L 14 Retina and Visual Pathway PDF

**L 14; The visual processing in the retina-Visual pathway** **Objectives:** 1. Explain the mechanism of phototransduction by the photoreceptors 2. Discuss the duplicity theory of vision by the retinal photoreceptors 3. Describe the visual pathways. 4. Understand how visual field deficits i...

**L 14; The visual processing in the retina-Visual pathway** **Objectives:** 1. Explain the mechanism of phototransduction by the photoreceptors 2. Discuss the duplicity theory of vision by the retinal photoreceptors 3. Describe the visual pathways. 4. Understand how visual field deficits indicate specific defects in the visual pathways. 5. Describe the organization of the visual cortex 6. Describe how the brain processes visual information, including colour information ***Receptor and neural function of the retina*** The retina is the innermost layer of the eyeball. It lines the posterior 2/3 of the eye ball. It extends anteriorly almost to the cilirary body. It contains photoreceptor cells (rods and cones) and two types of neurons, bipolar cells and ganglion cells. In histology, the membranes and cells of the retina are arranged in10 layers but from the functional point, the retina can be viewed to be arranged into 4 layers from outside to the inside as follows: 1 - Pigmented layer It is the outermost layer of the retina in direct contact with the choroid. - It contains black melanin pigments to prevent light reflection inside the eyeball. - It also stores large quantities of vitamin A. - The pigmented epithelium cells provide nutrition (glucose and essential ions) to photoreceptors and other cells associated with them. 2 - Layer of rods and cones They are the retinal receptors sensitive to light (photoreceptors). Each rod and cone is divided into: an outer segment, an inner segment, and a synaptic zone. The outer segment contains the photo- sensitive compounds that react to light. The inner segment contains the nucleus and is rich in mitochondria. Cones: There are about 6 million cones per each retina, they are pyramidal in shape. They are concerned with day, acute and colour vision. In the central part of the retina the cones have a diameter of only 1.5 micrometers while in the peripheral part their diameter is 5-8 micrometers. Rods: There are about 120 million rods per each retina. They are cylindrical in shape, and are concerned with night vision. 3 - Layer of bipolar cells (inner nuclear layer) They are the first order neuron of the visual pathway. They transmit the visual signals from the rods and cones to the ganglion cells. 4 - Layer of ganglion cells Their dendrites synapse with the fibers of the bipolar cells. Their axons pierce the choroids and sclera at the blind spot to form the optic nerve. **Specialized areas of the retina** I. **[The optic disc (blind spot)]** The optic nerve leaves the eye at a point 3mm medial to and slightly above the posterior pole of the globe. It is called the optic disc. It is composed of nerve fibers only. If light falls on this part, there will be no visual sensations.This is why it is called the blind spot. II. **[The macula lutea- Fovea centralis:]** Near the posterior pole of the eye, there is a small yellow area called the macula lutea in which there are few rods and the majority of receptors are cones. In the center of the macula lutea, there is an oval depression called the fovea centralis. It contains cones only. III. **[At the extreme retinal periphery there are rods only.]** In the area between the fovea and the periphery of the retina both cones and rods exist. 1\. The initial layers are pulled aside, this allows light to fall directly on the receptors and the eyes move so that the light rays always fall on this central part. There is no blood vessels. 2\. The foveal cones are very thin and densely packed. Each cone in the fovea is connected to one bipolar cell, and each bipolar cell leads to a single ganglion cell. Thus each foveal cone has its own private pathway to the cerebral cortex.***This is responsible for the acute vision of the fovea, and for the high light threshold of the cones.*** On the other hand, in other portions of the retina where rods predominate, many rods are connected to one bipolar cell and collections of bipolar cells synapse with a single ganglion cell. ***This is called convergence i.e signals summate to give intense stimulation.