Sensory Receptors

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Sensory Receptors

• Sensory receptors are either specialized endings
  of afferent neurons or separate cells that signal
  the afferent neuron

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Properties of Receptors

• Sensory transduction
• convert stimulus energy into membrane potential
  changes
• Receptor potential
• graded potentials in a receptor cell
• Adaptation
• conscious sensation declines with continued
  stimulation

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Receptors Transmit Information

• Modality - type of stimulus
• Location
• each sensory receptor receives input from its
  receptive field
• sensory projection
• brain identifies site of stimulation based on
  where APs of sensory neurons are sent
• Intensity
• frequency of APs and number of fibers firing APs
• Duration - change in AP firing frequency over
  time
• phasic receptor - adapt quickly, initial burst of
  APs that decrease in frequency over time
• smell and hair receptors
• tonic receptor - adapt slowly, generate APs
  continually
• pain receptor

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Receptive Fields

• The size of a receptive field determines the
  sensory acuity which allows the body to discern
  between discrete stimuli

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Phasic Receptors
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Overlapping Receptive Fields

• Overlapping stimulation between neighboring
  receptive fields provides general information
  about the location of a stimulus

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CNS Modulation of Pain

• CNS activity can screen out painful sensory
  information by inhibiting neurons in the afferent
  pathway through the secretion of opiods
• enkephalins, endorphins and dynorphins

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Somesthetic Projection (Ascending) Pathways

• Somatic sensory pathways to the cerebral cortex
  involve a 3 neuron pathway that crosses the
  midline
• 1st order neuron (afferent neuron)
• from body, enter the dorsal horn of spinal cord
  via spinal nerves
• from head, enter pons/medulla via cranial nerve
• 2nd order neuron
• cross to opposite side in spinal cord or medulla
• synapse in thalamus, except for proprioception
  (spatial location of limbs and joints) which
  synapse in the cerebellum
• 3rd order neuron
• extend from thalamus to primary somesthetic cortex

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Somesthetic Projection (Ascending) Pathways

• Sensory information regarding pain or temperature
  ascends via the anterolateral pathway
• Sensory information regarding body movement,
  touch and pressure ascends via the dorsal column
  pathway

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Somatosensory Cortex

• Somatosensory areas in the cortex of the brain
  are anatomically organized in relation to the
  source of information, with larger areas
  dedicated to parts of the body that process fine
  discriminations

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Vision and Light

• Vision is the perception of light emitted or
  reflected from objects in the environment
• Light
• form of electromagnetic radiation (energy)
• visible light wavelengths range from 400 to 750
  nm
• small fraction of electromagnetic radiation types
• Photochemical reactions in the eye produce a
  nerve signals for light in the visible range
• radiation below 400 nm
• UV, x-rays, gamma rays
• radiation above 750 nm
• microwaves, radar, radio waves

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Frequency and Wavelength
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Anatomy of the Eye
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Optical Components

• Light passes first through the cornea
• transparent cover on anterior surface of eyeball
• Then through the aqueous humor
• watery fluid posterior to cornea, anterior to
  lens
• Light then passes through the pupil, which is a
  hole in the center of the iris (colored portion
  of the eye)
• Light continues through the lens
• changes shape (accommodate) to focus light
• flattened due to pull of zonular fibers
  (suspensory ligaments) upon relaxation of ciliary
  muscle
• rounded when the ciliary muscle and the zonular
  fibers slacken
• Light then passes through the vitreous humor
• gel fills space between lens and retina
• Light is finally focused onto the retina where
  photoreception occurs

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Accommodation of Lens for Near and Distant Vision
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Neural Components

• Includes retina and optic nerve
• Retina
• photoreceptor cells
• Rods
• Cones
• neurons
• Bipolar cells
• Ganglion cells
• Light must first pass through the layers of
  ganglion and bipolar cells before it is detected
  by the photoreceptors
• Images are focused on to the fovea centralis, a
  small portion of the retina which has the
  greatest density of photoreceptor cells providing
  the greatest amount of visual acuity (percision)

