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Biophysics of Sensory Perception , Receptors , Biophysics of Vision Ján Jakuš

Biophysics of Sensory Perception , Receptors , Biophysics of Vision Ján Jakuš. Sensory perception. - it is an ability to distinguish , detect , utilize some feelings , and answer to many informa-tion that come to the brain through the reflex arc .

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Biophysics of Sensory Perception , Receptors , Biophysics of Vision Ján Jakuš

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  1. BiophysicsofSensoryPerception, Receptors,BiophysicsofVisionJán Jakuš

  2. Sensory perception - itisanability to distinguish, detect, utilizesomefeelings, and answer to manyinforma-tionthatcometothebrainthroughthe reflexarc. - The reflex arcconsistsofreceptor, afferent nerve pathway , thecentralnervoussystem (brain and spine),efferentpathway and effectors(muscle...) - Informationfromanenvironmentcome to our body and are processed by our 5 senses (Touch, Taste, Smell, Vision, Hearing). Reachingthe body theinformationare coded in 2 forms: as a localelectricresponse (localelectricpotential), and thegeneralactionpotential (AP)

  3. Reflex arc

  4. Local response (LR)and Action potential (AP) - characteristics Local electric response-takes local place, it does not spread to the vicinity, when its magnitude reaches more than 10 mV then, in turn AP is produced. This type of coding is so called „AMPLITUDE“. (i.e. the stronger is stimulus, the higher is amplitude of response (examples: the receptor potential (generator potential), EPSP and IPSP or end-plate potentials). Action potential is a generally spreading electricity, being govern by the Law All or None .This type of coding is named „ FREQUENCY“. The stronger is a stimulus the higher is a rate of APs from the receptors. The brain knows that a higher frequency of action potentials means a stronger stimulus(and vice versa)

  5. Coding of sensory information on Paccinian Corpuscle (a skin receptor for touch and pressure)

  6. Coding of Stimulus on Paccinian corpuscle Mechanical touch-pressure energy affects the receptor nerve membrane (without myelin) causing its local depolarization, that results in an appearance of RECEPTOR or “GENERATOR POTENTIAL“ (GP). When another touch pres-sure stimuli come on the receptor they causes creation of many local potentials and their summation . When the amplitude of GP is above10mV then the series of ACTION POTENTIALS rise up on the afferent nerve fibre (which is covered with myelin),that leaves the Paccinian corpuscle.

  7. Receptors- Definition and Properties - Sensory Receptors are special nerve endings, distributed throughout the body ( in the skin, muscles, vessels, bones and joints, in lungs, heart, and another organs). - They Convert Different Forms of Energy into Electrical Signals. Thus theyserve astransdu-cers,changing the particular form of energy ( e.g. mechanical, chemical, thermal, or electromagne-tic) into the electrical signal. - Our body contains20 types of receptors that can detect e.g. heat, pressure, stretch, acceleration, sound, light, smells, taste, partial pressure, con-centration of salts, hormons...and other forms of stimuli (Only receptors for ionizing radiation are missing)

  8. Receptors - Classification I.According to locality:Exteroreceptors- are placed within the skin, like receptors for touch, pressure, heat, cold or pain Proprioreceptors- are placed in muscles, in bones and joints -they inform a brain about the lengt of muscles and ligaments Interoreceptors – receptors within the organs (heart, lungs, kidney) They detect plasma osmolarity, partial pressure of O2 blood pressure.. II.According to type of energy:Mechanoreceptors- they transform mechanic energy into electric signal.E.g. exteroreceptorś, baroreceptors, pulmonary stretch receptors). Fotoreceptors- receptors containing photopigments (rods and cones at retina Chemoreceptors – taste receptors in the tongue, smell receptors within a nose, osmoreceptors in hypothalamus,.. Nociceptors- pain receptors - in skin, in organs ... III.According to complexity:simple receptors (skin)and complex ones (eye, ear)

  9. The Flight Reflex

  10. Laws for Sensory Perception: Weber-Fechner’s Law of Perception: is a basic psy-chophysical Law. “The bigger is the intensity of sti-mulation,the higher is the magnitude of sensation“. Magnitude of sensation E = log S , ( S - intensity of stimulation) or in a modified form : Stevenson’s Law: FAP= k . Sn (FAP is rate of APs from a receptor, k- constant, n=1 is valid for mechanoreceptors, n 1, for fotoreceptors, n 1, for pain receptors. The Law of Projection:Each sense occupies the uniq-ue and separated site within the brain cortex. Therefore, we are able to distinguish the individual stimuli - like touch, pressure, pain sensations, light or sound, at the same time!

