PHYSIO C9 – Visual System PDF

PHYSIO C9 – Visual system I.​ The eye 1.​ Internal chambers To understand the pathway of light, the anatomy must be clear: the eye is formed of a posterior and anterior cavity separated by the lend. -​ anterior cavity is separated into two subsectors by the iris:...

PHYSIO C9 – Visual system I.​ The eye 1.​ Internal chambers To understand the pathway of light, the anatomy must be clear: the eye is formed of a posterior and anterior cavity separated by the lend. -​ anterior cavity is separated into two subsectors by the iris: o​ anterior chamber o​ posterior chamber -​ posterior cavity, vitreous chamber, will be discussed in next lecture. The lens is important for its role in the light pathway to convey the light in the retina and activate the receptors. The lens is an even anatomical separation, between anterior and posterior cavity/segment. If the anterior cavity is considered, the iris is the separator between posterior and anterior chamber. The anterior chamber is between the cornea and iris, the posterior chamber is located between the iris and the lens. In the anterior cavity there’s a fluid known as aqueous humour which is reabsorbed by the sclera’s venous sinus. In the posterior chamber there is also fluid which is a sort of gel called vitreous fluid. 2.​ The anterior chamber The aqueous humour is produced by a capillary network within the ciliary body, a specialized anterior derivative of the choroid layer. This fluid fills the whole chamber and drains according to the arrows in the image into a canal, the sclera’s venous sinus, located at the edge of the cornea. It eventually reenter the blood. The aqueous humour is a fluid and provides the pressure on the surrounding structures. This pressure is exerted on the cornea anteriorly (provides the needed tension) and the lens posteriorly. This pressure keeps the curvature of the cornea and of the anterior chamber adequate. The cornea does not need to be regulated in terms of tension. The aqueous humour, addition to its mechanical function, will perform a metabolic function. Since the cornea and the lens possess no blood supply, due to the needed transparency of their structures, the aqueous humour provides the necessary nutrients for these structures. There is therefore a balance between the production and the absorption. If for some reason, the drainage doesn’t comply to the production of aqueous humour (due to a blocked canal for example), an accumulation of fluid occurs, causing an increase in pressure. This condition is called glaucoma. In more severe cases, the excess fluid will push against inner layer of the retina, causing retinal and optic nerve damage, thus leading to blindness. Do not mix aqueous humour w vitreous humour: the vitreous humour maintains the shape of the eyeball in the posterior chamber. 3.​ The light must enter the eye The route of the light is counter-intuitive: the receptors for light are located in the most posterior layer of the retina. This means that the light must not only cross the entire vitrous body, but also all the layer of the retina. For light to be detected, it must enter the eye. The amount of light entering the eye is controlled by the iris. This means that not all the light passing through the cornea reaches the light-sensitive retina. The iris is a thin, pigmented smooth muscle that forms a ringlike structure within the aqueous humour. It contains a specific pigmentation. Considering the eye in front there is: -​ the iris, -​ the sclera,  -​ the pupil, a hole in the middle. -​ As the iris is made by pigmented muscles, it can be contracted and activated. It is formed of a a circular layer and a radiant layer of muscle. When the muscle is used, according to the layer used it is possible to change the diameter of the pupil. The size of the pupil is adjusted by the variable contraction of the iris smooth muscle. The≠ layer of the iris are used for ≠ functions: -​ Contracting the circular layer, the diameter of the pupil is reduced, -​ Contracting the radiant layer, the diameter of the pupil is increased. These 2 layers are controlled by 2 branches of the autonomic system. -​ the sympathetic cause the radial contraction pupillary dilation -​ parasympathetic causes the circular contraction pupillary constriction 4.​ Pupillary light reflex Pupillary light reflex is what happens when the amount of light in the environment changes (using a torch for example). The reflex is a motor response to a stimulus. It is a consensual reflex: If one of the 2 eyes is covered and the amount of the light is changed on only one side, there will be a change in size of the pupil in both eyes; this feature is a. The pupillary reflex is usually considered as the modulation of the contraction of the circular pupillary muscles. The dilation of the pupil due to the dim light is in fact the reversion to the normal size. The dilation occurs in case of massive activation of the orthosympathetic system, such as in stressful conditions. This state of the pupil is important to be evaluated when a patient arrives to the emergency room. This is because the information form the retina travels in the afferent pathway, taking a shortcut to the pretectal nuclei, then to the Edinger Westphal nucleus (parasympathetic nucleus) where the preganglionic fibre go to the ciliary ganglion synapsing on the postganglionic neuron and reaching the constrictor muscle of the pupil. These structures are very important for their location. When a patient lacks this reflex, it means there is something wrong on this circuit. There can be fixed constriction or fixed dilation, and this is not normal. 5.​ The lenses of the eye refract the light A complex number of points (an object) will have to be transferred in the same way on the retina to maintain the shape. When the source of light is in the environment the rays will be divergent from the point and not parallel. The eye will be hit by these divergent rays and what would happen, if nothing is done, is that all the rays will spread around in different points of the retina. Therefore, the rays have to be made converge on one point in the retina where the photoreceptors are located. This convergence is done by lenses which converge the rays onto the retina in one point. There are different lenses in our eye. The lenses of the eye refract the entering light in order to focus the image on the retina. The two structures that are involved in the eye’s refractive ability are the cornea and the lens. The curved corneal surface, the first structure the light passes through, contributes to the eye’s total refractive ability because the ≠ in density between the air-cornea interface is greater than the ≠ in density in between the lens and the fluids surrounding it. 6.