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

1. Biophysics of Sensory Perception, Receptors,Biophysics of VisionJán Jakuš

2. 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. - The reflex arc consists of receptor, afferent nerve pathway , the central nervous system (brain and spine),efferent pathway and effector (muscle...) - Information from an environment come to our body and are processed by our senses (Touch, Taste, Smell, Vision, Hearing).Reaching the body the information are coded in 2 forms: as a local electric response (local potential), and the general action potential (AP)

3. Reflex arc

4. Local response (LR)and Action potential (AP) 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: receptor potential (generator potential), EPSP and IPSP or end-plate potential). Action potential is a generally spreading electricity, being under the Law All or NoneThis type of coding is named „ FREQUENCY“. I.e.the stronger is 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)

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

7. 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 it 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.

8. 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 astransducers,changing the particular form of energy ( e.g. me-chanical, chemical, thermal, or electromagnetic) 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, concentration of salts, hormons...and other forms of stimuli (Only receptors for ionizing radiation are missing)

9. 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 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. exte-roreceptorś, 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)

10. The Flight Reflex

11. Laws of 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.

12. The Law of Adaptation • Adaptation -is an internal electric property(caused by membrane properties of the receptor) to respond when the long-term stimulus of a constant intensity is applied. Actualy, it is a drop of the receptor excitabili-ty to give rise the GP and then the APs • Receptors with Rapid Adaptation of their BurstActivity- their fire just for a short time, during the constant (maintained ) stimulation ( as typically seen in touch, pressure, taste and smell receptors.) • Receptors with Low Adaptation of their Burst Activity-they fire for a long time with only a low drop of their firing activity (as seen in the pain, cold, heat receptors, baroreceptors, in pulmonary stretch recepors, the chemoreceptors, carotid baroreceptors or in the pulmonary stretch receptors). • The receptors with Low Adaptation are involved in a control of blood pressure, in control of breathing , in responses of body to the pain, etc.

13. Receptor Adaptation

14. 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 Light 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.

15. Anatomy of the Eye The ability to see depends on the actions of several structures in and around the eyeball:Sclera, cornea, chorioid, lens,pupil, iris, ciliari body with ciliary musc-les, macula, retina with rods and cones, vitreous body, optic nerve, optical cortex, etc.

16. Parts of the Eye

17. Scheme of the Eye

18. Optic analyzerconsists of three main parts: Eyeball, Optic nerves and pathways, Occipital cortex.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 a two liquids( the humor aquens and the vitrous humor)

19. 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 a pain sensitive nerve endings ). Just behind the CORNEA there is the ANTERIOR CHAMBER, filled with a clear watery liquid named HUMOR AQUENS.

20. Chorioidtakes place between the sclera and retina. It contains a lot of vessels with nutritivefunction. Anteriorly, thechorioid sets up two CILIARY BODIES

21. 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

22. 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 injury or diabetes mellitus), the lens is dimmed and this pathology is named as Cataract Lens

23. 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

24. VITREOUS BODY Vitreous body is a thick, 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 form and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. 

25. 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. RODS are responsible for night vision (Scotopic vision).CONES are responsible for day-light and color visions (Photopic vision).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 receptors at this site.

26. RETINAL BACKGROUND (searched by Ophtalmoscopy)

27. Remember: The light rays first pass through the layer of the Ganglionar Cells, then the Bipolar cells and finally strike the Rods and Cones. Photopigments within the rods and cones are bro-ken by light and electrons are released.The electro-nes create Generator Potentials inside the Ganglio-ar Cells, but not inside the RODES and CONES (because they are hyperpolarized at that time). Action Potentialsare produced on the efferent axons that leave the Ganglionar Cells.

28. Chemistry of Photopigments RODScontain pigment rhodopsine ( 11 cis-retinal-opsine) that undergos the fotochemic reaction. Light chemically changes the Rhodposine into the Opsine ( all-trans - retinal opsine) + 1 electron. This electrone from the photoreceptor induces the production of Generator Potential and the 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 photo-pigments . When there is vitamin A deficiency it results in a disorder-HEMERALOPY (Dark-blind-ness syndrom) 3 types of CONEScontain photopigments :Erytrolab, Chlorolab, Cyanolab,(sensitive for red,green,blue)

29. COLOR VISION (HELMHOLTZ -YUANG THEORY of Color Vision).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 DICHROMAT ,suffering from the particu-lar type of a color blindness e.g. deuteroanopy, protanopy or tritanopy. ( see practicals for details)When all three types of CONES are disabled one is MONOCHROMAT. Color blindness is hereditary disorder(For more details see Nave and Nave and Handouts).

30. Accomodation– is a process whenthe refrac-tory power of the eye rises up. Accomodation enab-les to focus our eye from the FAR POINT ( 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 muscles causes the relaxation of thesuspe-nsory 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 ref-ractory power of the eye ( degree of accomodation) is measured in unit named DIOPTRIA (diopter)D= 1 / focal distance (m)For whole eye the optical power is approx. 59 D, for cornea is 43D, for lens around 16 D.!

32. Failures of the Image Forming MechanismRefraction Failures:MYOPIA, HYPEROPIA, PRESBYOPIA, and ASTIGMATISM Normal eye is EMETROPIC - ( with the axial diameter = 2.5 cm).The light rays are refracted 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) It appears when either the eyball is too long (diame-ter is above 2.5 cm), or the refractory power of the cornea is higher. Then light rays are refracted in front the retina. The picture is blurry.One hasdifficulty to see distant objects .. Vision seems better when one squints.The correction is by glasses with biconcave lenses, that diverge the rays just to the retina.

34. Hyperopia- Farsightedness It appears when the eyball is either short (d < 2.5cm), or refractory power of cornea and lens are lower. Then rays are refracted behind the retina. One hasdifficulty to see up close. Also sym-ptoms like eye fatigue and eye strain and heada-ches may appear when reading .Correction with biconvex lenses improves this failure, focusing the rays directly on the retinal surface

35. PRESBYOPIA- OLDSIGHTEDNEES(iskind of Hyperopia i.e. farsightedness).Presbyopia is also known as the “short arm syndrome”The elasticity of LENS is age depended. The persons above 45 years, loose progressively the lens elasticity, therefore their refractive eye power and accomodation are steping down, Their NEAR POINT of vision reaches the distance more than 45 cm. In order to improve the sharpness of vision one has to take glasses with biconvex lenses.

37. Refraction Failures- scheme

38. All of mentioned above failures can be treated by wearing of glasses with bicon-cave or biconvex lenses , or using the conact lenses , or even by laser surgery. See Handouts for Practicals. Tasks: Determination ofVisual Aquity by Snellens Types, Determination of Near and Far Points of Vision

40. Thanks for Comming and Attention !