PERIPHERAL NERVOUS SYSTEM: Detects stimuli and transmits

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Chapter 8
Special Senses
Sensory system
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Sensory system serves as a protective
mechanism for an organism by
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detecting changes in the environment
An environmental change becomes a stimulus and
initiates nerve impulse to CNS by sensory neurons.
The stimulus is then interpreted by cerebral cortex
Senses are classified according to the
distribution of the receptors as
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somatic (general) senses: touch, pain, pressure,
temperature
special senses :vision, taste, smell, hearing and
balance
A special sense is located in a special sense organ
e.g.
Chemoreceptors: two types
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Taste : receptors on tongue
Smell : receptors in the upper nasal cavity
Photoreceptors :
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Vision : receptors of eye
Mechanoreceptors:
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Hearing and Equilibrium: receptors of internal ear
Chemical Senses – Taste and
Smell
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Both senses use chemoreceptors
Stimulated by chemicals in solution
 Taste has four types of receptors
 Smell can differentiate a large range of
chemicals
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Both senses complement each other and
respond to many of the same stimuli
Sense of Smell: or olfaction
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occurs in response to
odors
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The special nasal
epithelium of nasal
cavity is called
olfactory epithelium
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Airborne molecules
enter nasal cavity and
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stimulate chemo
receptors present in
olfactory epithelium
Sense of Smell: or olfaction
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Stimulation creates action potential sending impulses to
brain
through olfactory nerve (I) to cerebral cortex
Between frontal and temporal lobe above hypothalamus
These receptors degenerate with age
Sense of Smell: or olfaction
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During common cold, nasal mucous producing cells is
inflammated
preventing the odor from reaching the olfactory neurons of
nose and
thus preventing the sense of smell
Sense of Taste
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The sense structures that
detect gustatory or taste
stimuli are called taste
buds
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Taste buds are oval
structures embedded in the
epithelium of tongue and
mouth
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Known as papillae
Sense of Taste
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Food particles
dissolved in saliva
stimulate taste buds
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And send impulse to
brain through
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Medulla by
facial nerve (VII)
glassopharyngeal
nerve (IX)
vagus (X).
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Sense of Taste
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There are four primary taste
sensations detected by taste buds:
a. Bitter: e.g. coffee. Taste buds
are at the back of the tongue
b. Sour: due to acids . Taste buds
are on either side of tongue
c. Salty : salts like NaCl . Taste
buds are on the sides and front of
tongue
d. Sweet : Sugars give sweet
taste. Taste buds are in front of
the tongue
The Eye and Vision
70% of all sensory receptors are in the
eyes
 Each eye has over a million nerve fibers
 Protection for the eye
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Most of the eye is enclosed in a bony orbit
 A cushion of fat surrounds most of the eye
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Accessory Structures of the Eye
Eyelids and eyelashes
 Conjunctiva
 Lacrimal apparatus
 Extrinsic eye muscles
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Accessory Structures of the Eye
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Eyelids: Eyelids protect
eye from foreign
objects
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Tarsal glands in
eyelids lubricate the
eye
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Eyebrows: prevent
sweat from entering
eyes and help in
shading eyes
Accessory Structures of the Eye
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Conjunctiva
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Delicate membrane
that lines the eyelids
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Connects to the
surface of the eye
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Secretes mucus to
lubricate the eye
Accessory Structures of the Eye
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Lacrimal apparatus:
Consists of lacrimal gland
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Lacrimal gland— produces
lacrimal fluid, located above the
lateral end of eye
Properties of lacrimal fluid
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Dilute salt solution (tears)
Contains antibodies and lysozyme
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Protects, moistens, and lubricates
the eye
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Empties into the nasal cavity
Accessory Structures of the Eye
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Extrinsic eye muscles
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Six muscles attach to
the outer surface of
the eye
Produce eye
movements
Structure of the Eye
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Eye is enclosed in three layers
or tunics
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Fibrous layer
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Vascular layer
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Outside layer
Middle layer
Sensory layer
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Inside layer
Structure of the Eye: The Fibrous Layer
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Sclera
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White connective tissue
layer
Seen anteriorly as the
“white of the eye”
Cornea
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Transparent, central
anterior portion
Allows for light to pass
through
Repairs itself easily
The only human tissue that
can be transplanted without
fear of rejection
Structure of the Eye: Vascular Layer
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Choroid is a blood-rich nutritive
layer in the posterior of the eye
contains a dark pigment
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Light is focused by choroid
pigment onto retina thus
preventing scattering of the
light
Modified anteriorly into two
structures
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Ciliary body— smooth
muscle attached to lens
Iris— regulates amount of
light entering eye
 Pigmented layer that
gives eye color
 Pupil— rounded
opening in the iris
Structure of the Eye: Vascular Layer
Iris is regulated by:
a. sympathetic
nervous system by
dilating it
b.
parasympathetic
nervous system by
constricting it
Structure of the Eye: Sensory Layer
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Retina contains two
layers
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Outer pigmented layer
They contain photosensitive
pigments
Involved in the conversion
of light into nerve impulses
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Inner neural layer
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Contains receptor cells
Biopolar cells
Ganglion cells
Structure of the Eye: Sensory Layer
 Light sensitive
pigments are
 Rods: contain
rhodopsin pigment
differentiates
between light and
darkness
 Cones: detect colors
(red, blue and green)
of the visual light.
