Chapter 9.docx

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Matthias Niemeier

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Chapter 9 The eye Introduction Vision: Based on the light bounced into our eyes from objects, we make sense of a complex world. Light: Electromagnetic energy that is emitted in the form of waves. The waves are: 1. Absorbed 2. Scattered 3. Reflected 4. Bent Retina: Located at the back of the eye. Contains photoreceptors specialized to convert light energy into neural activity. Eye: Automatically adjusts to differences in illumination and automatically focuses itself on objects of interest. The eye can track moving objects and keep its surface clean (via blinking and tears). Each eye has two overlapping retinas: 1. One specialized for low light levels that we encounter from dusk to dawn 2. One specialized for higher light levels and for the detection of color from sunrise to sunset. Function of the retina: Specialized to detect differences in the intensity of light falling on different parts of it. Image processing begins in the retina before any visual information reaches the rest of the brain. Optic nerve: Axon bundles of retinal neurons which distribute visual information (in the form of action potentials) to several brain structures that perform different functions. Targets of the optic nerves: 1. Involved in regulating biological rhythms a. Synchronized with the light-dark daily cycle. 2. Involved in the control of eye position and optics. Lateral geniculate nucleus: The LGN serves as the first synaptic relay in the pathway that serves visual perception. It is located in the dorsal thalamus. From the LGN, visual information ascends to the cerebral cortex, where it is interpreted and remembered. Chapter summary: 1. Explore eye and the retina 2. See how light carries information to our visual system 3. How the eye forms images on the retina 4. How the retina coverts light energy into neural signals that can be used to extract information about luminance and color differences Properties of light Visible light: Light is the electromagnetic radiation that is visible to our eyes. Consists of wavelengths 400-700 nm. Electromagnetic radiation: Described as a wave of energy. Characterized by: 1. Wavelength a. The distance between successive peaks or troughs 2. Frequency a. The number of waves per second b. The energy content of electromagnetic radiation is proportional to its frequency i. Radiation emitted at high frequency (short wavelengths) has highest energy content ii. Radiation emitted at lower frequencies (longer wavelengths) have less energy 3. Amplitude a. The difference between wave trough and peak. Optics Optics: The study of light rays and their interactions. Interactions include: 1. Reflection 2. Absorption 3. Refraction Reflection: The bouncing of light rays off a surface. Most of what we see is light that is reflected off objects in our environment Absorption: Transfer of light energy to a particle or surface. 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. This bending occurs because of the speed of light between two mediums. Images are formed in the eye by refraction. The structure of the eye The eye: Specialized organ for the 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; it appears dark because of the light absorbing pigments in the retina. Iris: Surrounds the pupil. The pigmentation of the iris is what we call the eye color. The iris contains two muscles that can vary the size of the pupil: 1. One makes it smaller when it contracts 2. One makes it larger when it relaxes. Cornea: The glassy transparent external surface of the eye that covers the pupil and iris. Sclera: The white of the eye which forms the tough wall of the eyeball. It is continuous with the cornea. Inserted into the sclera are three pairs of extraocular muscles which move the eyeball in the orbit. These muscles are normally not visible because they lie behind the conjunctiva. Conjunctiva: A membrane that folds back from the inside of the eyelids and attaches to the sclera Eye’s orbit: The bony eye socket in the skull. Optic nerve: 1. Carries axons from the retina 2. Exits the back of the eye 3. Passes through the orbit 4. Reaches the base of the brain near the pituitary gland Ophthalmoscopic appearance of the eye Ophthalmoscope: Device that enables one to peer into the eye through the pupil to the retina. Retina through ophthalmoscope: Obvious feature is the blood vessels on its surface. These retinal vessels originate from a pale circular region called the optic disk. The optic disk is also where the optic nerve fibers exit the retina. Sensation of light: Cannot occur at the optic disk because there are no photoreceptors. Also cannot occur where large blood vessels exist because the vessels cast shadows on the retina. We are not aware of any holes in our field of vision because the brain fills in our perception of these areas. Macula: Located at the middle of each retina. It is a darker colored region with a yellowish hue. I is the part of the retina for central (as opposed to peripheral) vision. The macula is distinguished by the relative absence of large blood vessels. This absence of blood vessels improve the quality of central vision Fovea: A dark spot about 2mm in diameter. It lies in the center of each retina and is surrounded by the macula. It is classified as: 1. Temporal 2. Nasal 3. Superior 4. Inferior Cross sectional anatomy of the eye Aqueous humor: The fluid located between the cornea and the lens. Source of nourishment for the cornea. Lens: Suspended by ligaments (called zonule fibers) attached to the ciliary muscles which are attached to the sclera and form a ring inside the eye. Changes in the shape of the lens enable our eyes to adjust their focus to different viewing distances. The lens divides the two compartments of the eye containing different fluids. Vitreous humor: The fluid located between the lens and the retina; serves as pressure to keep the eyeball spherical. Image formation by the eye Image formation by the eye: The eye collects the light rays emitted by or reflected off objects in the environment and focuses them onto the retina to form images. Bringing objects into focus involves the combined refractive powers of the cornea and lens. The cornea is the site of most of the refractive power of the eyes. Refraction by the cornea: As light passes the cornea and into the aqueous humor, it is refracted and converges onto