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Schematic Layers of the Retina
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Photoreceptor Cells

• Photoreceptor cells contain visual pigments
  which are excited by different wavelengths of
  visible light
• rod cells
• night (scotopic) vision
• contains the visual pigment rhodopsin
• cone cells
• color (photopic) vision
• contains the visual pigment photopsin

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Color Vision

• Primates have well developed color vision
• nocturnal vertebrates have only rods
• Cones named for absorption peaks of different
  photopsins
• blue cones peak sensitivity at 420 nm
• green cones peak at 531 nm
• red cones peak at 558 nm (orange-yellow)
• Color perception based on mixture of nerve signals

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Visual Projection Pathway

• Bipolar and ganglion cells - 1st and 2nd order
  neurons
• Hemidecussation in optic chiasm
• 1/2 of fibers for each eye cross and are
  projected to the opposite side of the brain
  (contralateral)
• 1/2 of fibers for each eye do not cross and are
  projected to the same side of the brain
  (ipsilateral)
• the image on the left halves of each retina is
  projected to the left visual cortex
• the image on the right halves of each retina is
  projected to the right visual cortex
• Ganglion cells synapse at the thalamus
• 3rd order neurons in the thalamus extend to
  primary visual cortex where conscious visual
  sensation occurs

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Visual Projection Pathway

• Visual association areas in parietal and temporal
  lobes process visual data
• object location, motion, color, shape, boundaries
• store visual memories (recognize printed words)

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The Nature of Sound
Amplitude loudness
cycles/sec. frequency pitch

• Sound - audible vibration of molecules
• vibrating object pushes air molecules
• Sound waves are the zones of atmospheric
  rarefaction and compression

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Pitch and Loudness

• Pitch - frequency vibrates specific parts of ear
• hearing range is 20 - 20,000 Hz (cycles/sec)
• most speech is 1500-4000 Hz where hearing is most
  sensitive
• Loudness amplitude intensity of sound energy

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Outer, Middle and Inner Ear
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Outer, Middle and Inner Ear

• Outer Ear
• auricle
• focuses sound into the auditory canal towards the
  tympanic membrane (ear drum)
• Middle Ear
• auditory (eustachian) tube connects to
  nasopharynx
• equalizes air pressure on tympanic membrane
• ear ossicles
• malleus
• incus
• stapes
• Inner Ear
• cochlea
• organ of sound reception
• vestibular apparatus
• semicircular ducts, utricle and saccule
• organs of equilibrium and balance

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Anatomy of Cochlea
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Anatomy of Cochlea

• A fluid filled tube divided into 3 segments
• Scala media (cochlear duct)
• spiral organ (organ of Corti)
• contains hair cells that detect sound
• filled with endolymph (ECF)
• very high K concentration
• Scala vestibuli and Scala tympani
• filled with perilymph
• The vibrations in the stapes are transmitted to
  the oval window, which creates ripples
  (vibrations) in the cochlear fluid of the scala
  vestibuli

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Spiral Organ
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Spiral Organ

• Inner hair cells
• hearing
• Outer hair cells
• adjust to different frequencies to increase
  precision
• Basilar membrane
• vibrates due to sound waves and moves the hair
  cells attached to it
• Stereocilia of hair cells attach to gelatinous
  tectorial membrane
• bend as the hair cells are moves upward into the
  stationary tectorial membrane
• bending causes the opening of gated channels
  which initiate APs that are sent to the brain via
  the cochlear nerve

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Stimulation of Cochlear Hair Cells

• Vibration of ossicles causes vibration of basilar
  membrane under hair cells
• as often as 20,000 times/second

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Basilar Membrane Frequency Response

• The basilar membrane is narrow at the proximal
  end and gets progressively wider toward the
  distal end
• similar to the strings on a piano
• High pitch sounds cause the proximal portion of
  the membrane to vibrate while low pitch sounds
  cause the distal portion of the membrane to
  vibrate

