Special Senses I: Olfaction and Gustation

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Special Senses IOlfaction and Gustation
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5 Special Senses

• Olfaction smelling
• Gustation tasting
• Vision
• Hearing
• Equilibrium
• All special sensory receptors are housed in
  complex sensory organs or specialized epithelial
  structures
• Special sensory information travels via cranial
  nerves

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Olfaction
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Olfactory Epithelium

• Covers the superior nasal concha
• Pseudostratified columnar epithelium containing
  millions of bipolar olfactory receptor cells

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Olfactory Epithelium

• Contains 3 cell types
• 1) Olfactory receptor cells bipolar neurons
  with dendrite projecting to surface, and axon
  projecting through hole in cribriform plate to
  synapse in olfactory bulb
• 2) Supporting cells columnar epithelial cells
  of nasal mucosa
• 3) Basal cells stem cells that lie at base of
  epithelium and continually replace olfactory
  receptor cells (which live for only one month)
  Neuron replacement!
• Olfactory glands produce mucus to keep surface
  moist

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Olfactory Receptor Cells

• At epithelial surface, olfactory receptor cells
  have olfactory hairs projecting from the dendrite
  into the mucus. Chemoreceptors
• The olfactory hairs are nonmotile cilia that
  contain odorant binding protein receptors
• When an odorant molecule binds to the receptor, a
  generator potential (graded potential) is
  produced in the receptor cell
• 1,000 different smell receptor types.
• Each receptor binds to a variety of odor
  molecules that share a common structural part, so
  each odor molecule may bind to several different
  receptors, one for each part.

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Olfactory Pathways

• Olfactory receptor cells synapse onto the
  clustered dendrites of mitral cells in the
  olfactory bulb, lying over the cribriform plate.
  Cluster glomerulus
• All receptors of same type send their axons to
  the same glomerulus
• Each odor will activate several mitral cell
  glomeruli signature
• Mitral cells send axons through olfactory tract
  to limbic (emotional centers) and temporal
  regions of cortex (consciousness of smell)

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Olfaction

• Normally can recognize about 10,000 odors
• Very low threshold ( can smell even a few
  molecules), but adapts rapidly
• Anosmia absence of sense of smell. Caused by
  trauma to nerves, excessive mucus, blockage by
  polyps, zinc deficiency or use of zinc nasal
  sprays

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Gustation
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Taste Buds

• nearly 10,000 taste buds mainly on tongue, but
  also on soft palate, pharynx and epiglottis
• number declines with age (why children like bland
  food)
• Found on specialized epithelial structures on
  tongue - elevations called papillae

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Anatomy of Papillae

• Three types of papillae contain taste buds
• Circumvallate 12, in V shape, each
  contain100-300 taste buds
• fungiform scattered over entire surface of
  tongue 5 taste buds each
• foliate lateral margins of tongue most taste
  buds degenerate in early childhood
• (filiform papillae have no taste buds)

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Anatomy of Taste Buds

• Specialized epithelial strucuture within
  stratified squamous epithelium of papilla
• Contains gustatory receptor cells, surrounded by
  supporting cells. Basal cells can regenerate old
  receptor cells (live 10 days)
• Gustatory hairs project from gustatory receptor
  cells, through the taste pore, to sample the
  external surface

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Physiology of Gustation

• chemicals that stimulate gustatory receptor cells
  are known as tastants
• dissolved in saliva, they make contact with
  receptors on gustatory hairs
• receptor potential arises differently for
  different tastants tastants may enter cell
  directly (Na , H) or bind to receptor
• Encoding of distinct tastes is still unclear,
  since individual gustatory receptor cells respond
  to many tastes

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Taste Pathways

• Gustatory receptor cells synapse with dendrites
  of 1st-order neurons that project via cranial
  nerves VII, IX and X into brainstem
• From brainstem, 2nd-order neurons project to
  thalamus.
• 3rd-order neurons project to primary gustatory
  area in parietal lobe of cerebral cortex.

