Sensory%20Receptors

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Sensory Receptors

• Perceptions of world are created by the brain
  from AP sent from sensory receptors.
• Sensory receptors respond to a particular
  modality of environmental stimulus.
• Receptors transform (change) different forms of
  sensation to nerve impulses.

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• Sensory modalities
• Taste
• Smell
• Touch
• Hearing
• Vision

6. Internal sensory such as proprioceptors,
which monitor the positions of muscles and joints
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• Distinguish different types of sensory stimuli
• Each kind of receptor cell is highly selective of
  a specific kind of energy
• CNS sorts out the original stimulus (anatomic
  region in the brain to which information is sent)

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Structural Categories of Sensory Receptors

• Free
• Pain, temperature.
• Encapsulated
• Pressure.
• Meissners corpuscles
• Touch.
• Rods and cones
• Sight.
• Modified epithelial cells
• Taste.

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Functional Categories of Sensory Receptors

• Grouped according to type of stimulus energy they
  transform.
• Chemoreceptors
• Chemical stimuli in environment (taste buds,
  olfactory epithelium) and blood (pH, C02).
• Photoreceptors
• Rods and cones.
• Thermoreceptors
• Temperature.
• Mechanoreceptors
• Touch and pressure.
• Nociceptors
• Pain.
• Proprioceptors
• Body position.

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Sensory Adaptation

• Tonic receptors
• Produce constant rate of firing as long as
  stimulus is applied.
• Phasic receptors
• Burst of activity but quickly reduce firing rate
  (adapt).

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Common Mechanisms and Molecules of Sensory
Transduction
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Signal transduction 1. G-protein mediated 2nd
messenger system (e.g vision, olfaction) 2. Salt
passive movement of Na 3. Sour pH-sensitive K
channel
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Quality coding A single sensory receptor can
encode the intensity of the stimulus, but not the
the quality of the stimulus Patterns of activity
in many receptors cells encode the quality of the
stimulus. Therefore, sensory organs provide more
information about the stimulus than a single
sensory receptor.
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Range fractionation Individual receptors cover
only a fraction of total dynamic range of sensory
system, so the entire dynamic range of the
modality is divided among the different classes
of receptors.
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• Sensory adaptation during sustained stimulation
• Filter
• Receptor run down
• Accumulation of products
• change of electrical properties
• Less response of spike-initiating zone
• Adaptation in CNS

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Resting state
Pressure released
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• Mechanisms enhance sensitivity
• spontaneous activity of the receptors
• Distinction between noise and signal in CNS
• Feedback inhibition

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Taste
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• Taste receptors
• Insects-sensilla
• Vertebrate-taste buds

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Taste

• Gustation
• Sensation of taste.
• Epithelial cell receptors clustered in taste
  buds.
• Taste cells are not neurons, but depolarize upon
  stimulation and release chemical transmitters
  that stimulate sensory neurons.

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• Five distinct taste
• Salty
• Sweet
• Sour
• Bitter
• Umami

Each quality of the taste is transduced by a
distinctive mechanism
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Salty taste Na ion enter receptor through
amiloride sensitive Na channel, cause
depolarization of the receptor
Sour taste H act through amiloride-sensitive
Na channel or block K channel
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Smell
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Pheromones chemicals released by an organism
into its environment enabling it to communicate
with other members of its own species
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Brain
Olfactory bulb
Mitral cells
Glomeruli
To limbic system and cerebral cortex
Bone
Olfactory receptors
Cilia
Fig. 6-46, p.240
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• How can one kind of cell enable us to
  discriminate among thousands of different odors?
• The mammalian genome contains a family of about
  1000 related but separate genes encoding
  different odor receptors
• Each olfactory neuron expresses only a single
  type of receptor
• Each receptor is probably capable of binding to
  several different odorants
• Each odorant is capable of binding to several
  different receptors

• Odorant A binds to receptors on neurons 3, 427,
  and 886.
• Odorant B binds to receptors on neurons 2, 427,
  and 743.

The brain then would interpret the two different
patterns of impulses as separate odors
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Mechanical receptor Hearing
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Mechanoreceptor Sense physical contact with
surface of their bodies. The stimulus that
activates a mechanoreceptor cell is a stretch or
distortion of plasma membrane
Fig. 6-28, p.221
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Statocyst A gravity-sensing organ made up of
mechanoreceptive hairs cells and associated
particles called statolith
Statolith A small, dense, solid granule found in
a statocyst
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Vertebrate Ears
Two Primary Functions 1) Equilibrium--sense
position with respect to gravity and
acceleration through space 2) Hearing--sense
vibrational stimuli in environment
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Vestibular Apparatus and Equilibrium

• Equilibrium (orientation with respect to gravity)
  is due to vestibular apparatus.
• Consists of 2 parts
• Otolith organs
• Utricle and saccule
• Semicircular canals

