Chapter 12
Senses
Chapter Outcomes
Explain the difference between sensory
reception, sensation, and perception
Describe the process of sensory
adaptation
Distinguish between the major sensory
receptors in the human body
Describe the principal structures of the
human eye and their functions
Chapter Outcomes
Observe the principal features of the
mammalian eye and perform experiments
that demonstrate the functions of the
human eye
Describe several eye disorders and
treatments
Describe how the structures of the human
ear support the functions of hearing and
balance
Chapter Outcomes
Explain how humans sense their
environment through taste, smell, and
touch
Explain how small doses of neurotoxins
can be used as painkillers
12.1- Sensory Reception,
Sensation & Perception
What is the difference between these
three?
Reception
Sensation
Perception
Sensory Receptors
Our sensory neurons are attached to
receptors that are activated by specific
stimuli
These sensory receptors are highly
modified ends (dendrites) of sensory
neurons
We have a number of different types of
receptors in our body
Receptor
Stimulus
Information Provided
Taste
Chemical
Taste buds identify specific
chemicals
Smell
Chemical
Olfactory cells detect presence of
chemicals
Pressure
Mechanical
Movement of skin or changes in
body surface
Proprioreceptors
Mechanical
Movement of limbs
Balance (ear)
Mechanical
Body movement
Outer Ear
Sound
Signals sound waves
Eye
Light
Signals change in light intensity,
movement & colour
Thermoregulators
Heat
Detect flow of heat
Groups of Receptors
Often receptors are grouped in specific
organs which are specialized to respond to
a single stimuli (such as organs for taste,
smell, hearing and vision)
The sensations that we receive from these
receptors are actually produced in the
brain – if transmission from the sensory
neuron is blocked, the sensation stops
In general, the stimuli that we respond to
are those most relevant to our survival
For example, our range of hearing and
vision is limited compared to other
animals, even though the other stimuli are
present
Our senses can also undergo sensory
adaptation
This occurs when a receptor becomes
accustomed to a particular stimulus being
present
Sensory Adaptation Examples
Ever notice that some strong smells, over
time, seem to disappear?
However, if you leave that environment
and return, the smell has seemingly
reappeared
This phenomenon is due to your sense of
smell becoming accustomed to that strong
smell
We can also become accustomed to
temperature changes
For instance, before you step into a warm
shower, the bathroom might seem
relatively warm
However, after the shower, you step out
and feel very cold
This is because your body becomes
accustomed to the warmer temperatures
of the shower
12.2 - The Eye
The eye consists of three layers:
1. The Sclera
Outermost portion of the eye
Includes the cornea & aqueous humor
2. Choroid Layer
Contains pigments that prevent light from
scattering
Includes the:
Iris
Pupil
Ciliary Muscles
3. Retina
Composed of three layers of cells:
Rods & Cones
Bipolar Cells
Ganglion Cell Layer
The Retina
The Fovea Centralis
This is a region in the center of the retina
that contains a dense bundle of cones
The lens of the eye focuses the majority of
the light on this area
The fovea produces sharp colour images
Surrounding this area are rods which pick
up low-intensity black & white light
Vision – The Lens
Images form on the retina because of the
focal length of the lens
Unlike plastic or glass lenses, the lens of
the eye can change its shape, which
makes it able to focus on near and far
objects
Objects 6 m (20 ft) from the eye should be
focused without any change to the lens’
normal shape
The Chemistry of Vision
Rods and cones contain a light-sensitive
pigment known as rhodopsin
In the absence of light, rods release inhibitory
neurotransmitters that inhibits nearby nerve cells
When light hits this pigment it is split into two
components: Opsin (a protein) and retinene (a
form of vitamin A)
This division stops the release of the inhibitory
transmitter, allowing transmission of an impulse
to the optic nerve
Regeneration
As indicated by the previous diagram, the
breakdown of rhodopsin is much faster
than its regeneration
This is responsible for the afterimages that
are often seen after looking at a single
object for a long time or at a bright light
Bright light can cause temporary blindness
because the rhodopsin is not regenerated
in sufficient amounts to maintain vision
Colour Vision
The cones used for colour vision come in
three varieties – red, green and blue
Slight changes in the opsin component of
rhodopsin are responsible for the various
cones’ sensitivities to different colours of
light
The following diagram shows the subtle
differences in the opsin molecules
As you can see, there are subtle changes to the
amino acids that make up these proteins
Colour Blindness
Colour blindness is caused when one or
more of the colour cones are defective
This is caused by a mutation in the genes
that create the opsin molecules
These mutations alter the sequence of
amino acids that make up the opsin, and
therefore change its shape and function
Types of Colour Blindness
There are a number of different types of
colour blindness
A rare case, known as monochromacy,
occurs when a person lacks all three
colour pigments and can distinguish no
colour at all
More common is dichromacy, where one
of the pigments is absent – this is often
inherited and affects males more often
than females
Types of Colour Blindness
A third type of colour blindness is
anomalous trichromacy, where all three
pigments are present, but have altered
spectral sensitivity
It often results in a difficulty in
distinguishing between red and green
hues (most common) or yellow and blue
hues (very rare)
Types of Dichromacy
Protanopia – an absence of red colour
receptors; red will appear dark
Deuteranopia – green photoreceptors
are absent, and it affects red-green
colour distinction
Tritanopia – total absence of blue
receptors
What does dichromacy look like?