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Gustation

• five primary tastes bitter, sour, salty, sweet,
  umami (meaty or savory MSG)
• Other flavors are combinations of tastes, plus
  odors
• Thresholds are lowest for bitter tastes least
  for salty and sweet
• Specialized areas for reception of different
  tastes is partially, but not wholly true
• Rapidly adapting

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Special Senses II Vision
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Accessory Structures of the Eye

• palpebrae upper and lower eyelids
• palpebral fissure opening between lids
• lateral medial commissures corners
• lacrimal caruncle red bump in medial corner
  contains sebaceous and sudoriferous glands

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Eyelids

• Orbicularis oculi and levator palpebrae muscle
• Tarsal plates with Meibomian glands (sebaceous)
• Conjunctiva mucous membrane covering inner lids
  and sclera (white of the eye), but not cornea.
  Pinkeye conjunctivitis. Contains blood vessels
  (bloodshot eyes)

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Lacrimal Apparatus

• lacrimal glands produce tears, secreted via
  lacrimal ducts onto conjunctiva of the upper lid
• Tears wash over surface and drain through
  nasolacrimal duct into nasal cavity
• lacrimal fluid (tears) watery solution
  containing salts, mucus, and lysozyme
  (bactericidal).
• protects, cleans, lubricates and moistens the
  eyeball
• spreads from lateral to medial by blinking
• about 1ml of lacrimal fluid per day

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Extraocular Muscles

• Must be coordinated if not, diplopia may lead to
  loss of vision in one eye, or loss of depth
  perception

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Anatomy of the Eye
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Layers of the Eyeball

• Fibrous tunic most superficial
• Vascular tunic middle layer
• Retina inner coat

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1.Fibrous Tunic

• Avascular
• Cornea transparent layer that covers iris.
  Curved, to help focus light on retina. Receives
  oxygen from outside air. Can be transplanted no
  blood vessels
• Sclera white of eye covers entire eyeball
  except cornea. Gives shape to the eyeball
• Junction scleral venous sinus (canal of Schlemm)
  drains aqueous humor from anterior chamber
  (cavity)

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2.Vascular Tunic

• Choroid ciliary body iris
• Choroid highly vascular layer lining inside of
  sclera. Provides nutrients to retina
• Ciliary body contains a circular ciliary muscle
  with ciliary processes that secrete aqueous
  humor. The lens is suspended from the edges of
  the ciliary processes by suspensory ligaments
• Iris colored portion. Contains circular and
  radial smooth muscles that regulate the size of
  the pupil. Melanocytes determine color.

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Iris

• The iris regulates the amount of light entering
  the eye by adjusting the size of the pupil
• Pupillary reflex When bright light is shone
  into the eye, the circular muscles of the iris
  contract to constrict the pupil. Parasympathetic
  control.
• In dim light, or excitement, the radial muscle of
  the iris contract, dilating the pupil.
  Sympathetic control

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3. Retina

• Innermost layer
• lines posterior ¾ of eyeball
• contains visual receptors which are the beginning
  of visual pathway
• optic disc optic nerve (II) and central retinal
  artery and vein exit eyeball

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Retina - Layers

• Outer pigmented layer contains melanin to absorb
  stray light rays, preventing reflection and
  scattering, so image on retina remains sharp and
  clear
• Inner neural layer outgrowth of brain.
  Processes visual data before sending to optic
  nerve

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Photoreceptors

• 6 million rods 120 million cones
• Rods allow black-white vision in dim light
• Cones need brighter light produce color vision
• Feed information to bipolar cellsfor initial
  processing, then ganglion cells. Ganglion cell
  axons project through optic disc to brain.

