Special Senses

1
Special Senses

• Chapter 15

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Anatomy of an Eyeball

• Accessory structures
• 3 tunics (layers)
• Fibrous (cornea sclera)
• Vascular (choroid)
• Sensory (retina)
• Segments
• Anterior divided into chambers
• Posterior
• Filled with humors (fluid)
• Lens

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Accessory Eye Structures

• Eye muscles
• Rectus as named oblique opposite and lateral
• Diplopia muscle weakness/alcohol
• Strabismus uncontrolled rotation
• Eyebrows
• Eyelids
• Blink to spread secretions
• Eyelashes trigger blinking
• Conjunctiva
• Mucus prevents drying out
• Conjunctivitis
• Lacrimal apparatus
• Tears clean, protect, and moisten
• Excess secretions
• Emotional tears unique to humans
• Stuffy/runny nose when cry
• Watery eyes with cold

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Fibrous Tunic

• Avascular CT
• Sclera (white of the eye)
• Protects and shapes
• Muscle attachment
• Continuous with dura mater
• Cornea (transparent)
• Outer stratified squamous, why?
• Inner simple squamous maintain clarity
• Innervated
• Transplants not rejected between people

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Vascular Tunic

• Choroid
• Vascularized to supply nutrients
• Melanocytes to absorb light
• Ciliary body
• Smooth muscle ring ? ciliary muscles control lens
  shape
• Ciliary processes secrete aqueous humor
• Suspensory ligaments hold lens in place
• Iris
• Colored portion of ciliary body
• Brown pigment only (varies)
• Less scatters light blues/greens/grays (babies)
• Encircles the pupil (2 smooth muscle layers)

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Sensory Tunic

• Pigmented layer (outer)
• Prevents light scattering
• Phagocytize damaged photoreceptors
• Neural layer (inner)
• Photoreceptors, bipolar cells, ganglion cell
• Rods and cones
• Blind spot (optic disc) filled
• Macula lutea and fovea centralis
• Rapid eye movement for rapid scene changes
• Vascular supply from choroid and central
  vein/artery
• Opthalmologist examines
• Retinal detachment when layers separate
• Vitreous humor seeps in
• Photoreceptors lose nutrients blindness

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Humors

• Anterior segment with aqueous humor
• Similar to CSF
• Continual development
• Nutrients O2 to lens, cornea, retina
• Blocked drainage up pressure glaucoma
• Posterior segment with vitreous humor
• Transmits light, support lens, intraocular
  pressure
• Unchanged from embryonic development

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The Functioning Eye

• Light enters the pupil,
• regulated by the iris
• Passes through a convex lens
• Avascular
• Lens fibers added through life
• Cataracts clouding of lens due to loss of
  nutrients
• Lens is shaped by the ciliary body to focus light
  on the retina (accommodation)
• Refraction of light converges to a focal point
• Real image forms upside down and reversed

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Visual Pathway

• Visual field
• Overlap to provide depth perception 3D vision
• Ganglion cells
• Optic nerve
• Optic chiasm
• Nasal and temporal visual field
• Optic tract
• Thalamus
• LGN
• Primary visual cortex
• Conscious perception of images

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Olfactory Receptors

• Ciliated bipolar cells
• Located in olfactory epithelium
  (pseudostratified ciliated)
• Mucus captures and dissolves odorants
• Pass through cribriform plates
• Synapse in olfactory bulbs
• Odorant detection
• Humans can distinguish 10,000 odors
• Some is pain (ammonia, chili, methanol)
• Combinations of different odorant/receptor
  binding
• Replaceable, but responsiveness declines with age

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Olfactory Neural Pathway

• Olfactory receptors synapse with mitral cells
• Contained in glomeruli
• Receptor type specific
• Refines smell
• Mitral cells signal via olfactory tracts
• 2 pathways
• Olfactory cortex
• Hypothalamus, limbic system emotional
  connection

