Sense Organs
Anatomy and Physiology
Senses
Sense organs fall into two major
categories; sense organs and special
sense organs
– Sense organs refer to receptors that
function to produce general senses such
as touch, temperature, and pain to
initiate reflexes
– Special sense organs function to
produce vision, hearing, balance, taste,
and smell
Sense Organs
Sense organs are called sensory
receptors
The general function of receptors are
to respond to stimuli by converting
them to nerve impulses
Receptor potential occurs when an
adequate stimulus acts on a receptor;
when it reaches a threshold, it results
in an action potential (impulse) in the
sensory neuron
Sense Organs
Receptors can adapt to the stimuli,
meaning that the magnitude of the
receptor potential decreases over a
period in response to a continuous
stimulus
Receptors are classified according to
their location in the body, the
specialized stimulus that causes
them to respond, and their structure
Receptors Classified By Location
Exteroceptors are located near the body
surface and respond to external stimuli; these
are sometimes referred to as cutaneous
receptors
Visceroceptors or interoceptors are located
internally; they function in response to
internal organ stimuli
– Proprioceptors are specialized and are found in
skeletal muscle, joints, and tendons. They provide
information about body movement, orientation,
and muscle stretch
Tonic receptors allow us to locate parts of our body
without having to look at it
Phasic receptors allow us to feel change in body positions
Receptors Classified By Stimulus
Detected
Mechanoreceptors respond to
mechanical stimuli that change
position of the receptor itself
– Pressure applied to skin, stretching
muscles, etc
Chemoreceptors are activated by the
amount or the changing
concentration of certain chemicals
– Taste, smell, pH, blood glucose
levels,etc
Receptors Classified By Stimulus
Detected
Thermoreceptors are activated by
changes in temperature
Nociceptors are activated by intense
stimuli that results in tissue damage;
pain is produced
– May be caused by toxic chemicals, light,
sound, pressure, heat
Photoreceptors are only found in the
eyes; they respond to light
Receptors Classified By Structure
Free nerve endings are the simplest, most
common and most widely distributed
sensory receptors; these include
exteroceptors, visceroceptors, and
nocioceptors
Brain tissue lacks free nerve endings and
is incapable of sensing painful stimuli
Slightly modified free nerve endings are
responsible for itching, tickling, touch,
movement, and mechanical stretching
Receptors Classified By Structure
Stimulated free nerve endings almost
always result in the feeling of pain
and is a first indication of injury or
disease
– Acute pain fibers mediate a sharp,
intense and localized pain sensation
– Chronic pain fibers mediate a less
intense, but more persistent pain (dull/
aching)
Receptors Classified By Structure
Root hair plexuses are free nerve
endings associated with the hair
follicles and detect hair movement
Merkel discs are responsible for
sensing light touch
There are six types of encapsulated
nerve endings; they all have some
type of connective tissue capsule
that surrounds the dendritic ends
Receptors Classified By Structure
Meissner’s corpuscles are responsible
for sensations of light touch and lowfrequency vibration
– These are concentrated in the hairless
skin areas; lips, fingertips, etc
Krause’s end bulbs respond to light
touch and low-frequency vibrations;
it is also suggested that they are
stimulated in temperatures below
65°F
Receptors Classified By Structure
Ruffini corpuscles are located deep in
the dermis and mediate the sense of
crude and persistent touch; these
receptors are stimulated by
temperatures ranging from 85° to
120° F
Pacinian corpuscles are found deep in
the dermis and respond to deep
pressure, high-frequency vibrations
and stretching
Receptors Classified By Strucure
Muscle spindles consists of grouping
muscle fibers that are encapsulated and
contain sensory nerve fibers; these are
anchored to the connective tissue of
skeletal muscle
– these provide monitoring of the strength of
muscle contractions and stretching of muscles
Golgi tendon organs are located between
muscle tissue and tendons; they react to
excessive muscle contraction and cause
the muscle to relax
– This protects muscles from tearing and pulling
away from the tendinous points of attachment
Olfactory
The olfactory neurons are bipolar and have
cilia that touch the surface of the
epithelium lining the nasal cavity
Olfactory receptor neurons are
chemoreceptors
Olfactory receptors are replaced on a
regular basis by germinative basal cells in
the olfactory epithelium
Receptor potentials are generated when
receptor neurons are exposed to gas
molecules or chemicals dissolved in the
mucus covering the nasal epithelium
Olfactory
Olfactory neurons rapidly adapt to
continuous stimulation due to
inhibition of action potentials by
specialized granule cells in the
olfactory bulb
Olfactory Pathway
1. Chemicals dissolved in mucus surrounds