  11. The Law of Adaptation • Adaptation -isaninternalelectricproperty(caused by membranepropertiesofthe receptor) to respondwhenthelong-termstimulusof a constantintensityisapplied. • ReceptorswithRapidAdaptationoftheirBurstActivity- theirfire just for a shorttime, duringtheconstant (maintained ) stimulation ( asitistypicallyseen in touch, pressure, taste and smellreceptors.) • ReceptorswithLowAdaptationoftheirBurstActivity-theyfirefor a longertimewithonlyalow drop oftheirfiringactivity (asseen in thecold, heatreceptors, baroreceptors, in pulmonarystretchrecepors, thechemoreceptors, carotidbaroreceptors or in the pulmonarystretchreceptors). • ThereceptorswithoutAdaptation (likepainreceptors)are alwaysactive, firingtheAPs.

  12. Receptor Adaptation

  13. Biophysics of Visionis the most important human sense taking 80% of information from an environment.The stimulus for vision is electromagnetic waving of photons ( for Visible Lightthe wavelenght λ= 380 – 780 nm). For Ultraviolet Light λ is below 380 nm, for Infrared Light λ is above 780 nm. The velocity of visible lightin vaccum is approx. 186 000 miles/s = 300 000 km/s.

  14. Anatomy of the Eye Optic analyzerconsists of three main parts: Eyeball, Optic nerves and pathways, Visual cortex..

  15. Parts of the Eye

  16. Scheme of the Eye

  17. The Eyeball ( is spherically shaped with d = 2.5 cm), consists of three special tissue covers( from outside to inside ): 1.the sclera,2.the chorioid, 3. the retina, and with two liquids(the humor aquens and the vitreous humor)

  18. SCLERA – is a thin ligament layer, that runs forward, thereby to creat the transparent CORNEA (it is the most important refractive surface area, without vessels, but with plenty of pain sensitive nerve endings ). Just behind the CORNEA there is the ANTERIOR CHAMBER, filled with a clear watery liquid named HUMOR AQUENS.

  19. Chorioidtakes place between the sclera and retina. It contains a lot of vessels with nutritivefunction. Anteriorly, thechorioid sets up two CILIARY BODIES (at a vertical view)

  20. Ciliary Body The ciliary body lies just behind the iris.  Attached to the ciliary body are tiny fiber "guy wires" called zonules.  The crystalline lens is suspended inside the eye by the zonular fibers

  21. LENS is transparent and placed just behind the iris.Its role is to focus (refract)the light rays onto the retina. When the lens is patologically changed (as a result of thermal injury or diabetes mellitus), the lens is dimmed and this disorder is named as Cataract Lens

  22. IRIScontains pigments, vessels and two muscles which either contract or dilate the pupil (sphincter and dilator) which can adjust the pupil diameter to the light intensity.Bright light – smaller diameter, Dimmed light- bigger diameter

  23. VITREOUS BODY Vitreous body is a transparent,jelly matter substance that fills the center of the eye behind the lens. It is composed from vitreous humor and comprises about 2/3 of the eye's volume, giving it a form and shape. The viscous properties of the vitreous humor allow the eye a return to its normal shape, if compressed. 

  24. RETINA contains two kinds of special Photorecep-tors: RODS (120 millions/one eye) and CONES ( 6 millions / one eye ), as well as from the two layers of the BIPOLAR and GANGLIONAR CELLS. RODS are more sensitive for the light intensity than CONES. RODSare responsible for night vision and black-white vision (Scotopic vision).CONESare respon-sible for day-light and color visions (Photopic visi-on).Macula lutea or (the Yelow Spot) is the place with the best visual acuity (keenes of eyesight), because contains the highest number of cones. Contrary, the Blind Spot- where the optic nerve and the eye vessels leave the eyball- is “blind” because no fotoreceptors are placedhere.

  25. RETINAL BACKGROUND (searched by Ophtalmoscopy)

  26. Remember: The light rays first pass through the layer of the Ganglionar Cells, then the Bipolar cells and finally strike the Rods and Cones.Photopigmentswithintherods and cones are broken by light and ,thenelectrons are released.TheelectronescreateGenerator Potentials inside the Ganglionar Cells, butnot inside the RODES and CONES (because they are hyperpolarized at that time). Action Potentialsare produced on the efferent axons that leave the Ganglionar Cells.

