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The Senses
Overview of the Senses
Vision
Hearing
Taste
Smell
Touch
Visceral Senses
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receptors receives information about the
environment
receptors generate nerve impulses and
send information to the CNS
a coordinated response can be directed to
maintain homeostasis.
Types of receptors:
1. Photoreceptors
Vision – rods and cones found in the retina
of the eye
2. Chemoreceptors
Taste – taste-buds found in the tongue
Smell – olfactory cells found in the
olfactory cavity
Internal senses – osmoreceptors that
regulate blood pressure, CO2 balance,
etc.
3. Mechanoreceptors
Touch / pressure / pain – receptors in the
skin
Hearing – hair cells in the inner ear detect
sound waves
Balance – hair cells in the ear detect motion
Body position – proprioceptors and stretch
receptors in the muscles
4. Thermoreceptors
Temperature – receptors in the skin detect
changes in radiant energy
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Sensation – receiving and processing by
the brain of neural impulses from the
sensory receptors
Perception- interpretation of sensory
information by the cerebral cortex.
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Sensory
adaptation/fatigue
-neuron becomes
accustomed to a
stimulus and stops
firing.
-the adaptation
indicates that the
environment is not
dangerous.
The Eye- a photoreceptor
(I spy with my little eye)
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regulate and focus light
translate the wavelengths of light into
nerve impulses.
detects visible light spectrum
The External Eye
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Eyelid –protects the eye. (5)
Eyelash – keeps dust and small particles
from getting into the eye.
Conjunctiva – a thin protective layer on
the cornea. (4)
Lacrimal Glands
 located above and at the outer edges of
the eye socket.
 produce salty germicidal fluid that washes
dust out of the eyes and keeps them
lubricated.
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This fluid evaporates or drains into the
nasal chamber so it must be replaced.
Nasal Cavity – drains fluid from the
lacrimal glands into the nose. This is why
a sharp smell or pinch on the nose makes
your eyes water.
Internal Structures of the Eye
The Eye
External Layer:
Sclera (1) – tough elastic connective
tissue that encases and protects the
eyeball (this is the white posterior portion)
Cornea (1a)– transparent anterior portion
of the sclera, allows light in while keeping
other things out, helps focus light on the
retina
Intermediate Layer:
Choroid (2) – darkly pigmented tissue
through which blood vessels nourish the
back of the eye, pigment absorbs light and
prevents reflections
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Ciliary Body (2a) – thickened portion of
the choroid attached to the suspensory
ligaments
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smooth muscle contracts / relaxes to change
the shape of the lens, also secretes aqueous
humour.
Suspensory Ligaments (7a) – attach to
the lens and the ciliary muscles
Iris (2c)
 pigmented anterior portion of the choroid
 composed of circular and radial muscles
that control the amount of light that
enters the eye
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Pupil (9) – the opening of the iris that
allows light into the eye
Internal Layer:
Retina (3)
 two types of photoreceptor cells, rods
and cones
 two layers of cells (bipolar cells and
ganglionic cells) that organize visual
information before impulses are sent to
the brain
Fovea Centralis (3a) – slight depression
in the back of the eye, area of highly
concentrated cones, this is where light is
focused for clear, precise color vision
Rods – night vision, abundant in the eye,
mainly in the peripheral portion of the
retina, sensitive to slight movements in
dim light
Rhodopsin
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Pigment in the rods (low light levels)
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Is broken down (bleached) in high light
and must be replaced
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Vitamin A is used to make retinal
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Retinal + opsin = rhodopsin
Cones – color vision, located at the fovea
centralis, sensitive to color and bright light
Other:
Lens (7) – flexible, transparent body
responsible for focusing light into the retina
Optic Nerve (8) – carries visual information
to the occipital lobe of the brain for
processing
Aqueous Humour (10) – thin fluid between
the cornea and the lens that refracts light,
also absorbs nutrients for the eye tissue
Vitreous Humour (12) – jelly-like fluid in the
posterior cavity of the eye, gives structure to
the eye and prevents the retina from caving
in
Focusing and Accommodation
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Extrinsic eye muscles – control movements of
the eyeball within the socket (side to side or up
and down). There are three pairs of muscles.
Ciliary muscles – found within the ciliary body,
control the shape of the lens (contracted
muscles make the lens round, relaxed muscles
flatten the lens)
Accommodation – is the flattening or
rounding of the lens in order to focus
objects on the retina.
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a focused image is inverted (upside down)
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The brain rights the image
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Neurons join at the optic chiasma and then
split to the right and left visual cortex
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this allows for stereoscopic vision and depth
perception.
Visual Disorders
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Myopia (nearsightedness) – elongated
eyeball, light focuses in front of the retina
Hyperopia (farsightedness) –
shortened eyeball, light focuses behind
the retina
Corrective Lenses
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Astigmatism – irregular shape to the
eyeball or lens causing distortions, the light
rays do not converge at one point, instead
they scatter
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Cataracts – proteins in the lens change
(usually with age) because of a lack of
enzymes, causing the lens to become
opaque instead of transparent
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Glaucoma – pressure built up in they eye
due to the inability to absorb aqueous
humour, results in degeneration of the
cells in the back of the eye and eventually
blindness (common in untreated diabetics)
trabeculectomy
The Ear-mechanoreceptor
(“…all the better to hear you with my dear…”)
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sensitive to vibrations
The Outer Ear (1)
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Pinna (1a) – folds of the visible ear. Helps
to funnel sound waves into the ear.
