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Receptors Powered By Docstoc
					Sensory physiology

Instructor: DU Jing
            Department of Physiology
           Jining medical college
Office: 0850 physiological sciences
 Section A General Physiological
Properties of Receptors
   Sensory Receptors & Sensory Organs
   General physiological properties of receptors

 Section B Special Sensory systems
    Vision       Vestibularsystem
    Hearing      Somatic sensation
 Sensation and perception are reflection of
the objective world in the subjective
 Stimulus energy is transduced by Sensory
Receptors/Sensory Organs into nerve impulse
which travels along specific neuron pathways
to specific areas in the cerebral cortex of the
                                       Section A
     Receptors & Sensory Organs
1) Receptors (Sensory receptors, not proteins)
Sensory receptors is referred to the cells or
structures located on the surface of the body or
within tissues, the function of which is to detect
changes in internal or external environment and
transduce these changes to electrical response.
2) Sensory organs
Sensory organ is the special organ
that is composed of sensory receptor and
its subsidiary structures.
    For example:

   Receptors -- Rods and cones in the retina;
                the endings of sensory nerves;

   Sensory Organs – Eyes, ears, the vestibular
    apparatus in the inner ear, the taste buds on
    the tongue, the olfactory epithelium of the
    nasal cavity
                                       Section A

ⅡClassification of receptors
   By location

 By   types of
    stimuli        noticeptors (pain receptors)
                   electromagnetic receptors
 General physiological properties of
    1. Adequate stimulus of receptor
    2.Transducer function of receptor
    3. Encoding of receptor
    4. Adaptation of receptor
                                               Section A
 General physiological properties of
    1. Adequate stimulus of receptor:
     Each kind of receptors is very highly
     sensitive to one type of stimulus for which it
     is designed. This special stimulus is called
     adequate stimulus of the receptor.
    eg. electromagnetic waves with specific rangs of
    wavelength to the photoreceptor cells in the retina;
    mechanical vibration at a specific rang of frequency
    to the hair cells in the cochlea
 Receptors can also respond to

 inadequate stimuli;

 Each receptor has its own sensory

Perceptions of the world are created
 by the brain from action potentials
 (APs) sent from sensory receptors.

Receptors can transduce (change)
 different forms of stimulus energy to
 electrical signals that are conducted
 along the specific afferent nerve
 pathways to CNS.
                                      Section A

 General physiological properties of
2. Transducer function of receptor
    The process in which a stimulus energy is
    transduced into the electrical response is
    known as transducer function of receptor.

    stimulus→ electrical response → brain
Receptor potential
During the process of transducing, the
receptor firstly changes the stimulus energy
into a graded local electrical response
across the receptor membrane or the
sensory nerve ending, this transitional
potential that can later initiate the action
potential is known as a receptor potential.
Characters of receptor potential
  • Slow, local potential, can summate
  • Extends by electrotonic propagation,
  decreases with distance
  • Stimulus information is reflected
  by the magnitude, duration and
  direction of receptor potential.
           Steps of sensation
            receptors   (receptor potential →AP)
stimulus                 electrical information
           function         along afferent nerves

                   cerebral cortex

                 sensation or perception
   Mechanism of transducer function
                 Stimulus        Receptor
Through Second Messenger
                          Ion channel (+)
     Change in ion fluxes across receptor membrane

                   Receptor potential
(ending of afferent neuron)      (a separate cell)
                                    alters the release of
    AP on afferent nerves           neurotransmitter
  During the process in which stimulus is
 changed to AP by a receptor, not only the
 form of stimulus energy is transduced, but
 also the information brought by stimulus is
 transformed into the sequence of AP.

                 Transducing of energy form
Stimulus → AP
                Transformation of information
                                        Section A
 General physiological properties of

    3. Encoding of receptor:
    Receptor could transfer the messages of
    environmental changes brought by stimuli into
    the information conveyed in the action
    potential sequence on the afferent neuron.
    This function of information transformation is
    termed as encoding of receptor.
Encoding of stimulus characteristics:
 Special receptor, special neuron pathway,
 Special CNS location .