​ Recall the visible light The visible light is a spectrum of all the electromagnetic waves. Only a spectrum of wavelengths can be detected: in between 400nm and 700nm. Light does not have colour. Colour is the representation of our brain to distinguish between different wavelengths. Colours are products of our brain; they are not present in nature. There is no colour if there is not a visual system. Objects have a color because they absorb at some wavelengths and reflect at others. For example, if an object is: -​ White, it reflects all the wavelengths of light -​ Black: absorbs all the wavelengths of light -​ Red: reflects mostly all the wave lengths belonging to “red light”. The color of a light is its wavelength The color of an object is the wavelength “bent” by that object. Mixing up light from ≠ colors (blue-green-red) gives a white light. Mixing up ≠ pigments gives dark all wavelengths are absorbed 7.​ Recall refraction and lenses When light passes from one medium with a given density to another medium with a ≠ density, it’s speed is altered. This causes a refraction. The point source of the light, according to the distance of the object (for far distance, >6m) will emit divergent rays, but our eyes will receive parallel rays, but still separated distant vision. If the point source of light is closer the rays will arrive as divergent and separated close vision. If lenses are used to converge, we need a biconvex lens. The rays will converge in the principal focal point. This point is not random, the distance between the lens and the focal point (principal focal length) is always the same. The eye is not changing shape. There is a precise distance between the lens and the retina. In the close vision (image A2), the rays are divergent. There is still a convergence on a point, thanks to the lens, but what changes is the distance (focal length) of the principal focal point. It is further away as more space is needed to converge the incoming divergent rays. Humans can change the power of their lenses because the focal point must always fall on the same point which is the retina and cannot change. The lenses must change the curvature as it is not possible to elongate the eye. There are different lenses: cornea and the lens. The humour aqueous and vitreous do not have a strong refraction and is irrelevant compared to the others. 8.​ Refraction and lenses The cornea is convex, and the lens is biconvex. The cornea is the most external, the lens is internal. The thicker the lens the closer the focal point. The concave lenses increase divergence, extending the focal distance. Convex lenses are thickest in the center, light is bent so that it converges or comes together at a focal point. In general, the thicker the lens (more convex), the more the light is bent and the closer the focal point is. The distance between retina and lens is fixed. Concave lenses on the other hand are thicker at the edges and diverge light extending the focal distance: the work in opposite manner. In human eyes no concave lenses are present. Reflection of light by objects accounts for most of the light reaching the eyes. Light waves diverge (radiate outwards) in all directions from every point of a light source. The forward movement of a light wave in a particular direction is known as light ray. The action of the eye is similar to a camera: the light enters, bends on the cornea, refracts as it enters lens, second refraction as it leaves lens, and the focal point falls on retina. The result is that the waves reflected by objects are caught The image above shows an optical section of lenses. The eye has different lenses, but the curvature of the cornea is fixed. The aqueous humour holds it in position. The curvature of the lenses can be modulated, they are suspended by a ligament which is surrounded by the muscle. The action of the muscle causes it to be stretched or relaxed. The bending power of the cornea can’t be changed, but the one of the lenses can. Depending on whether close vision or far away vision is needed, the muscle adjusts the stretch that it exerts on the lens. The other aspect that the eye controls is the amount of light that enters. Accommodation includes both functions: the adequate length of focal point and correct amount of light entering. Normally, they work together, not because they are part of a single circuit (they aren’t) but because the system integrates them. In close vision the amount of light entering decreases and in distant vision it increases. The retina is the “film” where the projection goes. 9.​ The path of light: The light is modified as soon as it enters the eye, the steps it goes through from air to retina are: -​ Cornea (light is bent at the entry) -​ Aqueous humour -​ Lens (light is bent at entry and exit) -​ Vitreous humour And then it passes through the entire thickness of neural layer of retina before it can excite the photoreceptors that abut the pigmented layer. During this passage, light rays are bent 3 times: 1)​ on entry into the cornea 2)​ on entry to the lens 3)​ on exit from the lens Divergent light rays reaching the eye must be bent inwards to be focused back into a point (focal point) on the light-sensitive retina and provide an accurate image of the light source. 10.​Focus on the retina Two factors allow for this to happen: 1)​ The powerful lenses of the eye ​ Cornea is responsible for most bending even if it is unchanging, it is very powerful. ​ Lens (while elastic) can be actively changed to allow fine focusing and does accommodation. The cornea alone accounts for a focal distance of 17 mm so the strength is of 59 dioptres. Accommodation strength brings it up to 73 dioptres. Dioptres is the measure used to assess the power of the eye. Without the cornea we’d be blind because it allows for the minimum dioptres, sometimes cornea must be replaced with a new one. 2)​ The source of light: -​ object close to the eyes present us with light rays that are diverging rapidly. To see something close to us, we must focus these rapidly diverging rays. -​ Objects that are distant present light rays that have become more parallel – the angle between them has decreased to a great extent. In fact, if the distance is great compared to the size of our eyeball, the light rays for all intents are parallel to one another. 