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Chapter 12
Somatic and Special Senses
Somatic (General) Senses
• receptors that are widely distributed throughout the
body
• skin, various organs and joints
Special Senses
• specialized receptors confied to structures in the
head
• eyes, ears, nose, mouth
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Senses
Sensory Receptors
• specialized cells or multicellular structures that collect
information from the environment
• stimulate neurons to send impulses along sensory fibers to
the brain
Sensation
• a feeling that occurs when brain becomes aware of sensory
impulse
Perception
• a person’s view of the stimulus; the way the brain
interprets the information
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Pathways From Sensation to Perception
(Example of an Apple)
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Receptor Types - General
Chemoreceptors
• respond to changes in chemical concentrations
Pain receptors
• respond to tissue damage
Thermoreceptors
• respond to changes in temperature
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• Mechanoreceptors
• respond to mechanical forces
proprioceptors –tensions of muscles and tendons
baroreceptors – blood pressure
stretch receptors – inflation in lungs
Photoreceptors
• respond to light
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Sensory Impulses
•Stimulation causes a change in membrane potential
•Nerve fiberSensory receptors can be ends of nerve
fibers or other cells close to them
• generates an action potential and an impulse is sent
•Peripheral nerves transmit the impulse to the CNS
•CNS analyzes and interprets the impulse
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Sensations
• Feelings that occur when the brain
interprets sensory impulses
• All nerve impulses are the same
• Sensation depends on which area of the
cerebral cortex receives the impulse
• Receptors usually respond to specific
stimuli so the brain creates the correct
sensation
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• Cerebral cortex interprets the impulse to
come from the receptors stimulated
• Projection allows a person to pinpoint
the region of stimulation
Ex: eyes see and the nose smells things
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Sensory Adaptation
• Occurs when sensory receptors are
continuously stimulated
• As receptors adapt impulses leave at
decreasing rates and may stop entirely
• Impulses are only triggered if the strength of
the stimulus changes
Ex: adjusting to bad smell in a room
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Somatic (General) Senses
• senses associated with skin, muscles, joints, and viscera
• three groups
• exteroceptive senses – senses associated with body
surface; touch, pressure, temperature, pain
• visceroceptive senses – senses associated with
changes in viscera; blood pressure stretching blood
vessels, ingesting a meal
• proprioceptive senses – senses associated with
changes in muscles and tendons
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Touch and Pressure Senses
Senses mechanical forces that deform or displace
tissue
3 types
•Sensory nerve fibers – found in epithelial tissue
associated with touch and pressure
•Meissner’s corpuscles – small oval masses of
connective tissue found in hairless part of the
skin(lips, palms etc.)
sense light touch and texture
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• Pacinian corpuscles – large masses of
connective tissue fibers and cells
found in subcutaneous tissues
stimulated by heavy pressure
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Touch and Pressure Receptors
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Temperature Senses
Heat receptors
• sensitive to temperatures above 25oC (77o F)
• unresponsive to temperature above 45oC (113oF)
Cold receptors
• sensitive to temperature between 10oC (50oF) and 20oC
(68oF)
•Both cold and heat receptors rapidly adapt
Pain receptors
• respond to temperatures below 10oC
• respond to temperatures above 45oC
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Pain Receptors
• Consist of free nerve endings are located
in the skin and internal tissues, except the
nervous tissue of the brain
• Stimulated when tissue is damaged
• Usually specific to the type of pain
• Adapt very little, if at all
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Visceral Pain
• pain receptors are the only receptors in viscera whose
stimulation produces sensations
• pain receptors respond differently to stimulation
• not well localized
• may feel as if coming from some other part of the body
• known as referred pain
• an example would be heartburn
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Pain Nerve Pathways
Acute pain fibers
• A-delta fibers
•thin, myelinated
• conduct impulses
rapidly
• associated with
sharp pain
• well localized
Chronic pain fibers
• C fibers
•thin, unmyelinated
• conduct impulses more
slowly
