SENSES
Three types of senses:
 1. SOMATIC SENSES: Light touch
(being touched by a feather), heat, cold,
vibration, pressure, pain.
 These are routinely tested by doctors in a
physical exam, especially for people with
diabetes and lupus.
SENSES

2. PROPRIOCEPTORS are found in the
muscles, joints, and tendons. They measure the
amount of movement, force, and position of the
body.
 Proprioception is often tested by having the
patient close their eyes and saying if their
fingers are up or down.
 Proprioceptors send information to the
cerebellum. That’s how you know your legs are
crossed before you stand up.
 Somatic senses (including pain) and
proprioception are NOT considered special
senses.
SENSES
 3.
SPECIAL SENSES: Smell, taste,
vision, hearing, equilibrium (balance).
OLFACTORY SENSE (smell)




Olfactory receptors are CHEMORECEPTORS; a special
type of neuron which senses particular chemicals and
triggers an action potential.
Chemoreceptors are at the roof of the nasal cavity.
There are hundreds of thousands of types, and they can
smell a wide variety of substances.
They are extremely sensitive, and can detect parts per
billion, as in the scent of natural gas…just a few
molecules!
The olfactory nerve goes through the cribiform plate to
the OLFACTORY BULB (one of the shortest nerves in
the body) and into the limbic system.
OLFACTORY SENSE (smell)
 Scientists
who are trying to find a way
to make neurons divide to heal nerve
injuries often study the body’s only
mitotic neurons.
 These neurons are the olfactory
receptors.
 People who experience imaginary
odors have what are called “unicate
fits”.
Olfactory Receptors
Figure 16.3a, b
Some towns are old factory towns.
Some are ol’ factory towns!
GUSTATORY SENSE (taste)





Sensed on taste buds, which are located mostly on the
tongue surface, but are also on the palate, pharynx, and
a few on the lips.
Taste buds have specialized cells, which increase
surface area and have chemoreceptors.
They are surrounded by support cells (like glia). They
synapse on sensory neurons, which go to the facial
nerve.
Someone with a damaged facial nerve can not easily
taste sweet, sour, or salty substances. Taste buds
are the only parts of the nervous system that can
regenerate completely.
The taste information is sent to the primary
gustatory (taste) cortex, located in the parietal lobe
of the brain.
Taste Buds
Figure 16.1a, b
Taste Bud
GUSTATORY SENSE (taste)
 How
many different tastes are there?
Dozens. Salt, sweet, bitter, and sour are
only a few.
 Where are they located on the tongue? All
tastes are located all over the tongue.
 The picture in the book was drawn 120
years ago by an anatomist that knew his
drawing was not right; he just wanted to
use it as a starting point for further
experimentation.
GUSTATORY SENSE (taste)



Taste appreciation is also involved in texture (a mealy
apple is not as good), temperature (cold pizza tastes
different than warm), and smell (perfume or cigarette
smoke clog the senses and decrease taste).
There are dozens of taste receptors, hundreds of
thousands of smell receptors, so the subtly of taste is
from smell.
Foods people like are in opposite proportion to the
numbers of taste receptors for that. People that love
sweets have FEWER taste receptors for sweets, so they
crave more taste of sweet things. If you dislike
something, it’s because you have lots of receptors for it.
Also, as you get older, you become less tolerant of
sweets and more tolerant of bitter tastes (like beer and
coffee).
Fun Facts
 The
catfish has over 27,000 taste buds.
(What could be so tasty on the bottom of a
pond?)
 Flies
taste with their feet.
Tongue with bacteria
• What did the right eye say to the
left eye?
• Between you and me, something
smells!
THE EYE



Structures Surrounding the Eye
The eye is in the orbit of the skull for protection.
Within the orbit are 6 extrinsic eye muscles, which move
the eye.

There are 4 cranial nerves: Optic (II),
Occulomotor (III), Trochlear (IV), and Abducens
(VI).

