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Chapter 18: Senses
18-1
Sensory Receptors and
Sensations
Sensory receptors are specialized to
detect certain types of stimuli.
Each type of sensory receptor responds
to a particular kind of stimulus.
Exteroceptors (hearing, sight receptors,
for example) detect stimuli from outside
the body.
Interoceptors receive stimuli from inside
the body; they are directly involved in
homeostasis.
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Types of Sensory Receptors
Chemoreceptors respond to chemical
substances, such as changes in pH, or
the senses of taste and smell.
Pain receptors are chemoreceptors that
respond to chemicals from damaged
tissues.
Mechanoreceptors respond to mechanical
forces.
The senses of hearing and balance both
involve mechanoreceptors.
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Proprioceptors (mechanoreceptors) in
tendons around joints make us aware
of position; pressoreceptors in arteries
detect blood pressure changes, and
stretch receptors in lungs detect
degree of inflation.
Thermoreceptors respond to temperature
changes; there are both warm
receptors and cold receptors.
Photoreceptors respond to light energy.
Special photoreceptors called rods result
in black-and-white vision, while cones
detect color.
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How Sensation Occurs
Sensation occurs when nerve impulses
reach the cerebral cortex.
Perception is an interpretation of the
meaning of sensations.
The sensation that results depends on the
part of the brain receiving the impulses.
Receptors may integrate signals before
sending nerve impulses.
Sensory adaptation occurs when a
stimulus continues but the receptor
decreases its response.
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Sensation
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Proprioceptors and Cutaneous
Receptors
Proprioceptors
Proprioceptors help us know the position
of our limbs in space.
Proprioceptors include muscle spindles
that are stimulated when muscle fibers
stretch; a reflex is initiated and the
muscle tightens in proportion to the
degree of stretch.
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These proprioceptors allow the muscles
to maintain the proper length and
tension, or muscle tone.
The knee-jerk reflex involves muscle
spindles.
Signals to the CNS from muscle spindles
help maintain balance and posture.
Golgi tendon organs are proprioceptors
with the opposite effect.
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Muscle spindle
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Cutaneous Receptors
The dermis of the skin contains sensory
receptors for touch, pressure, pain, and
temperature (warmth and cold).
Three types of cutaneous receptors are
sensitive to fine touch:
1) Meissner corpuscles are concentrated
in finger tips, lips, tongue, nipples, and
genital areas;
2) Merkel discs are found where the
epidermis meets the dermis; and
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3) free nerve endings (root hair plexus)
around hair follicles all detect touch.
Three different types of pressure
receptors are Pacinian corpuscles,
Ruffini endings, and Krause end
bulbs.
Temperature receptors are simply free
nerve endings in the epidermis; some
are responsive to cold and others are
responsive to warmth, although there
are no structural differences between
them.
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Sensory receptors in human skin
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Pain Receptors
Nociceptors are pain receptors on
internal organs and may be sensitive to
temperature, pressure, or chemicals.
Referred pain occurs when stimulation of
internal pain receptors is felt as pain
from the skin.
Referred pain most likely happens
because of shared nerve pathways
between the skin and internal organs.
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Chemical Senses
Chemoreceptors in the carotid arteries and
aorta respond to the pH of the blood and
communicate with the medulla oblongata
to change breathing rate.
For example, when blood pH drops, these
chemoreceptors signal the medulla
respiratory center that triggers breathing
rate to increase; expiration of CO2 raises
the pH of the blood to normal.
Taste and smell are chemical senses.
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Sense of Taste
The taste buds located in papillae on the
tongue contain taste cells that
communicate with sensory nerve
fibers.
Microvilli on taste cells contain receptor
proteins that match chemicals in food.
The brain determines the taste according
to a “weighted average” of incoming
impulses from taste buds sensitive to
either sweet, sour, salty, or bitter tastes.
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Taste buds
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Sense of Smell
Olfactory cells (modified neurons) are
located in epithelium in the roof of the
nasal cavity.
After molecules bind to receptor proteins
on the varied cilia of olfactory cells,
nerve impulses lead to olfactory areas of
the cerebral cortex.
The perceived odor is determined by the
combination of olfactory cells
stimulated.
The effects of smell and taste combine.
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Olfactory cell location and
anatomy
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Sense of Vision
Vision is dependent on the eye and the
visual areas of the cerebral cortex.
It is estimated that at least one-third of
the cerebral cortex is involved in
processing visual information.
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Anatomy of the Eye
The eye has three layers.
The sclera is the outer layer seen as the
white of the eye and includes the
transparent bulge in the front of the eye
called the cornea.
The choroid is the middle, darkly
pigmented layer that absorbs stray
light rays; it also becomes the iris that
regulates the size of the pupil.
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Behind the iris, the choroid thickens and
forms the ciliary body.
The ciliary body contains the ciliary
muscle, which controls the shape of
the lens for near and far vision.
The lens divides the eye into two
compartments: the anterior
compartment (containing aqueous
humor) and the posterior compartment
(containing vitreous humor).
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Rod cells and cone cells are located in
the retina that forms the inner layer.
The retina lines the back half of the eye
and has cone cells densely packed in
one area called the fovea centralis.
Sensory fibers from the retina form the
optic nerve leading to the brain.
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Anatomy of the human eye
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Focusing
The cornea and the lens focus light rays
on the retina.
To see a close object, the ciliary muscles
change the lens shape to provide visual
accommodation.
After age 40, the lens is less able to
accommodate and near vision is less
acute.
