Slide 1

The Senses
Overview of the Senses
Visceral Senses
receptors receives information about the
environment (environmental stimuli)
receptors generate nerve impulses and
send information to the CNS
a coordinated response can be directed to
maintain homeostasis.
Types of receptors:
1. Photoreceptors (light stimuli)
Vision – rods and cones found in the retina
of the eye
2. Chemoreceptors (chemical
Taste – taste-buds found in the tongue
Smell – olfactory cells found in the
olfactory cavity
Internal senses – osmoreceptors that
regulate blood pressure, CO2 balance,
3. Mechanoreceptors (mechanical
enegy stimuli, touch, movement, etc)
Touch / pressure / pain – receptors in the
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)
Temperature – receptors in the skin detect
changes in radiant energy
Sensation – receiving and processing by
the brain of neural impulses from the
sensory receptors
Perception- interpretation of sensory
information by the cerebral cortex.
-neuron becomes
accustomed to a
stimulus and stops
-the adaptation
indicates that the
environment is not
Ex. Bed sheets at
night, itches that go
The Eye- a photoreceptor
(I spy with my little eye)
regulate and focus light
translate the wavelengths of light into
nerve impulses.
detects visible light spectrum
The External Eye
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
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
Intermediate Layer:
Choroid (2) – darkly pigmented tissue
through which blood vessels nourish the
back of the eye, pigment absorbs light and
prevents reflections
Ciliary Body (2a) – thickened portion of
the choroid attached to the suspensory
smooth muscle contracts / relaxes to change
the shape of the lens, also secretes aqueous
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
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 (destingusih light and dark)
Pigment in the rods (low light levels)
Is broken down (bleached) in high light
and must be replaced
Rhodospsin is rebuilt at night
Vitamin A is used to make retinal
Retinal + opsin = rhodopsin
No vitamin A can lead to night blindness
Cones (determine colors and shades)– color
vision, located at the fovea centralis,
sensitive to color and bright light
3 color pigment cones: red (long light
waves), blue (medium light waves), green
(short light waves)
When a person lacks one or more of types
of cones its called color blindness
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
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
Focusing and Accommodation
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.
a focused image is inverted (upside down)
The brain ‘rights’ the image
To do this neurons join at the optic chiasma
and then split to the right and left visual
this allows for stereoscopic vision and depth
Visual Disorders
Myopia (nearsightedness) – See things
near but no far away.
elongated eyeball, light focuses in front of the
Need a concave lens to correct
Hyperopia (farsightedness) – See
thing far away, but not close up
shortened eyeball, light focuses behind the
retina. Most common type of eye problem
Need a convex lens to correct
Corrective Lenses
Astigmatism – irregular shape to the
eyeball or lens causing distortions, the light
rays do not converge at one point, instead
they scatter
Cataracts – proteins in the lens change
(usually with age) because of a lack of
enzymes, causing the lens to become
opaque instead of transparent, mostly
caused by UV light (Think of Terminator
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)
The Ear-mechanoreceptor
(“…all the better to hear you with my dear…”)
sensitive to vibrations
Deal with hearing, balance, and equilibrium
The Outer Ear (1)
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).
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)
 Incus (anvil)
 Stapes (stirrup)
Eustachian tube
(2d)– extends from
the middle ear to
the naso-pharynx
allows for the
equalization of
pressure in the ear
when you swallow
Inner Ear (3)
Oval Window (3a)
– attached to the
 as the stirrup
moves the oval
window moves
creating vibrations
in the fluid of the
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
Cochlea (3c) –
coiled, fluid filled
tube that converts
vibrations into
nerve impulses
Organ of Corti – Hearing
projects from the basilar membrane lining
the bottom of the cochlear canal
Ex) basil membrane has stiff and narrow
hairs near the oval membrane (beginning
of the cochlea): activated by high
frequency wave = high pitch
The basil membrane has more elestic hairs
near the end wide part of the cochlea, low
frequency waves activate these= low
Loudness and pitch– related to the
amplitude of the sound waves coming into
the ear, increases movement of the basilar
Basilar membrane
 lines the bottom of the cochlear canal
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
Tectorial membrane
 sits on top of the hair cells in the basilar
moves up and down with the sound
causes brushing and bending of the hairs
in the basilar membrane which initiates a
nerve impulse
Auditory nerve (3d,e) – carries the
nerve impulses generated by the auditory
cells in the ear to the temporal lobe of the
1) Sound waves move through the
auditory canal
2) ear drum vibrates
3) ossicles go into motion
4) oval window vibrates
5) fluid in the cochlea moves
6) fluids moves against organ of corti
7) nerve impulse
8) auditory nerve receive nerve impulse
9) message sent to temporal lobe
Static equilibrium: movement in one
Ex) head movement
Dynamic Equilibrium: movement in
many planes
Ex. Being on a roller coaster ride
Static Equilibrium is maintained by:
Saccule and Utricle
fluid filled sacs with hair cells covered in
small carbonate crystals called otoliths
as the head is tilted, or the body inverted,
the crystals shift and move across the
hairs, the hairs bend and send signals to
the cerebellum
Structures for Balance
Vestibular apparatus (3f) –provide
information about balance
Dynamic Equilibrium is maintained by:
Semicircular canals (3g)
 3 fluid filled tubes in the X,Y, and Z planes
projecting from the cochlea
 gelatinous fluid moves when the head
moves from side to side or back and forth
to give information about dynamic
 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
Hearing Problems
Conduction Deafness:
congenital defect or infection
ossicles fuse
restricts the ability to magnify sound waves.
(hearing aids help)
Nerve Deafness:
exposure to loud noises and age
breakage or destruction of hairs.
neurons don’t fire. (hearing aids are ineffective)
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
on walls of the
papillae (folds) of
the tongue, palate,
pharynx and the
Taste buds can
There are 4 basic taste sensations:
Sour – produced by H+ ions in acids
Salty – produced by cations (+ ions) from
ionized salts such as NaCl
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:
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.
The Nose
(The nose knows. Phew!)
3000 times more receptors for smell than for
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.
Molecules with different shapes stimulate the
olfactory receptors to create the sensation of
different smells. These receptors then send
impulses to the cerebral cortex
Camphor – spherical molecules with diameter of 0.7 nm
Musk – disk shaped molecules with a diameter of 1.0
Floral – flexible, disk shaped molecules with long side
Etheral – thin, rod-shaped molecules
Peppermint – wedge shaped molecules
Olfactory fatigue: when exposed to a
smell, over a long period of time, one may
no longer sense the smell
The Skin
largest organ.
receptors for
touch, pressure,
pain and
Proprioceptors – provide information
about the position of body parts, joints,
tendons, muscles
Stretch receptors – found in muscles
and around the lungs
Visceral Senses
Receptors in internal organs
mediated by the autonomic nervous system to
promote internal homeostasis.
maintain blood pH, blood pressure, oxygen
levels, blood glucose levels, water re-absorption.
give sensations of thirst, hunger and nausea.

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