Image Formation and Phototransduction By Dr. Abdelaziz Hussein Lecturer of Physiology
Vision Vision is a complex process through which an image of the external environment is formed on the photosensitive retina of the eye due to stimulation of the photoreceptors by light waves. This image is conducted as a nerve impulse to the visual cortex where it is interpreted & recognized.
Vision
The Eye
Retina It is the innermost photosensitive layer. Histology of retina: It is formed of 10 layers, but 4 layers are of physiological significance Retina in mammals has around 55 types of cells. Contains about 70% of total sensory receptors Blood supply of retina: Receptors (rods & cones) supplied by the choroid blood vessels (retinal detachment is so damaging to receptor layer), Inner layers of retina supplied by central retinal artery.
Functional layers are : Functional Layers of Retina 1. Pigment Layer: Is outer layer of the retina in direct contact with the choroid. 2. Layer of photoreceptors: Are the photosensitive receptors (rods & cons). 3. Layer of bipolar cells: Form the first order neuron in the visual pathway. 4. Layer of bipolar cells: Form the second order neuron in the visual pathway. Their axons converge to form the optic nerve. N.B. Lateral cells include amacrine and horizontal cells
Pigmented Epithelium 1. It contain melanin granules which absorb excess of light prevent reflection of light into retina which cause glare 2. Produce extracellular matrix keep the outer segments of photoreceptors straight 3. Stores large amounts of vitamin A which is essential for regeneration of photosensitive pigments 4. It cause continual renewal of outer segments by phagocytosis of the tips of outer segments of photoreceptors
Each rod or cone is formed of: Photoreceptors (Rods & Cones) An outer segment Are modified cilia. It is short and conical in cone ; long and cylindrical in rod Contain a great number of disc shaped shaped shelves which are composed of folds of the cell membrane which contains; 1. photosensitive pigments e.g. rhodopsin 2. G protein called transducin 3. c GMP phosphodiestrase The membrane of the outer segment of rods and cones contains cgmp gated ion channels selective for Na+, Ca++ and Mg++. An inner segment Includes a nuclear region and a synaptic terminal. Contain many mitochondria. Have synapses with bipolar cells. The membrane of inner segment contains an active Na+ K+ pump
Rods & Cones Number Distribution Photosensitive pigment Connection Function Cones ~ 6 millions in each retina More in centre Present 3 types (iodopsin) No convergence (1:1; direct private line) 1. Less sensitive to light 2. More accurate 3. Can perceive colour vision 4. Responsible for day vision Rods ~ 120 millions in each retina More in periphery Non in Fovea Rhodopsin Convergence (300:1 connection) 1. More sensitive to light sensitivity 2. Less accuracy of visual acuity 3. Cannot perceive colour vision 4. Responsible for night vision
Bipolar and Ganglion Cells Bipolar cells It is the 1st order neuron in visual pathway All bipolar cells secrete excitatory transmitter (glutamate) Ganglion Cells The 2 nd order neuron in visual pathway. The only retinal neuron that respond to stimulation by a full action potential; depolarization 3 Types of Ganglion cells: 1 Magnocellular (M) Less common (10%) gross analysis of visual image & location of objects in visual field and motion 2 Parvocellular (P) Numerous (80%) Responsible for fine detailed vision (shape & texture) and color vision 3 Coniocellular Few (10%) Medium in size Controlling pupillary reflexes.
Lateral Cells Horizontal Cells Lateral connection between rods & cones, between bipolar cells Responsible for lateral inhibition giving a stop to lateral spread of excitation visual accuracy Amacrine Cells Lateral connection between BC & GC ~ 30 types Helping to analyse visual signals before leaving retina
Special Parts of the Retina Fovea Centralis (yellow spot) An oval depression lateral to optic disc about 0.4 mm diameter Present in the centre of a small yellow pit (Macula Lutea; 4.5 mm diameter). Macula present at posterior pole of eye ball. Fovea contains cones only highest visual acuity (bl.v, plexiform + nuclear layers are displaced aside light direct on cones).
Fovea Centralis Fovea is the most sensitive spot in retina because; 1. All layers are shifted aside leaving outer segments of photosensors to be hit directly by light 2. High density of small diameter Cones with long outer segments 3. 1:1 convergence (cone BC BC GC) 4. Wide presentation in occipital primary visual area
Optic Disc (Blind Spot) It lies 3 mm medial to post. pole of eye ball. It is the site of exit of optic nerve & retinal bl. vessels. It contains no photoreceptors blind spot.