*** Fig.2; Distribution of rods and cones across the retina **Duplicity theory of retinal function** **Comparison between the two retinal functions** There are two kinds of input to the CNS from the eye: input from the rods and input from the cones. The existence of two kinds of input, each working maximally under different conditions of illumina- tion is called duplicity theory of retinal function. +-----------------------+-----------------------+-----------------------+ | ***Property*** | ***Day vision | ***Night vision | | | (photopic, | (scotopic, | | | central)*** | peripheral)*** | +=======================+=======================+=======================+ | 1\. Receptor | Cones | Rods | +-----------------------+-----------------------+-----------------------+ | 2\. Locality | Central retina | Peripheral retina | +-----------------------+-----------------------+-----------------------+ | 3\. Sensitivity to | Low | High | | light | | | +-----------------------+-----------------------+-----------------------+ | 4\. Light threshold | High | Low | +-----------------------+-----------------------+-----------------------+ | 5\. Color perception | Present ( 3 types of | Absent ( they are | | | cones ) | achromatic) | | | | | | | | Colors are perceived | | | | as shades of grey. | +-----------------------+-----------------------+-----------------------+ | 6\. Visual acuity | Acute vision | Poor vision | +-----------------------+-----------------------+-----------------------+ | 7\. Details of | Well seen | Not well seen | | objects | | | +-----------------------+-----------------------+-----------------------+ | 8\. Adaptation time | Short | Long | +-----------------------+-----------------------+-----------------------+ ***PHOTOCHEMISTRY OF VISION*** ***Photosensitive compounds*** Both the rods and cones contain chemicals that decompose on exposure to light 1 - Rhodopsin (visual purple) It is the light sensitive pigment present in the rods. It is a combination of a protein called scotop- sin and a carotenoid pigment called retinine or retinal. In the dark the retinal is in the cis form (11 cis- retinal). This cis form is the only form which can combine with scotopsin to form rhodopsin. It makes up to 90% of total proteins in the membranes of rod disks. It is one of the G - protein coupled receptors. Its colour is purple because it absorbs the middle part of the spectrum leaving the red and the violet to be reflected back to the observer, maximum absorption occurs in the blue-green part of the spectrum at a wave length of 505 millimicrons. Decomposition of rhodopsin by light energy When light energy is absorbed by rhodopsin, the rhodopsin begins to decompose within a very small fraction of a second. The cause of this is photoactivation lead to instantaneous change of the cis form of retinal into an all- trans form that still has the same chemical structure but has a different physical structure- a straight molecule rather than an angulated molecule. The all trans retinal begins to pull away from the scotopsin. The immediate product is bathorhodopsin, which is a partially split combination of the all-trans retinal and scotopsin. Then several intermediates continue to be formed in seconds. It is the metarhodopsin II, also called activated rhodopsin that excites the electrical changes in the rods , then impulses are transmitted along the optic nerve to the central nervous system. ***Relationship between retinal and vitamin A*** Each of the two types of retinal can be converted into a corresponding type of vitamin A, and in turn the two types of vitamin A can be reconverted into the two types of retinal. Most of vitamin A of the retina is stored in the pigment layer of the retina. ***Night blindness:*** Night blindness occurs in any person with severe vitamin A deficiency. The simple reason for this is that without vitamin A, the amounts of retinal and rhodopsin that can be formed are severely depressed. This condition is termed night blindness because the amount of light available at night is too little to permit adequate vision in vitamin A deficient person. ***Phototransduction : Light transduction reactions in rods*** **How is the capture of light by visual pigments is converted into an electrical event?** ***A - Dark current*** There is movement of sodium ions in a complete electrical circuit through the inner and outer segments of the rod. - **[The inner segment]** continually pumps sodium from inside the rod to the outside and potassium ions are pumped to inside the cell (Na+/K+ ATPase).Potassium ions leak out of the cell through nongated potassium channels that are confined to the inner segment of the rod. As in other cells, this sodium- potassium pump creates a negative potential on the inside of the entire cell. - **[The outer segment]** , in the dark state, is very leaky to sodium ions that flow through cGMP-gated channels. In the dark state,cGMP levels are high permitting positively charged sodium ions to continually leak to the inside of the rod. Thus under normal dark conditions, when the rod is not excited, there is reduced electronegativity inside the membrane of the rod, measuring about -40 millivolts rather than the usual -70 to -- 80 millivolts found in most sensory receptors.This means that the rods are depolarized in the dark. - ***B-Transduction of light into an electrical signal*** When rhodopsin in the outer segment of the rod is exposed to light, it is activated and begin to decompose. The cGMP gated sodium channels are closed and the outer segment membrane conductance of sodium is reduced \(1) Light is