Physiology of Vision PDF 2024-2025

PHYSIOLOGY OF VISION PHYSIOLOGY OF VISION DR. NORHAZLINA BINTI ABDUL WAHAB DEPARTMENT OF PHYSIOLOGY, Ng Sook Luan, PhD FACULTY OF MEDICINE, [email protected] PPUKM [email protected] 012-9305208...

PHYSIOLOGY OF VISION PHYSIOLOGY OF VISION DR. NORHAZLINA BINTI ABDUL WAHAB DEPARTMENT OF PHYSIOLOGY, Ng Sook Luan, PhD FACULTY OF MEDICINE, [email protected] PPUKM [email protected] 012-9305208 03-9145 8612 / 019-333 4122  LEARNING OBJECTIVES 1. Explain optical mechanism and its aberration 1 corrections. and 2. Discuss pupillary reflex and accommodation and their clinical significance.  Refraction When light meets the surface of a different medium at an oblique angle, it will be refracted. Refraction depends on: angle – the greater the angle the greater the amount of bending. comparative densities of 2 media (Refractive index) The refraction of light through glass. When the light ray strikes a denser medium than the air, it is bent towards the normal. When it leaves the glass and re-enters the less dense medium it is bent away from the normal. The apparent depth of an object in water when viewed from above is less than the object's real depth because of refractive effects: the light wave bends as it passes from the denser water into the less dense air.  VISION The process through which light reflected through objects and then translated into a mental image.  Process Involves in Vision The light that enters the eye is bent 3 times: As it enters the cornea, enter & leaving the lens 4 1 Light is most Photoreceptors of refracted by the the retina transduce cornea (due to its light energy into shape and density electrical signals of the fluid) 3 The lens make a fine adjustment to focus an image on the retina. The lens is highly elastic. It 2 Light enters the eye is focused curvature & light-bending on the retina by the lens power can be adjusted.  Focusing For Distant Vision The far point of vision = distant at which the lens does not need to change its shape for focusing. For normal (emmetropic) eye, the far point is 6 m (20 feet). The ciliary muscles relax, the lens is stretched flat and at its lowest refractory power.  Focusing For Close Vision Light from objects less than 6 m away diverges as it approaches the eyes For close/near vision, the eyes need to make active adjustments (accommodation): Accommodation of the lenses. Constriction of pupils Convergence of the eyeballs Near point of vision = the closest point on which we can focus clearly. Emmetropic eyes of young adults, the near point is 10 cm (4 inches) from the eye. Common Visual Defects Myopia (near-sightedness) see clearly only those objects that are close up caused either by the eyeball being too long or by the cornea and lens system of the eye being too strong (over-converging the light), both of which cause the images of distant objects to be formed in front of the retina instead of on it. corrected by suitable glasses with diverging (concave) lenses. Concave lens Hyperopia / hypermetropia (far-sightedness) unable to focus on close objects (Emmetropia) caused by the failure of the lens to return to its normal rounded shape, or by the eyeball being too short, with the result that the image is focused on a point behind the retina corrected by wearing glasses fitted with converging (convex) lenses. Convex lens Presbyopia an increasing inability with advancing age to focus on near objects (non- accommodating). caused by thickening and loss of elasticity in the lens, which is no longer able to relax to the near-spherical shape required for near vision. Astigmatism Unequal curvature in different parts of the cornea/lens. the vertical and horizontal cannot be in focus at the same time. Correction - use of a cylindrical lens that reduces the overall focal length of one plane so that both planes are seen in sharp focus (other procedures: corneal implants, laser).  Retina Phototransduction = The process by which light energy is converted into a graded receptor potential. Rods ✓ very sensitive to light (respond to dim light) – best suited for night vision and peripheral vision. ✓ contain a single kind of photopigment (rhodopsin / purple pigment). Cones ✓ have low sensitivity for light ✓ used for colour vision & high acuity Rods and Cones Distribution Rods and Cones Arrangement and Connections  Photoreceptors 2 types: Rod Cone Outer segments- receptor region contain photopigments Inner segment – connect to the cell body  Phototransduction Photoreceptor activity in the dark: 11-cis-retinal Increase [cGMP]. cGMP binds to Na+ channels to keep them open. Na+ influx leads to membrane depolarization. Ca2+ channels open in the synaptic terminal. Increase inhibitory neurotransmitter release. Bipolar cells are inhibited (IPSP). No action potential formed in the axon of ganglion cell. Photoreceptor activity in the light: All-trans-retinal cause activation of phosphodiesterase Reduce [cGMP]. Na+ channels close. Photoreceptor hyperpolarized. Ca2+ channels closed No neurotransmitter released. Bipolar cells depolarized (EPSP) – neurotransmitter released. Action potential formed in the axon of ganglion cell.  