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Chapter 22
Special Senses
Copyright 2009 John Wiley & Sons, Inc.
Olfaction: Sense of Smell
Olfactory epithelium- 10–100 million receptors
Superior nasal cavity, inferior surface of the cribriform plate and
extending along the superior nasal concha.
Cell Types:
Olfactory receptors respond to chemical stimulation
(odorant) by initiating the olfactory response.
Olfactory Hairs (cilia) –end of receptors
Supporting cells of the mucous membrane line the
nose.
Physically support, nourish, and electrically insulate the
olfactory receptors, and help detoxify chemicals.
Basal cells continually undergo cell division to
produce new olfactory receptors
Olfactory (Bowman’s) glands produce mucus.
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Olfactory Receptors
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The Olfactory Pathway
Olfactory (I) nerves- right and left -unmyelinated axons
Terminate in the brain in paired masses of gray matter- olfactory
bulbs,
Axons of olfactory bulb neurons extend posteriorly and form the
olfactory tract.
Olfactory sensations are the only sensations that reach the
cerebral cortex without first synapsing in the thalamus.
Project to the primary olfactory area
located at the inferior and medial surface of the temporal lobe
and frontal area
Projects to the limbic system and hypothalamus
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Olfactory Pathway
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Gustation
Taste or gustation, is a chemical sense.
Five primary tastes:
sour, sweet, bitter, salty, and umami.
All other flavors, are combinations of the five
primary tastes, plus accompanying olfactory
and tactile sensations
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Anatomy of Gustatory Receptors
Taste buds- receptors for taste
10,000 taste buds on the young adult ‘s tongue, soft palate,
pharynx, and epiglottis. Declines with age.
Consisting of three kinds of epithelial cells:
supporting cells, gustatory receptor cells, and basal cells.
Supporting cells contain microvilli
1 Gustatory hair, projects from each gustatory receptor
cell to the external surface through the taste pore
(opening in the taste bud).
Basal cells produce supporting cells that develop into
gustatory receptor cells
Surround 50 gustatory receptor cells.
Have a life span of about 10 days
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Gustatory Receptors
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Taste buds
Papilla- elevations on the tongue.
Three types of papillae contain taste buds:
1.
Circular vallate papillae- form an inverted V-shaped row at
the back of the tongue.
2.
Fungiform papillae- mushroom-shaped elevations
scattered over the entire surface of the tongue, contain
about five taste buds each.
3.
Foliate papillae- located in small trenches on the lateral
margins of the tongue, but most of their taste buds
degenerate in early childhood.
Filiform papillae- contain tactile receptors but no taste buds.
They increase friction between the tongue and food, making
it easier for the tongue to move food in the oral cavity.
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The Gustatory Pathway
Tastant- chemicals dissolved in saliva that stimulate
gustatory hairs resulting in action potentials.
Three cranial nerves contain axons of sensory neurons
from taste buds .
The facial (VII) nerve- anterior two-thirds of the tongue
The glossopharyngeal (IX) nerve-posterior one-third of the tongue
The vagus (X) nerve- throat and epiglottis.
To Gustatory nucleus in the medulla oblongata to:
Limbic system and hypothalamus
Thalamus to Parietal lobe
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Vision: Accessory Structures of the
Eye
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Eyelids
Palpebrae- eyelids
protect the eyes from excessive light and foreign
objects, and lubricate the eyeballs.
The upper eyelid is more movable than the lower
Contains the levator palpebrae superioris muscle.
Palpebral fissure- space between the upper and lower
eyelids that exposes the eyeball.
The angles are the lateral commissure, and medial
commissure.
Lacrimal caruncle contains sebaceous (oil) glands and
sudoriferous (sweat) glands.
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Eyelids
Tarsal plate- thick fold of connective tissue that gives
form and support to the eyelids.
Tarsal or Meibomian glands- secrete fluid that helps
keep the eyelids from adhering to each other.