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Gustation

• Taste buds detect molecules in solution
• About 10,000
• Four familiar and 1 other found in papillae
• Sweet organic substances
• Alcohol, sugar, amino acids
• Sour acids, H in solution
• Salty inorganic salts
• Bitter alkaloids
• Aspirin, nicotine, caffeine
• Umami glutamate aspartate
• Meats, cheeses, and protein-rich foods (MSG)
• Each receptor responsive to a particular type of
  substance
• Often mixes
• Many tastes (80) are really smell (head colds)

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Papillae

• Fungiform
• Mushroom shaped
• Tops of, all over tongue
• Foliate
• Fold in side walls
• Circumvallate
• Largest, fewest, back of
• tongue
• Filiform
• Hair like projections all over tongue
• Do not have taste buds
• Roughness

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Gustatory Neural Pathway

• Cranial nerves (VII and IX) carry sensations to
  medulla
• Relay through the thalamus into primary gustatory
  cortex
• Pathway initiates digestive process too

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Regions of the Ear

• Outer ear
• Pinna, external auditory canal, and tympanic
• membrane (separates)
• Middle ear
• Pharyngotympanic tube equalizes pressure
• b/w middle ear and atmosphere (pop)
• Function of tympanic membrane
• Ossicles (malleus, incus, stapes) amplify
  signal
• Inner ear
• Membranous labyrinths w/i bony labryinth
• Cochlea houses the hearing organ
• Vestibule report on changes of head position
• Saccule and utricle for gravity and acceleration
• Semicircular canals for rotation of head

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The Cochlea

• Scala vestibuli
• Perilymph like CSF
• Oval window
• Scala Tympani
• Perilymph
• Round window
• Scala media (Cochlear duct)
• Endolymph K rich intracellular
• fluid
• Organ of Corti
• Contains hair cells embedded in a basilar
  membrane
• Vestibular membrane
• Tectorial membrane bends cells as basilar
  membrane moves
• Signal to auditory nerve

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Frequency and Amplitude

• Sounds detected as changes in APs
• Pitch depends on frequency
• High pitch higher frequency
• Basilar membrane responsive to certain
  frequencies
• 20 to 20,000 Hz 1500 4000 most sensitive
• Loudness depends on amplitude
• Louder sounds higher amplitude
• Vigorous vibrations in cochlea more bending
  more APs
• Hair cells easily damaged due to prolonged
  exposure to certain frequencies

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Physiology of Hearing

• Pinna collects sound waves
• Travel down auditory canal to tympanic membrane
• Moves ossicles with vibrations
• Stapes pushes on oval window, in and out
• Creates fluid pressure waves in scala vestibuli
  perilymph
• Pressure waves deform scala tympani to push round
  window in and out
• Pressure changes move endolymph
• Highest frequency at base (oval window), lowest
  at apex
• Pressure changes in endolymph, from perilymph
  changes, moves the basilar membrane
• Hair cells on Organ of Corti bend as they move
  against the tectorial membrane
• Generates nerve impulses that leave via the
  cochlear nerve

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Auditory Pathway

• AP signals from cochlea to medulla
• Cochlear nuclei
• Some fibers cross to olives (collection of nuclei
  in the medulla) , all ascend into MGN(medial
  geniculate nucleus) in the thalamus
• Pass through inferior colliculi (reflex area)
• Interactions with superior colliculi to turn
  toward sound
• Synapse in primary auditory cortex
• Localization utilizes relative intensity and
  timing

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Dynamic Equilibrium

• Maintain body position after initiation of movt
• Within semicircular canals
• Rotation within 1 of 3 planes
• Endolymph moves opposite direction of movt
• Reverse to signal stop
• Dizzy feeling

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Static Equilibrium

• Linear changes only
• E.g. elevator changes or car acceleration/decelera
  tion
• Vestibule
• Saccule vertical, hairs horizontal
• Utricle horizontal, hairs vertical
• Maculae overlaid by otoliths
• Movt displaces in opposite direction

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Motion Sickness

• Results from conflict between eyes and
  equilibrium sensors in the inner ear
• Feeling motion, but not seeing it (inside
  structure)
• One system is hallucinating, implying toxins in
  system vomiting
• Dramamine inhibits input from equilibrium sensors
• Astronauts learn to control