Download A.3: Perception of Stimuli

Sensory Receptors
 Organisms perceive information about their environment
via sensory receptors which can detect various stimuli
 Sensory organs are a window to the brain.
 When stimulated, the sense organs send a message to the
central nervous system.
 The nerve impulses arriving at the brain result in
sensation.
 We actually see, smell taste and feel with our brains rather
than our sense organs.
 Sensory cells also send messages to certain parts of the
brain that control emotion and memory.
 This is why we link tastes, sights, and sounds with
emotions and memories.
Sensory Receptors
 CHEMORECEPTORS
 Have proteins in their membranes that can bind to a
particular substance and initiate an action potential
 Chemoreceptors in the nose sense smell
 Chemoreceptors on our tongues (taste buds) detect
taste
 Chemoreceptors in our blood vessels detect blood pH
 Pain receptors are a type of chemoreceptors that
respond to chemicals released by damaged tissues.
Animations
 Smelling
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000117&ptid=17
 Tasting
 http://www.pennmedicine.org/encyclopedia/em_Disp
layAnimation.aspx?gcid=000129&ptid=17
Olfactory Receptors
 Olfaction in the sense of smell.
 Olfactory receptors are chemoreceptors in your nose.
 They have cilia which project into the air of the nose.
 Their membrane contains odorant receptor molecules
where are proteins that detect chemicals in the air.
 Odorants from food in our mouth can pass through
the mouth and nasal cavities to reach the nasal
epithelium
Olfactory Receptors
 Each olfactory receptor cells has just one type of
odorant receptor on its membrane (it can detect just 1
type of chemical or groups of chemicals).
 There are many receptor cells for each type of odor.
 Using these receptor cells, most animals can
distinguish a large number of chemicals in the air (or
water if the animal is aquatic).
 Mice for example have over 1000 different odorant
receptors.
 In some animals, the chemical can be detected in
extremely low concentrations
 Humans have a very insensitive and imprecise sense of
smell in comparison .
The Best Sniffer?
 With 1948 (types of) olfactory receptors
Thermoreceptors
 Detect changes in temperature
 Cold receptors can be found just under the skin
surface
 Warm receptors are located deeper.
 The hypothalamus contains thermoreceptors to
monitor blood temperature
Mechanoreceptors
 Detect movement
 Stimulated by mechanical force or pressure.
 Pressure receptors in your skin detect touch.
 Pressure rectors in your arteries detect changes in
blood pressure
 There is a system in our ears that involves fluid filled
canals and hairs that detect our body positions and
movement.
Photoreceptors
 Detect light
 Include the rods and cones in our eyes.
Label a Diagram of the Human Eye
Parts of the Eye
 CONJUCTIVA:
 Covers the sclera
 Keeps the eye moist
 CORNEA
 Made of a strong, transparent layer of tissue
 Covers iris and pupil
 Helps focus images, refracting light
 AQUEOUS HUMOUR:
 Clear fluid that supports the eyeball and transmits light
 PUPIL: the dark circle of the eye
 Actually a hole that allows light into the eye
 IRIS: the coloured part of the eye
 circular band of muscle surrounding the pupil
 regulates the size of the pupil
 In dim light, the iris opens  pupil dilates (becomes
wider) to allow more light in
 In bright light, the iris closes  pupil contracts
(becomes smaller)
 SCLERA: The white part of the eye
 The protective outer layer of the eye
 LENS:
 convex lens that focuses light rays and directs it to a
point.
 Your lens can change focus so that you can see
an object clearly regardless of whether it is right
in front of you, or far away.
 This is possible because it is surrounded by a
circle of muscles: ciliary muscles
 CILIARY MUSCLE
 muscles that surround the lens and control the
shape and therefore the focus of lens
 VITREOUS HUMOUR:
 Clear fluid that supports the eyeball and transmits light
 RETINA:
 inner lining at the back of the eye that acts as a
projection screen for light rays entering your eye
 Made of photoreceptors (rods and cones)
 ROD CELLS:
 photoreceptor cells of the retina that detect shapes and
movement in low light and shades of grey.
