VISUAL SYSTEM PHYSIOLOGY. Discipline of Physiology and Neuroscience, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania

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1 VISUAL SYSTEM PHYSIOLOGY Discipline of Physiology and Neuroscience, Carol Davila University of Medicine and Pharmacy, Bucharest, Romania

3 Outer layer of the eye Cornea - No bood vessels - Most powerful eye lens! - Dioptric power is not variable - Transparent - Very well innervated (pain) trigeminal nerve Sclera - continuation of the substantia propria of the cornea - tough connective tissue coating, protective layer The limit between the cornea and the sclera is called limbus Visible part of the sclera is covered with conjunctiva

4 Middle layer of the eye- uvea (uveal tract)= iris, cilliary body, choroid Iris - Pigmented disk shaped structure - Central opening- pupil- variable size- optical diaphragm - Pupilary light reflex- constriction/dilation controls the quantity of light reaching the retina Choroid - Many blood vessels - The blood vessels of the choroid supply the pigment epithelium of the retina

5 Pupillary light reflex Myosis= parasympathetic reflex Mydriasis= sympathetic reflex

Pupillary light reflex Light stimulates optic nerve (neuron 1) that synapses in the pretectal nucleus (neuron 2) project to Edingher-Westphal nuclei (BOTH SIDES)

6 Pupillary light reflex Light stimulates optic nerve (neuron 1) that synapses in the pretectal nucleus (neuron 2) project to Edingher-Westphal nuclei (BOTH SIDES) stimulating preganglionic parasympathetic neuron (neuron 3) synapses in the ciliary ganglia with postganglionic parasympathetic neuron (neuron 4) that CONSTRICT BOTH PUPILS

7 Middle layer of the eye- uvea (uveal tract)= iris, cilliary body, choroid Cilliary body - Wide ring- shaped structure adjoining the iris laterally - 2 parts: - Posterior= pars plana- muscle named orbiculus ciliarissphincter- like muscle - Anterior= pars plicata- 80 fringe like ciliary processes containing capillaries, from which thin fibers (suspensory ligament or Zinn zonule) pass to the lens

9 Ciliary body Two main functions: (1) It produces and secretes aqueous humor into the posterior chamber of the eye (2) it contains the smooth muscle that acts on the crystalline lens, via the zonular fibers, to shift the focus of the eye from far to near (accommodation)

10 Aqueous humor Secreted from the ciliary processes into the posterior chamber of the eye It passes through the pupil into the anterior chamber of the eye (between the iris and the cornea) Aperture of the iris influences flow- myosis loosens it

11 Aqueous humor drainage Most of the AH- absorbed into a critical element filtration angle Filtration angle- complex structure located where the sclera meets the root of the iris Here both the iris and the sclera are loosened 90 % of AH trabecular meshwork Schlemm s canal (dilated vessel which encicles the limbus) aqueous veins episcleral veins anterior ciliary veins

12 Intraocular fluids The eye is filled with intraocular fluid, which maintains sufficient pressure in the eyeball to keep it distended: -aqueous humor: a freely flowing fluid (protein-free ultrafiltrate) which lies in front of the lens, continually being formed by ciliary epithelium and reabsorbed by Schlemm s canal to maintain normal intraocular pressure (20 mmhg) -vitreous humor: a gelatinous mass held together by a fine fibrillar network composed of elongated proteoglycan molecules, in which water and dissolved substances can diffuse slowly. It fills the posterior chamber.

14 Aqueous humor Balance between secretion and elimination of the AHdetermines the intraocular pressure (IOP) Nutrition for avascular eye tissues (aminoacids and glucose)- posterior face of the cornea, lens, trabecular meshwork Immune function- presence of antibodies Refractive index

15 Glaucoma= rise in the IOP The average normal intraocular pressure is approximately 20 mm Hg (measured clinically by using a tonometer). Imbalance in the secretion and reabsorption of aqueous humor can increase the pressure in the eye (more frequently decrease in the reabsorbtion) this condition threatens the viability of the head of the optic nerve (mechanic and ischemic lesion)= blindness if left untreated

18 Normal disk ratio between the diameter of the cup compared to the one of the whole disk<1/2 Glaucomatous diskwide cupping, optic nerve atrophy

20 Ciliary body Two main functions: (1) It produces and secretes aqueous humor into the posterior chamber of the eye (2) it contains the smooth muscle that acts on the crystalline lens, via the zonular fibers, to shift the focus of the eye from far to near (accommodation)

21 Accommodation Far point of the normal eye is infinity. The eye is able to focus parallel rays, that originate at infinity, on the retina (see objects at infinity effortlessly)- beyond 7 m. Near point = minimum distance at which the eye can see objects clearly. By increasing its dioptric power the normal eye can see objects clearly as close as 10 cm (age 30). Young adults have their near point at the end of their nose (the closest distance humans are able to focus)