Other Common Vision Defects
There are a number of other common
vision defects
1. Glaucoma
Caused by increased pressure in the
aqueous humor
This pressure causes the blood vessels
in the retina to collapse
The rods and cones die because of a
lack of oxygen and other nutrients
Glaucoma can be treated with medication
or surgery
Medications aim at reducing the pressure
within the aqueous humor by either
helping it drain or reducing the production
of the aqueous humor
Laser or microsurgery can be used to cut
a small hole to relieve the fluid pressure,
but this is not a permanent solution
2. Cataracts
Cataracts are caused by the lens
becoming more opaque
This prevents light from coming through
and reaching the retina
Cataracts can be treated by replacing the
damaged lens with an artificial one using
surgery
http://upload.wikimedia.org
3. Astigmatism
Astigmatism occurs when the lens is
irregularly-shaped and only correctly
focuses in one plane
This can be countered by using an
external lens to compensate for the
irregular shape of the lens in the eye
4. Myopia (Nearsightedness)
Myopia occurs when the eyeball is “too
long” and the image from the lens
focuses in front of the retina
This is treated by using a biconcave lens
to diverge the light rays before they
reach the lens
5. Hyperopia (farsightedness)
The main contributing factor to hyperopia
is an eye that is “too short”, resulting in
the image being focused behind the
retina
A convex lens can be used to converge
the light rays before they reach the lens,
which refocuses the light on the retina
As well, as we age, our lens becomes
less elastic and we lose the ability to
focus on near objects
The Blind Spot
Where the ganglion cells merge, they form
the optic nerve
At the point where the optic nerve enters
the retina, it creates a region that has no
rods or cones
This is known as the blind spot
Visual Interpretation
Messages from the eyes travel through the optic
nerves to the brain
Once in the brain, the pieces of visual
information are sorted, processed, and
integrated to produce a 3-D image
Aspects of sight such as movement, colour,
depth, and shape are handled by different parts
of the occipital lobe
This speeds up the processing of the visual
image
Note that
images from
the right eye
are
interpreted
on the left
side of the
occipital
lobe
12.3 - The Ear
The ear carries out two functions – it is
used for balance and for hearing
Both of these senses use specialized hair
cells that are very tiny and respond to the
movement of fluids in the ear
Anatomy of the Ear
The Outer Ear
The outer ear consists of:
The pinna
The auditory canal
The Middle Ear
The middle ear produces the sound nerve
impulses that are sent to the brain
It consists of several parts:
The tympanic membrane (tympanum)
The ossicles
The oval window
The Eustachian tube
The Inner Ear
The inner ear contains:
The cochlea
The semicircular canals
The vestibule
Hearing and Sound
Our hearing can detect sound energy as
low as 1.0×10-12 Watts
Sound travels as pressure waves through
a material, and therefore will not pass
through a vacuum
Sounds travel most rapidly through solids,
and most slowly through gases
The Organ of Corti
The Organ of Corti consists of three
structures:
The basilar membrane, which contains
many hair cells
The hair cells, which have many tiny
projections known as stereocilia
The tectorial membrane, into which are
embedded the ends of the stereocilia
Production of a Sound Impulse
1. The tympanic membrane vibrates as
pressure waves hit it
2. These vibrations are passed on to the
ossicles, which amplify the sound
3. The vibrations of the ossicles move the
oval window; the round window moves
as well, producing waves of fluid in the
inner ear
4. These waves of fluid travel through the
cochlea
5. The movement of fluid causes a thin
membrane known as the basilar
membrane to move. This membrane is
attached to hair cells located in the organ
of Corti
6. The movement of the cilia of the hair
cells against the tectorial membrane
produces a nerve impulse which is sent
to the brain
Production of a Sound Impulse - Animation
Hearing and Pitch
Different pitches of sound can be heard by
the human ear (a range of about 20 –
20,000 cycles per second)
Low-pitched sounds stimulate the hair
cells near the far end of the cochlea, while
high-pitched sounds stimulate hair cells
near to the oval window
Hearing Loss
Hearing loss generally results from nerve
damage (generally damage to the hair
cells) or damage to the sound-conduction
system of the outer and middle ear
Repeated loud noise destroys stereocilia