11.​Distance vision Our eyes are best adapted for distance vision as the default condition of the eye is prepared for distant vision. Distant vision can be divided into a distance beyond which no change in lens shape is needed anymore for focusing, and emmetropic (normal) distance which is 6 m. -​ Ciliary muscles are completely relaxed during distance vision -​ the lens is stretched flat by tension due to the suspensory ligaments and it is as thin as it gets. It is counter intuitive, but when distant vision is needed and so the eye it’s in default condition, is when the lens is stretched (“flat condition”) and the muscle is relaxed. The ciliary muscle is in fact a circular muscle that contracts compressing the lens and making it thicker in its centre. When contraction stops lens goes back passively to the original position thanks to its elastic property. When the muscle contracts the ligament becomes floppy as the distance between the attachment of the ligament on the muscle and on the eye decreases (see image below for clarification). The curvature of the lens is modulated by the strength of contraction of the ciliary muscles. 12.​Focusing on close vision A near point of vision is the closest point possible to focus on and corresponds to the maximum bulge of the lens. Close vision demands for the eye to have an active adjustment. For close vision to occur three processes are needed and must occur simultaneously: 1)​ Accommodation of the lenses: ciliary muscles contract and light gets bent more sharply 2)​ Constriction of the pupils: prevents most of the divergent light from entering the eye, increases the clarity and depth of focus. 3)​ Convergence of the eyeballs: when looking at a near object the eyes converge The two arms of the vegetative system are the ones controlling the process: Relaxed situation corresponds to tonic sympathetic stimulation that keeps muscles relaxed, in contracted condition parasympathetic system is in action. The parasympathetic oversees the whole accommodation process. Objects are made of loads of light points. The light after hitting the object reflects light points, the R in the image goes through refraction three times and in this is way it will be upside-down and left-right turned when on the retina. In distant vision no action is required, in close vision the accommodation is needed. 13.​Homeostatic imbalances of refraction: The physiological condition is emmetropia: light rays from an object are focused properly on the retina. There are three types of imbalances: -​ Myopia: near sighted, image focused in front of retina. Perception of blurry objects when focusing on distant ones. It can be corrected using a concave lens. Light diverges  once more than it supposed, after it has converged too early. By using lens the divergence amount is adjusted and focal point will match with retina in the right point -​ Hyperopia: far sighted, image is focused behind the retina. Also consists of the perception of blurry images, it can be corrected using a convex lens. -​ The Snellen eye chart is used to know how many dioptres people have. -​ Astigmatism: unequal curvature in different parts of lens or cornea (mainky). A point image is perceived as dilated like a stain. It can be corrected using a cylindrical (thoric) lens In astigmatism some parts of the objects are seen well, and some are seen badly, it is in fact a local problem. Only a certain portion of the cornea is altered, the rest works fine. Parts of the chart below are seen as almost blurry or distorted by individuals with astigmatism. Some parts are going to be perfectly distinguished and some quadrant’s line might be confused. Other eye defects/imbalances increase as people grow older: The lens should be biconvex and transparent, but sometimes transparency is lost and elasticity as well. Accommodation won’t be properly performed. Cataract is the condition where the lens is “cloudy” which causes light to be bent in all directions. Cataract can be removed with non-dangerous procedure. Presbyopia: defect due to the loss of elasticity of the lens (near objects appear blurred). It can be corrected using a bifocal lens (combination of lenses with different refractive strengths). Diplopia = double vision: defect due to the failure in focusing the images on the retinas of the two eyes in the corresponding points. It’s not due to refraction problems, but usually related to deficits in ocular muscles activity or in a disruption of the alignment of the two eyes. RECAP Pupil reflex = regulation of the amount of light Accommodation = regulation of the refraction power Lenses then work on the rays to focus them onto the retina where they will trigger the rods and cones which give us vision. Diverging rays, the lens must be more curved. Parallel rays, the lens must be less curved. A set of muscles around the lens can contract to make it more rounded, as they relax, they allow the lens to become more rounded. 14.​The visual field: ​ Monocular visual field (monitored by one eye only): 160 deg (W) X 135 deg (H) ​ Binocular visual field (monitored by both eyes): 200 deg (W) X 135 deg (H) The periphery is monocular, the center binocular. The real image is formed by convex lenses it is received upside-down and reversed from left to right. Each point from the object is reproduced as a point on the retina, creating an image of the original object. II.​ The eye and the brain 1.​ The light enters the eye: sensory transduction In order to understand how the retina works, it is useful to remember the layers of the retina, which is considered an outpocketing of the brain, located into the eye. The retina is a delicate two-layered structure:  -​ The outer layer is pigmented, and located next to the choroid. It is one cell thick and it absorbs light. In this layer, cells act as phagocytes and store vitamin A, needed by photoreceptor cells. -​ The inner layer is the neural layer, and it extends anteriorly to the ora serrata. In this layer the phototransduction process occurs, by which the light hits the receptors and activates them. Light rays end up onto the retina, they hit the receptors and they are transduced into a variation of their membrane potential. This represents an exception, since normally receptors activate by depolarizing: in this case receptors activate by hyperpolarizing. 