Auditory Canal (1b) – carries sound
waves to the middle ear (eardrum).
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lined with hairs, sweat and glands that produce
earwax to prevent foreign bodies from getting
into the ear.
The Middle Ear (2)
Tympanic membrane
(1c) – eardrum
-a flexible membrane
that vibrates when hit
with sound waves
Ossicles – the
three bones in
the ear, amplify
sound vibrations
using leverage
Malleus (2a)
(hammer)
 Incus (anvil)
(2b)
 Stapes (stirrup)
(2c)
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Eustachian tube
(2d)– extends from
the middle ear to
the naso-pharynx
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allows for the
equalization of
pressure in the ear
when you swallow
Inner Ear (3)
Oval Window (3a)
– attached to the
stirrup
 as the stirrup
moves the oval
window moves
creating vibrations
in the fluid of the
cochlea
Round Window
(3b) – at the far
end of the cochlea
 puffs outwards to
relieve pressure
from the oval
window creating
waves in the
cochlear fluid
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Cochlea (3c) –
coiled, fluid filled
tube that converts
mechanical
vibrations into
nerve impulses
Organ of Corti – Hearing
Apparatus
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projects from the basilar membrane lining
the bottom of the cochlear canal
Basilar membrane
 lines the bottom of the cochlear canal
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about 25,000 small nerve fibers ending
in hair cells are arranged in order of
length along the organ of corti
each cell vibrates with a different
wavelength of sound giving the
sensation of pitch
Ex) basil membrane is stiff and narrow near
the oval membrane: activated by high
frequency wave = high pitch
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Basilar Membrane Oscillations
Loudness – related to the amplitude of the
sound waves coming into the ear,
increases movement of the basilar
membrane
Tectorial membrane
 sits on top of the hair cells in the basilar
membrane
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moves up and down with the sound
vibrations
causes brushing and bending of the hairs
in the basilar membrane which initiates a
nerve impulse
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Auditory nerve (3d,e) – carries the
nerve impulses generated by the auditory
cells in the ear to the temporal lobe of the
brain
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Hearing
Balance
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Static equilibrium: movement in one plane
Ex) head movement
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Dynamic Equilibrium: movement
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Structures for Balance
Vestibular apparatus (3f) –provide
information about balance
Semicircular canals (3g)
 3 tubes in the X,Y, and Z planes projecting
from the cochlea
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gelatinous fluid moves when the head
moves from side to side or back and forth
to give information about dynamic
equilibrium
Ampulla
 found in the semicircular canals
 hair cells embedded in a gelatinous
material bend with the flow of fluid in the
canals, creating a sense of dynamic
balance
Saccule and Utricle
 sacs with hair cells covered in small
carbonate crystals called otoliths
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as the head is tilted, or the body inverted,
the crystals shift creating a sense of
static balance
Hearing Problems
Conduction Deafness:
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congenital defect or infection
ossicles fuse
restricts the ability to magnify sound waves.
(hearing aids help)
Nerve Deafness:
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exposure to loud noises and age
breakage or destruction of hairs.
neurons don’t fire. (hearing aids are ineffective)
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Tinnitus: ringing in the ears caused by
damaged hair cells in the cochlea or by
random “static” within the brain
The Tongue
- chemoreceptors.
Taste buds
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on walls of the
papillae (folds) of
the tongue, palate,
pharynx and the
epiglottis.
There are 4 basic taste sensations:
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Sour – produced by H+ ions in acids
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Salty – produced by cations (+ ions) from
ionized salts such as NaCl
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Sweet – produced by compounds containing
hydroxyl (OH) groups such as alcohols,
amino acids, sugars, ketones and lead salts
Bitter – produced by alkaloids such as
quinine, strychnine and caffeine
Old thinking:
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Taste is often a combination of all four
types of receptors giving the
perception of different flavors.
Taste is enhanced by the sense of
smell.
Taste may also be genetic.
http://www.bbc.co.uk/science/humanbody/body/factfiles/t
aste/taste_ani_f5.swf
The Nose
(The nose knows. Phew!)
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3000 times more receptors for smell than for
taste.
receptors are located on tissue called the
olfactory epithelium.
The neurons have modified cilia on the ends
of their dendrites that trap and receive
airborne (gas or vapor) molecules.
http://www.bbc.co.uk/science/humanbody/b
ody/factfiles/smell/smell_ani_f5.swf
Molecules with different shapes stimulate
the olfactory receptors to create the
sensation of different smells.
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Camphor – spherical molecules with diameter
of 0.7 nm
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Musk – disk shaped molecules with a diameter
of 1.0 nm
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Floral – flexible, disk shaped molecules with
long side groups
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Etheral – thin, rod-shaped molecules
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Peppermint – wedge shaped molecules
The Skin
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largest organ.
receptors for
touch, pressure,
pain and
temperature.
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Proprioceptors – provide information
about the position of body parts
Stretch receptors – found in muscles
and around the lungs
Visceral Senses
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Receptors in internal organs
mediated by the autonomic nervous system to
promote internal homeostasis.
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maintain blood pH, blood pressure, oxygen
levels, blood glucose levels, water re-absorption.
give sensations of thirst, hunger and nausea.