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.