Eyelids are PALPEBRA, and eyelashes are CILIA.



Fun fact: there is a new medicine to make you grow longer
eyelashes!
People of Asian descent have an EPICANTHIC FOLD in
the upper eyelid; no functional difference.
Around the eyeball are glands.
Surface Anatomy of Eye
Extrinsic Eye Muscles
Figure 16.6a, b
FUN FACTS

Do eyelashes grow back?
 Eyelashes take about four to eight weeks to
grow back.
 What purpose do eyebrows serve?
 To keep the sweat and rain out of our eyes. The
arch shape of the eyebrow diverts rain or sweat
down our cheeks, keeping our eyes dry.
Eyebrows also help us to communicate.
Sometimes a raised eyebrow is more effective
than words.
Eyelashes
GLANDS OF THE EYE
 1.
LACRIMAL GLANDS are the largest
set. They are on the superior lateral eyelid
and they produce tears, which drain into
the nasal cavity via the LACRIMAL
DUCT.
 The function is to moisten and lubricate
the eye surface, and it has enzymes to kill
bacteria (which thrive in warm, moist
conditions).
Figure 16.5b
GLANDS OF THE EYE
LACRIMAL CARUNCLE (“little meat”)
is the spot on the medial corner of the eye.
It makes an oily secretion like a
sebaceous gland.
 The function is to lubricate the eye a little
bit for the eyelids. When the secretion
dries, it is called “sand” in the eyes.
 2.
GLANDS OF THE EYE

3. TARSAL GLANDS are sebaceous glands on
the inside of the eyelid, and produce sebum,
which is an oil to lubricate the eyeball.
 Therefore, the oil component found in tears
is produced by the tarsal glands.
 The tarsal glands and the lacrimal caruncle
make a waterproof surface so the eye won’t dry
out.
 When tarsal glands are clogged = CHALAZION
Chalazion
GLANDS OF THE EYE
 4.
CILLIARY SEBACEOUS GLANDS go
to only the cilia.
 When clogged = STY.
Sty
Dry Eye
Syndrome
FUN FACTS

An ostrich's eye is bigger than it s brain!
 Is the human eye fully grown at birth?
 A typical newborn's eye is around 18 millimeters
in diameter. A fully grown adult's eye measures
24-25 millimeters. A fully developed eyeball is
about two-thirds the size of a ping-pong ball.
 This means a human eye grows only about 28%
over the course of its life. Fish supposedly have
the ability to increase their eye size "steadily
over the course of their entire lives," a talent lost
on us.
Ora serrata
Rods and
cones
The Eyeball

1. CONJUNCTIVA is like a Saran Wrap
covering around the eye and under the eyelids.
It’s made of stratified columnar epithelium (the
first time in the body we’ve seen this tissue).
 It also has lots of goblet cells to secrete moisture
for those areas.
 Deep to the epithelium is loose connective tissue
with lost of small blood vessels, which are not
seen unless the conjunctiva becomes inflamed:


Blood-shot eyes: just from being tired
PINK EYE (layman’s term), known as
CONJUNCTIVITIS (from bacteria, very contagious).
The Eyeball

2. FIBROUS TUNIC is the next layer, and has 2 parts:
 A. SCLERA is the white of the eye, made of dense
irregular connective tissue. It is continuous with the dura
mater of the brain. The eye is part of the brain. The
sclera protects the eye.
 B. CORNEA is clear, and avascular (no blood supply)
except around the periphery. Therefore, there is no
tissue rejection when it is transplanted into another
person; there is also no need to find a donor match. It
has lots of pain receptors, so a scratched cornea is very
painful. Its function is to be the main focuser of light for
the eye. If damaged, need a corneal transplant.
Scleritis
Conjunctivitis
The Eyeball