2.​ Sensory transduction is a complex process computed by retinal circuit The picture shows the direction of light (yellow arrow) and the direction of the retinal visual processing (green arrow). The receptors are located at the back of the retina, while the sensory neurons are placed anteriorly. This is counterintuitive, since it would be expected to have the receptors in front, to be the first to detect the stimulus, as it is in all the other sensory organs. In the eye though, the light has to go through all the layers of the retina to reach the receptors. There are three main types of cells in the retina: -​ Photoreceptors: o​ Rods o​ Cones -​ Bipolar cells -​ Ganglion cells: are “the neurons of the retina”, they synapse with the bipolar cells and then emit the axons, which travel on the retina and convey in the optic nerve. The light goes through all the layers before reaching the rods and the cones, so through both the bipolar cells layer and the ganglion cells layer. Further in the back there is also the pigmented layer, then the choroid layer and finally the sclera. The light is dissipated at the level of the pigments by the absorption of the light.  The visual processing goes from the back toward the front; the light hits every structure before hitting the receptors, but these are the only ones to respond, since they have adequate devices to respond to the stimulus. 3.​ Pigmented layer The reason for the counterintuitive organization is the special relationship that exists among the outer segments of the photoreceptors and the pigment epithelium. The cells that make up the retinal pigment epithelium have long processes that extend into the photoreceptor layer, surrounding the tips of the outer segments of each photoreceptor The pigment epithelium plays two roles that are critical to the function of retinal photoreceptors. -​ The membranous disks in the outer segment, which house the light-sensitive photopigment and other proteins involved in phototransduction, have a life span of only about 12 days. New outer segment disks are continuously being formed near the base of the outer segment, while the oldest disks are removed. During their life span, disks move progressively from the base of the outer segment to the tip, where the pigment epithelium plays an essential role in removing the expended receptor disks. This shedding involves the “pinching off” of a clump of receptor disks by the outer segment membrane of the photoreceptor. This enclosed clump of disks is then phagocytosed by the pigment epithelium. -​ The epithelium’s second role is to regenerate photopigment molecules after they have been exposed to light. Photopigments are continuously cycled between the outer segment of the photoreceptor and the pigment epithelium. 4.​ Cones and rods Rods and cones are an example of third type receptors, cells (which are not neurons) connected to another cell, that transfers the message to a neuron: the ganglion cell. This is a three step process, but the ganglion cell is the only one able to induce an action potential. The other cells (photoreceptors and bipolar cells) are excited, by changing their membrane potential with graded potential, but they do not induce an action potential, so the output must originate in the ganglion cells. This system, made of these three cells with the contribution of amacrine and horizontal cells, works to allow not only the detection of the point source of light, but also shapes, objects, and to allow representations on the retina. 5.​ Photoreception – hyperpolarisation In most sensory systems, activation of a receptor by the appropriate stimulus causes the cell membrane to depolarize, ultimately stimulating an action potential and transmitter release onto the neurons it contacts. In the retina, however, photoreceptors do not exhibit action potentials: light activation causes a graded change in membrane potential and a corresponding change in the rate of transmitter release onto postsynaptic neurons. Indeed, much of the processing within the retina is mediated by graded potentials, largely because action potentials are not required to transmit information over the relatively short distances involved. Perhaps even more surprising is that shining light on a photoreceptor, either a rod or a cone, leads to membrane hyperpolarization rather than depolarization. In the dark, the receptor is in a depolarized state, with a membrane potential of roughly –40 mV (including those portions of the cell that release transmitters). Progressive increases in the intensity of illumination cause the potential across the receptor membrane to become more negative, a response that saturates when the membrane potential reaches about –65 mV. How does activation of rhodopsin cause hyperpolarization? The answer is that when rhodopsin decomposes, it decreases the rod membrane conductance for sodium ions in the outer segment of the rod causing hyperpolarization. The figure on the right ( ) shows movement of Na+ and K+ ions in a complete electrical circuit through the inner and outer segments of the rod. The inner segment continually pumps Na+ from inside the rod to the out- side, and K+ ions are pumped to the inside of the cell. -​ K+ 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 Na+/K+ pump creates a negative potential on the inside of the entire cell. However, the outer segment of the rod, where the photoreceptor discs are located, is entirely different. Here, the rod membrane, in the dark state, is leaky to Na+ ions that flow through cyclic guanosine monophosphate (cGMP)–gated channels. In the dark state, cGMP levels are high, permitting positively charged sodium ions to continually leak back to the inside of the rod and thereby neutralize much of the negativity on the inside of the entire cell. 