Encoding of stimulus intensity:
 Stimulus intensity is distinguished
 both by the frequency of AP generated
 on the afferent N and by the number
 of the fibers transporting the messages.
                                   Section A

 General physiological properties of

4. Adaptation of receptor:
 Receptors have the ability to reduce the
 frequency of AP generated on the afferent
 neuron in spite of a sustained stimulus
 Classification according to adaptation:
 Rapidly adapting receptor
 --important for the body to detect new stimuli
 eg. Touch and pressure receptors in the skin

 Slowly adapting receptor
 --important for monitoring the continuous
 functions of the body
 eg. Stretch receptors in muscles participating
 in maintaining posture
 Section B
  Special Sensory systems

       Vision
           Refractive system (optical portion)
                 Aqueous humor
Structures         Lens
in the eye           Vitreous humor
involved in
              Retina (photoreceptive system)
                   Photoreceptor cells (rods, cones)
                      Bipolar cells
                         Ganglion cells
 Adequate stimulus:
370--740nm electromagnetic waves
(visible wavelengths of light)
 Processes of vision:
 Formation of the image on the retina

--refractive system
 Phototransduction by the photoreceptors

--rods and cones
 Electrical signals is sent along the optic

never to the visual cortex in the brain
 Vision

  Ⅰ Refractive function of the eye
    and accommodation
                                 Vision
      Ⅰ Refractive function of the eye
        and accommodation

As in a camera,
image is up-
side down and
inverted. The
brain interprets
this correctly.
Refractive structures of the eye

           Aqueous humor
               Vitreous humor
          (Outside to inside)
Refractive structures of the eye



     ← Viewed through an ophthalmoscope
    Reduced eye                       Vision
 If all the refractive surfaces are algebraically
 added together and considered to be a single
 lens with its center placed 15cm in front of
 the eye, the normal eye may be schematically
 regarded as ‘reduced eye’ .

It’s an imaginative artificial model which has
the same refractive effect as the eye.
Under the condition of non-accommodation,
the focal point of normal human eye is on the
retina. Lights from an object at infinite distance
(>6m) can form a image on the retina.
        Visual accommodation  Vision
When focusing at objects at different distance
from the eye, some adjustments are made,
especially when focusing at near objects (<6m) --
visual accommodation.
Accommodation include
   shape changes of the lens
   diameter changes of the pupil
   convergence of two eyes
Ability of lens accommodation:
  Near point of vision is the nearest distance
  distinguished by the eye.
           Shape changes of the lens
Controlled by the zonular fibers
and the ciliary muscle .


                 Zonular   The periphery of the
                           lens is joined to the
                           ciliary muscle
                           (circular like a
                           sphincter) by the
                           zonular fibers.
 Shape changes of the lens
 Shape changes of the lens
                     Lessens      Lens
Near     Ciliary     tension on   becomes
object   muscle      zonular      more
         contracts   fibers       spherical

                     Increases    Lens
Far      Ciliary     tension on   becomes
object   muscle      zonular      more
         relaxes     fibers       flattened
Light rays from close objects diverge and
require more accommodation for focusing.
Since the lens must be elastic to assume a
more spherical shape during accommodation
for near objects, the increasing stiffness of the
lens that occurs with aging makes
accommodation for near vision increasingly
difficult --- presbyopia, a normal part of the
aging process in people around 45 years old.
Old people often wear
corrective glasses for close work.
 Diameter changes of the pupil
-- The amount of light entering the eye is
controlled by the diameter of the pupil, the
hole in the center of the iris through which
light enters the eye.
-- a reflexive process (light-sensitive reflex)
-- protect the retina from damage induced
by too bright light
-- also one of the symbols of deep
anesthesia or death
  Convergence of two eyes
Focusing on an object moving from the
distance to the near, the two eye balls
convergent towards the nasal sides, so that the
images could be focused on corresponding
points of the two retinas.
  Errors of refraction                  Vision

The normal condition, in which the eyes
has the full adjustment range that allows
the maximum capacity for accommodation
to keep objects in focus as they get nearer
and nearer, is termed as emmetropia.
The eyes of many individuals don’t
achieve this ideal. Errors of refraction
include myopia, hyperopia, astigmatism
and cataract (opacity of the lens with aging).