the back of the eye, towards the retina. Focal distance: The distance from the refractive surface to the point where parallel light rays converge. Focal distance depends on the curvature of the cornea – the tighter the curve, the shorter the focal distance. Diopter: Measure of refractive power: The reciprocal of the focal distance in meters. The cornea has a refractive power of about 42 diopters. This means that parallel light rays striking the corneal surface will be focused 0.024 m (2.4 cm) behind it (about the distance from cornea to retina). Refractive power: Depends on the slowing of light at the interface between two mediums. If the mediums are similar enough, the refractive power will be eliminated. E.g. Things look blurry when you view them underwater because the water-cornea interface has very little focusing power. Accommodation by the lens The lens: Also contributes around 12 diopters to the formation of a sharp image at a distant point. The lens is involved more importantly in forming crisp images of objects located closer than about 9 m from the eye. Accommodation: The process by which focusing power is obtained by changing the shape of the lens. 1. At far distances, a flat lens accommodates vision a. Relaxation of the cilary muscle increases the tension in the suspensory ligaments  the lens is stretched into a flatter shape 2. At near distances, a fat lens accommodates vision a. During accommodation, the ciliary muscle contracts and swells in size, making the area inside the muscle smaller and decreasing the tension in the suspensory ligaments  the lens become rounder and thicker  increases the curvature of the lens surfaces  increases refractive power. The pupillary light reflex: The pupil continuously adjusts for different ambient light levels. The pupillary light reflex involves connections between the retina and neurons in the brain stem that control the muscles that constrict the pupils. A property of this reflex is that it is consensual; shining a light into one eye causes the constriction of the pupils of both eyes (lack of a consensual pupillary light index is often taken as a sign for a disorder involving the brainstem). Constriction of the pupil has the effect of increasing the depth of focus; in this way, distant objects appear to be less out of focus. Visual field: 1. The left visual field is imaged on the right side of the retina and towards the right brain 2. The right visual field is imaged on the left side of the retina and towards the left brain Visual acuity: The ability of the eye to distinguish two nearby points. Depends on several factors: 1. Spacing of photoreceptors in the retina 2. Precision of the eye’s refraction 3. Distance across the retina is described in terms of degrees of visual angle Microscopic anatomy of the retina Neuroscience of vision: The conversion of light energy into neural activity Direct pathway for visual information: 1. Photo receptors 2. Bipolar cells 3. Ganglion cells 4. Optic nerve 5. Brain Retinal processing is influenced by two additional cell types: 1. Horizontal cells a. Receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors 2. Amacrine cells a. Receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells Two important points to remember: 1. Photoreceptors are the only light-sensitive cells in the retina. All other cells are influenced by light only via direct and indirect synaptic interactions with photoreceptors 2. The ganglion cells are the only source of output from the retina. No other retinal cell type projects an axon through the optic nerve The laminar organization of the retina Laminar organization: Cells are organized in layers inside out. From front to back of the retina: 1. Vitreous humor 2. Ganglion cells 3. Bipolar Cells 4. Photo receptors Reason for inside out laminar organization: 1. Pigmented epithelium that lies below the photoreceptor: a. Plays a critical role in the maintenance of the photoreceptors and photopigments. b. Absorbs any light that passes entirely through the retina, minimizing the reflection of light within the eye that might blur the image Cell layers of the retina: Named in reference to the middle of the eyeball: 1. Ganglion cell layer a. Contains the cell bodies of the ganglion cells 2. Inner plexiform layer a. Contains the synaptic contacts between bipolar, amacrine and ganglion cells. 3. Inner nuclear layer a. Contains the cell bodies of the bipolar, horizontal and amacrine cells 4. Outer plexiform layer a. Where photoreceptors make synaptic contact with bipolar and horizontal cells 5. Outer nuclear layer a. Contains the cell bodies of the photoreceptors 6. Layer of photoreceptor outer segments a. Contains the light sensitive elements of the retina. b. The outer segments are embedded in the pigmented epithelium Photoreceptor structure: Every photoreceptor has four regions: 1. Outer segment a. Contains a stack of membranous disks i. Light sensitive photopigments in the disk membranes absorb light 1. Triggering changes in the photoreceptor membrane potential a. There are two types of photoreceptors in the outer segment i. Cone photoreceptors 1. Short tapering outer segment 2. Fewer membranous disks ii. Rod photoreceptors 1. Long cylindrical outer segment 2. Many membranous disks 3. Outnumber cones 20:1 2. Inner segment 3. Cell body 4. Synaptic terminal The structural differences between cone and rod photoreceptors imply functional differences: 1. Rods a. The greater number of disks and higher photopigment concentration make them over 1000 times more sensitive to light than cones i. Under nighttime (scotopic) conditions, only rods contribute to vision b. All rods contain the same photopigment 2. Cones a. Under daytime (photopic) conditions, cones contribute the most to vision. b. Cones contain three types of photopigment i. Variations among pigments make the different cones sensitive to different wavelengths of light ii. Responsible for our ability to see color Duplex retina: A scotopic retina using only rods and a photopic retina using mainly cones. Regional differences in retinal structure: Retinal structure varies from the fovea to the retinal periphery: 1. Peripheral retina: a. Has a higher ratio of rods to cones b. Higher ratio of photoreceptors to ganglion cells i. Results in more sensitivity to light because: 1. Rods are specialized for low light 2. There are more photoreceptors feeding information to each ganglion
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