The eye
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The document summarizes key aspects of vision physiology including: 1. The visual pathway begins with light detection in the retina by rods and cones, followed by signal transmission through the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiation, and visual cortex. 2. Accommodation allows the lens to change shape for focusing on near and far objects via contraction of the ciliary muscle. This reflex involves pathways from the retina to brainstem and back. 3. Refractive errors like myopia and hyperopia result when light focuses in front or behind the retina, respectively, and are corrected using lenses.
The eye
- 1. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page1 The EYE -Dr. Chintan
- 2. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page2 INDEX 1. Visual Pathway & Lesion ………………………………………………………………….03 2. Accommodation reflex …………………………...………………………………………..08 3. Refractive errors ……………………………………………………………………………..10 4. Color blindness ……………………………………………………......................................12 5. Dark Adaptation ……………………………………………………………………………...13 6. Light Reflex …………………………………………………………………………………….15 7. Night blindness ……………………………………………………………………………….15 8. Layers of retina ……………………………………………………………………….………16
- 3. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page3 Q-1. Visual Pathway & Lesion Retina ↓ Optic nerve (Nasal fiber carry temporal half field of vision, temporal fiber carry nasal half of vision) ↓ Optic chiasm (Nasal fiber cross to opposite side, temporal fiber same side) ↓ Optic tract ↓ LGB ↓ Optic Radiation ↓ Visual cortex (Primary visual cortex 17, Visual association area 18, 19, Frontal Eye Field 8) - (1) from the optic tracts to the suprachiasmatic nucleus of the hypothalamus, to control circadian rhythms that synchronize various physiologic changes of the body with night and day; - (2) into the pretectal nuclei in the midbrain, to elicit reflex movements of the eyes to focus on objects of importance and to activate the pupillary light reflex; - (3) into the superior colliculus, to control rapid directional movements of the two eyes
- 4. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page4 LGB: - Layers II, III, and V receive signals from the lateral half-temporal fibers of the ipsilateral retina, - layers I, IV, and VI receive signals from the medial half-nasal fibers of the retina of the opposite eye - Layers I and II - magnocellular layers - large neurons – input large type Y retinal ganglion cells - rapidly conducting pathway - color blind, transmitting only black and white information. - Layers III through VI - parvocellular layers - large numbers of small to medium sized neurons - type X retinal ganglion cells that transmit color - moderate velocity of conduction
- 5. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page5 Visual Cortex (6 Layers):
- 6. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page6 Lesions of Visual Pathway:
- 7. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page7
- 8. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page8 Q-2. Accommodation reflex - In children, the refractive power of the lens of the eye can be increased voluntarily from 20 diopters to about 34 diopters; this in an “accommodation” of 14 diopters - The shape of the lens is changed from that of a moderately convex lens to that of a very convex lens - In a young person, the lens is composed of a strong elastic capsule filled with viscous, proteinaceous, transparent fluid - When the lens is in a relaxed state with no tension on its capsule, it assumes an almost spherical shape, owing mainly to the elastic retraction of the lens capsule
- 9. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page9 - Suspensory ligaments are constantly tensed by their attachments at the anterior border of the choroid and retina. - The tension on the ligaments causes the lens to remain relatively flat under normal conditions of the eye - Ciliary muscle - meridional fibers and circular fibers - when they contract - releasing the ligaments’ tension on the lens - The ciliary muscle is controlled almost entirely by parasympathetic nerve signals transmitted to the eye through the third cranial nerve from the third nerve nucleus in the brain stem - Presbyopia - lens grows larger and thicker and becomes far less elastic, partly because of progressive denaturation of the lens proteins – weak ciliary muscle - The power of accommodation decreases from about 14 diopters in a child to less than 2 diopters by the time a person reaches 45 to 50 years; it then decreases to essentially 0 diopters at age 70 years - no longer accommodate for both near and far vision - bifocal glasses - Accommodation reflex - Convergence of eyeballs, pupillary constriction, ant. Surface of lens becomes more convex - Pathway: Rods & Cones of retina – optic nerve – optic chiasm – optic tract – LGB – optic radiation – visual cortex area 17 – frontal eye field area 8 – Edinger Westphal nucleus (3rd nerve nucleus) – pre ganglionic fiber of oculomotor nerve - ciliary ganglion – post ganglionic fiber through short ciliary nerves – sphincter pupillae muscle of iris (constriction), ciliary muscle contraction (lens becomes more convex) & contraction of medial rectus muscle (convergence)
