Chapter 12
Sense Organs
I. Introduction
• The body has millions of sense
organs.
• A. General sense organs
– 1. Function
• a. To produce the general or somatic
senses such as touch, temperature, and
pain, hunger, thirst….
• b. Initiate various reflexes necessary to
maintain homeostasis (crucial to survival)
• B. Special sense organs
–1. Function
•a. To produce the special
senses of vision, hearing,
balance, taste and smell
•b. To initial various
reflexes necessary to
maintain homeostasis
II. Receptors
• A. Receptor response –
responding to stimuli by
converting them to serve
impulses
–1. Different types of receptors
respond to different stimuli (ie:
heat receptors do not respond to
light)
–2. when an adequate stimuli
acts on a receptor, a sensory
neuron’s axon is triggered.
Impulses travel over sensory
pathways to the brain where
they are interpreted as a
particular sensation such as
heat or cold
–3. Adaptation is a common
phenomenon experiences by
receptors – receptor will adapt
to a continuous stimuli and thus
the intensity of the resulting
sensation decreases. (ie: hot
water in a bath tub, feeling your
clothes when you first put them
on).
• C. Classification of receptors
–1. By location
•a. Externoreceptors – on or
near the surface of the body and
respond to external stimuli.
Sometimes called cutaneous
receptors (skin). Examples
include those that detect
pressure, touch, pain, and
temperature. Special sense
organs are also classified as
externoreceptors.
•b. Visceroreceptors (or
interoreceptors) – located
internally within the substance
of body organs (viscera).
When stimulated provide info
about internal stimuli such as
pressure (urinary bladder),
stretch, chemical changes,
hunger, thirst…..
•c. Proprioceptor – specialized
type of visceroreceptor – located
in skeletal muscles and joints.
Provide information on position of
body parts.
–i. Tonic proprioceptors – locate
body parts at resting without
looking
–ii. Phasic proprioceptors –
located body parts during
continuous movement.
–2. By stimulus detected
•a. Mechanoreceptors –
activated by stimuli that
“deform” the receptor or
change its position. Pressure
on the skin deform the
underlying receptor.
•b. Chemoreceptor – activated
by amount or changing
concentration of certain
chemicals (ie: smell and
taste)
•c. Thermoreceptors – activated by
change in temperature
•d. Nociceptor – activated by intense
stimuli that results in tissue damage.
Stimuli can be toxic, chemical, intense
light, sound, pressure, heat. The
resulting sensation is pain.
•e. Photoreceptors – activated by
light. Located only in the eye.
– 3. By structure
•a. Free nerve ending – simplest,
most common, widely distributed type
of nerve ending. Located throughout
the body (externoreceptor and
interoreceptor) except in the brain.
•The brain is incapable of sensing
painful stimuli (so patients are
typically awake during brain surgery).
Also the receptor that picks up on
itching, tickling, and touch.
–i. Nociceptor – primary
receptor for pain
aa. Acute free nerve
ending – mediates sharp
pain and localized pain
sensations
bb. Chronic free nerve
endings – mediates dull,
aching pains
–ii. Root hair plexus – a
web-like arrangement
around hair follicle which
detects hair movement (hair
standing up on the back of
your neck in fight or flight
mode)
–iii. Merkel discs – sensitive
to even the lightest touch
•b. Encapsulated nerve
endings – surrounded by
connective tissue and all are
mechanoreceptors
–i. Touch and pressure
receptors
aa. Messiner’s corpuscle
– found in hairless skin
(fingertips and lips)
(a).
Two anatomic
variations of Messiner’s
(i). Krause’s end
bulbs – less tightly coiled
dentritic endings
(ii). Ruffini’s –
deeply located in dermis
III. Special Sense Organs
• A. Vision
–1. Eye Anatomy
•a. Schlera – outer coat,
tough, white fibrous tissue
•b. Cornea – transparent
portion of the schlera that
covers the iris
•c. iris – colored portion of the
eye. Made up of muscular
tissue
•d. Pupil – hole which allows
light to pass through
•e. Lens – refracting
mechanism of the eye
(bending of light)
•f. Retina – innermost part of the
eyeball where the image forms.
Image is upside down…contains
these photoreceptors:
–i. Rods – specialized neurons
that look like rods. Function
best in dim light
–ii. Cones – specialized neurons
that look like cones. Function
best in bright light
•g. Accessory structures
–i. Lacrimal glands –
secretes tears that function
to wash debris form the
eye. They drain either out
the tear ducts or down into
the nose (which is why your
nose runs when you cry)
–ii. Eyebrows – keep sweat
from getting into eyes
–iii. Eyelashes – keeps
foreign objects from getting
into eyes
–iv. Eyelids – function as the
windshield wipers for the
eye (keeps the eyeball
moist)
–2. Neural Pathway of vision
•a. Fibers conduct impulses form
the rods and cones to the optic
nerve
•b. Optic nerves from each eye
cross in the optic chiasma
•c. Optic tracts take the visual
information to the visual cortex of
the brain for interpretation
–3. 20/20 vision means that a
person can see at 20 feet what
the “normal” eye should see
clearly at that distance. Some
people see better than the
“normal” eye: 20/15 means
that they can see at 20 feet
what the normal eye can see at
15 feet
–4. Red eye in photographs –
the flash reflects off the layer
of blood vessels that lie
between the retina and
schlera
• B. Smell
–1. Olfactory Anatomy
•a. Olfactory sense organs
consist of:
–i. Olfactory cilia – hair-like
projections that wave
around mixing the mucous
in the upper surface of the
nasal cavity (that is why
you have to “sniff” or
inhale big when you are
trying to smell something,
to get the gas molecules
past the cilia
ii.