Ⅱ Phototransduction of the retina


ⅡPhototransduction of
the retina

   Structure of
 the retina – four
  Pigment cells
  Photoreceptor cells
  Bipolar cells
  Ganglion cells

 outside →inside
 Back → front
n cell
   Structure of the photoreceptor cell

Inner                          Stacked layers of
    mitochon                   membrane--discs

    nucleus                      retinal
                                  lipid bilayer
Synaptic                          with proteins
terminal                          mosaiced in it
            Comparison of rods and cones
        rods                      cones
more sensitive             less sensitive
highest density--6mm       highest density--center
from the fovea centralis   of the fovea centralis,
                           only cones
more convergent            more single connections
connections to neurons     to neurons
one type (no color vision) three types (color vision)
Photochemistry of vision
   Chemical composition of photopigment

                    an opsin (membrane protein
                    binding to retinal)
a photopigment
four types--        chromophore (retinal, derivative
one called          of Vit A)
rhodopsin in
the rods and
one in each of
three different
Photochemistry of vision
  -- a reversible chemical reaction
In bright light, decomposing of rhodopsin
excels composing; in the dark ,composing
excels decomposing.
Vitamin A can derive 11-cis retinal which can
bind to opsin to produce rhodopsin.
The amount of retinal lost during the process
of decomposing and composing of rhodopsin is
replaced by Vit A from foods.
Serious Vit A deficiency leads to nyctalopia,
(night blindness, impaired vision in dim light
and in the dark .)
    Phototransduction of the retina
   Light stimlus         Electrical response
              (by photoreceptors)

A unique character:             , all-trans retinal
receptor potential


      Hyperpolarized receptor potential

     Inhibits release of
     neurotransmitter (glutamte)    is depolarized
(a) In light   (b) In darkness
2. Information modification of the retina

            Local potential on
              bipolar cells
                  Visual pathways
 Optic nerve carries
  information to
    Some processing
     edits information
 Visual cortex
  interprets information
    Creates visual
  phototransduction by the cones
and color vision


  Trichromatic theory
Trichromatic theory
                  Color blindness

It’s a condition in which certain
colors cannot be distinguished, and
is most commonly due to an
inherited condition. It is caused by
missing of one or more of the
cones, or by poor function of the
cones. Red/Green color blindness
is the most common form, about
99%, and causes problems in
distinguishing reds and greens.
Ⅲ Some vision phenomena

     Landolt arc   Snellen map
4) Binocular Vision and Stereopsis
Binocular vision: The ability to maintain visual focus on
an object with both eyes, creating a single visual image.
Lack of binocular vision is normal in infants. Adults
without binocular vision experience distortions in depth
perception and visual measurement of distance.

                      Binocular Vision –
 Monocular Vision--
                      with overlapped
 with no overlapped   optic fields
 optic fields
4) Binocular Vision and Stereopsis

Stereopsis is one of the processes of the human visual
system that extracts depth information from a viewed
scene and builds a three dimensional understanding of
that scene. It makes use of the slight difference in
perspective of one eye relative to the other.
 Section B
  Special Sensory systems

 Hearing
 Hearing

            most sensitive: 1000~3000Hz

Hearing threshold and hearing range
                Structure of the ear
• Middle ear cavity
separated from external
ear by eardrum and from
internal ear by oval &
round window
• Auditory tube leads to
nasopharynx--helps to
equalize pressure on both
sides of eardrum
• Membranous labyrinth
contains cochlea (organs
of hearing) and vestibular
apparatus (equilibrium)
 Hearing
 Sound waves are conducted through the auditory
    canal and cause vibration of the eardrum

  Sound waves are conducted and amplified by
  moving of the ossicular chain in the middle ear

   Vibration of sound waves is converted into receptor
    potential by hair cells in the organ of Corti of the
 cochlea. Receptor P triggers release of neurotransmitter
   and then cause AP which travels along the afferent
        nerves to the auditory cortex of the brain.
ⅠFunctions of the external ear

 External ear
   pinna, external auditory canal
      Collect sound waves
      Conduct sound waves
      Sound localization
      Resonant phenomenon
ⅠFunctions of the external ear

   Resonant phenomenon

  By revibrating the sides and the end of the external
  auditory canal, sound is amplified. When sound
  waves arrive the eardrum, the intensity is increased
  almost 10 desibel (dB).
ⅡFunctions of the middle ear

Middle ear
   eardrum , ossicular chain, auditory tube
  Conduct    sound waves
  Amplify   the sound pressure
  Protect   auditory apparatus
  in the inner ear
  Equilibrate   the pressure
ⅡFunctions of the middle ear

    Conduct sound waves
 The eardrum vibrates as the
 same frequency as the sound
 waves conducted from the
 the auditory canal.
 With the vibration of the
 eardrum, the ossicular chain
 moves forward and
ⅡFunctions of the middle ear
   Amplify     the sound pressure