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. When the rhodopsin in the outer segment of the rod is exposed to light, it is activated and begins to decompose. The cGMP-gated sodium channels are then closed, and the outer segment membrane conductance of Na+ to the interior of the rod is reduced by a three-step process: (1)​ light is absorbed by the rhodopsin, causing photoactivation of the electrons in the retinal portion. (2)​ The activated rhodopsin stimulates a G protein called transducin, which then activates cGMP phosphodiesterase, an enzyme that catalyzes the breakdown of cGMP to 5′-GMP. (3)​ The reduction in cGMP closes the cGMP-gated sodium channels and reduces the inward Na+ current. Na+ ions continue to be pumped outward through the membrane of the inner segment. Thus, more sodium ions now leave the rod than leak back in. Because they are positive ions, their loss from inside the rod creates increased negativity inside the membrane, and the greater the amount of light energy striking the  rod, the greater the electronegativity becomes—that is, the greater is the degree of hyperpolarization. At maximum light intensity, the membrane potential approaches −70 to −80 millivolts, which is near the equilibrium potential for potassium ions across the membrane. 6.​ Wide range of luminal intensity The two types of photoreceptors, rods and cones, are distinguished by their shape (from which they derive their names), the type of photopigment they contain, their distribution across the retina, and their pattern of synaptic connections. These properties reflect the fact that the rod and cone systems (i.e., the receptor cells and their connections within the retina) are specialized for different aspects of vision (will be seen later on). -​ The rod system has very low spatial resolution but is extremely sensitive to light: it is therefore specialized for sensitivity at the expense of seeing detail. -​ The cone system has very high spatial resolution but is relatively insensitive to light: it is specialized for acuity at the expense of sensitivity. The properties of the cone system also allow humans and many other animals to see color. Description on rod and cone system will be explained later on. Figure below shows the range of illumination over which the rods and cones operate. Scotopic vision: At the lowest levels of illumination, only the rods are activated. Such rod-mediated perception is called scotopic vision. The difficulty of making fine visual discriminations under very low light conditions where only the rod system is active is a common experience. The problem is primarily the poor resolution of the rod system (and to a lesser extent, the fact that there is no perception of colour because in dim light there is no significant involvement of the cones). Although cones begin to contribute to visual perception at about the level of starlight, spatial discrimination at this light level is still very poor. As illumination increases, cones become more and more dominant in determining what is seen, and they are the major determinant of perception under conditions such as normal indoor lighting or sunlight. The contributions of rods to vision drops out nearly entirely in photopic vision because their response to light saturate (the membrane potential of individual rods no longer varies as a function of illumination because all of the membrane channels are closed). Mesopic vision occurs in levels of light at which both rods and cones contribute—at twilight, for example. It is possible to see over a wide range of luminance intensity thanks to different mechanism: -​ Pupillary size changes (pupil reflex): it is possible to change the pupillary diameter, which means that in conditions of low luminance, the pupil is open (dilated) in order to have the highest possible amount of light entering the system. In the case of bright light, pupils are constricted, allowing less light to enter the eye. -​ Mechanisms related to properties of photoreceptors: Light adaptation and Dark adaption -​ Duplex retina: rods and cones 7.​ Light adaptation The light adaptation occurs when there is an abrupt passahe form darkness into bright light. Retinal sensitivity decreases dramatically and the retinal neurons switch from the rod to the cone system. Rods are very light sensitive but have a poor visual acuity. Cones have a low sensitivity but a high visual acuity. Therefore, in darkness, the most important elements are detected, but in order to have high vision acuity, light is needed to further activate the cones. In bright light the rods are completely saturated, whereas the cones are above their threshold modulating their excitation in relation to light intensity. By moving from a dark area to a well-lit one, the retinal sensitivity is increased dramatically, and the retinal neurons switch from the rod to the cone system. When the receptors are exposed to light, there is the hyperpolarization of the receptors: initially the amount of light is such that the cones are “overwhelmed”, they are completely saturated and go into their deepest hyperpolarization. This means that they are not able to modulate the excitation, distinguish different amounts of light, or detect the environment. After a persistent exposure (seconds in this case), the receptors adapt, so the cones, very stimulated, have a slight depolarization. Since Ca2+ enters with Na+, when light closes Na channels Ca also decreases and the cGMP formation increases. There is then an increase in the conductance of Na+ and so a slight depolarization occurs. (Given that the Ca2+ enters with the Na+, if the Na+ decreases, also the Ca2+ decreases, so the guanylate cyclase increases its activity and partially opens some Na+ channels). The slight depolarization removes the overactivation and leaves the system able to modulate the activity, responding to different intensity of light. In fact, in a few seconds it is possible to see more clearly in the room. 8.​ Dark adaptation In the case of dark adaptation, the cones stop functioning (it is unavoidable, the threshold is not even reached) and the rods are the only working ones. Rhodopsin accumulates in rods and keeps them more sensitive and performant. In light adaptation the sensitivity of cones is decreased (until they stop working), in the dark adaptation rods are sensitized, until their maximum sensitivity. These 2 mechanisms belong to the same domain and rely on the differences between the receptors. 9.