3. VASCULAR TUNIC is deep to the fibrous tunic. It
has several structures.
 A. CHOROID has lots of blood vessels and pigment.
The function of the pigment is to make sure light does
not enter from the sides. The blood vessels provide
blood supply to the other layers.
 B. CILIARY MUSCLES surround the lens.
 C. SUSPENSORY LIGAMENTS (also known as the
zonule) hold the lens in place.
 D. LENS functions to continue to focus the light after it
passes through the cornea. It changes shape to allow
you to distinguish close from far. The lens changes
shape by the ciliary muscles pulling on the suspensory
ligaments.
The Vascular Tunic
PLAY
Vascular Tunic (Uvea)
Figure 16.8
Rods and
cones
Figure 16.7a
Figure 16.9a
Ciliary Muscles
 When
you are looking far away, the ciliary
muscles are relaxed, the lens is stretched
into a wide circle, and the suspensory
ligaments are tight.
 When you look up close, the ciliary muscle
contracts and gets smaller, to the
ligaments relax. Constantly looking close
puts strain on the ciliary muscles = EYE
STRAIN.
Pupils
 Fun
Fact: -When you are looking at
someone you love, your pupils dilate,
and they do the same when you are
looking at someone you hate.
PROBLEMS WITH THE LENS
 With
age, the lens loses flexibility, and is
less likely to round up.
 It stays in the position for seeing far, so
there is trouble focusing on things that are
near = PRESBYOPIA (“old eyes”).
 Occurs around age 45-50. The lens
cannot accommodate.
PROBLEMS WITH THE LENS
 Clouding
of the lens leads to a clinical
condition known as CATARACTS.
 Treatment is to remove the lens and
replace it with a plastic one (which is not
flexible either).
 If the lens yellows, you can’t see the color
blue. After surgery, can see blue again.
Cataract
Cataract Vision
IRIS (the colored part of the eye)
 The
function is to constrict or dilate the
pupil (opening) to allow light in.
 Therefore, it regulates the amount of
light passing to the visual receptors of
the eye.
 If there is a lot of pigment, eye is brown; a
medium amount = green, small amount =
blue, no pigment = pink (albino).
FUN FACTS
Why are all babies born with blue eyes?
 Melanin is a brownish pigment that adds color to your
hair, eyes, and skin. At the time babies are born, melanin
hasn't yet been "deposited" in the eyes' iris. Hence, they
appear blue.
 After about six months, eyes change color depending on
the amount of melanin. If you have a lot of it, your eyes
will turn brown or black. If you have little, they'll stay
blue. And if you have no melanin, your eyes may appear
pink. Interestingly, as the site notes, human beings aren't
the only creatures with freaky color-morphing eyeballs.
Kittens experience the same phenomenon.
How can someone have two different colored eyes?
 Eye color is a polygenic trait. Many babies are born with
blue eyes. Their eyes change color later as they begin to
produce more melanin.
RETINA
 The
retina is on top of the choroid layer.
 The retina is made up of
PHOTORECEPTORS, which are sensors
for light.
Rods and
cones
Figure 16.7a
Optic Nerve
Fovea
centralis
Rods and Cones

Two types of photoreceptors: Rods and
Cones.
 1. CONES (red, green, and blue) they have less
light sensitivity (poor at night) but see colors
well. There is a region on the retina that has
the highest concentration of cones; it is
called the FOVEA CENTRALIS. The fovea
centralis is the very center point of a small
circular region called the MACULA. When you
want details, focus the light on the macula,
because there are a lot of cones there. The
other layers contain a mixture of both.
Figure 16.7a
Test for Colorblindness
Rods and Cones
 2.
RODS (chartreuse = yellowish green)
have more light sensitivity (can see well at
night) but does not see colors well.
 Above the photoreceptors are layers of
neurons whose axons become the optic
nerve.
Optic Nerve
Fovea
centralis
Retina
Figure 16.10a
Photoreceptors
Figure 16.11
Blind Spot at the Optic Nerve
 The
region where the optic nerve and
blood vessels goes in and out of the eye
has no photoreceptors = BLIND SPOT.
 Hold your hands out at 45° and that’s the
location of the blind spot.
 You can still see your hands because the
other eye sees it. Close your right eye
and look for your right hand and you’ll find
the blind spot.
Figure 16.7a
Find your blind spot!
Stare at the center of X and move head closer until one red spot disappears
Blind Spot

