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Chapter 9

Bear chapter 9.pdf

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Department
Neuroscience
Course
NROC64H3
Professor
Matthias Niemeier
Semester
Winter

Description
Bear chapter 9: of the world on the retina. It automatically adjusts to Introduction: differences in illumination and  Sensitivity to light enables automatically focuses itself on animals, including humans to objects of interest. The eye is detect prey, predators and about to track moving objects and keep its transparent mates. Light is an surface clean by tears and electromagnetic energy that is emitted in form of waves. blinking features not found in Waves crashes into objects and camera. are absorbed, scattered,  The retina is much more than a film. It is part of the brain. The reflected and bent. The visual system can extract information eyes has 2 overlapping retinas: about the world due to the one specialized for low light nature of the electromagnetic levels that we encounter from waves and their interactions dusk to down, another specialized for higher light with the environment.  It requires a lot of neural levels and for the detection of machinery and has provided color, from sunrise to sunset. new ways to communicate, The output of the retina isn’t a faithful reproduction of the given rise to brain mechanisms intensity of the light falling on for predicting the trajectory of objects, events in time and it. The retina is specialized to space. Which gives rise to the detect differences on the formation of mental imagery intensity of light falling on different parts of it and image and abstraction? Half of the human cerebral cortex is processing is well under way in involved with analyzing the the retina, before any visual visual world. information reaches the rest of  The mammalian visual system the brain.  Axons of retinal neurons are begins with the eye. The retina lies back the eye which bundled into optic nerves, contains photoreceptors which distribute visual specialized to convert light information in form of AP to energy into neural activity. The several brain structures that perform different functions. rest of the eye acts as a camera which forms crisp, clear images Some targets of the optic nerves are involved in frequency. Radiation regulating biological rhythms, emitted at high frequency which are synchronized with (short wavelength) has the the light-dark daily cycle; highest energy content e.g others are involved in the gamma and x-rays. control eye position and optics. Radiation emitted at low First synaptic relay in the frequency (long pathway that serves visual wavelength) has the lowest perception occurs in a cell energy content e.g radio group of the dorsal thalamus waves and radar greater called LGN (lateral geniculate than 1mm. visible light is nucleus). The visual the part of electromagnetic information ascends to the spectrum that is detectable cerebral cortex from the LGN by the visual system where it is interpreted and consisting of wavelength of remembered. 400-700nm. The mix of wavelengths in this range Properties of light emitted from the sun Visual system uses light to form appears to humans as images of the world around us. white, whereas light of a single wavelength appears Light as one of the colors of a  Light is the electromagnetic rainbow. Hot colors like radiation that is visible to “red and orange” consists of light with longer our eyes. Electromagnetic wavelength and less energy radiation is a kind of energy than a cool color like blue or wave which has a wavelength, the distance violet. Colors are “colored” between successive peaks by the brain. or troughs; a frequency, the OPTICS number of waves per second; amplitude, the  A ray is a wave of difference between wave electromagnetic radiation trough and peak. travelling in a straight line. Light waves travel in  Energy content of electromagnetic radiation is straight lines until they proportional to its interact with the atoms and molecules of the changes in membrane atmosphere and objects on potential. the ground. These  Refraction the bending of interactions include light rays that occur when reflection, absorption and they travel from one refraction. Optics is the transparent medium to study of light rays and their another which is how interactions. images are formed in the  Reflection is the bouncing eye. A ray of light passing of light rays off a surface. through air into a pool of This depends on the angle water will bend towards a at which it strikes the line that is perpendicular surface. A ray striking a the surface if it strikes the mirror perpendicularly is surface at an angle. This reflected 180 back and a ray occurs because the speed of striking 45 is reflected 90 light differs in the two  Absorption is the transfer media. Light passes through of light energy to a particle the air more rapidly than or surface. Surfaces that water. The greater the appear black absorb energy differences between the of all visible wavelengths. speeds of light in the two media, the greater the Some compounds absorb light only in limited range of angle of refraction. The wavelengths, then reflects transparent media in the the remaining wavelength. eye bend light rays to form This is the basis for the images on the retina. colored pigment of paints Structure of the eye e.g blue pigment absorb long wavelengths but Eye is an organ specialized for reflects a range of short detection, localization and analysis of wavelength centred on light. 