​ Duplex retina Rods and cones have ≠ functional propreties: -​ Spatial distribution: rods and cones are not homogeneously distributed in the retina -​ Spectral sensitivity: some cones are sensitive to some wavelengths (spectrum) and some to others, they can distinguish between different wavelengths and the product of  activation of different families is colour. Rods can not: they just translate, activation of rods relative to intensity of light is translated into degrees of grey. -​ Sensitivity to light: threshold is different -​ Contrast sensitivity: based on architecture more than on individual properties. Rods and each of three cone types contain unique visual pigments, they absorb different wavelengths of light and have different thresholds for activation. Rods: ​ Are very sensitive (respond to dim light) ​ Absorb all wavelengths of visible light ​ Wavelengths are only perceived in gray tones Cones: ​ Require very bright light for activation ​ Absorb specific wavelengths of visible light ​ Specific wavelengths are perceived in varied colours Specific distribution The distribution of rods and cones across the surface of the retina also has important consequences for vision. Despite the fact that perception in typical daytime light levels is dominated by cone-mediated vision, the total number of rods in the human retina (about 90 million) far exceeds the number of cones (roughly 4.5 million). As a result, the density of rods is much greater than that of cones through-out most of the retina. However, this relationship changes dramatically in the fovea, the highly specialized region in the center of the macula that measures about 1.2 mm in diameter. The increased density of cones in the fovea is accompanied by a sharp decline in the density of rods. In fact, the central 300 μm of the fovea, called the foveola, is totally rod-free. The extremely high density of cone receptors in the fovea, coupled with the one-to-one relationship with bipolar cells and retinal ganglion cells, endows this component of the cone system with the capacity to mediate the highest levels of visual acuity. ​ As cone density declines with eccentricity and the degree of convergence onto retinal ganglion cells increases, acuity is markedly reduced. Just 6 degrees eccentric to the line of sight, acuity is reduced by 75%, a fact that can be readily appreciated by trying to read the words on any line of this page beyond the word being fixated on. The restriction of highest acuity vision to such a small region of the retina is the main reason humans spend so much time moving their eyes (and heads) around—in effect directing the foveae of the two eyes to objects of interest. ​ Conversely, the exclusion of rods from the fovea, and their presence in high density away from the fovea, explains why the threshold for detecting light at a lower stimulus is lower outside the region of central vision. It is easier to see a dim object (such as a faint star) by looking slightly away from it that the stimulus falls on the region of the retina that is richest in rods. In the monocular peripheral portion, it is difficult to perceive colours: we continuously perform the saccades, a rapid movement to try to focus the peripheral stimulus on the fovea (As a matter of fact, the control of ocular movement is very complex). In animals the peripheral retina has this aim to call for attention: put in focus the object of interest, straight in the fovea where there are cones that can distinguish different colours, a way to be refined in perceiving contrasts. Objects are made of different point sources of light, the ability to perceive a shape and details is based on the ability to perceive contrast of luminance. Objects are not only hit by light, but they also reflect light and, depending on their shape, the luminance will be different. The higher is the ability to detect contrasts, the higher will be the acuity of vision, the retina is basically made of “pixels”, the smaller the dimension, the higher will be the ability to distinguish two different source points of light, which means difference of luminance between the two, which means high acuity. In the fovea there is a cone mosaic, where there are different types of cones, sensitive to different wavelengths. The amount of light matters a lot: sometimes too much light impairs acuity as much as a low intensity of light, so it is necessary to calibrate the amount of light. In scotopic vision, colours are lost, since only rods are working, but with so much light, the picture is too bright and colours cannot be detected as well, since all cones are activated and saturated, it is almost white. We can see that colours allow us to detect the shape: shape and colours are extremely correlated. As a matter of fact, at night colours are lost, but also acuity is lower. Acuity can be explained only by architecture, as in Merkel’s and Meissner’s receptors. (The most sensitive portions of the skin are tips of fingers and the tip of the tongue, because there is a high density of receptors, so they have tiny receptive fields and the pathway is not convergent. Receptors do not converge too much on a projecting neuron: keeping the receptive fields separated until the cortex, it is possible to distinguish two stimuli applied to very close patches of skin as separated). If the receptive field is very big or two converge, it is not possible to perceive them separated and so this is not refined. The same is true for the visual system applied for retina. Rods are cones are “wired” in different ways: Circuits to retina Each Cone in the fovea (or at most a few) has a straight-through pathway to its own ganglion cell. So, different flashes of light, especially in the fovea, where there is the highest concentration of cones with tiny receptive fields, will be perceived as separated. This accounts for the sharp, detailed, high resolution of colour vision. Rods participate in converging pathways and as many as 100 rods ultimately feed into one ganglion cell. Each rods has a tiny receptive field, but they have convergent pathways, this means that a ganglion cell can be activated by flashes of light coming from different receptive fields, so a huge patch of retina. Rods effects are summated and considered collectively fuzzy and indistinct. Visual acuity is related to the resolution of minimal differences in objects shape, since the shape is the result of the ability to perceive a contrast of luminance between two different sources of light. It relays on spatial discrimination (parallel with somatosensory system). The minimal visual angle is one degree, and the minimal distance perceived at retinal level is 4.5 μm (basically a cone diameter). In fovea you are able to distinguish two points as separated at this distance, that is the highest possible acuity. Two cones have to be stimulated to obtain spatial discrimination. RODS: ​ High light sensitivity (scotopic vision) ​ High pigmentus and high light absorbing capacity ​ Amplification of signal ​ A photon is enough to stimulate a rod ​ Scarce temporal resolution: slow responses and slow integration of time ​ High speed images are not well distinguished RODS SYSTEM ​ Scarce visual acuity ​ Private connections ​ Absent in fovea ​ High convergence ​ No colors (only one pigmentus) CONES ​ Low light sensitivity (phototopic vision) ​ Low pigmentation and low light absorbing capacity ​ No amplification of signal  ​ More than 1 photon (100) are needed ti stimulate a cone ​ High temporal resolution: fast responses, fast integration time ​ High speed images are well distinguished CONES SYSTEM ​ High visual acuity ​ Private connections ​ Maximal concentration in fovea ​ Chromatic vision (3 pigmenta) 10.