Any noise over 80 dB can damage hair
cells
Hearing Loss Treatment
For people who have conduction
deafness, hearing aids are often used
However, patients with nerve deafness
can have a device implanted in the ear
that picks up sounds and transmits them
directly to the auditory nerve
Scientists have also been able to use
viruses to insert genes that allow the
growth of new stereocilia in guinea pigs
Perception of Sound
Nerve transmissions from the ears
eventually reach the temporal lobes
Depending on the neurons stimulated, the
brain interprets the sounds as specific
pitches and intensities
As well, neurons in our temporal lobes can
also generalize the area from which the
sound originated
The Inner Ear – Equilibrium
There are two types of equilibrium – static
and dynamic equilibrium
Static equilibrium refers to the position of
the head, while dynamic equilibrium
provides information regarding the
direction of movement
The inner ear registers equilibrium for the
body
The Inner Ear
http://oto.ucsd.edu
Gravitational Equilibrium
Gravitational equilibrium is maintained by
two fluid-filled sacs known as the utricle
and the saccule
Inside these sacs are tiny hairs
suspended in a jelly-like substance, which
contains calcium carbonate granules
known as otoliths
When the head is in
its normal position,
the otoliths do not
move
If the head is tipped
sideways or
backwards, the
otoliths are pulled by
the force of gravity,
and brush against the
cilia
The movement of the
cilia produce nerve
impulses that are
sent to the brain,
indicating the position
of the head
http://www.qmw.ac.uk
Rotational Equilibrium
Rotational equilibrium is maintained by the
semicircular canals in the ear
Each canal contains a fluid-filled pocket known
as the ampulla
Rotational stimuli causes the fluid in the canals
to move
This causes the ampulla to move, bending hair
cells that are attached to them
This produces a nerve impulse that is carried to
the brain
The Ampullae
http://geoanalyzer.britannica.com/eb/art-538
Motion Sickness
Motion sickness is caused by contradictory
messages being sent to the brain
It is often caused by the balance centers
of the ears sending a different message
than what is perceived by the eyes
This results in a nervous system response
that often includes nausea
Preventing Motion Sickness
One way of preventing motion sickness is
to ensure that the eyes and the ears
receive the same information – you should
be able to register the motion visually
Certain drugs may be used to combat
motion sickness
Taste
Our sense of taste
enables us to
differentiate
between edible and
non-edible matter
Our taste buds are
arranged into
sections on our
tongue
http://www.diwinetaste.com
Our individual taste buds act in a similar manner
to other selective receptors in the body
Therefore, for a chemical to activate a nerve
impulse from a taste bud, it must correctly fit
into the receptor (the ‘lock & key’ principle)
Unlike other stimuli, taste requires water to
operate
The chemicals must be dissolved to enter
the taste buds
Therefore, saliva also plays a role in
whether or not we can taste something
that is put in our mouth
Smell
Our sense of smell is similar to our sense
of taste
Receptor sites on olfactory cells in our
nose are designed to combine with
molecules of a certain geometry
The messages that are produced by the
receptors are then sent to the olfactory
bulb of the brain
Smell and Taste Together
You may be familiar with the fact that both
taste and smell work together
For instance, when you have a cold, the
olfactory receptors in your nose do not
work as effectively, and therefore you have
a diminished sense of taste
People such as wine tasters use both of
these senses together
Touch
Mechanoreceptors for touch are located
throughout the body
Different receptors are sensitive to stimuli
such as light touch, pain, and high and low
temperatures
The receptors that release pain signals, as
we have seen, release impulses to the
brain which can be blocked by pain
medications
Sensation and Homeostasis
Our senses allow our bodies to maintain
homeostasis
Our senses of sight, touch, taste, smell,
and hearing give us information on which
to act
Neurotoxin Painkillers
Many animals (such as frogs, puffer fish, and
cone snails) produce neurotoxins
Many of these neurotoxins work by preventing
the transmission of an impulse through a neural
pathway
Scientists are currently studying whether or not
many of these compounds could be used to
prevent the transmission of pain messages
without the side-effects of morphine and other
opiate-based drugs