• associated with dull,
aching pain
• difficult to pinpoint
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• An event may trigger impulses on both
types of fibers
• causing a dull sensation
• a sharp pain then a dull aching one
• aching pain is usually more intense and may
worsen over time
• Awareness of pain occurs when the impulse
reaches the thalamus
• Cerebral cortex judges the intensity and
locates the source of pain
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• Midbrain, pons, and medulla oblongata
regulate the flow of impulses from the
spinal cord
• Neuropeptides (endorphins) inhibit pain
sensations
• enkephalins and endorphins can suppress
chronic and acute pain impulses
• serotonin stimulates other neurons to release
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enkephalins
Stretch Receptors
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Stretch Receptors
• Proprioceptors that sense length and
tensions of muscles - 2 main types
Muscle spindles
• found in muscles near junctions with
tendons
• responds to a muscle stretching (stretch
reflex)
• helps maintain the desired position of a limb
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Golgi tendon organs
• found in tendons close to muscle
attachments
• stimulated by increased tension
• helps maintain posture and protects muscles
from being torn away from attachments
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Special Senses
• sensory receptors are within large, complex sensory
organs in the head
• smell in olfactory organs
• taste in taste buds
• hearing and equilibrium in ears
• sight in eyes
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Sense of Smell
Olfactory Receptors
• chemoreceptors
• respond to chemicals dissolved in liquids
Olfactory Organs
• contain olfactory receptors (bipolar neurons) and
supporting epithelial cells
• cover parts of nasal cavity, superior nasal conchae,
and a portion of the nasal septum
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Olfactory Receptors
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Olfactory Nerve Pathways
Once olfactory receptors are stimulated, nerve impulses
travel through
• olfactory nerves
olfactory bulbs
olfactory
tracts
limbic system (for emotions) and
olfactory cortex in cerebrum(for interpretation)
•The only nerve cells in direct contact to the outside
environment
•the only neurons that are regularly replaced (subject to
damage) yet about 1% are lost every year
•sense of smell diminishes with age
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Olfactory Stimulation
• olfactory organs located high in the nasal cavity
above the usual pathway of inhaled air
• olfactory receptors undergo sensory adaptation
rapidly
• sense of smell drops by 50% within a second after
stimulation
Olfactory Code
• hypothesis
• each odor may stimulate a specific set of receptor
subtypes
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Sense of Taste
Taste Buds
• organs of taste
• located on papillae of tongue, roof of mouth, linings of
cheeks and walls of pharynx
Taste Receptors
• chemoreceptors
• taste cells – modified epithelial cells that function
as receptors
• taste hairs –microvilli that protrude from taste
cells; sensitive parts of taste cells
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Taste Receptors
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Taste Sensations
Primary Taste Sensations
• sweet – most plentiful at tip of tongue
• sour – margins of tongue
• salty – tip & upper front part of tongue
• bitter – back of tongue
• umami(savory) – taste of glutamate (MSG)
•Food must be dissolved in saliva to be tasted
•Taste cells are modified epithelial cells that are
replaced continually – taste does not diminish with
age
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Hearing
Ear – organ of hearing
Three Sections
• External
• Middle
• Inner
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External ear
• outer structure called auricle - collects sound
waves and directs them inward
• external auditory meatus leads inward about 2.5
cm
• lined with skin that has modified sweat glands that
secrete wax
• hairs guard the opening
• sound waves pass to the end and alter the pressure
on the tympanic membrane (eardrum)
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Ear
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Middle Ear
• Includes the tympanic membrane, tympanic
cavity, and 3 small bones called auditory
ossicles (malleus, incus, stapes)
• Tympanic cavity is an air-filled space in the
temporal bone that separates the external
and internal ears
• Tympanic membrane covered by a thin
layer of skin on outer surface and mucous
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membrane on the inside
• Auditory ossicles transmit vibrations
between the tympanic membrane and inner
ear
Malleus, incus and stapes
• Vibration on the stapes moves fluid within
the inner ear which stimulates hearing