Phototransduction Ionic Basis of Photoreceptor Potential
Phototransduction It consists of 4 steps; 1. Decomposition of photosensitive pigment in rods & cones 2. Excitation of the rod activated rhodopsin and generation of photoreceptor potential 3. Termination of excitation 4. Regeneration of photopigment
Decomposition of photosensitive pigment Rhodopsin (Visual purple) consists of protein (scotopsin) and pigment (11 cis retinal). Light covert the angulated 11 cis retinal to straight all trans retinal enhance splitting between scotopsin & all trans retinal, thus Rhodopsin bathorhodopsin lumirhodopsin metarhodopsin I metarhodopsin II (activated Rhodopsin).
Generation of photoreceptor potential Activated rhodopsin activates transducin cascade activation of the enzyme phosphodiestrase cascade hydrolysis of cgmp closure of cgmp dependent Na + channels Na entry to outer segment (with Na + & K + still pumped out of inner segment) hyperpolarization of rod to 80 mv ( at dark RMP in rod is about 40 mv) transmitter (glutamate) release in synaptic terminal stimulation of bipolar cells excitation of ganglion cells that respond by generation of action potential optic nerve visual pathway.
Dark current and Photoreceptor potential
3 Termination of excitation Within a sec the activated rhodopsin is inactivated by the enzyme rhodopsin kinase reversal of all reactions regeneration of cgmp reopening of Na + channels termination of excitation. 4 Regeneration of photopigment All trans retinal is taken up by pigmented epithelum which contain isomeraze enzyme that converts it again into 11 cis retinal sent back to rods to recombine with opsin Cones as rod except no involvement of pigmented epithelium
Visual Pathway
Photoreceptors bipolar cells (1 st order neuron) ganglion cells their axons form the optic nerve optic chiasma, where the nasal fibers cross to the opposite side while the temporal fibers pass in the same side optic tract (ipsilateral temporal fibers + contralateral nasal fibers of retina) lateral geniculate body (3 rd order neuron) in thalamus optic radiation visual cortex in the occipital lobe (1ry and 2ry visual areas). Visual Pathway
Role of Different Parts of Visual Pathway in Visual Perception
Retinal Cells Analysis of the visual image occurs early in the retina a. Role of Bipolar cells and lateral cells discussed early b. Role of Ganglion cells: Ganglion cells have the following functions: 1. Detection of 2 point discrimination in the visual scene 2. Detection of the contrast in the visual scene 3. Detection of the movement and its orientation in the visual scene 4. Colour analysis
Lateral Geniculate Body (LGB) It is a thalamic relay nucleus that present at the dorsal end of thalamus C shaped 6 defined layers Layers 1, 2 receive from large M ganglion cells Magnocellular division Layers 3,4,5 & 6 from small P ganglion cells Parvocellular division Layers 2,3,5 receive signals from ipsilateral retina Layers 1,4,6 receive signals from contralateral retina
1. Plays a part in fusion of retinal images from the 2 eyes 2. Plays part in stereoscopic vision by comparing the visual images from both eyes and detection of minimal differences 3. Gating the transmission of signals to visual cortex 4. Magnocelluar neurons white and black shape and motion 5. Parvocelluar neurons color vision and accurate pointpoint spatial information Functions of LGB
Visual Cortical Areas
Visual Cortical Areas
1ry Visual Area (area 17) Site; Surrounds the calcarine fissure on the occipital lobe Its neurons arranged in the form of columns (1x1x2 mm) forming 6 distinct layers Representation of retina in area 17 Peripheral part is represented in anterior part Macula is represented in posterior part Upper part of retina is resented above calcarine fissure Lower part of retina is represented below the calcarine fissure
Role of 1ry Visual area in Visual Perception 1. Detection of lines and borders 2. Detection of the orientation of lines and borders 3. Analysis of colors 4. Fusion of the images from the 2 eyes 5. Perception of luministy
2ry Visual Areas ( areas 18 and 19) Site: Occipital lobe around 1ry visual area parietal & temporal lobes Areas 18 is called area V 1, more distant 2ry visual areas are assigned V 2,V 3 and so no. Functions: Dealing with complex perception of patterns & forms responsible for object recognition 10% of cells in inferotemporal cortex have large complex receptive fields & selective for specific stimuli (familiar faces) Lesion visual agnosia
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