 (Many nocturnal mammals only have rods.)
 CONE CELLS:
 photoreceptor cells of the retina that detect colour.
 FOVEA:
 Area of retina where cone cells are densely packed
(vision is most acute here)
 OPTIC NERVE:
 connects your eye to your brain
 contains nerves that will send information collected by
the photoreceptors to the brain
 BLIND SPOT:
 the place where the optic nerve attaches to the retina.
 Therefore there are no photoreceptors here and light
cannot be detected.
 CHOROID:
 Vascular layer of the eye
 Contains blood vessels that will provide oxygen to eye
cells
 SCLERA:
 The white part of the eye
 Protective outer layer of the eyeball
How does the eye work to focus light and
detect images?
1. Light enters the eye at the cornea
2. Light passes through the aqueous humour to reach the
pupil
3. Light is then focused by the lens through the vitreous
humour to the retina.
The retina is composed of photoreceptors: cells that are
sensitive to light. There are 2 types: rods, and cones.
The RETINA
4. Light passes through a layer of transparent nerve
axons, then through the layer of “bipolar” neurons
(sensory neurons) before it reaches the rod and
cones cells (receptors)
5. The rods and cones transmit the information to
nerve cells in the retina (the bipolar cells)
6. The nerve cells transmit the information to the
optic nerve which takes the information to the
brain to be processed.
(The image formed on your retina is actually
inverted but your brain will flip it and interpret it
right side up!)
Processing Visual Stimuli
 When light “hits” the retina, it passes in between
various neurons (the ganglions (and their axons in the
optic fibre) and the sensory neurons) and then finally
“hits” the rods and cones.
 The rods and cones will receive the stimuli (the light)
and initiate and action potential in the sensory
(bipolar) neurons that will be sent to the brain via the
ganglion cells of the optic nerve.
 The axons of the ganglion cells travel to the visual area
of the cerebral cortex of the brain.
 The brain corrects the position of the image so that is
it rights side up and not reversed
 Annotate a diagram of the retina to show the cell types
and the direction of the light source.
 See page 529 of text for a good example of a labeled
diagram
 However, it’s not annotated!
The RETINA
Animations
 Eye/ Seeing
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000109&
ptid=17
 Glaucoma
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000060&
ptid=17
 Retina
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000106&
ptid=17
 Blinking
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000010&
ptid=17
 Cataract
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000024&
ptid=17
 Cornea Injury
 http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000035&
ptid=17
Rods vs Cones
Rods and Cones
 Rods are very sensitive to light and absorb a wide range of
visible wavelengths.
 They work well in dim light
 Think about when the lights are suddenly turned on in a
dark room.
 The pigment in them is temporarily bleached so for a few
seconds they don’t work and you may have problems seeing
 Cones are only stimulated by bright light and therefore
colour vision fades in dim light.
 Colour of light can be precisely determined by the brain
based on the relative stimulation of the 3 types of cone
cells.
Red-green colour-blindness
 X-linked recessive disorder in humans and some other
mammals
 (More common in men than females)
 Caused by the absence of defect in the gene for the
photoreceptor pigments for either red or green cone
cells.
Contra-lateral Processing
 This refers to the fact that some of the nerve fibres in
the optic nerve will cross before reaching the brain
(optic chiasma)
 Info from the left side of each visual field converge at
the optic chiasma and pass to the right side of the
brain.
 Info from the right side of each visual field converge at
the optic chiasma and pass to the left side of the brain.
Summary
 The visual cortex in the RIGHT side of the brain,
processes stimuli from the LEFT side of the visual field
of BOTH eyes.
 The visual cortex in the LEFT side of the brain,
processes stimuli from the RIGHT side of the visual
field of BOTH eyes.
Herman Grid Illusion
A
B
Herman Grid Illusion
 Why do you see grey blobs in the white area between
the black squares that vanish when you try to look at
them directly?
 Theory: the areas where you see grey are in your
peripheral vision where there are fewer light sensitive
cells than at your fovea.
