Vision (1) PDF

Special senses 1) VISION Dr. Marwa abdelrahman  Structure of the eye The eye is a spherical, fluid-filled structure. It enclosed by three layers: 1) The sclera. 2) The choroid. 3) The retina  The Sclera It is the outermost fibrous layer. It forms the...

Special senses 1) VISION Dr. Marwa abdelrahman  Structure of the eye The eye is a spherical, fluid-filled structure. It enclosed by three layers: 1) The sclera. 2) The choroid. 3) The retina  The Sclera It is the outermost fibrous layer. It forms the visible white part of the eye. Consist of connective tissue. Function : it maintains the shape of the globe & provide area of attachment of extra ocular muscles. Anteriorly, the outer layer consists of the transparent cornea.  Cornea has no blood supply , it gets its supply directly from the air. Function of cornea : Allows the light rays pass into the interior of the eye. Refract the entering light (together with lens)  Choroid This is the middle vascular layer underneath the sclera. Characterized by : 1) Highly vascular. 2) Contains dark melanin pigments. Function : 1) Contains blood vessels that nourish the eye. 2) The pigments absorb light after it strikes the retina to prevent reflection or scattering of light within the eye.  The choroid layer becomes specialized anteriorly to form the ciliary body and the iris. Ciliary body: It is an anterior specialization of the choroid layer. It has two major components: 1) the ciliary muscle. ▪ It is a circular ring of smooth muscle. ▪ attached to the lens by suspensory ligaments (zonule) ▪ For accommodation. 2) the capillary network. ▪ Produces the aqueous humor.  The interior of the eye consists of two fluid- filled cavities, separated by an elliptical lens. Theses fluids are transparent to allow light to passes through the eye from the cornea to the retina.  These fluid are : 1) Vitreous humor. ▪ It filled the larger posterior cavity between the lens and the retina. It contains a clear, jellylike substance. function of The vitreous humor : helps to maintain the spherical shape of the eyeball (structural support) 2) Aqueous humor. ▪ The fluid filled the smaller anterior cavity between the cornea and the lens. ▪ contains a clear, watery fluid. The aqueous humor is produced by : The capillary networks within the ciliary body & it drains into a canal ( schlemm’s ) at the junction of the iris & the cornea and eventually enters the blood. Function of the aqueous humor : it carries nutrients to the (a vascular parts of the eye) cornea & lens, both of which lack a blood supply.  If the aqueous humor is not drained as rapidly as it forms ,it accumulates in the anterior cavity, causing the pressure to rise within the eye ( ↑ IOP). This condition is known as Glaucoma. if it left untreated can lead to blindness.  The iris : thin, pigmented smooth muscle muscles. In the center of the iris there is the pupil (the opening at which light enters the eye) Function :of the iris: 1) Controls the The amount of light entering the eye.( how??) 2) The pigment in the iris is responsible for eye color.  The Iris has two muscles that control the amount of light entering the eye (by controlling the size of the pupil). These muscles are supplied by the autonomic nervous system. They are : 1) Sphincter pupillae (circular muscle) → constrict the pupil supplied by parasympathetic NS. 2) Dilator pupillae (radial muscle) → dilate the pupil supplied by sympathetic NS.  The Retina Retina is The innermost neural, pigmented layer. It lines the posterior 2/3 of the choroid. Function : it contains the photoreceptors (Rods & Cons) that convert light energy into nerve impulses.  The point on the retina (near to the center) at which the optic nerve leaves & the blood vessels pass is called the optic disc. This region is often known as the blind spot (why??) Because there are no rods and cones so no image can be detected in this area  The Fovea centralis : is a small depression located in the center of the retina. It contains cons only. Fovea is the point of most distinct vision (because cones have greater acuity than rods)  Macula lutea : Is the area immediately surrounding the fovea also has a high concentration of cones and high acuity. But Macular acuity is less than that of the fovea because of the overlying ganglion and bipolar cells in the macula. Macular degeneration is The condition that characterized by loss of photoreceptors with advancing age secondary to DM, malignant HTN… Its one of the common causes of blindness.  Waves length Light : is a form of electromagnetic radiation that travels in forms of waves. The distance between two wave peaks is known as the wavelength.  