22 Accomodation Function of the eye that enables it to focus images situated between 7m and 10 cm on the retina! 3 phenomena Changes in the dioptric power of the cristalline lens Pupillary reflex Convergence Ciliary muscle- sphincter-like (it also has radially disposed muscle fibres- low importance) Muscle contraction= zonular fibres relaxation= lens becomes more round= increased focal power Muscle relaxation= zonular fibres stretching= lens becomes more flat= lower focal power

25 Accomodation mechanism PS stimulation contracts circular ciliary muscle fibers releases tension in zonal fibers lens to become rounder and increases its focal power the eye focuses on objects nearer than when the eye has less refractive power. As a distant object moves toward the eye, the number of PS impulses impinging on the ciliary muscle must be progressively increased for the eye to keep the object constantly in focus. Lenses can increase their focal power up to D

$Refraction Refraction is the bending of a wave when it enters a medium where it's speed is different.$

27 Refraction Refraction is the bending of a wave when it enters a medium where it's speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction The Index of refraction of a substance is a measure of the speed of light within it

$Refractive system of the eye four refractive interfaces: (1) interface air - tears+anterior surface of the cornea, (2) interface posterior surface of the$

28 Refractive system of the eye four refractive interfaces: (1) interface air - tears+anterior surface of the cornea, (2) interface posterior surface of the cornea - aqueous humor, (3) interface aqueous humor - anterior surface of the lens of the eye (4) interface posterior surface of the lens - vitreous humor.

29 Two things determine how much the light is REFRACTED: difference in the refractive indices of the two media (for air it is , for cornea it is 1.376) Angle between incident light and the interface between the two media Simple convex lenses converge light rays on a distant surface. Focal power = n2-n1/r n2- refractive indices of the second medium n1- refractive indices of the first medium r- radius of curvature

30 The unit of the focal power is a diopter (1D=1 m ¹) Focal power is the reciprocal of focal length

$Refraction and the eye The refractive apparatus of the eye- collectively termed the dioptric media (has total focal$

31 Refraction and the eye The refractive apparatus of the eye- collectively termed the dioptric media (has total focal power 60D) and consist mainly of the cornea and lens. Cornea has the highest dioptric power = 48.2 D (n2-n1=0.376; r=7.8 mm)

33 Internal lens (crystalline) The lens, biconvex, onion-like shape, is covered by a capsule and consists of packed columnar cells. The lens capsule is anchored to the ciliary body by its suspensory ligaments, or ciliary zonule. Main functions- refraction and accommodation The lens, in addition to becoming increasingly yellow with age, also becomes harder and less elastic, as a result of which the power of accommodation is lost (presbyopia) and convex spectacles may be required for reading. An opacity of the lens is termed a cataract. It is commonly age-related and it may interfere with vision.

34 Cataract- opaque lens

36 Vitreous body transparent, gel-like body (mostly water and hyaluronan) fills the large vitreous space between the lens and the retina. some peripheral fibrils form its capsule= hyaloid it contains a few macrophages and hyalocytes stellate cells with oval nuclei that produce the fibrils and hyaluronan.

38 Normal refraction= emmetropia Parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed the emmetropic eye can see all distant objects clearly with its ciliary muscle relaxed. to focus objects at close range, the eye must contract its ciliary muscle and thereby provide appropriate degrees of accommodation. Correct ratio between eye refractive power and eyeball length; normal eyeball length ~ 24 mm.

39 Hyperopia (or hypermetropia)- farsightedness Usually due to an eyeball that is too short With the lens maximally accommodated a near object is focused behind the retina (appears blurry)

Hyperopia If the person has used only a small amount of strength in the ciliary muscle to accommodate for the distant objects, he or she still has much accommodative power left, and

40 Hyperopia If the person has used only a small amount of strength in the ciliary muscle to accommodate for the distant objects, he or she still has much accommodative power left, and objects closer and closer to the eye can also be focused sharply until the ciliary muscle has contracted to its limit. Correction with convex lens or in young persons through accommodation

41 Myopia= nearsightedness Due to too long eyeball. Distant objects focus in front of the retina. As an object moves nearer to a myopic s eye, it finally gets close enough that its image can be focused. Correction with concave lens.

42 Myopia

43 Astigmatism Due to uneven curvature of the cornea Correction with cylindrical lens

44 The third layer of the eye= RETINA anterior, nonsensitive portion, which lies over the ciliary body posterior functional, or optical, portion, the photoreceptor organ. The optical (neural) retina lines the choroid from the papilla of the optic nerve posteriorly to the ora serata anteriorly.

