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Chapter #10
Somatic and Special
Senses
10.1 Introduction
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Our senses are not only necessary for us to
enjoy life, but to survive.
Sensory receptors detect changes in the
environment and stimulate neurons to send
nerve impulses to the brain.
Sensory receptors vary greatly, but fall into
two major categories.
2 major categories
1.
2.
Receptors associated with somatic senses.
Receptors associated with specialized
sensory organs.
Receptors assoc. w/ somatic senses
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Receptors associated with the somatic
senses of touch, pressure, temperature, and
pain form one group.
These receptors are widely distributed
throughout the skin, muscles, joints and
deeper tissues.
They are structurally simple.
Receptors associated with specialized
sensory organs

Special senses of smell, taste, hearing,
equilibrium, and vision.
10.2 Receptors and Sensations
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Each receptor is more sensitive to a specific
kind of environmental change but is less
sensitive to others.
This selective response distinguishes the
senses.
Type of Receptors
1.
2.
3.
4.
5.
Chemoreceptors- are stimulated by changes in the
chemical concentration of substances. (Taste and
smell)
Pain receptors- by tissue damage.
Thermoreceptors- by changes in temperature.
Mechanoreceptors- by changes in pressure or
movement.
Photoreceptors- by light energy.
Sensations
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are feelings that occur when the brain
interprets sensory impulses.
All nerve impulses that travel away from
sensory receptors into the central nervous
system are alike, the resulting sensation
depends on which region of the brain is
stimulated.
Sensory adaptation
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When sensory receptors are continuously
stimulated, many of them undergo an
adjustment called sensory adaptation.
As receptors adapt, impulses leave them at
decreasing rates, until finally these receptors
may stop sending signals.
Example a person walks into a room with a
strong smell the longer the person is in the
room the less noticeable the smell is.
Projection
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Because the brain projects the sensation
back to its apparent source.
Projection allows a person to pinpoint the
location of stimulation; thus the eyes seem to
see, and the ears seem to hear.
10.3 Somatic Senses
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Receptors associated with the skin, muscles,
joints, and viscera make up the somatic
senses.
Pressure & Touch Figure 10.1
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1.
2.
3.
Three types of receptors detect touch and pressure.
Free ends of sensory nerve fibers in the epithelial
tissues are associated with touch and pressure.
Meissner’s corpuscles are flattened connective
tissue sheaths surrounding two or more nerve .
Meissner’s fibers and are abundant in hairless
areas that are very sensitive to touch, like the lips.
Pacinian corpuscles are large structures of
connective tissue and cells that resemble the layers
of an onion. They function to detect deep pressure.
Skin Cross Section
Temperature Senses
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Temperature receptors include two groups of
free nerve endings: Warm receptors and cold
receptors which both work best within a
range of temperatures.
Both types of receptors adapt quickly.
Temperatures near 45 oC stimulate pain
receptors; temperatures below 10 oC also
stimulate pain receptors and produce a
freezing sensation.
Pain
Pain receptors consist of free nerve endings that are
distributed throughout the skin and internal tissues,
except in the nervous tissue of the brain, which has
no pain receptors.
 Pain receptors adapt poorly.
 Visceral pain receptors are the only receptors in the
organs that produce sensations. Stretching or spasm
of smooth muscle.
 Referred pain occurs because of the common nerve
pathways leading from skin and internal organs. An
example would be a heart attack being felt as pain in
the arm or as heartburn.
Acute Pain
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Acute pain fibers are relatively thin,
myelinated nerve fibers.
They conduct nerve impulses rapidly and are
associated with the sensation of sharp shortduration pain.
Originates from a restricted area of the skin
and seldom continues after the painproducing stimulus stops.
Chronic Pain
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Chronic pain fibers are thin, unmyelinated
nerve fibers.
They conduct impulses slowly and produce
dull, aching pain that may be diffused and
difficult to pinpoint.
Regulation of Pain
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A person becomes aware of pain when impulses
reach the thalamus in the brain, but the cerebral
cortex judges the intensity and location of the pain.
Other areas of the brain regulate the flow of pain
impulses from the spinal cord and can trigger the
release of chemicals called neuropeptides which
inhibit the release of pain impulses in the spinal cord.
Other chemicals called endorphins released in the
brain provide natural pain control.
10.4 Special Senses
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1.
2.
3.
4.
Special senses are those whose sensory
receptors are within large, complex organs
in the head.
Smell –Olfactory organs
Taste– Taste buds
Hearing, equilibrium– ears
Sight -- eyes
10.5 Sense of Smell
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The sense of smell is associated with
complex sensory structures in the upper
region of the nasal cavity. (Superior nasal
concha)
We usually smell food at the same time we
taste it.
Chemoreceptors
Olfactory Organs
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Contain the olfactory receptors, are yellowish-brown
masses that cover the upper parts of the nasal
cavity, the superior nasal conchae, and a portion of
the nasal septum.
Olfactory receptors cells are bipolar neurons
surrounded by columnar epilthelial cells.
Olfactory impulses are interpreted in the temporal
lobe of the brain.
Anosmia is the partial or complete loss of smell.
10.6 Sense of Taste
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Taste buds are the special organ of taste.
Taste buds are located primarily on the
surface of the tongue but, are also scattered
in the roof of the mouth and the walls of the
throat.
