WEEK 1: Physiology of the eye Flashcards
Describe the Physiological events of vision.
- Refraction of light entering the eye
- Focusing of image on the retina by accommodation of lens
- Convergence of image
- Photo-chemical activity in retina and conversion into neural impulse
- Processing in brain and perception
- Refraction of light entering the eye:
What is refraction?
Describe the pathway of light refraction as it enters the eye before it reaches the retina.
Light wave travels parallel to each other but they bend when passes from one medium to another. This phenomenon is called refraction.
Before light reach retina it passes through cornea, aqueous humor, lens, vitreous humor so refraction takes place in every medium before it falls on retina.
In normal eye, where is the light wave focused?
In myopic eye (short sightedness), where is light focused? So, this defect can be treated by using which lens?
In case of far sightedness, where is light focused?
This defect can be treated by using which lens?
In normal eye, light wave focused on retina.
However in myopic eye (short sightedness) light focused in front of retina. So this defect can be treated by using concave lens.
In case of far sightedness light focused behind retina, so no image is formed. This defect can be treated by using convex lens.
- Accommodation of lens to focus image:
What is accommodation?
What happens for us to view closer objects?
What happens for us to view distant objects?
The normal eye is able to accommodate light from objects of what length?
Accommodation is a reflex process to bring light rays from object into perfect focus on retina by adjusting the lens.
When an object lying less than 6 meters away is viewed, image formed behind retina. But due to accommodation of lens image formed in retina and we can see the object.
For accommodation to view closer object, ciliary muscle contract and lens become thick which causes focus on closer object.
Similarly, when distant object is viewed, ciliary muscles relax, so the tension of ligament become greater which pull lens and lens become thinner, due to which image forms on retina.
The normal eye is able to accommodate light from object about 25 cm to infinity.
Discuss the process for focusing on nearer objects.
Ciliary muscle contract——-ciliary body pull forward and inward ———tension on suspensory ligament of lens reduced ——lens become thicker and round due to its elasticity ——possible to focus near object.
Discuss the process for focusing on far objects.
Ciliary muscles relaxes——ciliary body return to its normal resting state—–tension on suspensory ligament of lens increases——-lens become thinner and flat———possible to focus distant object
- Convergence of image:
What is convergence?
Human eye have binocular vision, it means although we have two eye, we perceive single image.
In binocular vision, two eyeball turns slightly inward to focus a close object so that both image falls on corresponding points on retina at same time. This phenomenon is called convergence.
4.Photo-chemical activity in retina and conversion into neural impulse.
Describe Photo-chemical activity in retina.
*Each eye contains 120 million rods which are located in neuro-retina.
*Rods contains light sensitive pigment-rhodopsin.
*Rhodopsin is a molecule formed by combination of a protein scotopsin and a light sensitive small molecule retinal (retinene).
*Retinene (retinal) is a carotenoid molecule and is derivative of vitamin A (retinol).
Retinal exists in two isomeric form- cis and trans according to light condition.
The extra cellular fluids surrounding rod cells contains high concentration of Na+ ion and low concentration of K+ ions, while concentration of Na+ is low and K+ is high inside rod cells. The concentration is maintained by Na-K pump.
In resting phase, K+ tends to move outside the rod cells creating slightly –ve charge inside. When light is falls on rod cell, it is absorbed by rhodopsin, and it breaks into scotopsin and 11 cis- retinal.
The process is known as bleaching.11 cis-retinal absorb photon of light and change into all trans-retinal which in turn activates scotopsin into enzyme. This reaction produces large amount of transducin which activates another enzyme phosphodiesterase.
Phosphodiesterase hydrolyses cGMP which causes to cease the flow of Na+ ion inside rod cell. This causes increased negative charge inside cell creating hyperpolarized state. Hyperpolarized rod cells transmit the neural signal to bipolar cell. Bipolar cell, amacrine cell and ganglion cell process the neural signal and generate action potential to transmit to brain via optic nerve.
Describe Photochemical activity in cones.
Each eye contains 6 million cone cells.
The neural activity in cone cell is similar to that of rod cell but there are three different types of cone cells and each cone cell contains different photo-pigment and are sensitive to red, green and blue.
The perception of colour depends upon which cone are stimulated.
The final perceived colour is combination of all three types of cone cell stimulated depending upon the level of stimulation.
The proper mix of all three colour produce the perception of white and absence of all colour produce perception of black.
DESCRIBE VISUAL SENSORY RECEPTORS
CONES :
*Concentrated in the posterior pole (central retina)
*Responsible for colour and high resolution vision (eg reading, face recognition)
*Mostly used in good light
cone-shaped
less sensitive
operate in high light
color vision
RODS:
*Mostly in the peripheral retina
*Responsible for detection of movement, low spatial resolution
*Mostly used in poor lighting situations
*Information from rods and cones travel through retinal ganglion cells to synapse in the lateral geniculate nucleus
*rod-shaped
highly sensitive
operate at night
gray-scale vision