Cone Sensitivity
There are three
types of cones
 Different cones are
sensitive to
different
wavelengths
 Color blindness is
the result of lack of
one cone type
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Structure of the Eye: Sensory Layer
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Fovea centralis: region
where there are more
cones
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Blind spot: region where
the optic nerve exits the
eye
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Lens is present behind
the pupil
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Lens focuses the light
on the retina
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Lens is made of
transparent intercellular
material
It is suspended by
ligaments
Lens regulates the
focusing with the help
of suspensory ligaments
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Lens
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Lens divides the eye
cavity into
Anterior cavity or
aqueous humor
Posterior cavity or
vitreous humor
It is made of water
and mucoprotein.
It maintains the
pressure to support
retina
Lens
Physiology of vision
· Light reflected from the object enters cornea towards lens
to retina.
· Retina has cones, which detect colors (red, blue and green),
and rods detect tone of object and night vision.
· Photoreceptors of rods and cones send impulse to brain
through bipolar cells to optic nerves
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optic nerve pass through thalamus
cross path at ( optic chiasma),
connecting to
occipital lobe of the cerebral cortex
Thus responding to light
stimulation
And visual interpretation, or seeing
occurs
Human vision is binocular
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Provides depth perception
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Lens Accommodation
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Light must be focused to a
point on the retina for
optimal vision
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The eye is set for distance
vision
(over 20 ft away)
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The lens must change
shape to focus for closer
objects
•Short sight or near sightedness: Eye ball is
stretched in length thus focusing the object in
front of retina rather than on retina.
concave lens correct the mistake
•Long Sight or far sightedness: Eye ball is compressed such that
the light is focused behind the retina instead of on the retina
Convex lens fixes the mistake
Ear
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Is the sense
organ important
for hearing and
balance
The ear is
divided into
three parts:
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Outer ear
Middle ear
Inner ear
a. Outer ear : Is composed of
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Auricle (pinna) which
directs sound waves into
ear
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And External auditory
maetus (canal) lined with
epithelial cells of skin
called ceruminous glands,
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They produce wax
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Wax collects the dust
particles
a. Outer ear :
 Tympanic membrane
(or eardrum) connects
outer ear to middle ear
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Sound waves reach
tympanic membrane
through pinna and
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cause it to vibrate
b. Middle ear:
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Connects the outer ear to inner ear
Filled with air cavity within the temporal bone
Consists of two passages:
One passage opens into Mastoid air cells of temporal bone and
Second passage opens into pharynx through auditory or Eustachian tubes
Auditory tube equalizes air pressure between outside air and middle air cavity
b. Middle ear:
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Is made of
three small bones
called auditory ossicles
Malleus (hammer)
Incus (anvil) and
Stapes (stirrup)
These bones are
connected to the inner
ear
They amplify the
vibrations from
tympanic membrane
and transfer the
vibration to malleus 
anvil  stirrup 
inner ear
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Inner ear
Includes sense organs for hearing and balance
consists of bony labyrinth:
vestibule
cochlea (snail like structure) and
3 semicircular canals
And filled with perilymph (plasma like fluid)
Organs of Hearing
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Organ of Corti
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Located within the cochlea
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Cochlea is a bony structure
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Has modified cells forming a
structure called spiral organs or
organ of corti.
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Organ of corti is specialized
sensory haircells and filled with
endolymph.
Physiology of hearing
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Vibrations of air medium generate sound waves.
The sound waves are collected by pinna and
directed to external auditory maetus.
Physiology of hearing
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The sound waves cause vibrations
of tympanic membrane,
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which moves ossicles
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resulting sound waves being
amplified.
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Amplified sound waves create
waves in endolymph of the
cochlea
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and activate haircells of spiral
organ
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This creates action
potential, which is
transmitted through
vestibulocochlear nerve to
pons( of brain stem)
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to thalamus.
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From thalamus , impulse
is directed to auditory
cortex of temporal lobe
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and sound is perceived
Sound is measured in decibels
Diseases related to sensory system
Sensory
function
disease
Specific
structure
Function
affected
Sense of smell
Common cold
Nasal epithelium
Cannot smell
visual
glaucoma
Build up of aqueous
fluid due to blockage
of ducts in the
choroid
Blindness caused
due to damage to
nerves
vision
Macular degeneration Degeneration of fova
centralis( cones) due
to age advancement
Blind spot, faded
colors
vision
Color blindness
Cone proteins not
made
Cannot distinguish
red, blue and green
auditory
Conduction deafness
Fused middle ear
bones
Cannot hear from
birth
auditory
Nerve deafness
Cochlear cilia of
nerve cell receptors
degenerate due to
age advancement
Cannot hear

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