Dark Adaptation When we go from well-lit area into a dark area, we can’t see things for a while. Time-course of dark adaptation is biphasic (transition from cone- dominant vision to rod-dominant vision during dark adaptation): Cones stop functioning in low- intensity light but adapt fast. Rod pigments have been bleached out by bright light and adapt slowly. In the dark, rhodopsin accumulates, and retinal sensitivity increases.  Light Adaptation When we move from darkness into bright light, we are momentarily dazzled. Large amounts of photopigments (rods & cones) are broken down which causes glare. Within 60 s, cones are sufficiently recover. Within 5-10 min. visual acuity and colour vision improve.  Night Blindness (nyctalopia) Rod function is impaired. Prolonged vitamin A deficiency leads to rod degeneration. Retinitis pigmentosa – pigment epithelial cells are unable to recycle the tips of the rods as they are destroyed.  Colour Vision Absorption spectra overlap. 1. Blue Respond maximally to wavelengths of 430 nm 2. Green Respond maximally to wavelengths of 530 nm 3. Red Respond maximally to wavelengths of 560 nm Perception of intermediate colour results from differential activation of more than 1 type of cone at the same time  Red-green colour blindness Colour Blindness Can’t perceive green Hereditary defect of vision that reduces the ability to discriminate certain colours, usually red and green. It is sex-linked and affects men DEUTERANOPIA more than women. In the most common types of colour blindness there is confusion among the red– yellow–green range of colours; for example, many colour-blind observers are unable to distinguish red from yellow or yellow from green. Red-green colour blindness Blue-yellow colour blindness can’t perceive red  Ishihara Chart A: Normal color vision= 74 B: Normal color vision= 42 Red-green color blindness= 21 Red blindness(Protanope)= 2 Green blindness(Deuteronope)= 4  In humans, the image of an object created by each of our eyes is slightly different because our eyes are in Visual Field different positions. The brain combines the two images to give a sense of Perimeter is used to map visual depth. fields. With one eye closed we lose some of our sense of depth and perspective. Binocular vision Temporal field Nasal field Left eye visual field Right eye visual field Optic nerve Optic tract  Visual Pathway 1. Photoreceptors (rods and cones) in the retina are stimulated by photons of light entering the eye. 2. Light-sensitive surface membrane proteins (rhodopsin) are stimulated to propagate second messenger responses which convert light energy into electrical signals. 3. The photoreceptors synapse with retinal bipolar cells, which transmit the electrical signals to retinal ganglion cells. 4. The retinal ganglion cells converge at the optic disc, forming the optic nerve. 5. Left and right optic nerves (CNII) converge at the optic chiasm. 6. The optic nerves cross over (decussate) to the contralateral optic tract at the optic chiasm. 7. The optic nerve synapses with the second-order neuron at the lateral geniculate nucleus in the thalamus. 8. Optic radiations travel through the parietal lobe corresponding to the upper half of the retina/lower visual field, while the radiations travel through Meyer’s loop in the temporal lobe corresponding to the bottom half of the retina/upper visual field. 9. The optic radiations terminate in the calcarine sulcus of the occipital lobe for retinal image processing. 10. The images from both eyes are finally collated and a final image (inverted) is formed. The brain has to re-invert the image. Bitemporal hemianopia Homonymous hemianopia Visual Field Defects and Optic Nerve Pathway (Rhesus Medicine) https://youtu.be/2ZbFBlwWm3Q?si=IGwadDwIWchU-VBP  Consensual light reflex Parasympathetic nerves Ipsilateral Edinger- Westphal nucleus Contralateral Edinger-Westphal nucleus  Pupils fail to response to light when: Visual signals to Edinger-Westphal nucleus are blocked. CNS syphilis, alcoholism, encephalitis. Argyll Robertson – late-stage syphilis (small, irregular pupils, small near accommodation, don’t react (constrict) when exposed to light). Horner’s syndrome: Sympathetic nerves to the eyes are interrupted. Pupils persistently constricted. Superior eyelid droops (ptosis).  D Visual Acuity (V) = d/D? Normal: V=6/6 20/15 Distance from chart; d = 6 m (20 feet) 20/13 20/10 Brain Interpretation Stare closely at this light bulb for 25 seconds. Then immediately stare at a white wall or sheet of paper. What do you see?  The Box and the Sphere Keep your eyes on the dot. Is it in the front or in the back of the cube?  Hermann Grid Illusion Count all the black dots you can see THANK YOU