  27. Chemistry of Photopigments RODScontain pigment rhodopsine( 11 cis-retinal-opsine) that undergoes the fotochemic reaction. Light chemically breaks the Rhodosine into the Opsine ( all-trans - retinal opsine) + 1 electron. This electroneescapes from the photoreceptor and induces the Generator Potential and thenthe AP.At night, OPSINE ( all- trans-form ) is reniewed into the original pigment RHODOPSINE ( 11-cis form) under the catalytic effect of Vitamin A. Thus, the VITAMIN A is important for synthesis of photopigments. When there is vitamin A deficiency it results in a disorder-HEMERALOPY (Darkblindnesssyndrom) 3 typesof CONEScontain 3 photopigments : Erytrolab, Chlorolab, Cyanolab,(sensitiveforred, green, bluecolorsrespectively).

  28. COLOR VISION (HELMHOLTZ -YUANG THEORY of ColorVision).Humans are able to perceive 3 basic colors: green, red, blue and a variety of mixing colors, because the existency of 3 kinds of special pigments in 3 different types of cones, within the retina. Normal color vision is a typical feature for TRICHROMATS.When one type of cone is missing or disabled, then patient is namedDICHROMAT ,suffering from the particular type of a color blindnesse.g. deuteroa-nopy, protanopy or tritanopy. ( seepracticalsfordetails)When all three types of CONES are disabled one is namedMONOCHROMAT. Colorblindnessisa kindofhereditarydisorder(For more details see Nave and Nave and Handouts).

  29. Accomodation– is a process whenthe refrac-tory power of the eye rises up. Accomodation enab-les to focus our eye from the FARPOINT ( approx. above 6 m, to the point named NEAR POINT of vision (the closest point on which one can focus sharply).Durind the accomodation , contraction ofciliary muscle causes the relax of thesuspensory ligaments. Because the lens own elasticity, the lens will be thicker - obtaining more spherical shape. It looks like the lens „moves forward“. The shorter is the distance between the subject and the eye, the greater has to be the accomodation. The refractory power of the eye ( degree of accomodation) is mea-sured in units named DIOPTERSD= 1 / focal distance (m)For whole eye the totaloptical power is approx. 59 D, for cornea is 43D, for lens around 16 D.!

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  32. Failures of the Image Forming MechanismRefraction Failures:MYOPIA, HYPEROPIA, PRESBYOPIA, and ASTIGMATISM Normal eye is named asEMETROPIC - ( with the axial diameter = 2.5 cm).The light rays arerefracted by the cornea and lens and focused to the macula lutea on the retina, thus creating the real, smaller, and turned round image of the object).

  33. MYOPIA (Nearsightedness) Itappears when eitherthe eyballis too long (diame-ter is above 2.5 cm), or therefractorypowerofthecorneaishigher. Then lightrays are refracted in front the retina. Thepictureisblurry.One hasdifficulty to seedistantobjects.Visionseemsbetterwhenonesquints.The correction is by glasses with biconcave (divergent) lenses, that diverge the rays just to the retina.

  34. Hyperopia- Farsightedness Itappears when the eyball is either short (d < 2.5cm), or refractorypowerofcornea and lens are lower. Then rays are refracted behind the retina. One hasdifficulty to seeupclose.Alsosympto-mslikeeyefatigue and eyestrainandheadachesmayappearwhenoneisreading. Correction with biconvex (convergent) lenses improves this failure, focusing the rays directly on the retinal surface

  35. PRESBYOPIA- OLDSIGHTEDNEES(iskindofHyperopiai.e. farsightedness).Presbyopiaisalsoknownasthe“shortarmsyndrome” . Theelasticity of LENSisagedepended. Thepersonsabove 45 years, looseprogressivelythelens elasticity, thereforetheirrefractiveeyepower and accomodationare stepingdown. Their NEAR POINT ofvisionreachesthedistance more than 45 cm. In order to improvethesharpnessofvisionone has to takeglasseswithbiconvex (con-vergent) lenses.

  36. Refraction Failures- scheme

  37. Allofmentionedabovefailurescanbetreated by wearingofglasseswithbicon-cave (divergent) or biconvex (convergent) lenses , or usingtheconactlenses , or even by laser surgery (myopia, astigma-tism). SeeHandoutsforPracticals. Tasks: DeterminationofVisualAquity by SnellensTypes, DeterminationofNear and FarPointsofVision

  38. . ThanksforComming and Attention !

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