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Cochlear Hair Cells

• Stereocilia of IHCs
• bathed in high K of the endolymph
• tips bend in response to movement of basilar
  membrane
• opens K channels at the base of the stereocilia
• K flows in
• causes depolarization of hair cell and release of
  neurotransmitter
• stimulates sensory neuron at base of hair cell

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Potassium Channels
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Sensory Coding of Sound

• Sensory neurons associated with the hair cells
  exit the cochlea via the cochlear nerve and
  ultimately project to the primary auditory cortex
• Vigorous vibrations excite more inner hair cells
  over a larger area
• triggers higher frequency of action potentials
• brain interprets this as louder sound
• Pitch depends on which part of basilar membrane
  vibrates
• at proximal end, membrane narrow and stiff
• brain interprets signals as high-pitched
• at distal end, membrane wider and more flexible
• brain interprets signals as low-pitched

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Equilibrium

• Control of coordination and balance
• Receptors (hair cells) in vestibular apparatus
  respond to changes in the position of the head
• semicircular ducts
• saccule and utricle
• Static equilibrium
• perception of head orientation by saccule and
  utricle
• Dynamic equilibrium
• perception of motion or acceleration
• linear acceleration perceived by saccule and
  utricle
• angular acceleration perceived by semicircular
  ducts

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Saccule and Utricle

• Hair cells with stereocilia buried in a
  gelatinous membrane filled with crystals of
  calcium carbonate called otoliths
• add to the density and inertia and enhance the
  sense of gravity and motion
• The hair cells of the utricle are vertical while
  the cells of the saccule are horizontal in
  someone who is standing

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Saccule and Utricle

• Static equilibrium - when head is tilted, weight
  of membrane bends the stereocilia generating APs
• Dynamic equilibrium in a car, linear
  acceleration detected as otoliths lag behind
  bending the stereocilia generating APs

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Semicircular Ducts

• 3 ducts filled with endolymph oriented at right
  angles to one another detect sudden changes in
  head position
• At the ends of each duct is an ampulla
• group of hair cells hair cells with stereocilia
  buried in a mound of gelatinous membrane called
  the cupula

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Semicircular Ducts

• The flat sheets indicate that the plane of each
  of the three semicircular ducts are perpendicular
  to one another
• Provides sensitivity to any head rotation

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Head Rotation

• As head turns, endolymph lags behind, pushes
  cupula, stimulates hair cells at one end of the
  duct, but inhibits the hair cells at the other
  end of the duct

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Activation and Inhibition of Vestibular Hair Cells

• Bending of stereocilia in opposite directions has
  opposite effects on their membrane potentials

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Taste

• Gustation
• sensation of taste
• results from action of chemicals on taste buds
• Lingual papillae
• fungiform
• at tips and sides of tongue
• vallate
• at rear of tongue
• contains 1/2 of taste buds

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Physiology of Taste

• Molecules must dissolve in saliva and bind to
  receptors/gated channels in the cell membrane of
  taste cell which depolarizes cell and releases
  neurotransmitter onto sensory neuron
• 5 primary sensations - throughout tongue
• Sweet - concentrated on tip
• Salty - lateral margins
• Sour - lateral margins
• Bitter - posterior
• Umami - taste of amino acids (MSG)
• Influenced by food texture, aroma, temperature,
  and appearance
• Spicy food (hot pepper) stimulates free nerve
  endings resulting in the perception of pain

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Smell

• Olfaction
• sensation of smell
• results from action of chemicals on olfactory
  cells
• highly sensitive
• up to 10,000 different odors can be detected

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Olfactory Cells

• Olfactory cells are neurons that contain hairs
  which bind odor molecules in thin layer of mucus
• axons pass through the ethmoid bone

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Physiology of Smell

• Molecules bind to receptor on olfactory hair and
  activate an intracellular signaling pathway
• opens ion channels for Na or Ca2
• creates an AP
• Receptors adapt quickly
• Olfactory neurons rarely persist more than two
  months stem cells undergo mitosis and
  differentiation to assure that there is no loss
  of smell ability