45 Retinal layers Pigment cell layer- OUTERMOST Photoreceptor outer segment layer External limiting membrane Outer nuclear layer Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Layer of optic nerve fibers Internal limiting membrane- INNERMOST

48 Retinal pigment epithelium Essential in phototransduction

49 Pigmented cells Absorb photons that are not first captured by photoreceptors Prevent light waves from reflecting (scattering) at the level of the retina (albinos don t have melanin- poor visual acuity) Are poorly transpaent structures (pigment epithelium) Help the renewal process of the photoreceptors Phagocytic role for the photor outer segments membranesconstant destruction of photor (photoox stress)

50 Horizontal cells Synapse in the outer plexiform layer Interconnect photoreceptors and bipolar cells to themselves and to each other Lateral inhibition- high contrast! Outputs of horizontal cells are always inhibitory

52 Amacrine cells Interneurons- at least subtypes Location- all of them inner layer of retina Interconnecting bipolar cells to ganglion cells At first- thought they lack axons- dendrites end presynapticaly in other cells, but they do have long axon- like processes

53 Bipolar cells Type of neuron located in the inner nuclear layer First order neuron of the optic pathway Dendrites receive info from the photoreceptor cells and horizontal cells and pass it to the ggl and amacrine cells Receive input from cones and rods

54 Ganglion cells Neurons located near the inner surface of the retina Final output neurons of the vertebrate retina SECOND ORDER NEURON Collect visual info in their dendrites from bipolar and amacrine cells and their axons form the OPTIC NERVE FIBRES Use action potentials to speed visual information along their axons (electronic spread of potentials along dendrites is enough for the rest of neurons in the retina)

55 Photoreceptors Two main types of photoreceptors: rods and cones Only one type of rod responsible for monochromatic dark-adapted vision Three types of cones responsible for color sensitive vision Rod:cones = 16:1

56 Photoreceptors- modified unipolar neurons Photoreceptor cells (rods and cones) are subdivided into three regions: - the outer segment: contains a stack of membranous discs that are rich in photopigment (rodopsin in rods); also, contains Na+ channels, - the inner segment: connects with the outer segment by way of a modified cilium and contains nucleus, mitochondria; it synthesizes photopigment, also contains K+ channels and a high density of Na-K pumps, - the synaptic terminal (AXON) to bipolar cells.

57 Rods and cones are light transducers Dark pigment layer absorbs stray light & reduces reflection Disks in rods & cones are the site of transduction Transduction process mediated by pigments in the disks (ex. is rod)

59 Rods/cones At the fovea, highest VA 1. there are just cones, at a high density, 2. neurons of the inner layer of retina are displaced laterally to the side of fovea to minimize light scattering 3. One cone synapses on only one bipolar cell, which synapses on one ganglion cell -> the receptive field of a foveal ganglion cell is SMALL. Information flow through the retina from photoreceptors to bipolar cells and then to ganglion cells. The ganglion cell axons form the optic nerve and provide the output of the retina to the methathalamus.

60 Cone density falls to very low levels outside the fovea, and rod density rises

61 Phototransduction Conversion of light into electrical signals Biological conversion of the photon into an electrical signal in the retina Photoreceptors contain photopigments PIGMENT= opsins (G- protein coupled receptors) plus a chromophore= 11- cis retinal- substance sensitive to light 11- cis retinal undergoes photoisomerization alltrans retinal signal transduction cascade

62 Phototransduction

63 Rods Rods photopigment rhodopsin RETINAL + OPSIN In the dark, retinal is bound to opsin in the 11-cis-retinal form. Absorption of light causes a change to the all-transretinal form, which no longer binds to opsin. The separation of all-trans-retinal and opsin is called bleaching. Before the photopigment can be regenerated, all-transretinal must be transported to the pigment cell layer, reduced, isomerized, and esterified.

64 Cones Cones contain the pigment IODOPSIN three different cone opsins (photopsins) found in the three different types of cones, each sensitive to a different part of the visible light spectrum PLUS 11 cis retinal ONE CONE TYPE RESPONDS BEST TO BLUE LIGHT (420 nm), ANOTHER TO GREEN (530 nm), AND THE THIRD TO RED (560 nm). trichromatic color vision light produced changes resemble the sequence in rods, but the reactions and the recovery are quicker.

65 Na+/K+ pump in inner segment Na+ channel in the outer segment Rods have a steady state potential of -40 mv (less negative than normal) Light stimulation Na+ ch closure= hyperpolarisation!! Various degree of hyperpolarisation -65 mv

66 Rhodopsin= retinal + opsin Retinal= aldehyde of vitamin A Opsin= protein Retinal has 2 forms= 11 cis and all-trans= only cis binds to retinal Metharodopsin II= activated Opsin

67 Light...metarhodopsin II= enzyme activates transducin activates phosphodiesterase cgmp GMP (by the action of phosphodiesterase) Without light presence=> cgmp is attached to the sodium channel and keeps it open ([cgmp]ᵢ is high) Light makes cgmp to dissapear and Na+ ch to close hyperpolarisation

