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Hearing & Sight
BIOL241Ears & Eyes – Chap. 15
How do we see?• Gross Anatomy• Micro anatomy• Physics
Figure 15.4a
Central arteryand vein ofthe retinaOptic disc(blind spot)
Optic nervePosterior poleFovea centralisMacula luteaRetinaChoroidSclera
Ora serrata
(a) Diagrammatic view. The vitreoushumor is illustrated only in thebottom part of the eyeball.
Ciliary bodyCiliary zonule(suspensoryligament)CorneaIris
Anterior polePupil
Anteriorsegment (containsaqueous humor)LensScleral venoussinusPosterior segment(contains vitreous humor)
Layers, again!!• Outermost layers:• dense avascular connective tissue• Sclera• Uvea• Cornea• Humors• more
Sclera– Opaque posterior region– Protects and shapes eyeball– Anchors extrinsic eye muscles
Cornea: – Transparent anterior 1/6 of fibrous layer– Bends light as it enters the eye– Sodium pumps of the corneal endothelium on
the inner face help maintain the clarity of the cornea
– Numerous pain receptors contribute to blinking and tearing reflexes
Uvea (Vascularized)• Middle pigmented layer• Three regions: choroid, ciliary body, and
iris1.Choroid region
• Posterior portion of the uvea• Supplies blood to all layers of the eyeball• Brown pigment absorbs light to prevent visual
confusion
Uvea (Vascularized) 2
Ciliary body– Ring of tissue surrounding the lens– Smooth muscle bundles (ciliary
muscles) control lens shape– Capillaries of ciliary processes secrete
fluid – Ciliary zonule (suspensory ligament)
holds lens in position
Figure 15.13a
Lens
Invertedimage
Ciliary zonule
Ciliary muscle
Nearly parallel raysfrom distant object
(a) Lens is flattened for distant vision. Sympatheticinput relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.
Sympathetic activation
Figure 15.13b
Divergent raysfrom close object
(b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.
Invertedimage
Parasympathetic activation
Problems of Refraction• Myopia (nearsightedness)—focal point is in front of
the retina, e.g. in a longer than normal eyeball– Corrected with a concave lens
• Hyperopia (farsightedness)—focal point is behind the retina, e.g. in a shorter than normal eyeball– Corrected with a convex lens
• Astigmatism—caused by unequal curvatures in different parts of the cornea or lens– Corrected with cylindrically ground lenses, corneal
implants, or laser procedures
Figure 15.14 (1 of 3)
Focalplane
Focal point is on retina.
Emmetropic eye (normal)
Figure 15.14 (2 of 3)
Concave lens moves focalpoint further back.
Eyeballtoo long
UncorrectedFocal point is in front of retina.
Corrected
Myopic eye (nearsighted)
Figure 15.14 (3 of 3)
Eyeballtoo short
UncorrectedFocal point is behind retina.
CorrectedConvex lens moves focalpoint forward.
Hyperopic eye (farsighted)
Functional Anatomy of Photoreceptors
• Rods and cones– Outer segment of each contains visual
pigments (photopigments)—molecules that change shape as they absorb light
– Inner segment of each joins the cell body
Figure 15.15a
Process ofbipolar cell
Outer fiber
Apical microvillusDiscs containingvisual pigments
Melaningranules
Discs beingphagocytized Pigment cell nucleus
Inner fibers
Rod cell body
Cone cell body
Synaptic terminalsRod cell body
Nuclei
Mitochondria
Connectingcilia
Basal lamina (borderwith choroid)
The outer segments of rods and cones are embedded in the pigmented layer of the retina.
Pigm
ente
d la
yer
Out
er s
egm
ent
Inne
rse
gmen
t
Rods• Functional characteristics
– Very sensitive to dim light– Best suited for night vision and peripheral
vision– Perceived input is in gray tones only– Pathways converge, resulting in fuzzy and
indistinct images– “Night vision” (1/2 hour to establish)
Cones• Functional characteristics
– Need bright light for activation (have low sensitivity)
– Have one of three pigments that furnish a vividly colored view
– Non-converging pathways result in detailed, high-resolution vision
Figure 15.7
Maculalutea
Centralarteryand veinemergingfrom theoptic disc
Optic disc
Retina
Chemistry of Visual Pigments• Retinal
– Light-absorbing molecule that combines with one of four proteins (opsin) to form visual pigments
– Synthesized from vitamin A– Two isomers: 11-cis-retinal (bent form) and all-trans-
retinal (straight form)• Conversion of 11-cis-retinal to all-trans-retinal
initiates a chain of reactions leading to transmission of electrical impulses in the optic nerve
β-carotene• Why β-carotene? (Where have we seen it before?)