the olfactory cilia and a threshold is
reached
2. Receptor potential activated
3. Action potential of neuron generated and
passed to the olfactory nerves in the
olfactory bulb
4. Impulses pass through the olfactory tract
to the thalamic and olfactory centers of
the brain for interpretation, integration
and memory storage
Gustatory
Gustatory receptors respond to taste
stimuli
Most taste buds are associated with
the elevated projections of the
tongue called papillae
– Filiform papillae do not contain taste
buds but allow you to experience food
texture
Gustatory
Taste buds are stimulated by chemicals
dissolved in saliva; taste buds house
chemoreceptors
Each taste bud contains gustatory cells
that have cilia; cilia project into a taste
pore that is bathed in saliva
All tastes can be detected in all areas of
the tongue; It is believed, however,
that each type of taste bud responds
most effectively to one of the four
primary taste sensations (bitter, sweet,
sour, and salty)
Gustatory Pathway
1. Taste sensation begins with the creation
of the receptor potential in the gustatory
cells
2. The generation of an action potential
then transmits sensory input to the brain
1. Nerve impulses generated in the anterior 2/3
of the tongue travel over the facial nerve
2. Nerve impulses from the posterior 1/3 of the
tongue are conducted by the
glossopharyngeal nerve
3. The vagus nerve also plays a minor role in
taste; these sensations would begin in the
walls of the pharynx and epliglottis
Gustatory Pathway
3. All three cranial nerves carry the
impulses to the medulla oblongata
4. Relays carry impulses to the
thalamus and then into the taste
areas of the cerebral cortex in the
parietal lobe of the brain
Auditory and Balance
The ear has a dual function; it plays a
role in hearing, but it also functions as
the sense organ for balance and
equilibrium
Specialized mechanoreceptors are
responsible for both functions; the
mechanoreceptors are called hair cells
The ear is divided into three anatomical
parts; external ear, middle ear, and the
inner ear
Auditory and Balance
The external ear has a flap called the
auricle or pinna
The tube leading from the auricle to the
temporal bone is named the external
auditory meatus (ear canal)
Modified sweat glands are located in the
auditory canal; they secrete cerumen
– When the glands become impacted it results in
pain and temporary deafness
At the end of the auditory canal is the
tympanic membrane (eardrum) it
stretches across the end of the ear canal
separating it from the middle ear
Auditory and Balance
The middle ear (tympanic cavity) contains
three small bones called the auditory
ossicles; malleus (hammer), incus (anvil),
and stapes (stirrup)
The malleus attaches to the inner surface
of the tympanic membrane, the other end
attaches to the incus, which then attaches
to the stapes; the stapes then connects to
the oval window
Auditory and Balance
The middle ear has 4 openings
– The opening from the ear canal that is covered
by the tympanic membrane
– The oval window which is a opening into the
inner ear, this is also where the stapes fits
– The round window which is an opening into the
inner ear, it is covered by a membrane
– The eustachian tube (auditory tube) connects
the ear to the back of the throat
The openings are routes for infection and
are a concern of physicians
Auditory and Balance
The inner ear is called the labyrinth
because of its shape; it consists of two
main parts the bony labyrinth and the
membranous labyrinth
The bony labyrinth consists of three parts;
the vestibule, cochlea, and semicircular
canals
The membranous labyrinth consists of the
utricle, saccule, cochlear duct, and the
semicircular canals
– The utricle, saccule, and semicircular canals
are involved in balance
– The cochlea is involved in hearing
Auditory and Balance
Endolymph refers to the fluid in the
membranous labyrinth
Perilymph refers to the fluid that surrounds
the membranous labyrinth and fills the space
between it and the bony walls around it
Cochlea means “snail”; it describes the shape
of this part; the cochlea consists of cell
bodies for sensory neurons
– The cochlear duct is the only part of the internal
ear concerned with hearing
– The roof of the cochlear duct is known as the
vestibular membrane or Reissner’s membrane
– The floor of the cochlear duct is know as the
basilar membrane
Auditory and Balance
The hearing sense organ, The Organs of
Corti, rests on the basilar membrane
along the entire length of the cochlear
duct
– Dendrites of sensory neurons are associated
with the Organs of Corti; the axons of the
sensory neurons extend to form the cochlear
nerve
– Cochlear implants are used to assist deaf
persons if the Organs of Corti are damaged
Auditory Pathway
Sound is created by vibrations in
air, fluid, or solid material
The amplitude of a sound wave
determines its perceived loudness
The number of sound waves during
a specific period of time (frequency)
determines pitch
Auditory Pathway
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Sound waves enter the external auditory canal
