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Special Senses
Chapter 15
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
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
Fibrous Tunic
• Avascular CT
• Sclera (white of the eye)
– Protects and shapes
– Muscle attachment
– Continuous with dura mater
• Cornea (transparent)
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–
–
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Outer stratified squamous, why?
Inner simple squamous maintain clarity
Innervated
Transplants not rejected between people
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)
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
Humors
• Anterior segment with aqueous humor
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–
–
–
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
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
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
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
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
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)
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
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
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
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
Frequency and Amplitude
• Sounds detected as changes in AP’s
– 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 AP’s
• Hair cells easily damaged due to
prolonged exposure to certain
frequencies
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
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
http://openlearn.open.ac.uk/file.php/3373/SD329_1_027i.jpg
Dynamic Equilibrium
• Maintain body position
after initiation of mov’t
• Within semicircular
canals
– Rotation within 1 of 3
planes
– Endolymph moves
opposite direction of
mov’t
– Reverse to signal stop
• Dizzy feeling
Static Equilibrium
• Linear changes only
– E.g. elevator changes or car
acceleration/deceleration
• Vestibule
– Saccule: vertical, hairs
horizontal
– Utricle: horizontal, hairs
vertical
• Maculae overlaid by otoliths
• Mov’t displaces in opposite
direction
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