​The receptive field Despite the aesthetic pleasure inherent in having color vision, most of the information in visual scenes consists of spatial variations in light intensity. The mechanisms by which central targets decipher the spatial patterns of light and dark that fall on the photoreceptors have been a vexing problem (Box 11D). To understand what the complex neural circuits within the retina accomplish during this process, it is useful to begin by considering the responses of individual retinal ganglion cells to small spots of light. Stephen Kuffler, working at Johns Hopkins University, pioneered this approach in mammals early in the 1950s when he characterized the responses of single ganglion cells in the cat retina. He found that each ganglion cell responds to stimulation of a small circular patch of the retina, which defines the cell’s receptive field. Based on these responses, Kuffler distinguished two classes of ganglion cells : -​ ON-center ganglion cells -​ OFF-center ganglion cells Turning on a spot of light in the receptive field enter of an ON-center neuron produces a burst of action potentials. The same stimulus applied to the receptive field center of an OFF-center neuron reduces the rate of discharge, and when the spot of light is turned off, the cell responds with a burst of action potentials. Complementary patterns of activity are found for each cell type when a dark spot is placed in the receptive field center. ​ Thus, ON-center cells increase their discharge rate to luminance increments in the receptive field center, whereas OFF-center cells increase their discharge rate to luminance decrements in the receptive field center. The receptive field is not made up by one photoreceptor, but by a multitude of photoreceptors. The receptive field is connected to one bipolar cell, which is connected to one ganglion cell, and near the bipolar cell there are the horizontal cells and amacrine cells. Structures in the center of the field are connected directly to the bipolar, structures in the surrounding are connected to the horizontal cells, which in turn talk with the bipolar, which in turn is connected to the ganglion cell. For each receptive field there are two bipolar cells, connected with two ganglion cells. For each receptive field, the photoreceptors are always the same (for example: 100 selected rods). There is a population in the center and one in the surrounding area, and they have different relationships with the bipolar cell. The bipolar cells will be, as the ganglion cells; one on-center, the other off-center, for that specific copy. If the On-center ganglion cell in a specific field is stimulated by light in the center, there is a background discharge which increases. If the flash is directed to the surround, this will stop. For the other ganglion cell, the Off-center, if light flashes on the center, it stops firing, but when light is flashed in the surrounds, firing is increased. The two different ganglion cells are connected with the same receptive field and they react in the opposite way to the same stimulus. Each receptive field, made of a community of receptors, will connect to two bipolar and to two ganglion cells. Each bipolar and each ganglion cell will react completely opposite if light is flashed on the periphery or in the center. It is like having a double retina: for each patch of retina we have a double copy, that does exactly the opposite. If the light is flashed in the center of a field the brain will detect an increase in discharge for the On-center ganglion cell, while the Off-center will decrease the discharge. The two pieces of information are matched. This mechanism relies on two factors. -​ the action of the NT released by photoreceptors on the bipolar cells: glutamate (The glutamate is released when the cell is depolarized, at rest). The effect is opposite in the two bipolar cells. -​ the direct or indirect relationship of the receptors in the center or, in the periphery with the bipolar. The bipolar cell talks directly with receptors in the center, while it needs horizontal cells to talk with receptors at the periphery (their dendrites).  11.​Glutamate effect The effect of glutamate on the ON-center bipolar and on OFF-center bipolar is opposite. Glutamate is released when the photoreceptor is not stimulated, when it is at rest, because it is depolarized, so the receptors work constantly in absence of stimulus, in the dark. When light arrives, photoreceptors hyperpolarize, which means that the amount of glutamate released decreases. In the darkness, there is a constant release of glutamate by receptors (depolarized) onto bipolar cells. -​ The bipolar ON-cell is hyperpolarized, as the effect of glutamate is inhibitory, inducing a closure of the Na channels. In darkness it is not excited. -​ The ganglion cell connected to the bipolar ON-cell will not discharge. -​ The bipolar OFF-cell is depolarized, as the effect of glutamate is excitatory, by opening the Na channels. In darkness it is excited. -​ The ganglion cell connected to the bipolar OFF-cell will discharge In the light, the release of glutamate by the photoreceptors (hyperpolarized) onto bipolar cells is decreased. -​ The bipolar ON-cell is depolarized, as the inhibition is reduced by the decrease of the release of glutamate. In light it is excited. -​ The ganglion cell connected to the bipolar ON-cell will discharge. -​ The bipolar OFF-cell is hyperpolarized, due to the decrease of glutamate. In light it is not excited. -​ The ganglion cell connected to the bipolar OFF-cell will not discharge.  12.