Sensory System
 Allows us to experience the world.
 We experience the world through
our senses.
 Allows us to keep track of what is
happening in our bodies.
 Acts as a danger signal when we
come in contact with a harmful
 Five types of sensory receptors:
Cells that
detect stimuli
specialized area of a
sensory neuron that
detects a specific
 Chemoreceptors-stimulated by
changes in chemicals such as H+,
calcium and food. Ex. Taste and
 Pain receptors or nociceptors Ex. Receptors in the
eye respond to light.
Ex. Receptors in the
tongue respond to
chemicals in food.
stimulated by tissue damage or
distention. Ex. Pain.
 Thermoreceptors -receptors
stimulated by changes in
temperature. Ex. Heat and cold
 Mechanoreceptors-receptors
stimulated by changes in pressure
or movement of body fluids.
Ex. Hearing and equilibrium.
 Photoreceptors-receptors
stimulated by light.
Ex. Sight.
 Four components of a sensation:
The conscious
awareness of
Ex. “Ouch”indicates that
you are aware
of a painful
(1) Stimulus-light will stimulate
sight. Absence of light, you
cannot see.
 (2) Receptor-light waves
stimulate photoreceptors in the
eye producing a nerve impulse.
 (3) Sensory nerve-the nerve
impulse is conducted to the
occipital lobe of the brain.
s of sensation
 Projection-the process by which
the brain, after receiving a
sensation, refers that sensation
back to its source. Ex. Pain
 Adaptation-adapting to the
stimulus. Ex. When you enter a
room with a strong odor, it may
be overwhelming at first and then
become less noticeable. Sensory
receptors adapt.
Two Groups of Senses
 General senses: called
general or somatic.
 Widely distributed
throughout the body.
 Special senses:
 Localized within a
particular organ in the
 Include:
 Include:
 Pain
 Taste
 Touch
 Smell
 Pressure
 Sight
 Temperature
 Hearing
 Proprioception
 Balance
Five Special Senses
 Five special senses:
 Smell
 Taste
 Sight
 Hearing
 balance
 Pain-receptors for pain,
called nociceptors consist
of free nerve endings that
are stimulated by tissue
 Pain receptors are widely
distributed throughout
the skin, the visceral
organs, and other
internal tissues.
 Serves a protective function, it
motivates the person to remove
its cause.
 Patient’s with diminished pain
response are at risk .
 Diabetic patients have a
decreased pain sensation due to
nerve damage. They are unable to
feel pain in lower extremities.
They require meticulous foot care.
Referred Pain
 Referred Pain-when pain
feels as if it is coming
from an area other than
the site where it
Ex. Heart pain radiates
down arm.
 Analgesics-drugs to
relieve pain.
 Three pain triggers:
Pain Triggers
 (1) Tissue injury promotes the
release of certain chemicals that
stimulate pain receptors.
 (2) A deficiency of oxygen
stimulates pain receptors.
 (3) Pain may be experienced
when tissues are stretched or
 Touch receptors or tactile
receptors are found mostly
in the skin.
 They allow us to feel.
 Particularly numerous in
the lips, fingers, toes,
tongue, and external
 Located in the skin,
subcutaneous tissue, and
deep tissue.
 Two types of thermoreceptors
are heat and cold.
 Found in free nerve endings,
and specialized sensory cells
beneath the skin.
 Cold receptors are stimulated
between 50 and 76 degrees F
and heat receptors are
stimulated between 76 and
112 degrees F.
 Sense of orientation or
 Allows you to locate your
body part without
looking at it.
 Plays an important role
in maintaining posture
and coordinating body
 The receptors for
proprioception are located
in muscles, tendons, and
 Proprioceptors are also
found in the inner ear,
where they function in
 The cerebellum receives
sensory information from
these receptors.
Sense of Smell
 Olfaction-sense of smell,
associated with the
sensory structures
located in the upper
 Classified as
stimulated by chemicals
that dissolve in the
moisture of the nasal
Sense of Smell
 The olfactory receptors
are stimulated , the
sensory impulses travel
along the olfactory nerve
(CN I).
 The sensory information
is eventually interpreted
as smell within the
olfactory area of the
temporal lobe.
Sense of Taste
 The sense of taste is also
called the gustatory
sense or gustation.
 The taste buds are the
special organs of taste.
 Taste receptors are
located in the tongue and
are classified as
Sense of Taste
 Four basic taste
 Sweet-tip of the tongue.
 Salty-near the tip of the
 Sour-located in the middle
of the tongue.
 Bitter-back of the tongue.
Sense of Sight
 Vision is one of the most
cherished senses.
 The eyes are the organs
of vision.
 Visual accessory organs
assist with function and
protecting the eye from
Visual Accessory Organs
 Visual accessory
organs include the:
 Eyebrows
 Eyelids
 Conjunctiva
 Eyelashes
 Lacrimal apparatus
 Extrinsic eye muscles
Visual Accessory Organs
 The eyebrows:
 Patches of hair located
above the eyes.
 Perform a protective
 Keep perspiration out of
eyes and shade eyes from
glaring sunlight.
