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Special Sense
Olfaction (sense of smell)
• Olfaction is a chemical sense
• It is the sense least understood
• Olfactory nerve fibers go to the cortex
association areas
• We have a better memory for olfaction
than any other sense
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Histology of the olfactory apparatus
• There are millions of receptors
• Located on the surface of the superior and
middle nasal concha
• Cover an area of 5 square centimeters
• There are three types of cells involved
The 3 types of cells involved
(1) Olfactory receptors bipolar neurons
with olfactory hairs. They interact with
chemicals to produce action potentials
(2) Support cells columnar epithelium
(3) Basal cells stem cells that divide by
mitosis to produce new receptor cells
Olfactory Receptors
• Bipolar neurons
• Dendrite is an olfactory hair where odors
or received and action potentials are
generated
• Can reproduce through mitosis
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Support Cells
• Simple Columnar Epithelium
• Hold olfactory receptors in place and
protects them
Basal Cells
• Stem cells that can divide by mitosis to
produce new olfactory receptors
Bowman’s Glands
• Produce mucous
• Dissolves gases with odor causing
molecules
• Moistens olfactory epithelium
• Washes away old odors so stimulation
does not continue
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Physiology of Olfaction
• Several to hundreds of primary scents
• Different smells are related to different
combinations of receptors being
stimulated at the same time
• Molecules to be scented must be water
soluble to dissolve in fluid around receptor
and lipid soluble to dissolve in the cell
membrane of the receptor cell
How an action potential is
generated in an olfactory cell
• A molecule dissolves in the watery
mucous solution around the hair cells and
comes in contact with a receptor protein
on the plasma membrane
• An action potential is generated in the
olfactory nerve and transmits the
information to the brain
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Adaptation to Olfaction
• Initial stimulus to any odor quickly fades
- decreases 50% in the first second
- complete insensitivity in one minute
Gustatory Sensation (taste)
• Enhances olfaction
• Any substance that will stimulate taste
receptors will also stimulate olfactory
receptors a 1000 times more
Histology of Gustatory Receptors
• Receptors located in taste buds of which
there are over 50 containing over 10,000
receptors
• Most are located on the tongue but are
also found on the soft palate, larynx, and
pharynx
• There are three types of cells involved in
taste
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The three types of gustatory cells
(1) Support cells form a capsule around
about 50 taste buds
(2) Gustatory cells have hairs where taste
occurs
(3) Basal cells produce support cells which
in turn become receptor cells
Taste buds are located in papillae
Physiology of Gustation
• Molecule to be tasted must be dissolved in
saliva
• When it makes contact with the receptor
an action potential is initiated and a
message is sent to the brain
• There are six primary tastes – salty, sour,
bitter, sweet, meat broth, and water
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Adaptation to Gustation
• Takes 1 to 5 minutes
Visual Sensations
• Accessory structures of the eye include:
(1) Eyelids
(2) Eyelashes
(3) Conjunctiva
(4) Lacrimal apparatus
Eyelids
• Act as a light and physical barrier
• Spread lacrimal fluid over the cornea
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Eyelashes
• Protect the eye from dust and sweat
• Have sebaceous glands that lubricate the
eyelid for blinking (when infected called a
sty)
• Have their own mites to eat dead skin cells
Conjunctiva
• Is a layer of stratified squamous epithelium
that lines the insides of the eyelids and
covers the surface of the cornea
• It protects the cornea and forms pockets
on all sides of the eye to prevent anything
from entering behind the eye
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Lacrimal Apparatus (tear glands)
• Secretes lacrimal fluid that lubricates and
moistens the eye
• Contains antibacterial substance
(lysozyme) that protects the outer eye
from infection
Anatomy of the Eye
• The eye contains three distinct layers
(called tunics)
(1) Fibrous tunic
(2) Vascular tunic
(3) Nervous tunic (retina)
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Fibrous tunic
• Makes up the Sclera and the Cornea
• Is composed of dense connective tissue
Vascular Tunic
• Has three portions:
(1) Choroid
(2) Ciliary process
(3) Iris
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Choroid
• Supplies nutrients (in blood) to the retina
• Is darkly colored to absorb light
Ciliary Process
• Secretes aqueous humor into the anterior
cavity (between the cornea and the lens)
and helps to maintain the shape of the eye
• Contains the ciliary muscle that changes
the shape of the lens to accommodate for
far and near vision
Iris
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Colored
Two muscular layers
Hole in center called the Pupil
Innervated by:
- parasympathetic N.S. contracts circular
muscle which decreases the pupil
- sympathetic N.S. contracts the radial
muscle which increases the pupil
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Notes on the preceding slide
• Note: the figure above illustrates the
dilation and constriction of the pupil. In
dim light, the radially arranged smooth
muscle fibers are stimulated to contract by
sympathetic stimulation, dilating the pupil.
In bright light, the circularly arranged
smooth muscle fibers are stimulated to
contract by parasympathetic stimulation,
constricting the pupil.