receptors
• Oval window is an opening in the wall of
tympanic cavity where the stapes is attached
by ligaments
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• Tympanic reflex reduces pressure from loud
sounds that might damage receptors –
ossicles become more rigid
• Auditory Tube
• Connects each middle ear to the throat
• Allows air to pass between the tympanic
cavity and outside the body
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• Helps maintain equal pressure on both sides
of the tympanic membrane- needed for
normal hearing
• air pressure changes with altitude causing
ears to pop
• usually closed by valvelike flaps in the
throat
• swallowing, yawning, and chewing help
open the valves and equalize the pressure
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Inner Ear
• Complex system of intercommunicating
chambers called labyrinth – each ear has 2
regions
• osseous labyrinth – bony canal in temporal
bone
• membraneous labyrinth – a tube within the
osseous labyrinth
• perylymph is fluid between the 2 chambers
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• endolymph is fluid in the membraneous
labyrinth
• cochlea contains the organ of Corti which
contains the receptors (hair cells) that
vibrations in the fluid stimulates
• different frequencies stimulate different
receptors
• humans can detect sound waves with
frequencies from 20 - 20,000
vibrations/second
• semicircular canals provide a sense of
equilibrium
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Inner Ear
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Organ of Corti
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Auditory Nerve Pathways
• Nerve fibers from hearing receptors travel
in the cochlear branch of the
vestibulocochlear nerves
• Impulses travel to the medulla oblongata,
midbrain, thalamus, and are interpreted in
the temporal lobes of the cerebrum
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Auditory Nerve Pathways
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Summary of the Generation of
Sensory Impulses from the Ear
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Sense of Equilibrium
Static Equilibrium
• Maintains the stability of the head and body
when they are motionless
• Organs of static equilibrium are the
vestibule (bony chamber between the
semicircular canals and cochlea)
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• Sensory receptors are hair cells in a
gelatinous material that sense changes in
position
• Movements tilt the gelatinous mass which
stimulates the hair cells to signal nerve
fibers
• Brain responds by sending impulses to
skeletal muscles to contract or relax in order
to maintain balance
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Vestibule
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Macula
• responds to
changes in head
position
• bending of hairs
results in generation
of nerve impulse
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Dynamic Equilibrium
• Balances the head and body when they are
moved or rotated suddenly
• Sense organs (hair cells) are located in the
ampullae of the semicircular canals
• Sudden movements cause the hair cells to
bend and send an impulse to the brain
• Parts of the cerebellum also interpret
information from the semicircular canals
and help maintain balance
• Visual information is also used to maintain
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balance
Semicircular Canals
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Crista Ampullaris
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Equilibrium
Static Equilibrium
• vestibule
• sense position of
head when body is
not moving
Dynamic Equilibrium
• semicircular canals
• sense rotation and
movement of head and
body
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Sight
Visual Accessory Organs
•The eye, eyelid, lacrimal gland, and extrinsic
muscles are located in the orbital cavity of the skull
•The orbit is lined with bone, fat, blood vessels,
nerves, and connective tissue
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Visual Accessory Organs
• Each eyelid is made of 4 layers: skin,
muscle, connective tissue, and conjuctiva
• The conjunctiva is a mucus membrane that
lines the inner surface of the eyelids and
fold back to cover the anterior surface of the
eyeball
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• The lacrimal gland secretes tears and a
series of ducts carry the tears into the nasal
cavity
• The extrinsic muscles are associated with
one primary action although eye movements
may use more than one muscle
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Lacrimal Apparatus
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Extrinsic Eye Muscles
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Structure of the Eye
• It is a hollow, spherical structure about 2.5
cm in diameter
• Has 3 distinct layers: outer fibrous tunic, a
middle vascular tunic, and an inner nervous
tunic.
