Biophysics of Sensory Perception Receptors Biophysics of Vision
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 information 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 effectors (muscle. . . ) - Information from an environment come to our - body and are processed by our 5 senses (Touch, Taste, Smell, Vision, Hearing). Reaching the body the information are coded in 2 forms: as a local electric response (local electric potential), and the general action potential (AP)
Reflex arc
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 m. V 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)
Coding of sensory information on Paccinian Corpuscle (a skin receptor for touch and pressure)
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 pressure stimuli come on the receptor they causes creation of many local potentials and their summation. When the amplitude of GP is above 10 m. V then the series of ACTION POTENTIALS rise up on the afferent nerve fibre (which is covered with myelin), that leaves the Paccinian corpuscle.
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 they serve as transducers, changing the particular form of energy ( e. g. mechanical, chemical, thermal, or electromagnetic) into the electrical signal. - Our body contains 20 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)
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 O 2 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: III. According to complexity simple receptors (skin) and complex ones (eye, ear)
The Flight Reflex
Laws for Sensory Perception: Weber-Fechner’s Law of Perception: is a basic psychophysical Law. “The bigger is the intensity of stichophysical Law. “T 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 (F ( AP 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 unique 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!
The Law of Adaptation n 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. n Receptors with Rapid Adaptation of their Burst Activity - their fire just for a short time, during the constant (maintained ) stimulation ( as it is typically seen in touch, pressure, taste and smell receptors. ) n Receptors with Low Adaptation of their Burst Activity -they fire for a longer time with only a low drop of their firing activity (as seen in the cold, heat receptors, baroreceptors, in pulmonary stretch recepors, the chemoreceptors, carotid baroreceptors or in the pulmonary stretch receptors). n The receptors without Adaptation (like pain receptors) are always active, firing the APs.
Receptor Adaptation
Biophysics of Vision is the most important human sense taking 80% of information from an environment. The stimulus for vision is electromagnetic waving of photons ( for Visible Light the 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 light in vaccum is approx. 186 000 miles/s = 300 000 km/s.
Anatomy of the Eye Optic analyzer consists of three main parts: Eyeball, Optic nerves and pathways, Visual cortex. .
Parts of the Eye
Scheme of the Eye
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)
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.
Chorioid takes place between the sclera and retina. It contains a lot of vessels with nutritive function. Anteriorly, the chorioid sets up two CILIARY BODIES (at a vertical
Ciliary body ( CB ) contains CILIARY MUSCLE, and SUSPENSORY LIGAMENTS (Zonules), both being extremely important for accomodation of the LENS. The LENS is attached to the CB by these LIGAMENTS. Ciliary Body
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 Lens (as a result of thermal injury or diabetes mellitus), the lens is dimmed and this disorder is named as Cataract
IRIS contains 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
VITREOUS BODY Vitreous body is a transparent, jelly matter substance that fills the center of the eye behind substance 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.
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 and blackwhite 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
RETINAL BACKGROUND (searched by Ophtalmoscopy)
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 broken by light and , then electrons are released. The electrones create Generator Potentials inside the Ganglionar Cells, but not inside the RODES and CONES (because they are hyperpolarized at that time). Action Potentials are produced on the efferent axons that leave the Ganglionar Cells.
Chemistry of Photopigments RODS contain pigment rhodopsine ( 11 cis-retinalopsine) that undergoes the fotochemic reaction. Light chemically breaks the Rhodosine into the Opsine ( all-trans - retinal opsine) + 1 electron. This electrone escapes from the photoreceptor and induces the Generator Potential and then 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 photopigments. When there is vitamin A deficiency it results in a disorder- HEMERALOPY (Darkblindness syndrom) 3 types of CONES contain 3 photopigments : Erytrolab, Chlorolab, Cyanolab, (sensitive for red, green, blue colors respectively).
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 named DICHROMAT , suffering from the particular 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 named MONOCHROMAT. Color blindness is a kind of hereditary disorder (For more details see Nave and Handouts).
Accomodation – is a process when the refrac- tory power of the eye rises up. Accomodation enables 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 of , ciliary muscle causes the relax of the suspensory causes 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 greater has to be the accomodation. power of the eye ( degree of accomodation) is measured in units named DIOPTERS D= 1 / focal distance (m) D= 1 / focal distance (m For whole eye the total optical power is approx. 59 D, for cornea is 43 D, for lens around 16 D. !
Failures of the Image Forming Mechanism Refraction Failures : MYOPIA, HYPEROPIA, PRESBYOPIA, and ASTIGMATISM Normal eye is named as 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).
MYOPIA (Nearsightedness) It appears when either the eyball is too long (diameter 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 has difficulty to see distant objects. Vision seems better when one squints. The correction is by glasses with biconcave (divergent) lenses, that diverge the rays just to the retina.
Hyperopia- Farsightedness It appears when the eyball is either short (d < 2. 5 cm), or refractory power of cornea and lens are lower. Then rays are refracted behind the retina. One has difficulty to see up close. Also symptoms like eye fatigue and eye strain and headaches may appear when one is reading. Correction with biconvex (convergent) lenses improves this failure, focusing the rays directly on the retinal surface
PRESBYOPIA- OLDSIGHTEDNEES (is kind 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 are steping down. reaches the distance more than 45 cm. In order to improve the sharpness of vision one has to take glasses with biconvex (convergent) lenses
Refraction Failures- scheme
All of mentioned above failures can be treated by wearing of glasses with biconcave (divergent) or biconvex (convergent) lenses , or using the conact lenses , or even by laser surgery (myopia, astigmatism). See Handouts for Practicals. Tasks: Determination of Visual Aquity by Snellens Types , Determination of Near and Far Points of Vision
. Thanks for Comming and Attention !
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