- 10. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page10 Q-3. Refractive errors Normal (Emmetropia) – parallel rays of light focus on retina Hypermetropia (Long sightedness) – rays focus behind retina – near vision defective – corrected by spherical biconvex (converging +) lens Myopia (Short sightedness) – rays focus in front of retina – distant vision defective - corrected by spherical biconcave (diverging -) lens CauseCa Myopiapia Hypermetropia Axial Eyeball length more Eyeball length less Curvatural Lens, Cornea more convex Lens, Cornea more flatter Positional Lens anterior Lens posterior Index Refractive index more Refractive index less Missel. Spasm of accommodation Aphakia
- 11. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page11 Astigmatism – irregular shaped cornea like keratoconus or eyeball – different planes, rays will focus differently & form line focus & not point focus – corrected by cylindrical lens
- 12. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page12 Q-4. Color blindness - Normal vision – trichromatic - Abnormal – dichromatic, monochromatic - Triatanomaly – defective blue Colour - Protanomaly - defective red Colour - Deutranomaly - defective green Colour - Triatanopia – complete blindness for blue - Protanopia - complete blindness for red - Deutranopia - complete blindness for green - Tested by, - Ishihara chart, edridge green lantern test, Holmgren wool test
- 13. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page13 - Most common – red green - genetic disorder that occurs almost exclusively in males - Color blindness almost never occurs in females because at least one of the two X chromosomes almost always has a normal gene for each type of cone. - Because the male has only one X chromosome, a missing gene can lead to color blindness - Because the X chromosome in the male is always inherited from the mother - color blindness is passed from mother to son, and the mother is said to be a color blindness carrier Q-5. Dark Adaptation - If a person remains in darkness for a long time, the retinal and opsins in the rods and cones are converted back into the light-sensitive pigments. - vitamin A is converted back into retinal to give still more light- sensitive pigments, - the final limit being determined by the amount of opsins in the rods and cones to combine with the retinal - This is called dark adaptation - the sensitivity of the retina is very low on first entering the darkness, but within 1 minute, the sensitivity has already increased 10-fold - At the end of 20 minutes, the sensitivity has increased about 6000-fold, and at the end of 40 minutes, about 25,000-fold - all the chemical events of vision, including adaptation, occur about four times as rapidly in cones as in rods
- 14. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page14 - Despite rapid adaptation, the cones cease adapting after only a few minutes, while the slowly adapting rods continue to adapt for many minutes and even hours - change in pupillary size - Entering dark – mydriasis - Adaptation 30-fold within a fraction of a second - Neural Adaptation
- 15. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page15 Q-6. Light Reflex - When light is thrown on an eye – there is constriction of pupil in same eye – direct light reflex - There is constriction of pupil in opposite eye also – indirect (consensual) light reflex - Pathway: Rods & Cones of retina – optic nerve – optic chiasm – optic tract – tectum of midbrain - Edinger Westphal nucleus (3rd nerve nucleus) of both side – pre ganglionic fiber of oculomotor nerve - ciliary ganglion – post ganglionic fiber through short ciliary nerves – sphincter pupillae muscle of iris – constriction of pupil in both eyes Q-7. Night blindness
- 16. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page16 - Nyctalopia - Night blindness occurs in any person with severe vitamin A deficiency - the amounts of retinal and rhodopsin that can be formed are severely depressed. - The amount of light available at night is too little to permit adequate vision - For night blindness to occur, a person usually must remain on a vitamin A–deficient diet for months, because large quantities of vitamin A are normally stored in the liver - Once night blindness develops, it can sometimes be reversed in less than 1 hour by IV injection of vitamin A Q-8. Layers of retina 1. Pigmented layer – melanin – prevent refraction of light 2. Layers of rods & cones – outer segment of rods & cones 3. External limiting membrane – Muller cells 4. Outer nuclear layer – nuclei of rods & cones 5. Outer plexiform – synaptic connection between rods, cones & horizontal, bipolar cells 6. Inner nuclear layer – nuclei of horizontal, bipolar, amacrine cells 7. Inner plexiform – synaptic connection between bipolar, amacrine cells & ganglion cells 8. Ganglionic cell layer - ganglion cells 9. Nerve fiber layer – optic nerve fibers 10. Internal limiting membrane – Muller cells
- 17. Physiology Of Vision – Dr.Chintan Important: Please refer standard textbook of PHYSIOLOGY for further reading… Page17