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THE SENSORY SYSTEM
GENERAL SENSES
AND
SPECIAL SENSES
Sensory Receptor
 Dendrite of sensory neuron or
specialized cell that monitors
conditions in or around the body and
sends information (sensation) to the
CNS
 Special Senses—receptors localized in
a specific organ
 General Senses—receptors are all
over the body.
Sensation and Perception
 Sensation- feeling or awareness of a
stimuli
 Perception-the conscious interpretation
of stimuli and involves the cerebral
cortex
 Adaptation-with all receptors, long term
stimulation leads to adaptation causing
the perception of sensation
to be less acute
Sensation and Perception
 Mental state affects perception. Think
of the perception of pain when we are
sad and the perception of pain when
we are happy, active and busy doing
something.
 Sometimes will see or feel things that
aren’t there (ex. Phantom limb pain;
auras before epileptic seizures)
General Senses
 1. Temperature – separate receptors for
hot and cold
 2. Pressure-we can feel pressure even
when skin is anesthetized. Receptors are
found deep within the body as well as on
the surface.
 3. Touch-Preceptors are mostly in the
dermis. Especially numerous on the
fingertips, toes, lips, and the tip of your
tongue
General Senses
 4.Pain- our most protective sense.
The two types use the same
receptors, but have different neural
pathways.
 Two types:
 A. acute, sharp pain
 B. slow, chronic pain
Analgesic- pain relieving drug
1. Non-narcotic analgesics (decrease
inflammation) also known as NSAIDS
example; acetaminophen (Tylenol,
aspirin, ibuprofen
2. Narcotics- affect CNS and are habit
forming- change the response to
pain. Example; morphine
and Demerol
Pain Relievers
 Anesthetics- block pain sensation
 Endorphins- natural brain chemicals
that inhibit pain centers in the brain
 Heat and cold application
 Relaxation and distraction
techniques
Pain Tolerance vs. Pain Threshold
 Everyone has about the
same pain threshold ( point
at which stimulation
produces pain), but
we have widely
varying tolerance to
pain
Referred Pain
 Pain felt in a part of the body other
than the place being stimulated.
Examples; heart attack pain is often
felt in the left arm rather than in the
chest. Appendicitis is sometimes felt
at the umbilicus rather than in the
lower right quadrant
5.Propriception
 Sense of body position
 Cerebellum is the main
coordinating center.
 Propriception monitors the position of
joints and muscles involved in
posture and balance
Special Senses





Vision
Hearing
Equilibrium
Taste (gustation)
Smell (olfaction)
The Eye and Vision
 Eye is protected by:
skull bones
eyelids and eyelashes
tears- lubricate and
wash debris away
conjunctiva- the membrane
covering the visible portion of the
eyeball and lining the eyelids
3 Layers (tunics/coats)
of eye
 1. Sclera- outer layer of
tough connective tissue
Contains the white of the eye (sclera) and cornea
(clear). Gives shape to the
eye. Muscles attached here
help eye to move.
 Cornea- clear covering
that bends light and is
the first step in focusing
3 Layers (coats/tunics)
of the eye
2.Choroid- middle layer,
Made of Connective tissue
with blood vessels. Contains
the iris (the colored Part of
eye- the muscle
surrounding the pupil)
3 layers (coats/tunics)
of the eye
3. Retina- inner layer. Contains visual
receptors and neurons. Rods and cones
are receptors for vision
Contains many layers of nerve cells that
transmit the impulses from the light
rays to the optic nerve
Rods- used for vision in dim or dark
light and give a less sharp image
Cones- used in bright light and are
sensitive to color (red, green, blue)
Retina continued
 Dark adaptation- going from dim to
bright light or bright to dim light. It
takes some time for vision to adjust
(adapt) to a change in available light.