The light takes a path through the lens to the
blood vessels, so this is the only place in the
body where you can see blood vessels directly.
 The doctor can diagnose hypertension.
 On a clear, bright day, look at the blue sky and
you can see the shadow of your own blood
vessels on the photoreceptors as criss-cross
lines in field of vision. The little moving dots are
your blood cells.
Normal Retina
 The
visual information travels from the
retina deep into the brain through the
optic chiasma (not visible with an
opthalmascope).
 From here, it goes into the occipital
lobe of the brain, where it is processed.
Cranial Nerves II, III, IV, VI
OPTHALMASCOPE
 An
opthalmascope is the instrument
used to look inside the eye.
 The doctor can see the optic disc, fovea
centralis, macula, the lens, retina, blood
vessels, but of course, not the optic
chiasma, since that is on the brain surface,
external to the eye.
PROBLEMS WITH VISION
 FLOATERS
are when a capillary breaks
and cells break off. Floaters don’t actually
move, the eye just tries to track them.
Floaters
Floaters
Floaters
can best be
seen when
the person
looks at a
clear blue
sky or white
wall.
RETINAL DETACHMENT




The retina separates from the underlying choroid.
Retinal detachment can be caused from a blow to the
eye, or may occur spontaneously. Usually caused by an
injury like a blow to the eye with a baseball, punch, or
airbag to the eye.
It may not cause blindness immediately. Although
the detached portion contains capillaries, it is
separated from the main blood supply, so if it is not
lasered back into place immediately, permanent
blindness can result.
Cells in the retina die from lack of oxygen. Manifests as
a shimmering light. This is considered a medical
emergency and needs immediate treatment. Those
who are most vulnerable to spontaneous detachment are
those who are nearsighted.
HYPEROPIA (far-sighted)
eyes are too short
MYOPIA (nearsighted)
eyes are too long
The Eye as an Optical Device
Figure 16.14a–c
Hyperopia and Myopia


Normal eyes are perfect spheres. When the
eyeball is not a perfect sphere, the lens has to
accommodate as much as possible, and
corrective glasses are usually needed.
Myopic eyes are elongated (overhead projector
is in focus, but move it backward, gets fuzzy).
Even badly nearsighted eyes are only 1mm from
normal. Treatments are glasses or Lasix, which
is laser surgery on cornea, when it’s shaved so it
focuses light farther back to reach the retina.
Lasik Surgery
Hyperopia and Presbyopia

Hyperopia and presbyopia have some
features in common, but a key
difference between these two
conditions is that in hyperopia the
lens can accommodate, but in
presbyopia it cannot.
ASTIGMATISM
• ASTIGMATISM is when the cornea has
an irregular shape. Part of the field of
view is out of focus.
• The eyeball changes shape until age 24.
Astigmatism Test
Astigmatism Vision
MACULAR DEGENERATION


The size of the macula is the size of the printed
letter “O” in 14 pt font. When the macula
degenerates, you lose a lot of sight. This is the
most common cause of blindness in the US.
It’s due to bleeding in the eye, causing scar
tissue. The retina does not get enough oxygen,
and the cells die. Macular degeneration allows
vision in the periphery, but they can’t read or
drive.
Macular Degeneration
DIABETIC RETINOPATHY



This is when the high sugar levels destroy
the photoreceptors in the retina.
The blood vessels also swell and rupture
and the clots block vision.
Some of this damage can be repaired by
using a laser to evaporate the blood clots,
but any damage to the photoreceptors is
permanent. It can lead to blindness.
Diabetic Retinopathy
Diabetic Retinopathy
Diabetic Retinopathy vision
INTERNAL STRUCTURES
OF THE EYE