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Fig. 6-31, p.224
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Equilibrium Organs
Two Chambers 1) Sacculus 2)
Utriculus Endolymph--fluid in chambers with high
K and low Na. (transduction) Cupula--covers
cilia of hair cells with a gelatin projection.
(when moves, Vm of hair cells change) Otoliths--
associated with maculae (hair cells), mineralized
concretions signal position (gravity)
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Fig. 6-33, p.226
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Flow of Sound
1) Pinna and Tragus? auditory canal 2) tympanic
membrane ?malleus,incus, stapes 3) oval window
?cochlea Cochlea--specialized inner ear
structure that transduces sound into neuronal
signals Organ of Corti--location of hair cells in
mammalian inner ear
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Fig. 6-39, p.231
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Fig. 6-40, p.232
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• Sensitivity to different frequency
• Mechanical characteristics
• Electrical properties
• tuning of inner hair cells by outer hair cells

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Sound ? Neuronal Signals
--Receptor currents from hair cells summed
?basilar membrane movements --Movement of
stereocilia trigger ion channels to open --hair
cells depolarized as K and Ca
flow --excitatory chemical synapse with auditory
neurons originating in the spiral ganglion.
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Vision
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Arthropods Vertebrate Basic
units ommatidium cons/rods Visual field
2-30 0.020 Perceiving the
Yes No polarized light Image Mosaic, not
inverted higher resolution,Invert
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• Focus of image in eyes
• Moving the lens (some bony fishes)
• Change focal length of lens

Iris The pigmented circular diaphragm located
behind the cornea of the vertebrate eye Pupil
The opening center of the iris through which
light passes into the eye.
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Changes in the Lens Shape

• Ciliary muscle can vary its aperture.
• Distance gt 20 feet
• Relaxation places tension on the suspensory
  ligament.
• Pulls lens taut.
• Lens is least convex.
• Distance decreases
• Ciliary muscles contract.
• Reduces tension on suspensory ligament.
• Lens becomes more rounded and more convex.

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Vision
Photoreceptor transduce the energy of visible
light into neuronal signals
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Optic nerve
Direction of light
Pigment layer
Choroid layer
Retina
Direction of retinal visual processing
Sclera
Back of retina
Front of retina
Rod
Cone
Fibers of the optic nerve
Amacrine cell
Photoreceptor cells
Bipolar cell
Ganglion cell
Horizontal cell
Retina
Fig. 6-16, p.209
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• Two classes of photoreceptor based on shape s of
  the cells
• Rods dim light, low resolution, black/white
  version
• Cones bright light, color vision, high
  resolution

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Visual Pigments
Rhodopsin Two Components 1) Opsin protein 2)
light-absorbing molecule--retinal or
3-dehydroretinal Light photons change the
configuration of light-absorbing molecules
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Fig. 6-19, p.211
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Effect of Light on Rods

• Rods are activated when light produces chemical
  change in rhodopsin.
• Bleaching reaction
• Rhodopsin dissociates into retinene
  (rentinaldehyde) and opsin.
• 11-cis retinene is converted to all-trans form.
• Initiates changes in ionic permeability to
  produce AP in ganglionic cells.
• Provide black-and-white vision.

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Dark current an inward Na current, maximal in
dark, found in vertebrate.
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Fig. 6-20, p.213
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Vertebrate Invertebrate Light
stimulus hyperpolarization depolarizaiton
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Electrical Activity of Retinal Cells in
Vertebrates

• Na channels rapidly close in response to light.
• cGMP required to keep the Na channels open.
• Opsin dissociation causes the alpha subunits of
  G-proteins to dissociate.
• G-protein subunits bind and activate
  phosphodiesterase, converting cGMP to GMP.
• Na channels close when cGMP converted to GMP.

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The cation channels of visual transduciton
belongs to same family of voltage-gated K
channels.
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Cones and Color Vision

• Cones less sensitive than rods to light.
• Cones provide color vision and greater visual
  acuity.
• High light intensity bleaches out the rods, and
  color vision with high acuity produced by cones.

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Color vision
Three genes
three opsins
Three cones
blue, green and red
The color vision depends on the opsin, not on
retinal.
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Cones and Color Vision

• Trichromatic theory of color vision
• 3 types of cones
• Blue, green and red.
• According to the region of visual spectrum
  absorbed.
• Each type of cone contains retinene associated
  with photopsins.
• Photopsin protein is unique for each of the 3
  cone pigment.
• Each cone absorbs different wavelengths of light.

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Visual Acuity and Sensitivity

• Each eye oriented so that image falls within
  fovea centralis.
• Fovea only contain cones.
• Degree of convergence of cones is 11.
• Peripheral regions contain both rods and cones.
• Degree of convergence of rods is much lower.
• Visual acuity greatest and sensitivity lowest
  when light falls on fovea.