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Figure 16.10c
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Central Fovea

• small depression in exact center of posterior
  retina, in visual axis of eye
• only cones (no rods)
• connections to bipolar or ganglion cells are
  pushed to the side
• area of highest visual acuity (macula)

Blind spot at optic disc, no photoreceptors
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Lens

• Thick, transparent, biconvex disc that changes
  shape to allow precise focusing of light on
  retina
• Ciliary muscles control shape of lens through
  suspensory ligaments.
• With age, lens becomes less elastic and cannot
  focus as well
• Cataract clouding of the lens. May be caused by
  genetic factors, sun, smoking, or medications
  such as aspirin and steroids. Can be replaced by
  artificial lens

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Circulation of Aqueous Humor

• The anterior cavity of the eye is divided into
  anterior chamber (between cornea and iris) and
  posterior chamber (between iris and lens).
• Aqueous humor (a watery fluid) is secreted by
  ciliary processes, and flows out through pupil
  and is absorbed into scleral venous sinus.
• Replaced every 90 minutes.
• Blockage glaucoma.
• Increased pressure in
• eye damages optic nerve

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Vitreous Chamber

• Behind lens
• Filled with vitreous body, a jelly-like substance
• Normally holds retina flush against choroid
  layer, but can shrink and pull away with age,
  leading to detached retina.

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Image Formation
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Refraction

• light is bent (refracted) as it passes through
  cornea and lens. Near objects must be refracted
  more.
• about 75 of refraction occurs at cornea, and is
  constant
• other 25 occurs at lens, and is adjustable

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Accommodation (focus)

• A rounder lens refracts light more powerfully, so
  the lens is rounded for near vision
• The shape of the lens is altered by the pull of
  the suspensory ligaments of the ciliary body.

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Refraction Abnormalities
Eyeball too long
Eyeball too short
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Visual Impulse Transmission
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Photoreceptors

• rods and cones named for shape of outer segment,
  filled with membrane discs
• renewed rapidly rods 1-3 discs per hour
• photopigments (proteins in plasma membrane of
  outer segment) absorb light

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Retina

• When photoreceptors are activated by light, they
  cease to inhibit bipolar cells, which then
  activate ganglion cells
• Initial processing of visual information is
  performed in the retina itself
• Ganglion cells send their axons through the optic
  disc to the optic nerve

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Visual Pathway

• Optic nerves partially cross at optic chiasm
• Enter brain and terminate in the thalamus
• Thalamic neurons project to visual cortex.
• Right visual field projects to left visual cortex

thalamus
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Figure 16.17a
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Special Senses IIIHearing and Equilibrium
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Anatomy of the Ear
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Anatomy of Ear

• external ear
• collects sound waves and channels them inward
• middle ear
• conveys sound vibrations to oval window
• internal (inner) ear
• houses receptors for hearing and equilibrium

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External Ear

• Auricle collects sound waves and channels them
  inward
• external auditory canal
• tympanic membrane (ear drum)
• ceruminous glands
• secrete cerumen (ear wax)

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Middle Ear

• small air-filled cavity in temporal bone
• separated from internal ear by thin bony wall
  containing two small membrane-covered openings
  oval window and round window
• auditory ossicles malleus, incus, stapes
• tiny skeletal muscles
• tensor tympani muscle
• stapedius muscle
• anterior wall has auditory (Eustacian) tube which
  connects to nasopharynx, to equalize air pressure
  across the tympanic membrane

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Figure 16.19
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Ossicles

• Smallest bones in the body
• Connected by synovial joints
• Amplify power of sound vibrations (20X) by acting
  as levers
• Stapedius and tensor tympani muscles attach to
  stapes and malleus, and dampen loud sounds

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Internal Ear

• Houses receptors for hearing and equilibrium
• receptors for hearing
• cochlea
• receptors for equilibrium
• semicircular canals
• vestibule

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Internal Ear

• A bony labyrinth in the temporal bone
• Lined by a membranous labyrinth. The membranous
  labyrinth is surrounded by perilymph (similar to
  CSF), and encloses endolymph

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Vestibule and semicircular canals

• The vestibule is the central room in the bony
  labyrinth
• It contains two membranous sacs, the utricle and
  the saccule, connected by a small duct
• Connected to the vestibule are the semicircular
  canals, containing semicircular ducts lying at
  right angles to each other each contains an
  ampulla
• Receptors for equilibrium lie here

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Cochlea

• Receptors for hearing are located in the cochlea,
  a bony spiral canal
• Cross sections of the cochlea reveal that it is
  divided into three long chambers cochlear duct
  (membranous labyrinth, containing endolymph), and
  the scala tympani and scala vestibuli (perilymph)
• The scalae communicate at the tip of the spiral,
  at the helicotrema

