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8/6/2019 BIO 201-The Special Senses
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Chapter 15
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Eye and Associated Structuresy 70% of all sensory receptors are in the eye
y Most of the eye is protected by a cushion of fat and the
bony orbity Accessory structures include eyebrows, eyelids,
conjunctiva, lacrimal apparatus, and extrinsic eyemuscles
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Eyebrowsy Coarse hairs that overlie the supraorbital margins
y Functions include;
y Shading the eyey Preventing perspiration from reaching the eye
y Orbicularis muscledepresses the eyebrows
y Corrugator musclemove the eyebrows medially
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Palpebrae (Eyelids)y Protect the eye anteriorly
y Palpebral fissureseparates eyelids
y Canthimedial and lateral angles (commissures)y Lacrimal carunclecontains glands that secrete a
whitish, oily secretion (Sandmans eye sand)
y Tarsal plates of connective tissue support the eyelids
internally
y Levator palpebrae superiorisgives the upper eyelidmobility
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Palpebrae (Eyelids)y E yelashes
y Project from the free margin of each eyelid
y Initiate reflex blinkingy Lubricating glands associated with the eyelids
y Meibomian glands and sebaceous glands
y Ciliary glands lie between the hair follicles
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Palpebrae (Eyelids)
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Conjunctivay Transparent membrane that:
y Lines the eyelids as the palpebral conjunctiva
y Covers the whites of the eyes as the ocular conjunctivay Lubricates and protects the eye
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Lacrimal Apparatusy Consists of the lacrimal gland and associated ducts
y Lacrimal glands secrete tears
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Tearsy Contain mucus, antibodies, and lysozyme
y Enter the eye via superolateral excretory ducts
y Exit the eye medially via the lacrimal punctumy Drain into the nasolacrimal duct
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Lacrimal Apparatus
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Extrinsic Eye Musclesy Six straplike extrinsic eye muscles
y Enable the eye to follow moving objects
y Maintain the shape of the eyebally Four rectus muscles originate from the annular ring
y Two oblique muscles move the eye in the vertical plane
y Superiordownward laterally
y Inferiorupward laterally
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Extrinsic Eye Muscles
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Summary of Cranial Nerves and
Muscle Actionsy Names, actions, and cranial nerve innervation of the
extrinsic eye muscles
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Structure of the Eyebally A slightly irregular hollow sphere with anterior and
posterior poles
yThe wall is composed of three tunicsy Fibrous
y Vascular
y Sensory
y The internal cavity is filled with fluids called humorsy The lens separates the internal cavity into anterior and
posterior segments
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Structure of the Eyeball
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Fibrous Tunicy Forms the outermost coat of the eye and is composed
of:
y
Opaque sclera (posteriorly)y Clear cornea (anteriorly)
y The sclera protects the eye and anchors extrinsicmuscles
y The cornea lets light enter the eye
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Vascular Tunic: Ciliary Bodyy A thickened ring of tissue surrounding the lens
y Composed of smooth muscle bundles (ciliary muscles)
y Anchors the suspensory ligament that holds the lensin place
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Vascular Tunic: Irisy The colored part of the eye
y Pupilcentral opening of the iris
y Regulates the amount of light entering the eye
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Pupily Close vision and bright lightpupils constrict
y Distant vision and dim lightpupils dilate
y Changes in emotional statepupils dilate when thesubject matter is appealing or requires problem-solving skills
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Pupil Dilation and Constriction
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Sensory Tunic: Retinay A delicate 2-layered membrane
y Pigmented layerthe outer layer that absorbs light
and prevents its scatteringy Neural layer which contains:
y Photoreceptors that transduce light energy
y Bipolar cells and ganglion cells
y Amacrine and horizontal cells
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Sensory Tunic: Retina
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The Retina: Ganglion Cells and the
Optic Disc
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The Retina: Photoreceptorsy Rods:
y Respond to dim light
y
Are used for peripheral visiony Cones:
y Respond to bright light
y Have high-acuity color vision
y Are found in the macula luteay Are concentrated in the fovea centralis
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Blood Supply to the Retinay The neural retinal receives its blood supply from two
sources:
y
The outer third receives its blood from the choroidy The inner two-thirds is served by the central artery and
vein
y Small vessels radiate out from the optic disc and can be
seen with an ophthalmoscope
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Lensy A biconvex, transparent, flexible, avascular structure
that:
y
Allows precise focusing of light onto the retinay Is composed of epithelium and lens fibers
y Lens epitheliumanterior cells that differentiate intolens fibers
y Lens fiberscells filled with the transparent proteincrystallin
y With age, the lens becomes more compact and dense,and it loses its elasticity
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Chemical Sensesy Gustation (taste) and olfaction (smell)
y Their chemoreceptors respond to chemicals in
aqueous solutiony Tasteresponse to substances dissolved in saliva