and strike the tympanic membrane
Vibrations from the tympanic membrane move
the malleus, that then moves the incus, then the
stapes
The stapes fits against the oval window, so when
it moves it exerts pressure on it ; this causes a
ripple effect in the perilymph of the scala
vestibuli
The ripple is transmitted through the vestibular
membrane to the endolymph inside the cochlear
duct and then to the Organ of Corti and the
basilar membrane
The ripple then moves through the perilymph
(scala tympani side) to the round window (the
end point)
Auditory Pathway
6. The movement of the hair cells and
thus the stimulation of the organ of
corti results in an impulse being
conducted from the cochlear nerve
to the brainstem
7. Auditory impulses go through relay
stations in the medulla, pons,
midbrain, and thalamus before it
reaches the auditory area of the
temporal lobe of the brain
Balance
Sense organs involving balance, or
equilibrium, are found in the vestibule and
the semicircular canals
The utricle and the saccule function in
static equilibrium, a sense that explains
the position of the head relative to gravity
and the sense of acceleration and
deceleration of the body
The sense organs associated with dynamic
equilibrium are the semicircular canals;
they function to maintain balance
Static Equilibrium
The macula is a specialized epithelium
containing receptor hair cells and supporting
cells that are covered with a gelatinous
matrix
– Movement of the macula provides
information relating to head position or
acceleration
– Otoliths are composed of calcium carbonate
and are located in the maculas; turning the
head causes the otoliths to change position
and thus stimulates hair cells and neurons
Dynamic Equilibrium
The crista ampullaris is located in the
ampulla of each semicircular canal. It is a
specialized sensory epithelium
– The crista is composed of hair cells that are
embedded in a gelatinous cap, the cupula
– The cupula serves as a float that moves with
the flow of the endolymph
– As the cupula moves it creates receptor
potential, then an action potential. The
impulse passes through the vestibular portion
of the 8th cranial nerve. It reaches the medulla
oblongata and then to other areas of the brain
and spinal cord for interpretation, integration,
and response
Optic
The eyeball has three layers the
sclera, choroid, and retina
The sclera is made up of tough white
fibrous tissue
The anterior portion of the sclera is
the cornea, it lies over the iris.
– The iris is the colored part of the eye
– The cornea is transparent, whereas the
rest of the sclera is white and opaque
Optic
The cornea and the lens lack blood vessels
The Canal of Schlemm is a venous sinus
that drains the aqueous humor in the eye
– If the aqueous humor is produced faster than
it can be drained, glaucoma results
The choroid coat of the eye contains blood
vessels and pigment. It is modified into
three separate structures; the ciliary body,
the suspensory ligament, and the iris
Optic
The ciliary body contains small ciliary
muscles and ligaments that hold the lens
suspended in the correct place
The iris is the colored part of the eye; the
hole in the middle of the iris is called the
pupil
– The iris is attached to the ciliary body
The retina is made up of three layers of
neurons; photoreceptor neurons, bipolar
neurons, and ganglion neurons
Optic
The photoreceptor neurons are found in
the rods and cones
– Cones are more dense in the fovea centralis;
and become less dense outward from the
fovea centralis
– Rods are absent from the fovea centralis and
increase density toward the periphery of the
retina
Axons of the ganglion neurons extend
back to the posterior eyeball into an area
called the optic disc; this is where the
optic nerve emerges from the eyeball
– The optic disc is also called the blind spot. It
does not contain rods or cones
Optic
The eye is divided into two cavities;
– The anterior cavity is filled with aqueous
humor; a substance that is clear and
watery
– The posterior portion is larger and it is
filled with vitreous humor; a substance
with the consistency of a soft gelatin
– Both the aqueous and the vitreous
humors play a role in maintaining
intraoccular pressure to prevent the
eyeball from collapsing
Optic
The aqueous humor is formed from capillaries in
the ciliary body
Eyes have extrinsic and intrinsic muscles
– The extrinsic eye muscles are skeletal muscles
that attach the eye to the bones of the orbit
– The intrinsic eye muscles are smooth muscles
located within the eye; these are the iris and
the ciliary muscles
The iris regulates the size of the pupil
The ciliary muscles control the shape of the
lens
Optic
The eyebrows, eyelashes, eyelids, and
lacrimal apparatus are accessory
structures of the eye
– Eyebrows and eyelashes protect the eye from
entrance of foreign objects and they shade the
eyes from direct light
The eyelash contains small glands that secrete
lubricating fluid, if they become infected a sty
develops
– Eyelids are lined with conjuctiva, which are
mucous membranes, it continues over the
surface of the eyeball.