There are two cavities


Anterior Cavity
Posterior Cavity
Anterior Cavity

1. ANTERIOR CAVITY is anterior to the
lens, and is filled with AQUEOUS
HUMOR, similar to plasma, supplies
nutrients to the cornea and lens.
Figure 16.7a
GLAUCOMA



GLAUCOMA is increased pressure within
the anterior chamber of the eye. It leads
to blindness.
This form of blindness is more common in thirdworld countries because we have tests to detect
it and treat it.
The test measures how much pressure there is
here by seeing how easily the cornea is
deformed, either with air or direct pressure.
How many of you have had this test?
Glaucoma


2. POSTERIOR CAVITY is filled with
VITREOUS HUMOR, which is jelly-like,
and helps give shape to the eyeball. It
leaks out from a cut, you’ll go blind
because the body can’t replace it.
The four main things that cause
blindness are macular degeneration,
cataracts, glaucoma, and diabetic
retinopathy.
AMBLYOPIA



AMBLYOPIA = Lazy Eye. In a child, one eye
will track and focus, the other won’t.
If untreated in children, eventual blindness in
weak eye because the brain will shut down in
the occipital lobe.
Treatment is to patch the good eye to force the
bad eye to make the connections, or a surgery
to weaken the muscle to make the strong side
just as weak.
Other Eye Problems
Cancer of the Eye:
Choroidal Melanoma
Live worm in
the eye
NOTE



If a child is blind until age 4-5, and then
you restore the sight, he will still be blind
because the brain doesn’t form properly.
With kids who have astigmatism or weak
eye muscles, one eye stops seeing (or
sees double).
The thalamus in the brain will shut off all
the signals from the bad eye.
NOTE



To protect your eyesight throughout
your life, use the 20-20-20 rule:
Every 20 minutes look up for 20
seconds at something 20 feet away.
The rest of this lecture is not on the
exam!
The red lines are straight!
Look at
the dot
and
move
your
head
forward
and
back
Look at
the dot
and
move
your
head
forward
and
back
Stare at the + for 30 seconds and
the pink dots will disappear!
FUN FACTS ABOUT VISION




Why do you cry when you cut onions?
It releases an enzyme that reacts with the
amino acids in the onion that create an acid that
diffuses into the air and irritates your eyes.
Scientists have tried to make a non=crying
onion, but it turns out that enzyme is needed for
the onion flavor. You can heat the onion before
chopping or chop under running water. Or order
take-out.
Do cucumbers relieve puffy eyes?
No, they are just cool and filled with water, so
the cooling effect reduces the swelling.
FUN FACTS ABOUT VISION


Can carrots help improve your vision?
This myth dates back to WWII when the British Royal Air
Force was attempting to hide the fact that they
developed a new radar system to shoot down German
bombers. They bragged that the great accuracy of thie
British fighter pilots at night was a result was a result of
them being fed an enormous amount of carrots. It is
true that carrots contain beta carotene which converts to
vitamin A which is needed to prevent night blindness,
but we only need a small amount, and having more than
that will not improve vision. In fact, too many carrots
can cause your skin to turn orange.
FUN FACTS ABOUT VISION




Can you lose a contact lens in the back of
your head?
No; the underside of the eyelid is connected to
the sclera (white part of the eye).
Why do you get bags under your eyes
when you are tired?
The skin under the eyes is the thinnest in the
body, and it allows the dark, venous blood to
show through. Dark rings tend to be genetic and
get worse as you age. Good rest and nutrition
minimizes the rings.
FUN FACTS ABOUT VISION




Should you put a steak on a black eye?
It doesn’t do any more good than an ice pack
Does hysterical blindness really exist?
Yes. There is a device that can check for
someone faking blindness; they put a black and
white spinning pinwheel in front of the eyes, and
if you can see, your eyes will automatically move
a little back and forth.
FUN FACTS ABOUT VISION