​Light in the center of the receptive field In darkness, with the release of glutamate, the ON-bipolar is inhibited, the OFF-bipolar is excited. The ganglion cell connected with the Off bipolar will discharge, the ganglion cell connected with the On bipolar will not discharge. During light stimulation upon hyperpolarization, the release of glutamate is decreased onto bipolar cells. Bipolar ON-cell is depolarized and excited, bipolar OFF-cell is hyperpolarized and inactive. The ganglion cell connected with the ON-bipolar will discharge, while the ganglion cell connected with the OFF-bipolar will not discharge. Summarizing: In absence of light: ​ On-bipolar: inactive ​ Off-bipolar: active With light: ​ On-bipolar: active ​ Off-bipolar: inactive  13.​Light in the surround of the receptive field When we stimulate the surroundings the On center hyperpolarizes, and the Off center depolarizes. 14.​The role of the horizontal cells The horizontal cells connect the receptors in the surrounds to the bipolar cells, since they have no direct contact. If light is flashed onto the center, the cone in the surroundings depolarizes and glutamate release increases. The horizontal cell is activated and releases GABA, which acts on the bipolars. In this case, the bipolar cells receive less glutamate, due to activation in the center, and GABA, released by horizontal cells. Horizontal cells act a lot on OFF-center bipolar cells that hyperpolarize, inhibited by the lack of glutamate and the GABA as well. If light is flashed in the surroundings, the cone in the surrounding hyperpolarizes and the amount of glutamate onto the horizontal cells decreases, which in turn decreases the amount of GABA released. As a matter of fact, the activity of horizontal cells is decreased, so the OFF-center bipolar is activated, since the inhibition is removed. When the center is stimulated, the signal is determined by the neurotransmitter, that is decreased from the receptor to the bipolar, plus the action of the horizontal cells on the bipolar cells. The center receptor is hyperpolarized due to the light and even more due to the fact that the surroundings are in darkness. When the surrounds are stimulated, the inhibition from GABA is removed, so the excitation of the Off-center bipolar that were already active in darkness increases (they are depolarized), they increase their activation. When the surroundings are in darkness, the horizontal cells are fed by the glutamate, released by photoreceptors. The horizontal cells increase the hyperpolarization of the connected photoreceptors and increase the activation of the others. Their role is to inhibit the release of glutamate. Not only is the center the receptor hyperpolarized due to light, but it is hyperpolarized even more due to the fact that in the surroundings there is darkness, (the horizontal cells are activated and increase the hyperpolarization, now the amount of glutamate is even lower). If the surroundings are in light, the ON-center is off, because the glutamate inhibits it, and the OFF-center is in a (background) discharge. The surroundings are hyperpolarized, so the activity of the horizontal is decreased. The horizontal cells amplify the effect of glutamate on the Off-center, these cells are the only ones that can explain the effect on the surroundings, they do not receive the glutamate from the receptors of the periphery and so they are inhibited. The action of the horizontal is to improve even more the hyperpolarization; so the depolarization, in this case, is due to two factors: ​ The tonic inhibition due to horizontal cells is removed by light in the surrounds ​ Dark in the center This leads to an empowered action on the Off-center. When the light is in the center, the cell is excited due to the effect of the glutamate on the bipolar directly. If light hits the receptor, it hyperpolarizes, decreasing the neurotransmitter, which is inhibitor on the On-center bipolar. Thus, decreasing the amount of glutamate, the On-bipolar is excited, while the other one is in background discharge. When the light hits the surroundings, it is important to understand not only why the On-center is off, since there is no light but also why the other ones in the surroundings are switched on. The reason is that the horizontal cells lack in their action, the bipolar Off-center is excited even more and the release of glutamate is even more. The role of the horizontal is more crucial in the surroundings than in the center, where the stimulation is simpler. Amacrine cells have similar effects to the horizontal cells and they are responsive to transients.  15.​Retinal computing Looking at the situation A, in the graph below, we can see that if we spot a light in the center, the On center switches on, and the Off center is off. In situation B the opposite pattern is observed. The glutamate inhibits the On center and the release in the Off center is excited by the horizontal cells, the Off center fires. In case C with only a spot on the surround, there is a mild activation of the Off-center, it depends on the amount of receptors that are hyperpolarized, on how big the light spot is. Observing the behaviour of an On-center ganglion cell: (1)​ If we stimulate the center with a spot of light the ganglion fires, at 90 Hz, (2)​ then if we move the light to the periphery the frequency of discharge decreases because the center is still stimulated. (3)​ if we only stimulate the surroundings, the frequency decreases and passes under the background discharge. (4)​ By removing the light, background discharge is observed. By flashing a bar of light on the On-center ganglion cells we create a complex information. -​ Field A fires no response, -​ in field B the Off surrounds are in light and the On center is off. -​ Field C is at rest because it is completely split in two opposite halves. -​ The field D is excited because the light is on the On center and the surroundings are not very stimulated anyway. -​ The E field is almost at rest, because the Off surround is completely stimulated and it inhibits the signal. The brain always asks for confirmation, so it is important to rely on both the On-center and on the Off-center. If the information collected is coherent, the brain can reconstruct the shape of the light that is stimulating the retina. The absence of stimulus for one field is the onset of stimulus for another, in this way it is possible to tell the position of the spot of light.