occurs in a cell group of the dorsal thalamus called the lateral geniculate nucleus (LGN)  cerebral cortex  interpreted and remembered Properties of Light:  The visual system uses light to form images of the world around us Light:  Electromagnetic radiation that is visible to our eyes – can be described as a wave of energy  Wavelength – the distance b/w successive peaks or troughs  Frequency – the # of waves per second  Amplitude – the diff b/w wave trough and peak  Energy content of electromagnetic radiation is proportional to its frequency  Radiation emitted at high frequency (short wavelengths) has the highest energy content  Lower frequencies = longer wavelengths = less energy  Only a small part of the electromagnetic spectrum is visible to our eye  Visible light: 400-700nm  Colours themselves are coloured by the brain Optics:  Light rays travel in straight lines until they interact with atoms and molecules of the atmosphere and objects on the ground  These interactions include: reflection, absorption and refraction  Reflection – the bouncing of light rays off a surface o Strikes at 180, reflected back; strikes at 45, reflected at 90  Absorption – transfer of light energy to a particle or surface o black surfaces absorb all visible wavelengths o blue absorbs long wavelengths but reflects short ones  light sensitive photoreceptor cells in the retina contain pigments and use the energy absorbed from light to generate changes in membrane potential  refraction – the bending of light rays that can occur when they travel from one transparent medium to another o light passes through air more rapidly than water The Structure of The Eye:  organ specialized for detection, localization and analysis of light Gross Anatomy of the Eye:  pupil – the opening that allows light to enter the eye and reach the retina; appears dark b/c of the light-absorbing pigments of the retina  Iris – surround the pupil, pigmentation provides what we call the eye’s colour o Contains 2 muscles that can vary in size: one makes it smaller when it contracts and the other larger  Cornea – glassy transparent external surface of the eye, surround the pupil and iris  Sclera – continuous with the cornea, “white of the eye” which forms the tough wall of the eyeball  Eye’s orbit – bony socket in the skull in which the eye sites  Extraocular muscles – inserted into the sclera are three pairs of these; which move the eyeball in the orbit  Conjuctiva – a membrane that folds back from the inside of the eyelids and attaches to the sclera  Optic nerve – carrying axons from the retina, exits the back of the eye, passes through the orbit, and reaches the base of the brain near the pituitary gland Opthalamoscopic Appearance of the Eye:  Opthalmoscope – device that enable one to peer into the ye through the pupil of the retina  Optic disk – pale circular region from which the retinal blood vessels originate o Also where the optic fibers exit the retina  Sensation of light cannot occur at the optic disk b/c there are no photoreceptors nor can it occur where the blood vessels are  Our brain fills in any holes of our vision  Macula – in the middle of each retina, darker-coloured region with a yellowish hue, part of the retina for central vision  Vessels arc from the optic disk to the macula  Macula has a relative absence of blood vessels which improves the quality of central vision  Fovea – a dark spot about 2 mm in diameter o Latin for the word pit o Retina is thinner in the fovea than elsewhere o Marks the centre of the retina o Part that lies near the nose = nasal, near temple = temporal, above = superior, below = inferior Cross-Sectional Anatomy of the Eye:  The cornea lacks blood vessels and is nourished by the fluid behind it, the aqueous humor  Lens – behind the iris  Lens suspended by ligaments (zonule fibers) attached to the ciliary muscles, which are attached to the sclera and form a rind inside the eye  Changes in the shape of the lens enable our eyes to adjust their focus to different viewing distances  Lens also divides the interior of the yes into 2 compartments containing slightly diff fluids: o Aqueous humor – watery fluid that lies b/w the cornea and the lens o Vitreous humor – more viscous, jelly-like, lies b/w the lens and the retina, its pressure serves to keep the eyeball spherical Box 9.1 – Demonstrating the Blind Regions of your Eye:  Hole in the retina = regions where the optic nerve axons exit the eye and the retinal blood vessels enter the eye, the optic disk is completely devoid of photoreceptors  Blood vessels coursing across retina are opaque and block the light from falling on photoreceptors beneath them  Mechanisms in the visual cortex appear to “fill in” the missing regions Image Formation by the Eye:  Eye collects the light rays emitted by or reflected off objects in the environment and focuses them onto the retina to form images  Cornea is the site of most of the refractive power of the eyes Refraction by the Cornea:  Light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye  Focal distance – the distance from the refractive surface to the point where parallel light rays converge, depends on the curvature of the cornea, the tighter the curve the shorter the focal distance  Dipoter - measurement for the focal distance in meters  Cornea has a refractive power of 42 dipoters, parallel light rays striking the corneal surface will be focused 0.024 (2.4cm) behind it  Refractive power depends on the slowing of light at the air-cornea interface  Water-cornea interface has very little focusing power Accommodation by the Lens:  The lens also contributes to the formation of a sharp image at a distant point  Lens is involved more in the formation of crisp images of objects located closer than about 9 m from the eye  Accommodation – additional focusing power provided by changing the shape of the lens  Ciliary muscle forms a ring around the lens  During accommodation the ciliary muscle contracts and swells in size thereby making the area inside the muscles smaller and decreasing the tension in the suspensory ligaments o Lens become rounder and thicker (b/c of its natural elasticity)  rounding increases the curvature of the lens  increasing refractive power  Conversely the relaxation of the cilary muscle increases the tension in the suspensory ligaments and the lens is stretched into a flatter shape  Ability to accommodate changes with age Box 9.2 – Eye Disorders:  Imbalance in the extraocular muscles in the eyes, the eyes will point in different directions = strabismus (2 types of this) o Esotropia – directions of gaze of the 2 eyes cross and the person is said to be cross-eyed o Exotropia – directions of gaze diverge and person is said to be wall-eyed  Strabismus is congenital – it can and should be corrected in early childhood  Treatment involved the use of prismatic glasses or surgery to the extraocular muscles to realign the eyes  Without treatment one eye is dominant the other is suppressed  The supressed eye will become amblyopic – poor visual accuracy  If medical attention delayed, the damage may be permanent  Cataract – clouding of the lens o Many people over 65 have some degree of cataract o Surgery may be required o In the operation the lens is removed and replaced with an artificial plastic lens o Cannot adjust the same but vision will be good  Glaucoma – progressive loss of vision associated with elevated intraocular pressure, leading cause of blindness o Pressure in aqueous humor plays an important role in maintaining the eyes shape o As this pressure increases the entire eye is stressed, ultimately damaging the weak point where the optic nerve leaves the eye o Optic nerve axons are compressed and vision is generally lost from the periphery inward o Early detection and treatment with medication/surgery to reduce intraocular pressure are essential  Detached retina – retina pulls away from the underlying wall of the eye from a blow to the head or by shrinkage of the vitreous humor o Abnormal perception of shadows and flashes of light o Treatment – laser surgery to scar the edge of the retinal tear, reattaching retina to the back of the eye  Retinis pigmentosa – progressive degeneration of the photoreceptors o First sign: loss of night vision o Cause of this is unknown o In some cases it seems to have a strong genetic component o Currently no cure but taking vitamin A can slow progression  Macular degeneration – lose only central vision o Affects 25% of Americans over 65 years of age o Peripheral vision is usually normal o The ability to read, watch television and recognize faces is lost as central photoreceptors gradually deteriorate o Laser surgery can sometimes help prevent further vision loss, but no known cure Box 9.3 – Vision Correction:  Emmetropic – when ciliary muscles are relaxed and lens is flat; if parallel light rays from a distant point source are focused sharply on the back of the retina o Focuses parallel light rays on the retina without the need for accommodation  Hyperopia/farsightedness – light rays focused on point behind the retina, the eye can focus on far objects but the lens cannot accommodate enough to form an image on near point o Can be corrected by placing a convex glass/plastic lens in front of the eye  If eyeball is too long, parallel rays will converge before the retina cross and again be imaged on the retina as a blurry circle = myopia/nearsightedness o Amount of refraction provided by the cornea and lens is too great to focus distant objects o Artificial concave lenses must be used to move the point image back to the retina  Astigmatism – curvature and refraction in the horizontal and vertical planes is different o Corrected using an artificial lens that is curved more along one axis than others  Presbyopia – hardening of the lens that accompanies the aging process o While new lens cells are generated throughout life, none are lost o The hardened lens is less elasti
More Less

Related notes for NROC64H3

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.