Conjunctiva- thin, protective mucous membrane
composed of nonkeratinized stratified columnar
epithelium with goblet cells that is supported by areolar
connective tissue.
Palpebral conjunctiva- lines the inner aspect of the
eyelids
Bulbar conjunctiva passes from the eyelids onto the
surface of the eyeball.
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Eyelashes and Eyebrows
Eyelashes and eyebrows- help protect the eyeballs
from foreign objects, perspiration, and direct rays of the
sun.
Sebaceous ciliary glands- at the base of the hair
follicles of the eyelashes
Release a lubricating fluid into the follicles.
Infection of these glands, by bacteria cause a painful
pus-filled swelling called a sty.
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The Lacrimal Apparatus
Lacrimal apparatus produces and drains lacrimal fluid
or tears.
Lacrimal ducts empty tears onto the surface of the
conjunctiva of the upper lid
Supplied by parasympathetic fibers of the facial (VII)
nerves.
Tears pass medially over the anterior surface of the
eyeball to enter the lacrimal puncta.
Tears then pass into the lacrimal canals, which lead
into the lacrimal sac and the nasolacrimal duct.
This duct carries the lacrimal fluid into the nasal cavity just
inferior to the inferior nasal concha.
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Extrinsic Muscles
Six muscles:
Superior Rectus
Inferior Rectus
Lateral Rectus
Medial Rectus
Superior Oblique
Inferior Oblique
Innervated by Cranial Nerves III, IV, VI
Periorbital fat- surrounds the orbit
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Layers of the Eye Wall
Fibrous Tunic
Vascular Tunic
Retina
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Anatomy of the Eyeball
Fibrous Tunic - outer layer of the eye, strong dense collagenous
connective tissue layer.
Sclera- forms the white part of the eye.
Provides protection, maintains the shape of the eyeball and the
site of muscle attachment for the extrinsic muscles of the eyes.
Cornea- anterior surface of the outer tunic becomes transparent.
Both the sclera and cornea consist of dense collagenous connective
tissue
The sclera is opaque.
The cornea is transparent and avascular and has regularly
spaced collagen fibers that are smaller than the wavelength of
light.
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Anatomy of the Eyeball
Vascular Tunic- middle layer of the eyeball.
It is composed of three parts: choroid, ciliary body,
and iris.
Choroid- highly vascularized, provides nutrients to the
posterior surface of the retina.
Contains melanocytes- melanin in the choroid absorbs stray
light rays, which prevents reflection and scattering of light
within the eyeball.
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Anatomy of the Eyeball
Ciliary body- anterior portion of the vascular tunic, from the
choroid
Consists of ciliary processes and ciliary muscle.
Ciliary processes contain blood capillaries that secrete
aqueous humor.
Extending from the ciliary process are zonular fibers
(suspensory ligaments) that attach to the lens.
Ciliary muscle are arranged longitudinal, oblique, and
circularly.
Contraction or relaxation of the ciliary muscle changes
the tightness of the zonular fibers, which alters the
shape of the lens, adapting it for near or far vision.
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Anatomy of the Eyeball
Iris- colored portion of the eyeball, and its opening is called
the pupil.
Pupil consists of melanocytes and circular and radial
smooth muscle fibers.
The two distinct fiber arrangements of the smooth muscle
form the:
sphincter pupillae muscle- circularly oriented muscle
fibers at the pupillary boundary
dilator pupillae muscle- attaches to the circumference of
the sphincter pupillae and projects like the spokes of a
wheel toward the base of the iris.
A principal function of the iris is to regulate the amount of light
entering the eyeball through the pupil.
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Anatomy of the Eyeball
Optic disc- site where the optic (II) nerve exits the eyeball.
Bundled together with the optic nerve are the central retinal
artery, a branch of the ophthalmic artery, and the central
retinal vein.
The retina consists of a pigmented layer and a neural layer.
The pigmented layer is a sheet of melanin-containing
epithelial cells.
The melanin in the pigmented layer of the retina, helps to
absorb stray light rays.