y Smellresponse to substances dissolved in f luids of thenasal membranes
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Sense of Smelly The organ of smell is the olfactory epithelium, which
covers the superior nasal conchae
y
Olfactory receptor cells are bipolar neurons withradiating olfactory cilia
y Olfactory receptors are surrounded and cushioned by supporting cells
y Basal cells lie at the base of the epithelium
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Olfactory Receptors
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Taste Budsy Most of the 10,000 or so taste buds are found on the
tongue
y
Taste buds are found in papillae of the tongue mucosay Papillae come in three types: filiform, fungiform, and
circumvallate
y Fungiform and circumvallate papillae contain taste
buds
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Taste Buds
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Structure of a Taste Budy Gourd-shaped
y Consists of 3 major cell types:
y Supporting cellsinsulate the receptory Basal cellsdynamic stem cells
y Gustatory cellstaste cells
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Taste Sensationsy Sweetsugars, saccharin, alcohol, and some amino
acids
y
Saltmetal ionsy Sourhydrogen ions
y Bitteralkaloids such as quinine and nicotine
y Umamielicited by the amino acid glutamate
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Physiology of Tastey In order to be tasted, a chemical:
y Must be dissolved in saliva
y
Must contact gustatory hairs
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Influence of Other Sensations on Tastey Taste is 80% smell
y Thermoreceptors, mechanoreceptors, and nociceptors
also influence tastesy Temperature and texture enhance or detract from taste
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Hearing and Balance
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The Eary Three parts of the ear:
y Inner ear
y
Outer eary Middle ear
y Outer and middle ear are involved with hearing
y Inner ear functions in both hearing and equilibrium
y Receptors for hearing and balance:y Respond to separate stimuli
y Are activated independently
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Outer Eary The auricle (pinna) is composed of:
y The helix (rim)
y
The lobule (earlobe)y External auditory canal
y Short, curved tube filled with ceruminous glands
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Outer Eary Tympanic membrane (eardrum)
y Thin connective tissue membrane that vibrates inresponse to sound
y Transfers sound energy to the middle ear ossicles
y Boundary between outer and middle ears
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Middle Ear (Tympanic Cavity)y A small, air-filled, mucosa-lined cavity
y Flanked laterally by the eardrum
y
Flanked medially by the oval and round windowsy Epitympanic recesssuperior portion of the middle
ear
y Pharyngotympanic tubeconnects the middle ear to
the nasopharynxy Equalizes pressure in the middle ear cavity with the
external ear pressure
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Ear Ossicles
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Inner Eary Bony labyrinth
y Tortuous channels worming their way through thetemporal bone
y Contains the vestibule, the cochlea, and the semi-circular canals
y Filled with perilymph
y
Membranous labyrinthy Series of membranous sacs within the bony labyrinth
y Filled with a potassium-rich fluid
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Inner Ear
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The Vestibule
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The Semicircular Canalsy Three canals that each define two-thirds of a circle and
lie in the three planes of space
y
Membranous semicircular ducts line each canal andcommunicate with the utricle
y The ampulla is the swollen end of each canal andhouses equilibrium receptors in a region called the
crista ampullarisy These receptors respond to angular movements of the
head
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The Semicircular Canals
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The Cochleay A spiral, conical, bony chamber that:
y Extends from the anterior vestibule
y
Coils around a bony pillar called the modiolusy Contains the cochlear duct, which ends at the cochlear
apex
y Contains the organ of Corti (hearing receptor)
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The Cochleay The cochlea is divided into three chambers:
y Scala vestibulioval window
y
Scala mediacochlear ducty Scala tympaniround window
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Sound and Mechanisms of Hearingy Sound vibrations beat against the eardrum
y The eardrum pushes against the ossicles, which
presses fluid in the inner ear against the oval andround windows
y This movement sets up shearing forces that pull onhair cells
y
Moving hair cells stimulates the cochlear nerve thatsends impulses to the brain
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Deafnessy Conduction deafnesssomething hampers sound
conduction to the fluids of the inner ear
y E.g., impacted earwax, perforated eardrum,osteosclerosis of the ossicles
y Sensorineural deafnessresults from damage to theneural structures at any point from the cochlear haircells to the auditory cortical cells
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Deafnessy Tinnitusringing or clicking sound in the ears in the
absence of auditory stimuli
y
Menieres syndromelabyrinth disorder that affectsthe cochlea and the semicircular canals
y Causes vertigo, nausea, and vomiting
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Developmental Aspectsy All special senses are functional at birth
y Chemical sensesfew problems occur until the fourthdecade when these senses begin to decline
y Vision is not fully functional at birthy Babies are hyperopic, see only gray tones, and eye
movements are uncoordinated
y Depth perception and color vision is well developed by
age fivey With age the lens loses clarity, dilator muscles are less
efficient, and visual acuity is drastically decreased by age70