Nervous Tunic (retina)
• Where the optic nerve enters into the eye
the retina is non-functional. It appears as
a disc at the back of the eye and is
referred to as the optic disc or blind spot
• The rest of the retina is divided into two
major portions:
(1) The Pigmented Layer
(2) The Optical portion
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The Pigmented Layer of the retina
• This is a nonoptical black layer that
absorbs incoming light and prevents
reflection
The Optical portion of the retina
• Is responsible for vision
• Has 9 layers
• 6 million cones and 120 million rods are
found in the innermost layer
- rods = vision in dim light
- cones = color vision in bright light –
concentrated in the fovea centralis
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Fovea Centralis (Central fovea)
• Found inside a depression called the
macula lutea
• On the visual axis where light enters the
eye
• Area of acute vision because neurons are
bent out of the way allowing light to reach
the cones in the bottom layer
Notice on the photo below the
macula lutea in the center with the
fovea in the middle.
A light micrograph of the fovea
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The Lens
• The function of the lens is to focus the light
image on the fovea and make adjustments
for distance (accommodation)
Convergence
• Called “single binocular vision”
• As an object moves closer the pupils move
closer together (as the eyes rotate
inwards) to maintain depth of field
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Physiology of Vision
• Photoreceptors (rods and cones)
transduce light stimuli into a receptor
potential which is relayed to the bipolar
neurons where an action potential is
initiated and a message is sent to the
visual portion of the brain (in the occipital
lobe)
Color Vision
• The Trichromatic theory of color vision:
- there are three types of cones, blue,
green and red
Color perception is dependent on the
relative degree to which each cone is
stimulated by a wavelength of light that
hits it.
Auditory Sense (Hearing)
• The ear is divided up into three major
regions:
(1) The External ear
(2) The Middle ear
(3) The Inner ear Tympanic Membrane
to vibrate
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The External Ear
• The Auricle (pinna) collects sound waves
and directs them into the External Auditory
Canal
• The sound waves cause the Tympanic
Membrane to vibrate
• This vibration is transmitted through the
ear bones to the inner ear
The Middle Ear
• This is an air filled cavity
• Connects via the Eustachian (Auditory)
tube with the nasopharynx
• Contains the 3 auditory ossicles (ear
bones) that connect the tympanic
membrane to the oval window of the inner
ear
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Auditory Ossicles (ear bones)
•
Transmit the vibration from the tympanic
membrane to the oval window of the
inner ear. There are three bones:
(1) Malleous (hammer) attaches directly to
the tympanic membrane
(2) Incus (anvil) connects the malleous to
the stapes
(3) Stapes attaches directly to the oval
window
The Oval Window
• Is located under the stapes
• It transmits vibrations from the stapes into
the endolymph of the inner ear
The Round Window
• Connects the middle ear cavity to the inner
ear
• Absorbs vibrations in the inner ear
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The Inner Ear
• The Inner ear has two divisions:
(1) An outer bony Osseous Labyrinth
(2) An inner Membranous Labyrinth
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The Bony (Osseous) Labyrinth
• Consists of three areas:
(1) The Semicircular Canals
(2) The Vestibule
(3) The Cochlea
Semicircular Canals
• There are 3 semicircular canal
• Each canal has a swelling at is end called
an ampulla
• Each ampulla contains an organ of
equilibrium called the crista ampullaris
• The membranous portion inside is called
the semicircular duct
Crista Ampullaris
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The Vestibule
• Contains the Utricle and Saccule
• Organs of both static and dynamic
equilibrium
Utricle and Saccule
Cochlea
• Contains the Organ of Corti (the organ of
hearing)
• Consists of canals spiraling around a
modiolus
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Organ of Corti
• Sits on the basilar membrane
• Is the organ of hearing
• Has 16,000 hair cells that synapse with
the cochlear nerve
Organ of Corti
Organ of Corti
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Physiology of Hearing
• Auricle collects the sound waves and reflects
them down the external auditory meatus toward
the tympanic membrane
• The tympanic membrane vibrates
• The vibration is transmitted to the oval window
by the malleus, incus, and stapes
• The oval window vibrates causing the hair cells
of the Organ of Corti to vibrate
• The vibration causes the round window to bulge
outward and absorb the sound
• An action potential develops in the cochlear
nerve
Physiology of Equilibrium
• Two types of equilibrium – Dynamic and Static
• Utricle and Saccule function for both static and
dynamic equilibrium
• Have macula perpendicular to each other
• Each has hair cells
• Each mucula has otoliths which pull the macula
down due to gravity when the head is bent
• Bent hair cells produce an a receptor potential
• Action potential develops in the vestibular nerve
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Semicircular Ducts
• Functions for dynamic equilibrium only
• Each duct has a crista with hair cells
• When endolymph is set in motion it bends
the crista which bends the hair cells
• A receptor potential develops
• An action potential develops in the
vestibular nerve
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