 Participate in facial
expression as in the
“raised eyebrow look”.
Visual Accessory Organs
 Eyelids:
 Also called palpebrae.
 Protect the eyes.
 Prevent the entrance of
foreign objects.
 Wash tears over the
surface of the eye.
 The medial inner canthus
and the lateral outer
canthus are the corners of
the eye where the upper
and lower eyelids meet.
Visual Accessory Organs
 Skeletal muscle opens
and closes the eyelids.
 The muscle that opens
the eyelid is called the
levator palpebrae
(levator means to raise,
like an elevator).
 The muscle that closes
the eyelid is the
obicularis oculi.
Visual Accessory Organs
 Conjunctiva-thin mucous
membrane that lines the
inner surface of the
 The conjunctiva also
folds back to cover a
portion of the sclera
(discussed later) on the
anterior surface of the
eyeball and is called the
white of the eye.
Visual Accessory Organs
 The conjunctiva secretes
a substance that
moistens the surface of
the eye.
 The anterior surface of
the eye must be kept
moist or it will ulcerate
and scar.
 The conjunctiva is very
Visual Accessory Organs
 The “Bloodshot”
appearance of the eyes is
caused when the blood
vessels dilate.
 Eye drops that get the
red out, cause the blood
vessels of the conjunctiva
to constrict.
 Conjunctivitis-”pink eye”
inflammation of the
conjunctiva caused by a
bacterial infection.
Visual Accessory Organs
 Eyelashes:
 Line the edges of the
eyelid and help to trap
 Touching the eyelash
stimulates blinking.
 Sty or hordeolum -
infection of the hair
follicle caused by staph.
Visual Accessory Organs
 Lacrimal Apparatus:
 Composed of the lacrimal
gland and a series of
ducts called tear ducts.
 The lacrimal gland is
located in the upper
lateral part of the orbit
and secretes tears which
flow across the surface
toward the nose.
Visual Accessory Organs
 Tears drain through
small openings called
lacrimal puncta and then
into the lacrimal sac and
nasolacrimal ducts.
 The nasolacrimal ducts
eventually empty into the
nasal cavity.
Visual Accessory Organs
 Normal tears flow to the back
of the throat and are
 Increase in tears, as in crying,
nose begins to run.
 Tears perform several
functions, moisten, lubricate
and cleanse the surface of the
 Tears contain losozyme which
helps destroy pathogens and
prevent infection
The Eyeball
 Has a spherical shape and
is approximately ¾ to 1
inch in diameter.
 Most of the eyeball sits
within the bony orbital
cavity of the skull,
partially surrounded by a
layer of orbital fat.
 Composed of three layers:
sclera, choroid, and the
The Sclera
 The outermost layer.
 Made up of tough, fibrous,
connective tissue that
covers most of the eyeball.
 Helps contain the
contents of the eye.
 Shapes the eye and is the
site of attachment for
extrinsic muscles.
The Cornea
 Cornea-transparent
extension of the sclera.
 Covers the area over the
 Cornea is avascular and
transparent, light rays can
go through this structure.
 Called the window of the
The Cornea
 The cornea has a rich
supply of nerve fibers ,
sensitive to touch.
 Corneal reflex-protective
function, if the surface of
the cornea is touched
lightly, causes blinking.
The Choroid
 The Choroid is the middle
layer of the eye located
between the retina and the
 Highly vascular.
 Attached to the innermost
layer of the retina.
 Extends toward the front of
the eyeball to form the ciliary
The Choroid
 The ciliary body secretes
a fluid called the aqueous
 Gives rise to the ciliary
 Iris-most anterior
portion of the choroid,
the colored portion of the
The Pupil
 Pupil-opening or hole in
the middle of the iris.
 Pupil size is regulated by
two sets of intrinsic
muscles located in the
 Iris regulates the amount
of light entering the eye.
The Retina
 Innermost layer of the
 Nervous layer containing
visual receptors which
are sensitive to light,
called photoreceptors.
 Two types of
photoreceptors are rods
and cones.
The Retina
 Rods, scattered
throughout but most
abundant along
 Cones, most abundant in
the central portion of the
 Fovea centralis area that
contains the highest
concentration of cones.
The Retina
 The Optic Nerve forms
where the neurons of the
retina converge in a small
circular area in the back of
the eye called the optic disc.
 Because there are no
photoreceptors there,
images formed there are not
seen by the brain. This area
is called the “Blind Spot”
The Eye
Cavities of the Eye
 Anterior Cavity
 Posterior Cavity
 Located between the
 Larger and located
Lens and Cornea
 Filled with Aqueous
 Aqueous Humor helps
maintain shape of
anterior eye and
nourishes the cornea
between the Lens and
 Filled with Vitreous
 Vitreous Humor pushes
on the retina creating
good contact with the
Choroid ensuring a rich
supply of oxygenated
Eye Muscles
 Extrinsic eye muscles:
 Skeletal muscles.
 Located outside the eye.
 Move the eyeball in
various directions.
 Eyes move together in a
coordinated way.