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Windows

• The stapes ossicle of the middle ear fits snugly
  into the oval window of the vestibule. When the
  stapes pulses from sound waves, it pushes on the
  perilymph in the scala vestibuli.
• Pressure travels through the cochlea and is
  relieved at the round window, connected to the
  scala tympani, which bulges back out into the
  middle ear

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Spiral Organ

• In the cochlear duct lies the spiral organ of
  Corti (spiral organ), which houses the receptors
  for hearing

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Spiral Organ

• A specialized epithelium sitting on a basilar
  membrane, containing supporting cells and hair
  cells
• Hair cells have bundles of microvilli projecting
  from apical end hair bundles.
• Hairs project into endolymph and are of graded
  heights. Rub against a gelatinous tectorial
  membrane, projecting from vestibular membrane
• Synapse with 1st-order sensory neurons of spiral
  ganglion

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Figure 16.21d
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Figure 16.21e
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Sound Reception
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Sound Waves

• alternating high and low pressure regions
  traveling in same direction through a medium (air
  or fluid)
• originate from a vibrating object
• frequency number of waves that arrive in a given
  period (hertz)
• interpreted as pitch
• intensity or amplitude
• more intense vibrations higher pressure waves
  louder sound
• measured in decibels (dB)

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Physiology of Hearing

• auricle directs sound to tympanic membrane
• tympanic membrane vibrates
• vibration type determined by both intensity and
  frequency of sound waves
• Vibrates slowly to low frequency waves
• rapidly to high frequency waves
• forceful vibrations when loud
• gentle vibrations in softer sounds
• tympanic membrane vibrates ossicles malleus
  vibrates incus, then to stapes
• stapes pushes membrane of oval window

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Physiology of Hearing

• pressure waves in perilymph vibrate scala
  vestibuli in cochlea
• scala vestibuli pressure waves vibrate scala
  tympani and round window
• pressure waves in perilymph set up pressure waves
  in endolymph in cochlear duct, vibrating the
  basilar and vestibular membranes
• basilar membrane moves hair cells against the
  tectorial membrane
• leads to generation of nerve impulses to
  vestibulocochlear nerve

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Hearing
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Auditory Pathway

• The hair cells synapse on sensory fibers of
  cochlear branch of vestibulocochlear nerve.
• Cell bodies in spiral ganglion
• Project to nuclei in pons, then to thalamus and
  auditory cortex in temporal lobe
• Some inputs cross and some dont differences in
  timing allow localization of sound
• Hair cells may be damaged by loud sounds, or lost
  with age, and cannot regenerate

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Figure 16.21b
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Figure 16.23
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Equilibrium
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Equilibrium

• The receptors for equilibrium are found in the
  saccule, the utricle and the semicircular ducts
  the vestibular apparatus

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Otolithic Organs

• Within the walls of the utricle and saccule lie
  thickened epithelia called maculae.
• Receptors for static equilibrium sense of body
  position in relation to gravity
• The maculae contain hair cells with hair bundles
  projecting into a gelatinous otolithic membrane.
• A layer of calcium carbonate crystals, otoliths,
  cover the membrane
• As head tips, otoliths move, stimulating hair
  cells

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Semicircular Ducts

• Within the ampullae of the semicircular ducts lie
  cristae
• Each crista contains hair cells that project into
  gelatinous cupula.
• When head moves in the plane of a particular
  semicircular duct, the endolymph of that duct
  swirls deforms the cupula, stimulating the hair
  cells
• Yes stimulates anterior canal, no stimulates
  lateral canal, and lateral flexion stimulates
  posterior canal.
• Sense of dynamic equilibrium sense of movement

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Equilibrium Pathways

• The hair cells synapse on 1st-order sensory cells
  with cell bodies in the vestibular ganglion
• Project to nuclei in brainstem and the
  cerebellum, to control balance
• Strong input to nuclei that control eye movements
  the vestibulo-ocular reflex