 When you directly at the “grey” area, you are using the
center of the retina, your fovea, which has a high
concentration of light-sensitive cells.
Edge Enhancement
 The Hermann grid fools your eye because of the
extreme contrast between black and white edges.
 You have a special mechanism for seeing edges known
as edge enhancement
 Theory: light sensitive receptors in your eye switch off
their neighbouring receptors.
 This makes the edges look more distinct, because of
the extreme contrast between dark and light.
 When you look at and intersection in the grid (such as
A) there is a lot of white surrounding it compared to
looking at an area such as B which is surrounded by
black.
 Your brains receives the info that the contrast at A is
less than that at B.
 So B is seen as a white spot, and A is seen as a grey
spot.
Blind Spot
 is the one place on the retina of every healthy eye in
which there are no photoreceptors.
 Since there are no photoreceptors – light cannot be
detected here.
 There are no photoreceptors because this is where the
optic nerve attaches to the retina. You do not notice
your blind spot because your brain fills it in.
Find your blind spot
 Draw a small plus sign and a small dot on a piece of paper, at least 5 cm
apart.
 Cover your LEFT eye, and stare at the plus sign. Slowly move away (or
forward).
 When the black spot has disappeared, you have found your blind spot.
+
●
The Ear
 See diagram
The Ear
Ear Animations
 http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter19/a
nimation__effect_of_sound_waves_on_cochlear_struc
tures__quiz_1_.html
 http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter19/a
nimation__effect_of_sound_waves_on_cochlear_struc
tures__quiz_2_.html
How is sound perceived by the
ear?
 The pinna (the outer ear catches sound waves)
 Sound is directed to the eardrum by the pinna.
 The eardrum is a thin, taut sheet of flexible tissue.
 Sound causes the eardrum to vibrate
 This causes the small bones of the middle ear
(malleus, incus, and stapes) to vibrate and be
amplified (they amplify it more than 20x!)
 The stirrup/stapes is attached to the oval window (the
thin tissue covered opening to the inner ear).
 The oval window will create vibrations in the cochlea
(which is filled with fluid)
 As the fluid in the cochlea moves, this causes tiny
hairs that line the cochlea to move.
 These hairs are receptor cells
 The movement of the cochlea hairs cause the release
of a chemical message to stimulate an action potential
in the sensory neurons in the auditory nerve that will
be sent to the brain.
 Selective activation of different hair cells enables us to
distinguish between sounds of different pitches
 The vibrations in the fluid of the cochlea dissipate as
they reach the round window (another thin tissue)
 Different frequencies cause different parts of the
cochlea to vibrate, causing different neurons (with
different threshold values) to become depolarized.
 Loud noises cause the fluid to vibrate to a higher
degree and the hair cells bend even more. The brain
interprets this as a higher volume.
Cochlear Implants
 There are a variety of different causes for deafness and




hear deficiencies
In many cases, hear aids can amplify sounds and
overcome the problem.
However, if the hairs in the cochlea are defective
hearing aids won’t help.
If the auditory nerve is functioning properly, a
cochlear implant may help.
They do not fully restore normal hearing, but they
improve is and allow for recognition of speech
Cochlear Implant
Cochlear Implant
 External Part
 A microphone detects sound
 A speech processor filters out other sounds and selects
the speech frequencies
 A transmitter sends the processed sounds to the internal
parts
Cochlear Implant
 Internal Part
 Implanted in the bone behind the ear
 A receiver picks up sounds signals from the transmitter
 A stimulator converts the signals into electrical impulses
 Electrodes carry impulse to the cochlea
 The electrodes stimulate the auditory nerve directly,
bypassing the non-functioning hairs of the cochlea
Detecting Head Movements
 There are 3 fluid filled semicircular canals in the inner
ear.
 They detect movement of the head
using the sensory hair cells in each
canal
 When you move your head, the fluid
in the canals move, bending the hairs
which will send impulses to the
brain.
Detecting Head Movements
 The semicircular canals are at right angles to each
other, so each is in a different plane
 The brain deduces the direction of the movement by
the relative amount of stimulation of the hair cells in
each canal.