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Comprehensive Study on Molecular Physiology of Vision and Retinal Function

Explores eye anatomy, photoreceptor structure, phototransduction cascade, and neural pathways involved in vision processing.Molecular Physiology of Vision Explore the fascinating molecular mechanisms underlying vision—from photon absorption by visual pigments and activation of rhodopsin to phototransduction, ion-channel regulation, and signal transmission in retinal photoreceptors. This presentation provides an overview of how light is converted into electrical signals at the molecular level, helping us understand the complex physiology of visual perception.

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JAI NARAIN VYAS UNIVERSITY DEPARTMENT OF ZOOLOGY Molecular Physiology of vision SUBMITTED BY : Mitali Singh Panwar
Overview • Anatomy of eye • Introduction • Function of retina • Photoceptor structure and function • Phototransduction cascade • Neural Pathways to the brain
Anatomy of eye
• Eye is protected in a bony cavity, orbit, formed by facial bones. • Six extrinsic eye muscle along with skeletal muscle attached to outer surface of eyeball and control eye movement. • Lacrimal apparatus,system of gland and duct as tear glands. • EYE itself a hollow sphere divided into two compartments by lens, suspend by ligaments zonules. • A anterior chamber in front of eye filled with aqueous humor (moist),a low protein plasma -like fluid secreted by ciliary epithelium supporting lens. • Behind lens is much larger chamber, vitreous humor (glass) a clear, gelatinous matrix helps in maintaince of eyeball shape. • Outer wall of eyeball, the sclera , is composed of connective tissue.
INTRODUCTION • The eye is a sensory receptor that focuses light on a light-sensitive surface (the retina) and using a lens and an aperture or opening (pupil) . • VISION is the process through which light reflected from objects in our environment is translated into a mental image.
THIS PROCESS CAN BE DIVIDED INTO 3 PARTS: 1. Light enters the eye and the lens focus it on the retina. 2. Photoreceptors of the retina transduce light energy into an electrical signal. 3. Neural pathways from the retina to brain process electrical signals into visual image.
LIGHT ENTERS THE EYE THROUGH PUPIL: • Light enters the eye through cornea , which refracts it , then passes the pupil (controlled by iris) is further focused by the lens onto the retina. • Light enter the eye is refracted twice : first it passes cornea ;again when it passes lens. Two-third total refraction occurs at cornea and remaining one third at the lens. • Refraction occur through lens because it is capable of changing its shape, and keep in focus is known as accommodation. • Change in lens shape are controlled by ciliary muscle.
FUNCTION OF RETINA • The retina is responsible for transducing light energy into electrical impulses.It has following key functions: 1. Capture photons: the retina contains millions of light sensitive cells called photorecptors. Two types of photoreceptors are found in the retina of a human eye : rod and cones. Photon hits rods and cones and start the process of transduction . 2. Produce electrical signals: The photoreceptors together with other layers of cells in the retina convert into electrical energy. 3. Process visual signals: There are layers of cells that perform various processing functions such as motion detection,image sharpening and balancing the colour.
PHOTORECEPTOR •Anatomy of retina: • The retina contains multiple layers of cells, each layer perform different functions. • At the centre of the of the retina is a pitted region known as the fovea. The fovea contains only cones which are photoreceptors sensitive to bright colour lights. • The fovea is responsible for detailed colour vision.
RETINA CONSIST OF MANY DIFFERENT LAYERS: 1. RODS AND CONES: reasponsible for transducing or converting photon into electrical impulse. 2. HORIZONTAL CELL:it connect neighbouring receptors. It produce inhibitory neurotransmitter , when one receptor is activated and reduce output of its neighbours. It improves contrast between dark and light regions.
3. BIPOLAR CELL: These are intermediate neurons that collects signals from photoreceptors and relays them to the ganglion. 4.AMACRINE CELL: these are interneurons which refine signals from cells. They perform aspect of spatial processing including sharpening edges , identifying changes such as movement, help processing colours . 5. GANGLION CELLS: this is output layer of cells that combine electrical signals from bipolar and amacrine cell and send this to brain via optic nerve fibres. Some GANGLION CELLS helps to regulate circadian cycle (sleep – wake cycle)
Neuron and synapse: SYNAPTIC CONNECTION IS KEY TO THE ABILLITY OF NEURAL SYSTEM TO FUNCTION. The axon transmit the electrical signal in form of action potential (nerve impulse),which trigger the axon terminal to release chemical messengers called neurotransmitter.
TRANSDUCTION CASCADE Both types of photoreceptor cells (rods and cones) work in a similar way. They have four primary structures and functional regions: the outer segment, the inner segment, the cell body which has the nucleus and the synaptic terminal. Outer segment: It is filled with a dense stack of membrane discs spaced at intervals of around 30nm. The discs contains photopigments, rods have rhodopsin and cones have cone pigment.
• These discs are where the main phototransduction process occurs. • Inner segment: this is where much of metabolism and regulation of membrane potential takes place. • Synaptic connection: this performs the usual function of synapses in a neuron. It transmits electrical signals from the photoreceptor cell to other retinal neurons. • THE PROCESS OF PHOTOTRANSDUCTION IS SIMILAR FOR RHODOPSIN (IN RODS) AND THREE COLOUR PIGMENTS (IN CONES).
STRUCTURE OF RHODOPSIN A rod disc contains photopigment known as rhodopsin. RHODOPSIN composed of 2 molecules: opsin ,a protein embedded in membrane of rod disk. Retinal ,a vitamin A derivative that is light absorbing portion of the pigment. In absence of light , retinal binds snugly into a binding site of opsin. When activated by a little photon of light, retinal changes shape into new configuration.
Rods contain 3 types of channels: 1. cyclic nucleotide channel (cng channels) 2. potassium channel. 3. calcium channel in synaptic terminal. 1. THE ACTIVATED RETINAL NO LONGER BIND TO OPSIN AND IS RELEASED FROM THE PIGMENT IN THE PROCESS KNOWN AS BLEACHING. 1. When there is no light received, the sodium channel kept opened by a chemical called cyclic GMP (cGMP). 2. Na+ ions flows freely across the membrane through the channel and constantly depolarizing. 3. When even a photon hits the rhodopsin molecule, it changes its shape and releases a messenger called transducin.
VISUAL PATHWAY • Visual pathway shows how visual information outside the eye reaches the visual cortex in the brain. • There is a cross over point in the optic nerve bundle where right and left side signals cross each other. That is called the optic chiasm.
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