Why do you see stars when you are hit in
the head?
It happens to Wile E. Coyote every time he gets
hit on the head with an anvil by the Road
Runner. Yes, it can happen, and it indicates
there is a concussion. The visual area of the
brain has hit against the inside of the skull.
Will staring at an eclipse make you go
blind?
The intense light burns some of the cells in the
retina, but it doesn’t cause complete blindness.
FUN FACTS ABOUT VISION



Why is it impossible to sneeze with your eyes
open?
Sneeze impulse affects a variety of body parts, including
the abdomen, chest, neck, and face. During a sneeze,
the impulses that travels through your face causes your
eyelids to blink. This response is entirely automatic.
There's nothing you can do about it. Sneezing puts a lot
of pressure on your head and respiratory system, so
blinking is probably a protective mechanism.
The point is that all of these responses (the abdominal
contraction, the sharp burst of air out of your lungs, the
general lunging movement) are intertwined.
FUN FACTS ABOUT VISION


Can a person who is blind from birth
"see images" in their dreams?
People who are visually impaired from
birth appear to lack visual imagery in their
dreams but have a “very high percentage
of gustatory, olfactory, and tactual
sensory references," something very
unusual for sighted dreamers to
experience.
FUN FACT OR FICTION?


Extensive computer usage can cause dry eyes.
Fact. Extensive computer use can lead to dry
eyes. A person experiencing minor amount of
dryness will feel much worse after prolonged
computer use. Studies have shown that
computer users tend to stare at the screen
without blinking for a long time, which may
cause dry eyes. Dry eyes are one of the factors
leading to Computer Vision Syndrome. Thus
frequent blinking is essential to lubricate the
eyes and prevent them from drying.
Eating carrots will
improve your vision

Fallacy. It is true that carrots are rich in
Vitamin A, which is an essential vitamin
for our eyes. However, we require only a
small amount of this vitamin for good
vision. A well- balanced diet, with or
without carrots, provides all the Vitamin A
necessary for good vision.
FUN FACT OR FICTION?


Sitting close to a television set, movie or
computer screen can harm your eyes.
Fallacy. Our eyes are not harmed by
viewing these at a short distance. There
is, however, a greater likelihood of
experiencing eye fatigue or a headache.
FUN FACT OR FICTION?


Reading in dim light is harmful to your
eyes.
Fallacy. For centuries, all night time
reading and sewing was done by
candlelight or with gas or kerosene lamps.
Reading in dim light does not damage the
eyes. However, good lighting does make
reading easier and prevents eye fatigue,
especially for people who wear bifocals.
Computers and eyestrain








Computers can put stress on the eyes causing headaches, blurred
vision and eye fatigue. The following steps can be taken to reduce
eyestrain:
Computer screen is at least 18 to 26 inches away from your eyes.
Arrange the monitor so that the top line of on-screen text is at eye
level. Placing a monitor too high exposes more of the eye, causing it
to dry out.
Arrange lighting to minimize glare and reflections.
Keep the computer screen clean and dust-free to minimize glare.
Take frequent vision breaks to stretch your body and rest your eyes.
Remember to blink often to keep your eyes moist.
Place reference material alongside and as close to the computer
screen as possible to avoid frequent head and eye movements and
focusing changes.
Television and your eyes






Never watch TV in a completely darkened room.
The best lightning conditions consist of a back
light and dim general light in the room.
Place the set to avoid glare and reflections from
lamps, windows and other bright sources.
View from a distance at least five times the
width of the television screen.
Have the set at approximately eye level.
Wear lenses prescribed for vision correction.
Avoid staring at the screen. Briefly look away
from the picture, around the room.
Avoid eye strain while reading

Keep the reading material at the best distance:
place knuckles under your chin and book under
the elbow. Try to avoid reading while lying on
your back, stomach, or side. Make sure that
there is good light on close work tasks and good
room light as well. A light positioned behind and
over one's shoulder works well for reading. Also,
looking up from time to time from your reading
can help keep your eyes relaxed.