• Spaces within the eye are filled with fluid to
provide support and maintain shape
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The Outer Tunic
• Cornea is transparent and helps focus light
rays - anterior portion of eye
- contains few cells and blood vessels
- many nerve fibers
• Sclera is the white part of the eye
- protects the eye and is an attachment for
muscles
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• Optic nerve is located in the back of the eye
- pierces the sclera
- carries impulses to brain
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Structure of the Eye
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The Middle Tunic
• The choroid coat loosely joins the sclera
• many blood vessels to nourish tissues
• contains melanocytes (produces pigment)
that absorb excess light and helps keep
inside of eye dark
• The ciliary body extends forward from the
choroid coat
• ligaments extend to hold the lens in place
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• ciliary muscles contract to view a close
object and relax to view a distant object
• The lens is a clear membraneous structure
• light passes through
• cataracts form when the lens becomes
cloudy and clear images can’t be focused on
the retina
• Iris controls amount of light entering pupil
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Lens
• transparent
• biconvex
• lies behind iris
• largely composed of
lens fibers
• elastic
• held in place by
suspensory ligaments
of ciliary body
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Ciliary Body
• forms internal ring around front of eye
• ciliary processes – radiating folds
• ciliary muscles – contract and relax to move lens
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Accommodation
• changing of lens shape to view objects
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Iris
• composed of connective
tissue and smooth muscle
• pupil is hole in iris
• dim light stimulates
radial muscles and pupil
dilates
• bright light stimulates
circular muscles and
pupil constricts
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Aqueous Humor
• fluid in anterior cavity of eye
• secreted by epithelium on inner surface of the ciliary body
• provides nutrients
• maintains shape of anterior portion of eye
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Inner Tunic
• retina
• contains visual receptors
• continuous with optic nerve
• ends just behind margin of the ciliary body
• composed of several layers
• macula lutea – yellowish spot in retina
• fovea centralis – center of macula lutea; produces
sharpest vision
• optic disc – blind spot; contains no visual receptors
• vitreous humor – thick gel that holds retina flat against
choroid coat
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Posterior Cavity
• contains vitreous humor – thick gel that holds retina
flat against choroid coat
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Major Groups of Retinal Neurons
• receptor cells, bipolar cells, and ganglion cells - provide
pathway for impulses triggered by photoreceptors to reach the
optic nerve
• horizontal cells and amacrine cells – modify impulses
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Layers of the Eye
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Light Refraction
Refraction
• bending of light
• occurs when light waves pass at an oblique angle into
mediums of different densities
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Types of Lenses
Convex lenses cause
light waves to converge
Concave lenses cause
light waves to diverge
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Focusing On Retina
• as light enters eye, it is refracted by
• convex surface of cornea
• convex surface of lens
• image focused on retina is upside down and reversed from
left to right
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Visual Receptors
Rods
• long, thin projections
• contain light sensitive
pigment called rhodopsin
• hundred times more
sensitive to light than cones
• provide vision in dim light
• produce colorless vision
• produce outlines of objects
Cones
• short, blunt projections
• contain light sensitive
pigments called
erythrolabe, chlorolabe,
and cyanolabe
• provide vision in bright
light
• produce sharp images
• produce color vision
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Rods and Cones
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Visual Pigments
Rhodopsin
• light-sensitive pigment in rods
• decomposes in presence of
light
• triggers a complex series of
reactions that initiate nerve
impulses
• impulses travel along optic
nerve
Pigments on Cones
• each set contains different lightsensitive pigment
• each set is sensitive to different
wavelengths
• color perceived depends on
which sets of cones are stimulated
• erythrolabe – responds to red
• chlorolabe – responds to green
• cyanolabe – responds to blue
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Stereoscopic Vision
• provides perception of distance and depth
• results from formation of two slightly different retinal
images
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Visual Nerve Pathway
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Life-Span Changes
Age related hearing loss due to
• damage of hair cells in organ of Corti
• degeneration of nerve pathways to the brain
• tinnitus
Age-related visual problems include
• dry eyes
• floaters (crystals in vitreous humor)
• loss of elasticity of lens (presbyopia)
need reading glasses
• glaucoma
• cataracts
• macular degeneration
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Clinical Application
Refraction Disorders
• concave lens corrects
nearsightedness (myopia)
• convex lens corrects
farsightedness (hyperopia)
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• Astigmatism
- curvature of cornea is elliptical rather than
spherical
- some portions of the image are in focus
and some are blurred
- eyes try to accommodate which leads to
ciliary muscle fatigue and headaches
• Presbyopia (farsightedness of age)
- eye losses refractive power with age
- unable to accommodate to see close
objects
- need reading glasses