absorbed by rhodopsin causing change of the cis form of retinal into an all- trans \(2) The all trans retinal begins to pull away from the scotopsin forming metarhodopsin II, also called activated rhodopsin \(3) The activated rhodopsin stimulates a G-protein called transducin which then activates cGMP phosphodiestrase (PDE). This enzyme catalyzes the breakdown of cGMP to 5\'GMP \(4) The reduction in cGMP closes the cGMP-gated channel and reduces the inward sodium current. Because they are positive ions, their loss from inside the rod creates increased negativity inside the membrane. The greater the amount of light energy striking the rod, the greater the electronegativity becomes-that is the greater the degree of hyperpolarization. At maximum light intensity the membrane potential approaches -70 to -- 80 millivolts. \(5) Hyperpolarization that is graded with stimulus intensity leads to inhibition of release of Gutamate ***C-Termination of events in the rods after excitation by light*** 1. Rhodopsin kinase inactivates metarhodopsin II. 2. Resynthesis of cGMP: Light reduces the concentration of Ca2+ as well as that of Na+ in photoreceptors. The decrease in Ca2+ activates the enzyme guanylyl cyclase which generates cGMP. It also inhibits the light activated phosphdiestrase. ***Further Processing of visual information in the retina*** **A. Bipolar cell output:** Bipolar cells are the first data processing step. They receive input from either group of rods or a single cone. The glutamate receptor is coupled via inhibitory G-protein to nonselective cation channel, but because the relationship is inhibitory, the channel is prevented from opening in the dark (presence of Glutamate). When photoreceptor upstream is illuminated, glutamate decreases and the channel is released from its inhibitory influence and the bipolar cell depolarize in a graded manner. They are excited when light is turned ON. **B. Ganglion cell output:** Ganglion cells generate action potential to transmit visual information to the brain. PHOTOTRANSDUCTION Wednesday, June 8, 2016 **2- Cone pigments** The photochemicals in the cones have almost exactly the same chemical composition as that of rhodopsin in the rods. The only difference is that the protein portions, or opsins- called photopsins in the cones- rather than scotopsin of the rods. The retinal portion of the visual pigment is the same in the rods and cones. There are three types of color pigments, but only one type of them is present in each of the different cones, thus making the cones selectively sensitive to different colors: blue, green or red. ***COLOR VISION*** Color vision is the sense of discrimination of the different wave lengths that constitute the visible spectrum. The sensations aroused are described as red, orange, yellow, green, blue, indigo and violet arranged in the order of magnitude of their wage lengths. This is the function of cones only, and so they are most developed at the fovea. The limits of visible spectrum are between 400-730 mu. ***Retinal mechanism of color vision Trichromatic mechanism of color vision by the retina (Young-Helmholtz trichromatic theory )\ ***- The retina has 3 kinds of cones, each containing a different photopigment that are maximally sensitive to one of the three primary color: Red ,green and blue.\ - One pigment (the blue sensitive or short wave length) absorbs light maximally in the blue-violet portion of the spectrum at 445 mu { S-type cones}. - Another (the green sensitive or the middle wave pigment) absorbs maximally in the green portion at wave length at 535 mu{M-type cones}. - The third (the red sensitive or long wave pigment) absorbs maximally in the yellow portion of the spectrum at 565mu {L-type cones}. - Each of the three photochemical pigments is affected mostly by its specific wave length, but is also affected to a varying extent by other wave lengths of the chromatic series.\ - If the three types of cones are equally stimulated, the sensation of white is perceived. If the three types of cones are not equally stimulated, any of the chromatic series of colors may be perceived depending on the relative frequency of impulses from each of these cone systems. ***RETINAL ADAPTATION*** The retina can adjust its sensitivity to different intensities of light. This is called retinal adaptation. It depends on the concentration of the photosensitive chemicals in the rods and cones. **1. Dark adaptation** When a person enters a dark room after being previously exposed to bright light for a few hours, he can not at the start see anything in spite of the rapid pupillary dilatation. After few minutes, he becomes accustomed to the dark and he can see better i.e. the retina becomes more sensitive to light. **[Changes occurring during dark adaptation ]** 1- Dilatation of the pupils.\ 2- Regeneration of the pigments of rods and cones (all the retinal and opsins in rods and cones are converted into light sensitive pigments also vitamin A is converted into retinal). Regeneration of cones\' pigments is rapid and stops after 10 minutes while that of rhodopsin is slow and continues for hours. 