 Color blindness caused by the lack of
one or more types of cones
Retina
Choroid
Cornea
Sclera
Fovea
Lens
Aqueous
humor
Iris
Optic Disc
Vitreous
humor
Zonules
Ciliary
Body
Refraction
 Bending of light—the path light follows through the
eye
 1. Cornea-part of sclera
-transparent
 2. Aqueous humor-fluid
between the cornea and
lens
 3. Lens-clear-changes shape to
focus light on the retina
 4. Vitreous humor-gelatnous-helps give eyeball its
shape
Eye Muscles
 Intrinsic-within the eye
 Iris-colored part-the
muscle controlling the size of the pupil
(opening) which regulates the amount
of light entering the eye
 Ciliary Body-surrounds the lens and
changes the shape of the lens, helps
to focus eyes
Eye Muscles
Accommodation-allows the eye to focus on
near objects- the muscle contracts, the lens
changes shape- gets thicker
Accommodation decreases with aging
Extrinsic- 6 muscles around the outside the
eye. They move the eye
Nerve Supply to Eye
 Optic Nerve
(Cranial Nerve II)-takes info from the retina to
the brain
 Optic disc-area of retina where the optic nerve
attaches- no rods no cones -“blind spot”
 Fovea centralis-near optic disc-indented area on
retina. Contains only cones. Area of sharpest vision
 1 Branch of Trigeminal Nerve (Cranial Nerve V)takes sensation of pain, touch and temperature from
the eye to the brain
Accessory Structures of the Eye
 Conjunctiva-membrane that covers the
inner surface of the eyelids and the front of the
eye
 Lacrimal glandmakes tears
Lacrimation=crying
Lubricates the conjunctiva and
outer part of eye, decrease friction,
Prevents infection and nourishes the conjunctiva
Opthalmologist
 Physician who deals with the diseases
and disorders of the eye
Surgical specialty
Optician- makes glasses
Optometrist- gives eye exam
Eye Disorders
 Conjunctivitis-inflammation of
the conjunctiva. “Pink Eye”
very contagious.
Cause: viruses, bacteria, irritants, allergies
The ‘pink’ color is due to the dilation of
blood vessels within the eye
 Stye- bacterial infection of
a sebaceous gland along the
eyelashes
Eye Disorders
 Chlamydia trachomatis-
chlamydia organism causes STD and
inclusion conjuctivitis. Scars the
conjunctiva and cornea and leads to
blindness. In developing countries this is
called ‘trachoma’ and is chronic.
Treatment: antibiotics and good hygiene
 Ophthalmia neonatorum- eye
infection newborns can get during birthcaused by gonorrhea, chlamydia or
other STD
Eye injuries
 Scratch or laceration of the corneavery common in contact lens wearers
(don’t sleep in contacts or wear them
multiple days in a row without
removing and cleaning)
 Enucleation-removal of the eyeball
Vision Defects
 Hyperopia- farsightedness.
Eyeball is too short or
shallow. Focal Point is behind
the retina. See far objects
well and near objects poorly
 Myopia- nearsightedness.
Eyeball is too long. Focal
point is in front of the retina
Visual Defects
 Astigmatism –cornea and /or
lens surface is irregular
resulting in incorrect
refraction and blurred vision
 Presbyopia –(old eye) lens
loses its elasticity as people
age resulting in
farsightedness. The Lens does
not accommodate well
Visual Defects
 Treatment for all:
 corrective lens
 radial keratotomy (RK),
 photorefractive keratectomy (PRK)—
laser surgery.
 RK and PRK involve making radial
incisions into the cornea to correct
refraction.
Vision Defects (continued)
 Strabismus-eyes don’t work together. Extrinsic
muscles are not coordinated. Convergent
 Eye wanders toward the median.
 Divergent strabismus- eye wanders to the side.
If untreated, eventually the brain will not ‘see’
images from the affected eye. Treatment- eye
patch, glasses, surgery
Vision Defects
 Cataracts-lens becomes cloudy,
Not transparent. Leads to a
gradual loss of vision and
blindness. Treatment- surgery to
remove the old lens and implant
artificial lens
Vision Defects
 Glaucoma- excess pressure in
aqueous humor.
 Most common in those over 40.
 Those who have this disorder will often see bluegreen halos around light.
 Increased pressure leads to blindness because
of pressure on the optic nerve.
