Biophysics of Sensory Perception, Receptors, Biophysics of Vision J

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

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Reflex arc
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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 None
  This 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)

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Coding of sensory information on
Paccinian Corpuscle (a skin receptor
for touch and pressure)
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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 above 10mV then the series
  of ACTION POTENTIALS rise up on the afferent
  nerve fibre (which is covered with myelin),that
  leaves the Paccinian corpuscle.

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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. me-chanical, 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)

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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-roreceptors,
  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)

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The Flight Reflex
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Laws of Sensory Perception

• Weber-Fechners 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
• Stevensons Law FAP k . Sn (FAP is rate of
  APs from a receptor, k- constant, n1 is valid
  for mechanoreceptors, n? 1, for fotoreceptors, n
  ?1, for pain receptors.
• The Law of ProjectionEach 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.

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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 Burst
  Activity - 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.

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Receptor Adaptation
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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 ? 380 780 nm). For Ultraviolet Light
? is below 380 nm, for Infrared Light ? is
above 780 nm. The velocity of visible light in
vaccum is approx. 186 000 miles/s 300 000 km/s.
 
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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.

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Parts of the Eye
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Scheme of the Eye
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Optic analyzer consists 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)
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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.
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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
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Ciliary bodies CB (one on each side) contain two CILIARY MUSCLES, 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
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
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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
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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
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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. 
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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.
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RETINAL BACKGROUND
(searched by Ophtalmoscopy)
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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 Potentials are produced on
the efferent axons that leave the Ganglionar
Cells.
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Chemistry of Photopigments

• RODS contain 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 CONES contain photopigments
  Erytrolab, Chlorolab, Cyanolab,(sensitive for
  red,green,blue)

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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).
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Accomodation is a process when the 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 of
ciliary muscles causes the relaxation of the
suspe-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 43 D, for
lens around 16 D. !
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Failures of the Image Forming Mechanism
Refraction 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).

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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 has difficulty 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.

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Hyperopia- Farsightedness

• It appears when the eyball is either short (d lt
  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
  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

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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 reaches the distance more than 45 cm.
In order to improve the sharpness of vision one
has to take glasses with biconvex lenses.
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Refraction Failures- scheme
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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 of Visual Aquity by Snellens
Types , Determination of Near and Far Points of
Vision
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• Thanks for Comming
• and
• Attention !