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Neural Sensory Layer of Retina
Neural (sensory) layer of the retina is a multilayered outgrowth of
the brain.
Three distinct layers of retinal neurons—
photoreceptor layer, bipolar cell layer, and ganglion cell
layer.
Separated by two zones, outer and inner synaptic layers.
Light passes through the ganglion and bipolar cell layers and both
synaptic layers before it reaches the photoreceptor layer.
Two other types of cells in the bipolar cell layer of the retina are
called horizontal cells and amacrine cells.
Two types of photoreceptors: rods and cones.
Rods allow us to see in dim light.
Bright lights stimulate the cones, which produce color vision.
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Macula Lutea
Macula lutea- in the exact center of the posterior portion
of the retina, at the visual axis of the eye.
Fovea centralis- small depression in the center of the
macula lutea, contains only cones.
Area of highest visual acuity or resolution (sharpness
of vision).
The optic disc is also called the blind spot.
Cranial nerve II enters
Contains no rods or cones
Cannot see an image that strikes the blind spot
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Lens
Within the cells of the
lens, proteins called
crystallins, arranged
like the layers of an
onion, make up the
refractive media of the
lens.
Surrounded by a
connective tissue
capsule
Cataracts- cloudy lens
due to changes in
proteins
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Anatomy of the Eyeball
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Interior of the Eye
Interior of the eye- two fluid-filled cavities, separated by the lens,
permits light to pass through the eye from the cornea to the retina.
Anterior cavity- between the cornea and lens
Contains a clear watery fluid- aqueous humor.
Consists of two chambers.
Anterior chamber- between the cornea and the iris
Posterior chamber- behind the iris and in front of the zonular
fibers and lens.
Aqueous humor continually filters out of blood capillaries in the
ciliary processes of the ciliary body, enters the posterior chamber.
Flows forward between the iris and the lens, through the pupil, and
into the anterior chamber.
Aqueous humor drains into the scleral venous sinus (canal of
Schlemm) and then into the blood.
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Interior of the Eye
Vitreous chamber- Posterior cavity between the lens
and retina
Vitreous body- jelly-like substance
Maintains the shape of the eyeball
Outer layers form the Zonulafr fiber- support the
lens
Hold the retina against the choroid
Intraorbital Pressure- from aqueous humor and
vitreous body
16 mmHg
Glaucoma- increased aqueous humor production
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The Visual Pathway
Processing of Visual Input in the Retina
In the retina, input from several cells converge on a smaller
number of postsynaptic neurons or diverge to a larger
number.
Convergence predominates: 1 million ganglion cells receive
input from about 126 million photoreceptor cells.
Chemicals (neurotransmitters) released by rods and cones
induce changes in both bipolar cells and horizontal cells
that lead to the generation of nerve impulses.
Amacrine cells synapse with and transmit signals to
ganglion cells, which then initiate nerve impulses.
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Pathway in the Brain
Optic chiasm- crossing point of the optic nerve axons.
Become part of the optic tract, enter the brain.
Terminate in the lateral geniculate nucleus of the
thalamus.
Synapse with neurons form optic radiations, which
project to the primary visual areas in the occipital lobes
of the cerebral cortex.
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Visual Pathway in the Brain
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Hearing and Equilibrium
Anatomy of the Ear:
The ear is divided into three main regions:
External ear- collects sound waves and channels
them inward;
Middle ear- conveys sound vibrations to the oval
window; and
Internal ear- houses the receptors for hearing and
equilibrium
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The External Ear
Auricle (pinna) made of elastic cartilage
Helix and lobe
External auditory canal- in the temporal bone
Contains ceruminous glands and hair
Tympanic membrane- semitransparent membrane
between the external auditory canal and middle ear.
Fibrocartilage ring attached to the temporal bone and
across a three layered window
Umbo- apex of the tympanic membrane
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Anatomy of the Ear
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The Middle Ear
Small, air-filled cavity in the temporal bone that is lined by epithelium
Auditory ossicles- connected to one another by synovial joints and
attached by ligaments.