430nm perceived as blue. Gross anatomy of the eye Light sensitive photoreceptor cells in the  The pupil is the opening that retina contain pigments and use the energy absorbed allows light to enter the eye and reach the retina; it appears from the light to generate dark because of the light- absorbing pigments in the through the pupil to the retina. retina. Iris surrounds the pupil The blood vessel on the surface whose pigmentation gives the of the retina is the most eye its color. The iris contains 2 obvious feature viewed using muscles that can vary the size the device. Optic disk a pale of the pupil; one making it circular region which is where smaller when it contracts and the retinal vessels originate the other bigger. Cornea is the from, also where the optic glassy transparent external nerve fibers exit the retina. surface of the eye that covers  Sensation of light cannot occur the iris and the pupil. Sclera at the optic disk because there the white of the eye that forms are no photoreceptor located the tough wall of the eyeball there, nor can it occur where which is continuous with the the large blood exist because cornea. The eyeball sits on a the vessels cast shadows on bony eye socket in the skull, the retina. The brain fills in the also called the eye’s orbit. perception of the holes in our Extraocular muscles are eyes making it impossible for muscles inserted in the sclera us to notice them. which moves the eyeball in the  Macula a darker- colored orbit. These 3 muscles are not region with a yellowish hue visible because they lie behind found at the middle of the the conjunctiva which a retina. This part of the retina membrane that folds back for central vision. It is from the inside of the eyelids distinguished by the relative and attaches to the sclera. absence of large blood vessels. Optic nerve carries axons from The absence of large blood the retina, exiting the back of vessels in this region is one of the eye, passing through the specialization that improves orbit and reaching the base of the quality of central vision. the brain near the pituitary Another specialization of the gland. central retina is the fovea a Ophthalmoscopic appearance of the dark spot about 2mm in diameter and the retina is eye thinner in the fovea than elsewhere. The fovea makes  Ophthalmoscope a device that enable one to peer into the eye the center of the retina and it’s a convenient anatomical ligament called zonule fibers reference point. The part of the attached to the ciliary muscles retina that lies closer to the which are attached to the nose than the fovea is called sclera and form a ring inside nasal, the part near the temple the eye. Changes in the shape is called the temporal. Part of the lens enable the eye to above the fovea is called adjust its focus to different superior and below is called viewing distances. inferior.  The lens divides the interior of  The region where the optic the eyes into two nerve axons exit the eye and compartments containing the retinal blood vessels enter slightly different fluids. the eye is devoid of Aqueous humor is the watery photoreceptors beneath them. fluid that lies between the Holding a book about 1.5ft cornea and the lens. Vitreous away and closing your right eye humor a more viscous jellylike and fixating it on your left eye fluid lies between lens and the and moving the book around retina and its pressure serves slightly, you will eventually find to keep the eyeball spherical. a position where the black circle disappears. This is where IMAGE FORMATION BY THE EYE the spot is imaged on the optic The eye collects the light rays disk of the left eye, which is emitted by or reflected off objects in called the blind spot. Why the environment, and focuses them does the visual world appear onto the retina to form images. uninterrupted and seamless? Bringing the objects into focus This is because the mechanism involves the combined refractive in the virtual cortex appears to powers of the cornea and lens. The fill in the missing regions. lens is the site of most refractive power of the eyes. Cross-sectional anatomy of the eye REFRACTION BY THE CORNEA  Aqueous humor the fluid behind the cornea that  We see the star because the nourishes it due to its lack of eye focuses the star’s light to a blood vessels. Lens a point on the retina. The light transparent cell behind the iris rays striking the surface of the which is suspended by eye from a distant star are virtually parallel, so they bent restores the air-cornea due to refraction interface.  