The wavelengths in the electromagnetic spectrum range from 10 nm (extremely short) to 100 km or more ( as in long radio waves).  The photoreceptors in the eye are sensitive only to wavelengths between 400 - 700 nm. This means that the visible light is only a small portion of the total electromagnetic spectrum. The shorter visible wavelengths are sensed as → violet while the longer wavelengths are seen as → red  Refraction of light Light waves diverge in all directions from a light source. The light rays tend to bend when it transfer from media to another different in density. This s known as Refraction. When the light rays travel across a perpendicular plane → no refraction occur. But when the light strikes new media at any angles → refraction occur  The lens When the light ray reaching the eye and in order to see, it must be focused into a point on the retina. Lens & cornea are the eye refractory surface. lens has an important function in converging the light into the focal point in the retina. In general ,There are two types of lens : 1) Convex lens It converges light rays. The lens of the eye is convex. 2) Concave lens Diverges light rays. A concave lens is useful for correcting certain refractive errors in the eyes.  Accommodation The ability to adjust (increase) the strength of the lens is known as Accommodation. The strength of the lens depends on its shape, which in turns is regulated by the ciliary muscle. When the ciliary muscle is relaxed → the suspensory ligaments are taut(tense), they pull the lens to be flat (weak refractive shape)  But as the muscle contracts → the tension in the suspensory ligaments decreases so the ligaments are not pulling on the lens, and the lens becomes more spherical (rounded) The more rounded lens ,more strength lead to more bending of light rays.  For the Far vision (more than 6 meters) → the light rays are coming parallels (perpedindular plans), ciliary muscles are relaxed. For the Near vision (less than 6 meters) → the light rays here is coming diverging (at angles) so many changes occur in the eye to focus the near object in the retina (not behind). This is called the near point response It is includes : Accommodation & pupillary constriction.  The ciliary muscle is controlled by the autonomic nervous system. Sympathetic stimulation causing relaxation of the muscle (for far vision) Parasympathetic causes contraction (for the near vision).  why most people by middle age (45 -50 ys) requiring a corrective lenses for near vision (reading) ??( Presbyopia ) Cataract ?? Error of Refractions  Myopia (near sightedness)) 1 ▪ Means seeing the near object better. ▪ When the eyeball is larger than normal ,or the lens is too strong. ▪ The light rays are focused in front of the retina.  Correction : Corrected by concave lenses.  2) Hypermetropia (hyperopia( farsightedness Seeing the far objects better. Duo to the eyeball is shorter than normal, or the lens is too weak. The ligth rays focused behined the retina  Correction : Corrected by using convex lenses many people now correct the refractive errors with laser eye surgery (LASIK) which permanently change the shape of the cornea.  The Retinal cells Retina is a specialized neural layer. Microscopically, it contains photoreceptors & other neuronal cells that arranged in layers. These neuronal cells include : 1) Bipolar cells (B) 2) Horizontal cells (H) 3) Ganglion cells (G) 4) Amacrine cells (A)  These cells are arranged into layers , forming the layers of the retina, which are: 1) Pigmented cell layer. ▪ Lies just beneath the choroid. ▪ Have pigmented cells. 2) Photoreceptors cells layer. ▪ Consist of rods & cones. 3) Outer nuclear layer. ▪ Contain the nuclei of the photoreceptors. 4) Outer plexiform layer Contains the synaptic connections (photoreceptors –H & B cells) 5) Inner nuclear layer (nuclei of B cells) 6) Inner plexiform layer Synaptic connections (G -A & B cells) 7) Ganglion cell layer Contains the ganglion cells bodies 8) Optic nerve layer ▪ The axons of the ganglion cells form the optic nerve. ▪ These axons pass through the retina enters the optic disc & leave the eye as optic nerve. i  Photoreceptors ▪ They are receptors located in the retina, stimulated by light. Photoreceptors are two types : 1) Rods 2) Cones Structure : Each rods and cons have outer segment, inner segment & synaptic connection. 1) The outer segment : It is rod- shaped in rods & cone -shaped in cones. It membrane contains an abundance of light- sensitive photo pigments. Theses pigments are made up of two proteins: 1) Retinal. (a derivative of vitamin A) 2) Opsin. ▪ Cones has 3 pigments each of which respond to one color. ▪ While rods has one pigment (Rhodopsin). 