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Taste buds include a group of modified cells.
Each Taste bud has 50 to 150 receptor cells.
Receptor cells are replaced every three
days.
Tiny projections called taste hairs protrude
from the outer ends of the taste cells and
extend from the taste pore. Taste hairs are
the most sensitive structure of the taste bud.
Taste Sensations
1.
2.
3.
4.
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Sweet
Sour
Salty
Bitter
Are the 4 primary taste sensations.
Saliva enhances the taste of food by
dissolving the chemicals that cause taste.
10.7 Sense of Hearing
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1.
2.
External ear consists of 2 parts.
The outer, funnel like structure called the
auricle.
The S-shapted tube called the external
auditory meatus.
Cross Section of
the Ear
Middle Ear
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1.
2.
includes an air-filled space, an eardrum,
and three small bones.
Tympanic cavity is an air-filled space in the
temporal bone.
Eardrum is semitransparent membrane
covered by a thin layer of skin on its outer
surface and by mucous membrane on the
inside.
3. Auditory ossicles (bones) are the malleus,
incus, and stapes. The three smallest bones
in the body. The main function of the bone
is to transmit and intensify the vibrations
transmitted to the inner ear.
4. Oval Window- opening in the wall of the
tympanic cavity. The ossicle (Bone) that sits
in the oval window is the stapes.
5. Auditory Tube connects the middle ear to the
throat.
 The main function of the auditory tube is to
equalize the pressure on both sides of the
eardrum.
Inner Ear
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The inner ear is a complex system of communicating
chambers and tubes called the labyrinth.
The labyrinth includes 3 Semicircular canals which
provide a sense of equilibrium and a cochlea which
functions in hearing.
The apex of the cochlea to a membrane-covered
opening in the wall of the inner ear called the round
window.
Hearing receptor cells are located in the organ of
Corti.
10.8 Sense if Equilibrium
1.
2.
Static equilibrium senses the position of the
head, maintaining stability, and posture
when the head and body are still.
Dynamic Equilibrium senses the motion of
the head and aid in balancing the head and
body during sudden movement.
Semicircular canals are the organs that
detect motion.
10.9 Sense of Sight
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Eyelid has four layers. Skin, muscle,
connective tissue, and conjunctiva.
The skin of the eyelid is the thinnest skin of
the body.
The eyelids are moved by the orbicularis
oculi muscle.
Conjunctiva is a mucous membrane that
lines the inner sufarces of the eyelids and
folds back to cover the eyeball, except for the
cornea.
Figure 10.16 & Table 10.2
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1.
2.
3.
4.
Lacrimal gland which secretes tears.
6 extrinsic muscles move the eye in a
various directions.
Superior Rectus move the eye upward.
Inferior Rectus move the eye downward.
Lateral Rectus move the eye outward.
Medial Rectus move the eye inward.
Structure of the Eye
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The eye is hollow.
The eye has 3 distinct layers.
Outer Tunic
Middle Tunic
Inner Tunic
Outer Tunic
1.
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2.
3.
Cornea which is the window of the eye and
helps focus entering light rays.
The transparency of the cornea is due to
the small number of cells and the lack of
blood vessels.
Sclera white portion of the eye.
Optic nerve
Middle Tunic
1.
2.
3.
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Choroid coat contains pigment-producing
cells that help keep the inside of the eye
dark.
Ciliary body which is the thickest part of the
middle tunic.
Lens is clear, membrane like structure.
Accommodation allows the shape of the
lens to change as the eye focuses on a
close object.
Middle Tunic Continued…..
4.
5.
6.
Iris colored portion of the eye.
Aqueous humor watery fluid that helps the
eye maintain shape.
Pupil circular opening in the center of the
iris.
Inner Tunic
1.
2.
3.
Retina which contains the visual receptor
cells (photoreceptors). It is a sheet of tissue
in the back of the eye.
Macula lutea is a yellowish spot on the
retina.
Fovea centrailis is a depression in the
center of the macula lutea. This is the
region that produces the sharpest vision.
Inner Tunic Continued…..
4.
5.
Optic disc is on the retina it lacks receptor
cells, it is commonly known as the blind
spot.
Vitreous humor supports the internal parts
of the eye and helps maintain its shape.
Visual Receptors
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Rods long, thin projections found in the
retina. Colorless vision, in dim light.
Cones short, blunt projects found in the
retina. Detect color, provide sharp images.
A Human eye has 125 million rods and 7
million cones.
A Human eye has 150,000 cones in the
fovea centralis and bird eye has 1 million
cones.
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“Skin Cross Section”. March 27, 2007.
http://www.owlnet.rice.edu/~psyc351/Images/Wolfe-Fig-12-02-0.jpg
“Human Ear.” March 27, 2007.
http://images.google.com/imgres?imgurl=http://www.infj.ulst.ac.uk/~pni
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“Eye”. March 27, 2007.
http://images.google.com/imgres?imgurl=http://library.thinkquest.org/J
002508/images/eyeball.gif&imgrefurl=http://library.thinkquest.org/J002
508/eyeballs.htm&h=251&w=392&sz=19&hl=en&start=1&tbnid=Rm4w
O8m6xYd5aM:&tbnh=79&tbnw=123&prev=/images%3Fq%3Dhuman
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