Light strikes the cornea and ACCOMODATION BY THE LENS passes from the air into the aqueous humor. Light rays that  The lens is involved in forming strike the curved surface bend crisp images of objects located so that they converge on the closer than about 9m from the back of the eye, they enter the eye. As an object approaches, centre of the enter passing the light rays originating at a through the retina. Focal point can no longer be distance the distance from the considered to be parallel. refractive surface to the point These rays diverge and greater where parallel light rays refractive power is required to converges. It depends on the bring them focus on the retina. curvature of the cornea, the Accommodation is an tighter the curve, the shorter additional power provided by the focal distance. Diopter unit changing the shape of the lens. of measurement for the During accommodation, the reciprocal of the focal distance ciliary muscle contracts and in meters. The refractive power swells in size, thereby making of the cornea is about 42D the area inside the muscle meaning that the parallel light smaller and decreasing the rays strike the corneal surface tension in the suspensory about 0.024m behind it, about ligaments. The lens becomes the same from the cornea to rounder and thicker because of the retina. Refractive power its natural elasticity. This depends on the slowing light at rounding increases the the air-cornea interface. If air is curvature of the lens surfaces, replaced with a medium that increasing their refractive light passes at the same speed power. The relaxation of the as the eye, the refractive index ciliary muscle increases the will be eliminated. That’s tension in the suspensory reason why things look blurry ligament and the lens is when you open your eyes stretched into a flatter shape. because the water-cornea Accommodation changes with interface has very little age. Infants have more focusing. A scuba mask accommodation ability than clear image and glasses can be adults. used for near and far vision. Glaucoma is a progressive loss  Eye disorder if there is an imbalance in the extraocular of vision associated with muscles of the two eyes, the elevated intraocular pressure is eyes will point in different the leading cause of blindness. directions. This is known as the Pressure in the aqueous humor plays a crucial role in strabismus and there are two varieties; esotropia the maintaining the shape of the direction of the gaze of the two eye. The eye is stressed as the eyes cross, so the person is said pressure increases thereby, to be crossed eyed. Exotropia damaging the relatively weak the direction of the gaze is said point where the optic nerve to be diverged and the person leaves the eye. Optic nerves is said to be wall eyed. In most are compressed and vision is cases, it is said to be gradually lost from the congenital. Treatment is peripheral inward. The light usually involves prismatic sensitive retina at the back of glasses or surgery to the the eye is the site of numerous extraocular muscles to realign disorders that pose a them. Without correction, significant risk of blindness. A conflicting images are sent to detached retina results from the brain from the two eyes the retina pulling away from degrading depression the underlying wall of the eye perception. The suppressed due to a blow to the head of eye will become amblyopic shrinkage of the vitreous humor. This results into trauma meaning that it has poor visual acuity. Cataract is the most and symptoms include common eye defect in older abnormal perception of adults meaning a clouding lens shadows and flashes of light, and it is usually seen on adult treatment includes laser over the age of 65. During the surgery. Retinitis pigmentosa correction procedure, the lens is characterized by a is usually removed and progressive degeneration of replaced with an artificial photoreceptors. Symptoms plastic lens. Although, it cannot include loss of peripheral vision adjust its focus it provides a and night vision. It has a strong genetic component and more when the eyeball is too short than 100genes have been that the parallel rays will identified without a cure but converge before the retina taking Vit. A slows its effect. cross, and becomes a blurry People with macular circle on the retina. A concave degeneration only lost central lens is used to move the vision. Affect more than 25% of images on the retina. people over 65. While Astigmatism is the difference peripheral vision is normal, in the curvature and refraction ability to read, recognize faces in the horizontal and vertical is lost as central plane and can be corrected photoreceptors is lost. Laser using artificial lens that is surgery minimizes effect but it curved more along one axis has no cure. than others. Presbyopia is the hardening of the lens that VISION CORRECTION accompanies the aging process and is thought to be explained  The eye is said to be emmetropic when the ciliary by the fact that while new lens muscles are relaxed and the cell generate, none are lost. lens is flat and the parallel light The correction is the use of rays focuses on the retina bifocal lens which are concave on top to assist far vision and without the need for accommodation. convex at the bottom to assist Hyperopia/farsightedness is near vision. Radial keratotomy, when the eyeball is short from a procedure t
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