3- Increase in retinal sensitivity to light. This means a decline in visual threshold. 4- Visual acuity is small. **2. Light adaptation** Light adaptation occurs when a person passes from a dark place into bright\ daylight. At the start even moderate light seems to be very intense, irritating and painful in spite of the rapid pupillary constriction. After few minutes the retina becomes less sensitive to light i.e. light adaptation is the reduction in the retinal sensitivity to light. It is a rapid process mostly completed in 5 minutes. It is due to the break down of the photosensitive chemicals in the rods and cones. **[Changes occurring during light adaptation:]** 1. Constriction of the pupils. 2. Breakdown of the photosensitive pigment in the rods and cones into retinal and opsins and the retinal is converted into vitamin A. So, the concentrations of the photosensi- tive pigments are reduced. 3. Decreased in retinal sensitivity. The visual threshold rises. 4. Visual acuity is great. The visual nerve signals leave the retinae through the optic nerves. At the optic chiasma, the optic nerve fibers from the nasal halves of the retinae cross to opposite sides, where they join fibers from the opposite temporal retinae to form optic tracts. The fibers of each optic tract then synapse in dorsal lateral geniculate nucleus of the thalamus , and from 1-Suprachiasmatic nucleus of the hypothalamus to control circadian rhythms. 2-To pretectal nuclei of the midbrain which is the center of the pupillary light reflex. 3-To superior colliculus serving visual reflexes as eye and head movement and for control of rapid directional movements of the eyes. **Right and left visual hemifields** The full visual field is the entire region of space that can be seen with both eyes looking straight ahead. The visual field is divided into left and right halves. Objects in the left visual hemifield will be imaged on the nasal retina of the left eye and on the temporal retina of the right eye. Because the fibers from the nasal portion of the left retina cross to the right side at the optic chiasma, all the information about the left visual hemifield is directed to the right side of the brain through the right optic tract. Each optic tract fibers represents the contrlateral half of the visual field. **(C) Visual cortex** **I- Primary visual cortex, visual area 1(V1), striate area ( Brodman\'s area 17).** \- It is located in the superior and inferior banks of calcarine sulcus on the medial side of the occipital lobe and receives projections from LGB of the thalamus \- Signals from the macular area terminate near the occipital pole while signals from the peripheral retina terminate more anterior. The fovea has a wider representation than peripheral portions. \- Neurons in the visual cortex are: \- Blob neurons receive the projection about color. These blobs are the primary areas for analysis of color. \- Interblob neurons are classified in function into 2 groups: a\) Motion sensitive neurons; receives information of magnocellular stream. They detect object movement, direction and velocity of movement. b ) Orientation selective neurons: They are involved in analysis of fine object shape or form. These cells are not motion or direction sensitive. \- To summarize functions of area 17 or VI; 1- Perception and begin analysis of visual details as shapes, lines, borders. 2- Color vision by specific vertical column like areas called color blobs.\ 3- Movement of objects and its direction.\ 4- Fusion of the visual images from the 2 eyes for perception of depth. **II- Secondary visual areas (visual association areas)** Located lateral, anterior, superior and inferior to primary visual cortex, these areas are also called extrastriate cortex. Secondary signals are transmitted to these areas from the primary visual area V1. Secondary visual areas include V2 (Brodman\'s area 18). The other more distant visual areas have specific designations V3,V4 and V5 and so forth up to more than dozen areas. The importance of all these areas is that various aspects of visual image are progressively dissected and analyzed to have a meaning. Two major pathways to the secondary visual areas for analysis of informa- tion which are: 1- Dorsal pathway for analysis of motion and the third dimensional position This pathway is concerned with analysis of object movement and third dimension position. This analysis takes place in middle temporal gyrus V5 or (MT) and the adjoining region of the parietal lobe, called the medial superior temporal gyrus 2- Ventral pathway for analysis of visual details, forms and colors. This pathway is concerned with analysis of colors and details. This pathway starts in inferior temporal lobe known as (area IT). Neurons in this pathway are specifically color sensitive and shape sensitive cells. Some neurons in IT area are important in face recognition.