Lens• Biconvex, transparent, flexible, elastic, and avascular• Allows precise focusing of light on the retina• Cells of lens epithelium differentiate into lens fibers that
form the bulk of the lens• Lens fibers—cells filled with the transparent protein
crystallin• Lens becomes denser, more convex, and less elastic with
age• Cataracts (clouding of lens) occur as a consequence of
aging, diabetes mellitus, heavy smoking, and frequent exposure to intense sunlight
Figure 15.9
Figure 15.1a
Eyelashes
Sclera(covered byconjunctiva)
Site whereconjunctivamerges withcornea
Lateralcommissure
Iris
Medialcommissure
Lacrimalcaruncle
Eyelid
Eyelid
Eyebrow
Pupil
Palpebralfissure
(a) Surface anatomy of the right eye
Figure 15.1b
(b) Lateral view; some structures shown in sagittal section
Levator palpebraesuperioris muscleOrbicularis oculi muscleEyebrowTarsal platePalpebral conjunctivaTarsal glandsCornea
Palpebral fissure
EyelashesBulbar conjunctiva
Conjunctival sac
Orbicularis oculi muscle
Figure 15.2
Lacrimal glandExcretory ducts of lacrimal glands
Lacrimal punctumLacrimal canaliculus
Nasolacrimal duct
Inferior meatusof nasal cavityNostril
Lacrimal sac
How do we hear?• Gross Anatomy• Micro anatomy• Physics• What is sound?
Figure 15.29
Area ofhigh pressure(compressedmolecules)
Crest
Trough
Distance Amplitude
Area oflow pressure(rarefaction)
A struck tuning fork alternately compresses and rarefies the air molecules around it, creating alternate zones of high and low pressure.
(b) Sound waves radiate outward in all directions.
WavelengthAi
r pre
ssur
e
A pressure disturbance (alternating areas of high and low pressure) produced by a vibrating object: A sound wave
Moves outward in all directionsIs illustrated as an S-shaped curve or sine wave
Properties of Sound• Pitch
– Perception of different frequencies– Normal range is from 20–20,000 Hz
• Bass - Treble (Hertz =?)
– The higher the frequency, the higher the pitch• Loudness
– Subjective interpretation of sound intensity– Normal range is 0–120 decibels (dB)
Figure 15.31a
Scala tympani
Cochlear duct
Basilarmembrane
1 Sound waves vibratethe tympanic membrane. 2 Auditory ossicles vibrate.
Pressure is amplified.
3 Pressure waves created bythe stapes pushing on the oval window move through fluid in the scala vestibuli.
Sounds with frequenciesbelow hearing travel through the helicotrema and do not excite hair cells. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.
Malleus Incus
Auditory ossicles
Stapes
Ovalwindow
Scala vestibuliHelicotrema
Cochlear nerve
32
1
Roundwindow
Tympanicmembrane
(a) Route of sound waves through the ear
Figure 15.27
Anterior
Semicircularducts insemicircularcanals
PosteriorLateral
Cristae ampullaresin the membranousampullaeUtricle investibuleSaccule investibule Stapes in
oval window
Temporalbone
Facial nerve
Vestibularnerve
Superior vestibular ganglion
Inferior vestibular ganglion
CochlearnerveMaculaeSpiral organ(of Corti)Cochlearductin cochlea
Roundwindow
Figure 15.33
Medial geniculatenucleus of thalamus
Primary auditorycortex in temporal lobeInferior colliculusLateral lemniscusSuperior olivary nucleus(pons-medulla junction)
Spiral organ (of Corti)Bipolar cell
Spiral ganglion of cochlear nerve
Vestibulocochlear nerveMedulla
Midbrain
Cochlear nuclei
Vibrations
Vibrations
Where is the Vestibulocochlear nerve?
Homeostatic Imbalances of Hearing
• Conduction deafness– Blocked sound conduction to the fluids of the
internal ear• Can result from impacted earwax, perforated
eardrum, or otosclerosis of the ossicles• Sensorineural deafness
– Damage to the neural structures at any point from the cochlear hair cells to the auditory cortical cells
Homeostatic Imbalances of Hearing
• Tinnitus: ringing or clicking sound in the ears in the absence of auditory stimuli– Due to cochlear nerve degeneration,
inflammation of middle or internal ears, side effects of aspirin
• Meniere’s syndrome: labyrinth disorder that affects the cochlea and the semicircular canals– Causes vertigo, nausea, and vomiting
Figure 15.34
Macula ofsaccule
Otoliths
Hair bundle
KinociliumStereocilia
Otolithicmembrane
Vestibularnerve fibers
Hair cellsSupportingcells
Macula ofutricle
Figure 15.36a–b
Fibers of vestibular nerve
Hair bundle (kinociliumplus stereocilia)
Hair cell
Supportingcell
Membranouslabyrinth
Cristaampullaris
Cristaampullaris
Endolymph
Cupula
Cupula
(a) Anatomy of a crista ampullaris in a semicircular canal
(b) Scanning electron micrograph of a crista ampullaris (200x)
Figure 15.36c
Fibers ofvestibularnerve
At rest, the cupula standsupright.
Section ofampulla,filled withendolymph
(c) Movement of the cupula during rotational acceleration and deceleration
Cupula Flow of endolymph
During rotational acceleration,endolymph moves inside thesemicircular canals in thedirection opposite the rotation(it lags behind due to inertia).Endolymph flow bends thecupula and excites the haircells.
As rotational movementslows, endolymph keepsmoving in the directionof the rotation, bendingthe cupula in theopposite direction fromacceleration andinhibiting the hair cells.