69 In the dark photoreceptor cells are depolarized at about -40 mv. in the dark in the dark, cgmp levels are high and keep cgmpgated sodium channels open allowing a steady inward current, called the dark current Dark current- depolarisation Ca2+ voltage gated channels open increased intracellular concentration of Ca2+ causes TONIC (constant) release of the neurotransmitter into the synaptic cleft (GLUTAMATE)

70 Under light conditions Light turns 11 cis retinal into 11 all-trans retinal phototransduction cascade initiated... Progressive closure of Na+ channels in the outer segment, BUT Na+/K+ pump continues to pump Na+ ions out increased electronegativity inside the cell!! Hyperpolarisation= RECEPTOR POTENTIAL is proportional to light intensity maximum light intensity= -65 mv Neurotransmitter release DECREASES proportional to the ammount of light- no more tonic release of glutamate excitation of the optic pathway

A. In darkness, photoreceptor cells have open Na + channels, which result in a dark current and consequently a tonic release of neurotransmitter (glutamate) onto bipolar cells and horizontal cells.

71 A. In darkness, photoreceptor cells have open Na + channels, which result in a dark current and consequently a tonic release of neurotransmitter (glutamate) onto bipolar cells and horizontal cells. Dark current in a photoreceptor is caused by passive influx of Na +, which is returned to the extracellular space by pumping. Light closes the Na + channels and thus reduces the dark current. B. Second-messenger system underlying phototransduction. When light reacts with rhodopsin (RH), G protein transducin (T) is activated. This in turn activates phosphodiesterase (PDE), which breaks down cgmp into GMP. The dark current depends on cgmp, and thus a fall in cgmp concentration reduces the dark current, which causes hyperpolarization of the photoreceptor. GC, guanylyl cyclase.

73 Light and dark adaptation Light- photopigments are consumed a lot of vita is formed= light adaptation Dark- more photopigments are formed= dark adaptation Image- when switching from bright light to dark= dark adaptation- retinal sensitivity raises with more pigments being formed!

74 A dark-adapted retina relying on rods can have a light threshold that is 500 times lower than retina relying on fully dark-adapted cones.

75 Photopic/scotopic vision Photopic vision relates to human vision at high ambient light levels (e.g. during daylight conditions) when vision is mediated by the cones. Scotopic vision relates to human vision at low ambient light levels (e.g. at night) when vision is mediated by rods. Rods have a much higher sensitivity than the cones. However, the sense of color is essentially lost in the scotopic vision regime. At low light levels such as in a moonless night, objects lose their colors and only appear to have different gray levels.

77 Color vision There are three types of pigments in three types of cones All three pigments are necessary for correct colour vision. We need at least two pigments for color vision by color mixing Dyschromatopsia is produced by the absence of one of the pigments, the subject still sees almost all colours using only two pigments Absence of the red pigment protanopia Absence of the green pigment deuteranopia Absence of the blue pigment tritanopia.

78 Color vision The visible spectrum includes 300 wavelengths ( nm), and in some portions we can discern color differences of 1 wavelength. The ability to see so many colors depends on: a. a separate cone for each wavelength. b. optic nerve fibers for each color. c. visual cortex neurons sensitive to each color. d. difference in stimulation of red, green and blue sensitive cones.

79 human eye can detect almost all gradations of colors when only red, green, and blue monochromatic lights are appropriately mixed in different combinations Interpretation at the CNS level At one moment, a colored object stimulates the 3 cones differently color code for the brain! ORANGE= R:G:B= 99:42:0 BLUE= R:G:B=0:0:97 White- equal stimulation of all three types Black- no stimulation

80 Color blindness-dyschromatopsia Ishihara plates

83 After the retina Once the ganglion cell axons leave the retina, they travel through the optic nerve to the optic chiasm, a partial crossing of the axons. At the optic chiasm the left and right visual worlds are separated. After the chiasm, the fibers are called the optic tract. The optic tract wraps around the cerebral peduncles of the midbrain to get to the lateral geniculate nucleus (LGN). The LGN is a part of the thalamus. Almost all of the optic tract axons, therefore, synapse in the LGN. The remaining few branch off to synapse in nuclei of the midbrain: the superior colliculi and the pretectal area.

85 Reference: Boron 3 rd edition chapter Sensory transduction - Visual Transduction, pp

10/8/ dpt. n 21 = n n' r D = The electromagnetic spectrum. A few words about light. BÓDIS Emőke 02 October Optical Imaging in the Eye

10/8/ dpt. n 21 = n n' r D = The electromagnetic spectrum. A few words about light. BÓDIS Emőke 02 October Optical Imaging in the Eye A few words about light BÓDIS Emőke 02 October 2012 Optical Imaging in the Eye Healthy eye: 25 cm, v1 v2 Let s determine the change in the refractive power between the two extremes during accommodation!