Physiology of Vision PDF

Physiology of vision - Light  Electromagnetic radiation  Wave length of visible light is 400-700nm  Invisible light: ultraviolet 700nm  Light moves in air (vaccum) at speed of 300,000Km/sec  When the light is transmitted from one medium to another of diff...

Physiology of vision - Light  Electromagnetic radiation  Wave length of visible light is 400-700nm  Invisible light: ultraviolet 700nm  Light moves in air (vaccum) at speed of 300,000Km/sec  When the light is transmitted from one medium to another of different density  Light speed changes  Light is refracted  Refractive index (RI)  The power of reflection  How much the light speed decreases  RI = light speed in air / light speed in medium  RI of  Cornea = 1.38  Aqueous humor = 1.33  Vitreous humor = 1.34  Lens = 1.4  Most of refraction of light in the eye occurs at cornea (air-cornea interface) - Lenses refract light  Convex lens: converging light rays & the focus is a point  Concave lens: diverging light rays - The power of lens is expressed by diopter  Diopter = refractive power = 1/focal length (m)  Positive in convex lenses  Negative in concave lenses  If the eye is considered a single lens, the refractive power = 1/0.017 = +59D Refractive errors of the eye - Myopia  Near sightedness  The image of far objects falls in front of retina  The image of near objects falls on retina  Caused by  Too strong lens  Long eye ball  Correction is done by concave lenses - Hyperopia  Far sightedness  The image of near objects falls behind the retina  The image of far objects falls on the retina  Caused by  Too weak lens  Short eye ball  Correction is done by convex lenses - Astigmatism  The image in one plane falls at different distances from the plane which is on right angle of that plane  Caused by uneven curvature of the cornea  Correction is done by cylindrical lens - Keratoconus  Bulging of the cornea  Due to abnormal shape of the cornea  The image isn’t focused correctly on the retina  Correction is by contact lenses Accommodation - A process by which the refractive power of the eye changes rapidly - Done to focus the object, at different distances from the eye, clearly on the retina - If accommodation fails, the image appears blurry (not clear) - It is done by changing the curvature of the lens - The refractive power of the lens inside the eye equals to +20 diopters - It can be changed by this process to +34 diopters in young people - When there is no accommodation  Ciliray muscles are relaxed  Suspensory ligaments are tense  Lens is more flat & less convex  Refractive power is +20D - When there is an accommodation  Ciliary muscles are contracted  Suspensory ligaments are slack  Tension in ligaments is released  Lens is more convex (spherical)  Refractive power is +34D - Accommodation process  It is a reflex action  Done by stimulation of PSNS fibers to ciliary muscles  PSNS causes contraction of ciliary muscles  The stimulus is the blurred image of near objects on the retina  The reflex components  Sensory fiber = optic nerve (CN II)  Motor fiber = PSNS fiber in oculomotor nerve (CN III)  PSNS fibers to ciliary muscles originate in Edinger-Westphal nucleus  Edinger-Westphal nucleus will send the preganglionic fiber to the ciliary ganglion form which the postganglionic fiber (short ciliary neuron) will be sent to ciliary muscle  Accommodation is linked to  Eye convergence  Pupil constriction (meiosis)  - Presbyopia  Reduction in the power of accommodation gradually with aging  Takes place due to  Decreases lens capsule elasticity  Lens hardening  Proteins degeneration in the lens capsule Pupil - Opening formed by the iris - Controls the amount of light that enters the eye - Diameter ranges from 1.5mm at maximum constriction to 8mm at maximum dilation - The amount of light enters the eye is proportional to the square of pupil diameter - Amount of light that enters the eye at the maximum constriction = (1.5)2 = 2.25 - Amount of light that enters the eye at the maximum dilation = (8.0)2 = 64 - 2.25/64 = 1/30  there is 30-fold change in the amount of light that enters the eye - Diameter of pupil is controlled by the iris muscles  Sphincter (sphincter iridis): supplied by PSNS & causes pupil constriction  Dilator (dilator pupillae): supplied by SNS & causes pupil dilation Aqueous humor - Found in the anterior and posterior chambers - Produced by ciliary processes  Very vascular structure  Produce aqueous humor at the rate of 2-3 µL/min  Found in the posterior chamber - Circulation  Aqueous humor is in the posterior