Olfactory cells –
chemoreceptors that respond to
gas molecules or chemicals in
the air.
iii. Olfactory epithelium –
located in the superior portion
of the nasal cavity
iv. Olfactory receptors –
extremely sensitive and easily
fatigued when continuously
stimulated (that is why smells
that appear strong at first fade
away…perfume, cologne, etc)
– 2. Olfactory Physiology
•a. Examples of primary scents are
peppermint, putrid, floral and musk.
•b. Most people can identify several
hundred scents. Some people can
identify as many as 1000’s of scents.
Animals have a keen sense of
olfaction. Dogs can pick up scents
32 times better than humans, and
sharks can pick up scents 11,000
times better than humans.
–3. Olfactory neural pathway of
smell – if level of odor producing
chemicals is sufficient, then the
cilia will pick up the scent and
pass the nerve impulse onto the
olfactory nerve in the olfactory
bulb. AFrom there, the impulse
passes to the brain (thalamus to
the cortex) for recognition and
memory storage
Olfactory nerves
• C. Gustation (taste)
–1. Gustatory anatomy
•a. Taste buds –
projections or papillae on
tongue, lining of mouth,
throat, roof of mouth, and
lips
–b. children have approximately
10,000 taste buds but nearly
half of them will die by the
time a person reaches age 60.
–c. the white “stuff” on your
tongue in the morning is skin
cells that have died during the
night. The “bad breath” is the
decayed material still in your
mouth
–2. Gustatory Physiology
•a. Taste buds are
chemoreceptors that are
stimulated by chemicals
dissolved in saliva
•b. Four primary taste sensations
– bitter (back of tongue), sour
(back sides of tongue), salty
(front sides of tongue) and
sweet (tip)
•c. Taste cells are sensitive, but
fatigue quickly with constant
stimulation (ie: you don’t taste
gum after chewing it for a little
while)
–3. Gustatory neural pathway –
the taste sensation begins
when the substance binds to
the taste buds. From the taste
buds the nerve impulse is
carried via one of te cranial
nerves to the medulla
oblongata then to the thalamus
and then on to the taste area
of the cerebral cortex
–4. Olfactory and Gustatory
process interrelated as sense
of taste is highly correlated to
sense of smell. When a person
has a head cold the nasal
cavity becomes swollen and
the pathway is blocked so that
you can’t pick up the chemical
signals resulting in tasteless
food
• D. Hearing
–1. Hearing Anatomy
•a. External ear
–i. Pinna (auricle) acts
as a funnel to bring
soundwaves into ear
–ii. External auditory
meatus (ear canal) secretes ear wax (AKA:
cerumen) which traps
dirt and dust
–iii. Tympanic membrane
– (ear drum) stretches
across the end of the
auditory canal and
separates the external
ear from the middle ear
–iv. Inflammation of the
external ear is called
swimmer’s ear. AKA:
external otitis
•b. Middle ear
–i. Auditory ossicles (the
bones in the ear) –
malleus, incus and
stapes
–ii. Openings into the
middle ear
(a) one from the
external auditory
canal (covered by the
tympanic membrane)
(b)
one from
Eustachian tube (the
tube that leads from
the throat to the ear)
(c) two from the
inner ear – round
window and oval
window
•iii. Eustachian tube – purpose is
for equalization of pressure
between the inner and outer
surfaces of the tympanic
membrane….preventing rupture.
As you swallow or yawn, it forces
air in through the throat which
equalizes the pressure on the
eardrum from the other side.
•iv. Inflammation of the
middle ear is called otitis
media
•C. Inner ear – functions
for hearing and for
maintaining balance….it is
very intricate
–i. Vestibule – involved
with balance
–Ii. Semicircular canals –
involved with balance
–iii. Chochlea – (means
snail). The Organ of Corti
is the organ inside the
chochlea involved with
hearing
IV. Disorders of senses
• A. Tinnitus – ringing in the ear
• B. Sinusitis – inflammation of the
sinus pathways
• C. Cataracts – cloudy spots on
the lens as we age
• D. Glaucoma – increased
pressure in the eye which
damages the retina
• E. Floaters – semitransparent
specks that float in the field of
vision. Some are red blood cells
that have leaked out into the
retina area and float around the
retina
• F. Vertigo – dizziness – primary
symptom for inner ear infection
• G. Visual acuity –
clearness/sharpness of visual
perception
• H. Myopia – nearsightedness
• I. Hyperopia - farsightedness
V. Doctors of senses
•A. Opthalmologist
•B. Optometrist
•C. ENT (ear, nose and
throat)
VI. Clinical surgeries
• A. Radial keratomy (RK) – makes 6
small incisions around the cornea to
allow it to flatten out for myopia
patients
• B. Astigmatic keratomy (AK) –
transverse cuts across the cornea
help astigmatism
• C. Automated lamellar
keratoplasty (ALK) - cuts a thin
cap off the cornea surface to
shave and reshape the tissue
underneath
• D. Photorefractice keratectomy
(PRK) – “cool” lasers vaporize the
cornea tissue to help correct
myopia
• E. Laser-assisted in situ
keratomileusis (LASIK) – cuts
the cornea like a lid and bends
it back, then lasers are used to
vaporize and reshape the
tissue underneath. At the end,
the cornea cap is replaced.