Cataracts occur when the lens becomes
opaque; sun exposure might be a factor
in developing cataracts.
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Focusing
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Photoreceptors
Both rod cells and cone cells have an
outer segment with membranous disks
containing embedded pigments.
Rods contain a deep purple pigment
called rhodopsin that is composed of
retinal (made from vitamin A) and the
protein opsin.
Rods are numerous and provide
peripheral vision, perception of motion,
and vision in dim light at night.
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When a rod absorbs light, rhodosin splits
into opsin and retinal, leading to a
cascade of reactions and the closing of
rod membrane ion channels.
Inhibitory neurotransmitters are no
longer released from the rod.
Breakdown of rhodopsin in rods thus
initiates nerve impulses.
Cones have three different pigments (red,
green and blue) made from retinal and
opsin; opsin varies between the three.
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Photoreceptors in the eye
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Integration of Visual Signals in
the Retina
The retina has three layers of neurons:
rods and cones are near the retina,
bipolar cells are in the middle, and the
innermost layer contains ganglion cells
that carry impulses to the optic nerve.
The rod and cones synapse with the
bipolar cells, which in turn synapse
with ganglion cells that initiate nerve
impulses.
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As signals pass from one layer to the next,
integration occurs because each layer
contains fewer cells than the previous
layer.
However each cone connects directly to
one ganglion cell, while a hundred rods
may synapse with only one ganglion cell.
It is likely that much processing occurs in
the retina before impulses are sent to the
brain.
There are no rods and cones where the
optic nerve exits the retina; this is the
blind spot.
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Structure and function of the
retina
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Integration of Visual Signals in
the Brain
The visual pathway begins with the retina
and passes through the thalamus
before reaching the cerebral cortex.
The visual pathway and the visual cortex
split the visual field apart, but the
visual association areas rebuild it so
we correctly perceive the entire visual
field.
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Optic chiasma
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Abnormalities of the Eye
Color Blindness
The most common abnormality is a lack of
red and/or green cones.
Distance Vision
Nearsighted individuals (elongated eyeball)
cannot see distant objects; this is
corrected by a concave lens.
Farsighted individuals (shortened eyeball)
see distant objects well but not up close;
this is corrected by a convex lens.
Astigmatism occurs with an uneven cornea
or lens.
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Common abnormalities of the
eye
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Sense of Hearing
The ear has two sensory functions:
hearing and balance.
The sensory receptors for both senses
are located in the inner ear, and both
use a type of mechanoreceptor
consisting of hair cells with stereocilia
(long microvilli).
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Anatomy of the Ear
The ear is divided into three parts.
The outer ear consists of the pinna and
the auditory canal, which direct sound
waves to the middle ear.
The middle ear begins at the tympanic
membrane (eardrum) and contains the
ossicles: the malleus, incus, and
stapes that amplify sound waves.
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The malleus is attached to the tympanic
membrane, and the stapes is attached
to the oval window, which is covered
by membrane.
The inner ear contains semicircular
canals and vestibule involved in
equilibrium, and the cochlea for
hearing.
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Anatomy of the human ear
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Process of Hearing
Sound waves enter the auditory canal and
vibrate the tympanic membrane.
If the vibrations are strong enough, the
outer and middle portions (ossicles) of
the ear convey and amplify the sound
waves about 20 times and vibrate
against the oval window.
These vibrations set up pressure waves
within the fluid of the cochlea.
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The cochlea contains the spiral organ
consisting of hair cells on the basilar
membrane whose stereocilia are
embedded within the tectorial
membrane.
Vibrations within the cochlea cause the
sterocilia to vibrate against the
tectorial membrane, thus generating
nerve impulses.
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Different regions are sensitive to
different frequencies or pitch.
When the stereocilia of the hair cells
bend, nerve impulses are generated in
the cochlear nerve and are carried to
the brain.
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Mechanoreceptors for hearing
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Sense of Equilibrium
Rotational Equilibrium
Rotational equilibrium depends on the
stimulation of hair cells within the
ampullae of the semicircular canals.
Continuous movement of fluid within the
canals can cause motion sickness.
Vertigo is dizziness from a sensation of
spinning.
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Gravitational Equilibrium
Stimulation of hair cells within the utricle
and the saccule, two sacs located in
the vestibule, by the slippage of
calcium carbonate granules or otoliths,
provide impulses that tell the brain the
direction of movement of the head.
The movement of the otoliths provides a
sense of gravitational equilibrium.
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Mechanoreceptors for
equilibrium
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Chapter Summary
Sensory receptors respond to specific
environmental stimuli.
Sensation occurs in the brain when
sensory receptors send nerve impulses
to the brain.
Senses are divided into exteroceptors
that detect stimuli from outside the
body, and interoceptors that receive
stimuli from inside the body.
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Proprioceptors in muscles and joints
help the body maintain balance and
posture.
Cutaneous receptors in the skin respond
to touch, pressure, pain, and
temperature (both warmth and cold).
In the mouth, the microvilli of taste cells
have membrane protein receptors that
respond to certain molecules.
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Olfactory cells within the olfactory
epithelium respond to molecules and
result in a sense of smell.
Photoreceptors for sight contain visual
pigments, which respond to light rays.
Some integration occurs in the retina of
the eye before nerve impulses are sent
to the brain.
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Sensory receptors for hearing are hair
cells in the cochlea of the inner ear that
respond to pressure waves.
Sensory receptors for balance are hair
cells in the vestibule and semicircular
canals of the inner ear that respond to
the tilt of the head and to the movement
of the body, respectively.
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