2) The inner segments : Contain the nucleus and other organelles (rich in mitochondria) 3) The synaptic terminal. Synapse with the H & B cells.  Properties of Rods & Cones Cones Rods 6 million per retina 120 million per retina Concentrated in the center (fovea) More numerous in the periphery Low sensitivity (need bright light To be High sensitivity (can be activated by activated) dim light) Works only in Day vision Works in Night vision High acuity (sharp vision with details) Low acuity (outlines, not details ) Color vision )3 types of )shades of gray /black & white), one photopigments) pigment only  Mechanism of Photoreception (Phototransduction( Phototransduction: Is the process of converting light energy into electrical signals. This occur secondary to the conformational changes that occur to Photopigments when exposed to light.  Photoreception in rods: The pigment in rods is called rhodopsin. Rodopsin consist of : 1) Retinal 2) Opsin (scotopsin) In the absence of stimulation (dark): Na+ channels are open & Na+ ions influx continuously. This lead to depolarization of the photoreceptors & release of inhibitory NT (glutamate) This inhibits the visual rods pathway.  Activation of Rhodopsin by exposure to light ↓ change 11-cis retinal to the all-trans retinal )Metarodpsin( ↓ closure of the Na+ channels (by↓ intracellular cGMP) ↓ Hyperpolrization (duo to lack of Na influx) ↓ inhibits the release of glutamate at the synaptic terminal of rods.  Release of Stimulatory NT from the bipolar cells ↓ AP generated in the G cells. ↓ Conducted though the optic nerves to the brain. ✔ Rods photoreceptor are hyperpolarized by light & depolarized by dark.  Color Vision Color vision is the perception of the colors. Photoreception in cones is similar to rods. Cones have 3 types of pigments Each photo pigment absorbs different wavelengths of light make them sensitive to one of the primary color.(red, green & blue)  An object perceived as blue → absorbs the longer red and green wavelengths of light and reflects the shorter blue wavelengths, which can stimulate by the photopigment in the eyes (blue cones) & activate them  The input from one type of cones give sensation of one of the primary color. When the input is added from other cones give sensation of other different colors. Young-helmholtz theory??  White color is a mixture of all wavelengths of light, black color is the absence of light  Color blindness: is blindness of one or two colors duo to lack of cones pigments. These people can not distinguish between red & green colors because they determined by gene on chromosome x. Ishihara chart used to test for color blindness.  Retinal is a derivative of vitamin A, that’s why adequate amounts of this vitamin must be available (for synthesis of Rhodopsin ) Night blindness occurs as a result of deficiency of vitamin A. The person can see in the day using cones but cannot see at night because rods are not functioning.  Visual Pathway AP (which has been generated in the photoreceptors) are transmitted through the pathway to the cerebral cortex for perception of vision. Visual field is the field of view that can be seen without moving the head. Divided into nasal & temporal fields.  Visual pathway is as follow : Each Optic nerves carry the nasal fibers (medial side of the eye) & the temporal fibers (from the lateral side of the eye) The two optic nerves meet at the optic chiasm located underneath the hypothalamus. Within the optic chiasm, the fibers from the nasal side of each retina crossed to the other side & ascend contralatrally , while fibers from the temporal side remain uncrossed, ascend in epsilatrally. Thses forms the optic tracts.  Each optic tract synapse in the lateral geniculate body of the thalamus. Then the optic fibers travel through the deep white matter of the cortex , known as optic radiation. the fibers terminates in the primary Visual cortex in the occipital lobe. Some fibers go to the visual associated areas for processing. Visual filed defect  Eye movement Not all fibers in the visual pathway terminate in the visual cortex. Some are projected to other areas in the brain for non- sight activities like eye movement (e.g while reading) eye movements are controlled by (extraocular muscles).  Light Reflex When the light is directed to the eye, pupil of the same eye is constricted (direct light reflex) The pupil in the opposite eye is constricted too (consensual or indirect light reflex)