 Ophthalmologists and optometrists test pressure
in the eye by bouncing a tiny blast of air off the
surface of the eye.
 Treatment- drugs- eye drops
Vision Defects
 Retinal Problems
 Diabetic Retinopathy- one of the leading
causes of blindness in the U.S.
 Diabetes damages the retina because of
changes in blood vessels
 Retinal detachment- retina separates from
choroid.
 Cause: trauma or increased fluid between
choroid and retina.
 Leads to blindness.
 Treatment: surgery using laser in spots to
create scar tissue to connect retina to choroid
Vision Defects
 Macular degenerationhardening and blockage of
blood vessels in retina leading
to blindness.
 New vessels form to replace
damaged vessels- but the new
vessels form on the retinal
surface blocking light to the
cones
Vision Defects
 Floaters- spots in your
vision
 Gray or black specks
 Vitreous humor becomes
more liquid
 Microfibers within vitreous
tend to clump together
casting shadows on the
retina
The Ear and Hearing
 Organ of hearing and equilibrium
The Ear
 3 main parts
 External ear
 Middle ear
 Inner ear
External Ear
 Pinna (auricle)- directs sound waves into ear
 External auditory canal-directs sound waves
deeper into the ear
 Ceruminous glands
produce cerum(ear wax)
 X = external
meatus(opening)-
entrance to the auditory
canal
Pinna
auditory
X
External
auditory
canal
Incus
Semicircular
canals
Pinna
Cochlea
Malleus
Eustacian
or
Pharyngotympanic
or auditory
tube
Tympanic
membrane
vestibule
stapes
External
Auditory
Canal
Middle Ear
 Tympanic Membrane-dividing point
between external and middle ear.
Vibrates with sound
waves.
 Eustacian/auditory/
pharyngotympanic tubetube goes from the
tympanic membrane to
the pharynx (throat)equalizes pressure
Middle Ear
Auditory Ossicles- sends sound waves to
the inner ear
 3 tiny bones that
amplify sound waves
 Malleus
 Incus
 Stapes
Inner Ear
 Within temporal bone
 3 parts:
1. Vestibule-connected
to the oval window
2. Semi-circular canalsorgan of equilibrium
3. Cochlea-organ of
hearing
Ear Disorders
 Otitis Mediainflammation of the
middle ear. Often caused by
an upper respiratory infection. Pathogen goes up
Eustacian tube- attracts white blood cells which
leads to the ear filling with pus. If untreated,
tympanic membrane can rupture leading to
hearing loss. Treatment: antibiotics
Ear Disorders
 Mastoiditis- pathogens from middle
ear get into air pockets in the
mastoid process of the temporal
bone. Leads to severe earache, fever,
swelling behind the ear. Untreated
can spread to brain.
Hearing Loss
 Conductive Deafness-problem with
the passage of sound waves through
the ear.
Treatment: hearing aid, surgery to repair
tympanic membrane or free auditory
ossicles. Sometimes artificial ossicles
are implanted
Hearing loss
 Sensorineural Deafness- problem is in
the cochlea or along the neural
pathway.
 Treatment: few effective treatments.
Cochlear implants
Taste (gustation)
 Taste buds—clusters of taste
receptors
 Gustatory cells live only about 10
days
 Receptors more responsive to bad
tastes than good
4 Primary Tastes




Sweet
Salt
Sour
Bitter
Gustation and olfaction work
together. We are more
aware of tastes when we
can smell
Taste
 Tasting ability decreases with age-elderly
people sometimes find food to be tasteless
which leads to poor eating habits. They can
often taste sweet thing when other taste
sensations have diminished.
 Children have lots of very active taste
receptors, so they often don’t like spicy or
strong flavored foods.
Smell (olfaction)
 Olfactory receptors located
in upper part of nasal cavity
With inhalation , a small amount of
air passes over the olfactory receptor. The chemicals
in the air diffuse into the mucus of the nasal cavity
and then stimulate the olfactory receptors
 At least 50 primary smells
 With age the # of receptor decreases, so
smell becomes less acute