Inflammation of the conjuctiva is called pinkeye
Optic
The lacrimal apparatus consists of
structures that secrete tears and
drain them from the surface of the
eyeball
The Process Of Seeing
Four process focus light rays so that
they form a clear image
– Refraction means bending light rays.
Rays are refracted by the cornea,
aqueous humor, lens, and vitreous
humor
– If there are errors in refraction,
nearsightedness (myopia),
farsightedness (hyperopia), and
astigmatism occurs
Process of Seeing
Accommodation for near vision
requires changes in the curvature of
the lens, the constriction of the
pupils and the convergence of the
two eyes
– Curvature of the lens takes place to
achieve greater refraction (bending
light) for near vision because light rays
from nearer objects are divergent and
not parallel (as in further vision)
Process Of Seeing
Contraction of the ciliary muscle pulls the
choroid layer closer to the lens and
loosens tension of the suspensory
ligaments allowing the lens to bulge
– Near vision the ciliary muscle is contracted so
the lens will bulge
– Far vision the ciliary muscle is relaxed so that
the lens is flatter
– Presbyopia is a condition where people become
farsighted as the lenses lose their elasticity as
the person ages
Process Of Seeing
The iris constricts the pupil to prevent
divergent rays from entering the eye
through the periphery of the cornea and
lens; if divergent light entered the pupil it
could not be bent enough to provide a
clear image
Convergence of eyes refers process where
images must fall on the same location
within each retina in order to see a clear
image. This is accomplished by moving
the eyeball so that the visual axes are
parallel
Process Of Seeing
Diplopia occurs when objects fall on
noncorresponding points of the two
retina; double vision
Strabismus (cross-eye or squint) is a
condition that cannot be corrected by
neuromuscular effort. The person
learns to suppress one of the images
Photopigments
Photopigments can be broken down into a
glycoprotein called opsin and a vitamin A
derivative called retinal
Rods are named rhodopsin; they are highly
sensitive to dim light and cause a rapid
breakdown of the photopigment into opsin and
retinal components
Cones are responsible for the ability to see
colors; there are three different types of cones in
the eye
– Neural input from a varying number of cones creates
color
– Cones produce vision in bright light
Neural Pathway For Vision
Nerve fibers that conduct impulses
from rods and cones reach the visual
cortex in the occipital lobes of the
brain via the optic nerves, the optic
chiasma, optic tracts, and optic
radiations
Sense Disorders
Otosclerosis is an inherited bone
disorder that impairs conduction of
sound waves by causing structural
irregularities in the shape of the
stapes
Tinnitus is ringing of the ears and
could be a sign of otosclerosis
Excess cerumen can block sound
wave conduction
Sense Disorders
Otitis refers to a temporary ear infection
Presbycusis is the progressive loss of
hearing associated with aging and
degeneration of nerve tissue in the ear
and vestibulocochlear nerve
Meniere’s disease is a chronic inner ear
disease with an unknown cause; it results
in tinnitus, progressive nerve deafness
and vertigo (spinning sensations)
Sense Disorders
Myopia is nearsightedness; it can be
corrected using concave contact
lenses, glasses or refractive surgery
– The image focuses in front of the retina
Hyperopia is farsightedness; it can
be corrected by convex lenses and
refractive eye surgery
– The image focuses behind the retina
Sense Disorders
Astigmatism refers to irregularity in the
curvature of the cornea or lens
Cataracts are cloudy spots in the eye’s
lens that develop in the lens as we age, it
can interfere with focusing
Conjuctivitis is pink eye and is usually
caused by bacteria
– Chlamydial conjuctivitis, or trachoma, is
caused by the same bacteria that cause the
reproductive infection
– Conjuctivitis can be caused by other bacteria
such as Staphylococcus and Haemophilius
Sense Disorders
Retinal detachment occurs when part of
the retina falls away from the tissue
supporting it
– Usually results from aging, tumors, or blows to
the head
Diabetic retinopathy is a disorder that
causes small hemorrhages in the retinal
blood vessels that disrupt the flow of
oxygen to the photoreceptors
Glaucoma results from excessive
intraoccular pressure
Nyctalopia is night blindness that can be
caused by a vitamin A deficiency or
degeneration of the retina
Sense Disorders
Scotoma refers to the loss of only
the center of the visual field often
associated with multiple sclerosis
Colored blindness is an inherited
condition where one of the
photopigments in the cones are
missing