chamber  Leaves to the anterior chamber through the pupil  Drained into Schlemm canal after passing through a meshwork of trabeculae  Drained then to aqueous vein and then to the general circulation  - Functions  Maintains intra-ocular pressure (IOP)  Maintains the shape of the eye ball  Acts as a refractory medium  Supplies nutrients  Drains metabolic end-products - Intra-ocular pressure (IOP)  Pressure of the aqueous humor in the anterior segment of the eye  The mean IOP = 15 mmHg  Increased IOP is glaucoma that causes damage to the retina  Glaucoma is the most common cause of blindness Retina - The neural part of the eye - Contains 10 layers histologically - Contains 5 types of cells physiologically  Photoreceptors (rods & cones)  Bipolar cells  Ganglion cells  Horizontal cells  Amacrine cells - Blood supply  Inner two thirds by the central retinal artery  Outer one third by diffusion from choroid layer - Landmarks of the retina  Optic disc  Area where optic nerve and retinal BVS enter & leave retina  In the nasal part of the retina  Fovea  Temporal to the optic disc  Composed mainly of cones and few rods  The part with the highest visual acuity  Fovea centralis: in the center of fovea and contains cones only  Using fovea the vision is more acute because  High cones density  Layers are pushed aside (shorter pathway & less scattering)  No blood vessels  Small cones/ganglion ratio  Absence of chromatic aberration due to yellow pigment  - The basic neural connection  Photoreceptors  bipolar cells  ganglion cells  Cones  bipolar cells  ganglion cells  Rods  bipolar cells  amacrine cells  ganglion cells - Notes  Horizontal cells  Inhibitory (inhibit bipolar cells)  Helps in enhancement of contrast borders of image  Amacrine cells  Respond to light turn off/on  Respond to movement of light spot Rods Cones Number in each retina 100 million 3 million Distribution Periphery In fovea Receptive field Large Small Acuity Low High Photopigment Rhodopsin Color pigment Protein of pigment Scotopsin Photopsin Sensitivity to light Very sensitive Less sensitive Peak of wavelength sensitivity 500 nm Depends on type of cones Adaptation to dark Slow Fast Vision Night (grey scale) Daytime (color) Receptor/ganglion ratio High Low Neural circuit Rods  bipolar cells  Cones  bipolar cells  amacrine cells  ganglion ganglion Excitation Needs more time Needs less time Types One Three Process of the vision - Vision is done by converting light energy into neural signals in the retina - The process of conversion is called phototransduction - Phototransduction stages  Stage I  Activation of rhodopsin or color pigment by the light energy (photons)  Rhodopsin (11-cis-retinal & protein)  Metarhodopsin II (all-trans-retinal & protein)  Stage II  Metarhodopsin II stimulates transduction  Metarhodopsin II stimulates phosphodiesterase  Phosphodiesterase ↓ c-GMP  Stage III  Closure of sodium channels  Hyperpolarization  Inhibition of secretion of inhibitory neurotransmitters  Stimulation of ganglion cells - Summary  In the dark: sodium is moving and photoreceptor is  Highly permeable to sodium  Depolarized  Continuous release of inhibitory neurotransmitter  In the light: photoreceptor  Has low permeability  Sodium is out  Hyperpolarization  Low or absent release of inhibitory neurotransmitter - Vitamin A  Lipid soluble vitamin  Can’t be produced in the body  Present in the  Cytoplasm of photoreceptor  Pigment layer of retina  Stored in large quantities in the liver  After exposure of rhodopsin or color pigment to light  Activation of rhodopsin and color pigment  Decomposition into protein & all-trans-retinal  All-trans-retinal is dissociated into  All-trans-retinol (vitamin A)  11-cis-retinal  All-trans-retinol is converted into 11-cis-retinol  11-cis-retinol is converted into 11-cis-retinal  11-cis-retinal is recycled to be used to produce rhodopsin & color pigment  Vitamin A deficiency  Night blindness  Degeneration of neuronal layers of retina  Adaptation - Dark  ↑ Sensitivity of retina to light  Cones recover the sensitivity before the rods  Rods continue to adapt for longer time than cones  Full adaptation  Cones = 10 minutes  Rods = 30-50 minutes  Mechanism  ↑ Photochemicals concentration  Vitamin A is converted to retinal form  ↑ Size of pupil - Light  ↓ Sensitivity to light  ↓ Photochemicals concentration  Photochemicals are converted to retinal and protein  Retinal is converted into vitamin A  ↑ Vitamin A concentration W ganglion cells Y ganglion cells - 40% of ganglion cells - 5% of ganglion cells - Small cells (