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Sense Organs
The sense organs are divided
into two categories:
- General sense organs
- Special sense organs
General sense organs sense touch,
temperature, pain.
Special sense organs function to
produce vision, hearing,
balance, taste, and smell.
Both the general and the special
sense organs also function to
maintain homeostasis
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Receptors
Distribution of receptors
- Receptors for special senses of smell, taste, vision,
hearing, and equilibrium are grouped into localized
areas or into complex organs
- General sense organs of somatic senses are
microscopic receptors widely distributed
throughout the body in skin, mucosa, connective
tissue, muscles, tendons, joints, and viscera
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Classification of Receptors: By Location
Exteroceptors: detect pressure, touch, pain,
and temperature. Located on or near body
surfaces.
- Visceroceptors: detect pressure, stretch,
chemical changes, hunger, and thirst from
internal organs.
- Proprioceptors: Provide information on
body movement, orientation in space, and
muscle stretch. Found in skeletal muscle,
joint capsules, and tendons.
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Classification of Receptors: By Stimulus Type
- Mechanoreceptors: pressure
receptors.
- Chemoreceptors: activated by
chemicals
- Thermoreceptors: temperature
receptors
- Nociceptors: pain receptors
- Photoreceptors: light receptors of the
eye
- Osmoreceptors: receptors for
electrolytes in extracellular fluids
Classification by structure
1- Free nerve endings
2- Encapsulated nerve endings
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Free nerve endings
- Most widely distributed type of sensory receptor
- Include both exteroceptors and visceroceptors
- Called nociceptors: receptors for pain
- Also detect itching, tickling, touch, movement, and
mechanical stretching
- Primary receptors for heat and cold
- Two types of nerve fibers carry pain impulses from
nociceptors to the brain:
- Acute (A) fibers: mediate sharp, intense, localized pain
- Chronic (B) fibers: mediate less intense, but more persistent,
dull or aching pain
Slide
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Free nerve endings
Other free nerve ending
receptors
- Root hair plexuses:
weblike arrangements of
free nerve endings
around
hair follicles
- Merkel discs: mediate
sensations of
discriminative touch
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Encapsulated nerve endings
- Meissner’s corpuscle: Tactile disks
involved in touch.
- Krause’s end bulbs: detect low-frequency
vibrations Carry cold impulses.
- Ruffini’s corpuscles: mediate crude and
persistent touch; may be secondary
temperature receptors for heat (85° to
120° F).
- Pacinian corpuscles: mechanoreceptors
that respond quickly to sensations of
deep pressure, high-frequency vibration,
and stretch.
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Stretch Receptors
There are two types of
stretch receptors:
1- Muscle spindles
2- Golgi tendon
These receptors operate to
provide body with
information concerning
length and strength of
muscle contraction
Slide
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Special Senses
Characterized by receptors grouped closely together or
grouped in specialized organs; senses of smell, taste,
hearing, equilibrium, and vision
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Taste
Your taste buds have
receptors for different
kinds of chemicals: sugar,
salt, sours, and bitters.
This diagram shows how
a sugar molecule can
enter a taste bud and
bind to an ion channel in
the membrane of a
receptor cell. The
receptor cell then sends
neurotransmitters to
activate the sensory
neuron that goes to the
brain.
Slide
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Taste
Each taste bud has only one kind of taste receptor,
either sweet, salty, sour, or bitter. Each taste bud sends
this information into the brain, where it is integrated.
Increasing taste increases the frequency of action
potential to the brain.
Slide
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Sense of Smell
Olfaction works similarly to
taste. Receptors in the
nose can bind to a certain
kind of chemical, which
triggers the release of
neurotransmitters to the
nerves that send action
potentials to the brain.
Again, stronger, more
pungent smells send
more frequent potentials.
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Olfactory pathway
When level of odor-producing chemicals reaches a
threshold level, the following occurs:
 Receptor potential, and then action potential, is generated
and passed to the olfactory nerves in the olfactory bulb
 The impulse then passes through the olfactory tract and
into the thalamic and olfactory centers of brain for
interpretation, integration, and memory storage
lide
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The Ear: Organ of Hearing and Balance
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Hearing and balance
The ear is not only responsible for
hearing, but also for balance! It
is an organ that can distinguish
between volume and pitch of
sounds, as well as rotation in all
three planes. The outer ear, or
pinna, is responsible for
channeling sound into the
auditory canal. The tympanic
membrane, commonly referred
to as the “eardrum” receives the
sound pressure waves in the ear,
and converts them into
mechanical vibrations. These
vibrations are passed along
through three small middle ear
bones: the malleus, incus, and
stapes.
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Hearing and balance
The stapes then hits another
membrane called the oval
window. The oval window is
the opening to the fluid
filled, spiral shaped cochlea.
Vibrations are sent along the
cochlear fluid and are sensed
by the movement of “reedlike” cells on the basilar
membrane. The basilar
membrane motion receptors
send the information to the
brain via sensory neurons.
The further the wave moves
along the cochlea, the lower
pitch of sound we hear
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Hearing and balance
There is another part of the inner ear,
above the cochlea, that senses
position and balance of the head.
This region contains the semicircular canals. These canals are three
fluid-filled, half circle shaped canals
oriented orthogonally to each other.
When the head moves in the other
direction (just as if you quickly jolt a
glass of water forward, it might
splash backwards towards your
hand). The fluid motion moves hairs
on the membrane (like on the skin),
which stimulate nerve fibers to send
action potentials to the brain to let
you know which way you are moving.
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The Eye: The organ of vision
Slide
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Vision
The eye is a very interesting organ
that is able to sense a fairly large
spectrum of light. The eye is
filled mostly with water
(aqueous humor and vitreous
humor). Light is initially
refracted by the cornea, and
enters the eye through the
pupil. The lens then focuses the
light and targets it to the retina:
the location of light sensors in
the back of the eye.
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Vision
In the retina, there are sensory cells called rods and cones which
can sense light. Rods are used for night vision, whereas cones
can sense color and detail.
Slide
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Vision
The fovea centralis is in the center
of the retina and contains only
cones for the most detailed color
vision. The rods and cones
activate visual pigments
according to different
wavelengths of light, which
stimulate the nerves to fire
action potentials into the brain.
It is interesting that the rods and
cones are actually in the deepest
layer; so light must cross the
neurons to reach the
photoreceptor cells, before the
signal is transmitted in the
opposite direction.
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Vision: The Eye
Neuronal pathway of vision
- Fibers that conduct impulses from rods and cones
reach the visual cortex in occipital lobes via optic
nerves, optic chiasma, optic tracts, and optic
radiations
- Optic nerve contains fibers from only one retina, but
optic chiasma contains fibers from the nasal portion
of both retinas; these anatomical facts explain
peculiar visual abnormalities that sometimes occur
Slide
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