 Light waves enter your
eye, are refracted and
focused on the
photoreceptors of the
 Photoreceptors of the
retina translate light
impulses, to a nerve
impulse, which is then
transmitted from the
retina, along the optic
nerve to the occipital lobe
of the brain.
Muscles of the Eye
 Strabismus:
 Also called cross-eyed.
 Eyeballs are not
aligned, do not focus
on a desired point.
Muscles of the Iris
 Iris contains two muscles:
Radial muscle
 Circular muscle
 Contraction of the radial
muscle causes the pupil to
 Contraction of the circular
muscle causes pupil to
 Mydriasis-sympathetic nerve
stimulation that causes pupillary
 Mydriatic agents-drugs that cause
the pupils to dilate.
The Eye
Night vision
 Rods respond to dim
 Provide us with black
and white vision.
 Deficiency of vitamin A
can cause night
Day vision
 Image produced by the
stimulation of cones.
 Three types of cones that
produce different colors,
green, blue and red.
Stimulation of
combinations of these
colors produces different
The Eye
Not seeing
 Light waves enter your
 Defect along the
 Photoreceptors translate
light signal to nerve
impulses and then
transmitted from retina,
along optic nerve to
occipital lobe.
pathway from the
cornea to the brain can
interfere with vision.
 Ex. Scarred cornea or
cloudy lens may block
Conditions of the Eye
 Glaucoma-increased
intraocular pressure may
squeeze the blood vessels
of the choroid, depriving
the retina of adequate
blood supply.
 Can lead to blindness.
Conditions of the Eye
 Detached retina-retina
falls away from the
choroid and it’s blood
 Macular Degeneration
Abilities of the Eye
 Accommodation- the ability of the lens to change
shape in order to focus on close objects
Presbyopia- the inability of the lens to change shape
in order to focus on close objects
Convergence- movement of the eye medially (toward
the nose) to stay focused on an object
Photopupillary Reflex- When the pupil of the eye
changes size to either limit or increase the amount of
light entering the eye
Refraction- the bending of light rays
Consensually- what one pupil does…so does the other
Conditions of the Eye
 Diabetic retinopathy-
damage of the retinal
blood vessels of diabetic
 Vessels may develop
micro aneurysms which
rupture and cause
bleeding and scar
formation throughout
the retina.
The Ear
 Ear is divided into three
 (1) external ear
 (2)middle ear
 (3)inner ear
External Ear
 Part of the ear you can
 Composed of the auricle
and the external auditory
 Auricle or pinna-
composed of cartilage
covered by a layer of
loose fitting skin.
The Ear
 Auricle opens into the
external auditory canal.
 Canal provides a
passageway for sound
waves to enter the ear.
 External auditory is
hollowed out of the
temporal bone.
The Ear
 External auditory canal
is about 1 inch long and
½ inch wide and extends
to the eardrum
(tympanic membrane).
 Tympanic membrane
separates the external
ear from the middle ear.
The Ear
 External auditory ear
canal is lined with tiny
hairs and glands that
secrete cerumen.
 Hairs help prevent dust
and foreign objects from
entering the ear.
 Objects inserted to
remove cerumen may
damage the tympanic
Middle Ear
 Small air-filled chamber
located between the
tympanic membrane at
one end and a bony wall
at the other end.
 Contains:
 Tympanic membrane
 Three tiny bones
 The eustachian tube
Middle Ear
 Tympanic membrane is
composed of connective
tissue and has a rich
supply of nerves and
blood vessels.
 Tympanic membrane
vibrates in response to
sound waves that enter
the ear through the
external auditory canal.
Vibration is passed on to
tiny bones.
Middle Ear
 Middle ear contains the
three tiniest bones in the
 (1)malleus (hammer)
 (2)Incus (anvil)
 (3)Stapes (stirrup)
The Ear
 If pressure becomes
unequal, tympanic
membrane bulges.
 Membrane streches
causing pain. Ex. Pain
and pressure in an
Inner Ear
 Consists of an intricate
system of tubes hollowed
out of the temporal bone
called the bony labyrinth.
 Inner ear has three parts:
 (1) vestibule
 (2) semicircular canals
 (3) cochlea
Inner Ear
 Cochlea-snail shaped
part of the bony
 Inside the cochlea are the
receptors for hearing,
they are tiny cells that
contain tiny hairs and
are called the organ of
 Hearing:
 Sound waves travel
through the external
auditory canal and hit
the tympanic membrane.
 Vibration causes malleus,
incus and stapes to
 Stapes causes the fluid in
the inner ear to move.
 This causes the hairs to
 The bending of the hairs
triggers the nerve
impulse carried by the
cochlear branch of the
vestibulocochlear nerve,
(CN VIII) to the brain.
 Temporal lobe of the
cerebrum interprets the
impulse as sound.
 Receptors for balance are
 Vestibule contains the
receptors that provide
information about the
position of the head at
 Semicircular canalas
provide information
about the position of the
body as it moves.
 Receptors sense
changing positions of the
 When position changes
the hairs are bent.
 Receptor cells send
nerve impulses to the
 Brain sends the signal to
restore balance.

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