special senses chapter 15. anatomy of an eyeball accessory structures 3 tunics (layers) – fibrous...
TRANSCRIPT
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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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• 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
Fibrous Tunic
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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 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
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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
http://openlearn.open.ac.uk/file.php/3373/SD329_1_027i.jpg
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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
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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
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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