 Ossicular chain--Three flexibly
 linked ossicles include malleus
 (hammer), incus (anvil) and stapes
 (stirrup) .
 The ossicles form a ‘lever’ which
 can conduct sound waves with
 high effectiveness when moving
 with vibration of the eardrum.
ⅡFunctions of the middle ear
   Amplify the sound pressure
 The sound force of per unit is increased when
 conducted from the eardrum to the oval window.
 The area of eardrum is
 17 times larger than that                        Oval
 of oval window and the                           window
 long arm of the ossicles
 is 1.3 times longer than
 the short arm.
 Thus total amplification
 is about 22 times.
ⅡFunctions of the middle ear
    Protect the apparatus in the inner ear
 The amount of the energy transmitted to the inner ear
 is lessened reflexively by the contraction of two small
 skeletal muscles in the middle ear. This alter the
 intension of the eardrum and the position of the stapes
 in the oval window.
 The delicate receptor
 apparatus in the inner
 ear is protected from
 continuous intense
 sound stimuli.
ⅡFunctions of the middle ear
 Equalize the pressure on both sides of
the eardrum
 The auditory tube
 which connects the
 middle ear to the
 nasopharynx helps
 to equalize the
 pressure on both
 sides of the eardrum.
• Sound conduction to the inner ear


                  The round
 Middle ear       window

Air conduction -- normally, main way of sound
• Sound conduction to the inner ear

  Less sensitive than air
  Bone conduction plays
  very minor role in
  normal hearing.
Identify the causes of hearing loss:

Conductive Hearing Loss is caused by
pathologies in the eardrum or middle ear -- the
sensitivity of air conduction is obviously lessened.
While the bone conduction is still normal and
eventually more sensitive than air conduction.
Sensorineural Hearing Loss is caused by
pathologies in the cochlea or the auditory nerve –
the sensitivity of air conduction and bone
conduction are both lessened.
  Ⅲ Functions of the inner ear
   The inner ear is also called labyrinth ----
a system of coiled, membranous tubes filled with
                          Sound conduction
   cochlea for hearing
                          Transducer function

   vestibular apparatus
   for equilibrium
   Structure of the cochlea
Three coiled tubes side by side which
are separated by two layers of
membranes ----
Scala vestibuli (filled with perilymph,
connected with oval window )
Scala media (filled with endolymph)
Scala tympani (filled with perilymph,
connected with round window)
The cross section of the cochlea

                       (Reinssner’s membrane)
Transducer function of the cochlea
1. Vibration of basilar membrane
Waves in the fluid of the cochlea caused by movement of
the stapes produce distortions of the basilar membrane.
 Transducer function of the cochlea
1. Vibration of basilar membrane

   Traveling   wave theory of sound
The region nearest to
the middle ear vibrates
most easily in response
to high-frequency
tones (undergoes the
greatest movement).
Progressively more
distant regions of the
basilar membrane
vibrate maximally in
response to
progressively lower
 Transducer function of the cochlea
2. Excitation of hair cells and receptor P
   Transducer function of the cochlea
   2. Excitation of hair cells and receptor P

Vibration of basilar          Bending of the stereocilia of
membrane                      the hair cells (receptors)

               K+ channel (mechanically
               gated ion channel) is opened

               Influx of K+ depolarizes
                                                   Receptor P
               the hair cells
(Bending in the other direction hyperpolarizes the hair cells)
   Transducer function of the cochlea
   3. Receptor P of the hair cells to AP
Receptor P opens Ca2+             Ca2+ influx triggers
channel (voltage-gated, on        release of neurotransmitter
the flank membrane of hair        (Glutamate)

                     Glutamate binds to and activates
                     protein binding sites on the
                     terminals of the afferent neurons

Auditory cortex in     Auditory
the temporal lobe                      AP
 Section B
  Special Sensory systems

  Vestibular system

  Somatic sensation
 Please study by yourself.
You need to know the following content:
1. Types of receptor
2. The refractive system of the eye
3. The structure characters of retina
4. Information transforming in retina
5. Color blindness
6. Binocular vision and stereoscopic vision
7. Function of outer ear
You need to know well the following content:
1. Color vision
2. Light adaptation and dark adaptation
3. Visual field
4. Visual acuity
5. Functions of the middle ear
You need to hold the following content:
1. The general properties of receptors: adequate
   stimulus, transducer function, encoding, and
2. visual accommodation
3. Phototransduction of rods
4. Air conduction and bone conduction.
5. Transducer function of the cochlea

 Diameter changes of the pupil
Air conduction

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