Three ossicles: malleus, incus, and stapes
Head of the malleus articulates with the incus.
Incus articulates with the stapes
Footplate of the stapes fits into the oval window (fenestra
vestibuli).
Below the oval window is the round window (fenestra cochlea).
Below the round window is a secondary tympanic membrane
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The Middle Ear
Tensor tympani muscle limits
movement and increases
tension on the eardrum.
Stapedius muscle dampens
large vibrations of the stapes
due to loud noises, to protects
the oval window.
Auditory tube
(pharyngotympanic tube),
commonly known as the
eustachian tube connects the
middle ear with the
nasopharynx.
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Internal Ear (Labyrinth)
Structurally two main divisions:
The outer bony labyrinth that encloses an inner
membranous labyrinth.
Bony labyrinth- lined with periosteum and contains
perilymph.
Membranous labyrinth- contains endolymph
The three main areas:
(1) the semicircular canals
(2) the vestibule,
(3) the cochlea, receptors for hearing.
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Internal Ear
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The Cochlea
Cochlea- bony spiral canal divided into three
channels:
Cochlear duct or scala media- filled with
endolymph.
Scala vestibuli ends at the oval window.
Scala tympani ends at the round window.
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Organ of Corti
Vestibular membrane separates the cochlear
duct from the scala vestibuli
Basilar membrane separates the cochlear
duct from the scala tympani.
Basilar membrane contains the Organ of Corti
(Spiral Organ)
Contains receptors for hearing; hair cells and
supporting cells.
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Mechanism of Hearing
Auricle directs sound to the tympanic membrane which
vibrates against the malleus, incus, and stapes which
vibrates against the membrane that covers the oval
window.
Vibrations initiate fluid pressure waves in the perilymph
of the scala vestibuli; transmitted to the scala tympani
and to the round window.
Pressure waves in the endolymph of the cochlear duct
cause the basilar membrane to vibrate and the hair
cells move against the tectorial membrane; bending of
the stereocilia results in nerve impulses being
generated in cochlear nerve fibers (CN VIII).
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Mechanism of Hearing
Transmission proceeds to the medulla
and to either:
Inferior colliculus of the midbrain
Superior olivary nucleus in the pons
followed by the Inferior colliculus
Both ascend to the thalamus and then the
auditory center in the temporal lobe
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Mechanism of Equilibrium
The receptor organs for equilibrium of the vestibular
apparatus includes the saccule, utricle, and
semicircular ducts.
Two types of equilibrium:
Static equilibrium- maintenance of the position of the
body relative to the force of gravity.
Dynamic equilibrium- maintenance of body position in
response to sudden movements.
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Saccule and Utricle
Otolithic Organs: Saccule and Utricle
Walls of both contain maculae- receptors for static equilibrium and
contribute to dynamic equilibrium
Maculae consist of two kinds of cells:
Hair cells- the sensory receptors
Have one stereocilia of graded height, plus one kinocilium,
anchored to the basal body and extending beyond the longest
stereocilium.
Supporting cells- columnar cells
Secrete otolithiuc membrane- glycoprotein layer over the hair cells
Otoliths- calcium carbonate crystals embedded in the membrane
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Maculae of the Right Ear
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Semicircular ducts
Three semicircular ducts function in dynamic equilibrium.
Lie at right angles to one another in three planes.
Permits detection of rotational acceleration or
deceleration in the ampulla at the crista.
The crista contains hair cells and supporting cells
covered by gelatinous material called the culopa.
Bending of the hair bundles produces responses that
lead to nerve impulses. that pass along the ampullary
nerve.
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Semicircular Ducts
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Equilibrium Pathway
Nerve impusles pass along the ampullary nerve
to the Vestibulocochlear nerve (CN VIII) to the
medulla and pons, and then to the cortex
CN III, IV, VI, and XI coordinate information
Cerebellum- maintains static and dynamic
equilibrium
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Equilibrium Pathway
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