Perception of Stimuli

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Perception of Stimuli

• E2

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E1 - Stimulus and Response

• E1, p. 277

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Definitions

• Stimulus
• Any change that is detected by a receptor
• Can be internal or external
• Response
• Action resulting from a stimulus
• Many responses are collectively called behavior
• Reflex
• Rapid, unconscious response

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Receptors and neurons

• E.1.2, p. 277

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Receptors

• Before sensory neurons fire, receptors must be
  stimulated
• Receptors receive information (internal or
  external)
• Information is turned into action potentials
• APs passed onto CNS
• CNS decodes the information as sight, sound, etc.

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Types of receptors

• Information can be light, heat, sound, blood
  glucose levels, blood CO2 levels, etc.
• Chemoreceptors, thermoreceptors, photoreceptors,
  mechanoreceptors

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Neuron review

• Sensory
• Receive info from receptors
• Transmit Aps to relay neurons in the CNS
• Relay
• Receive Aps from sensory neurons
• Transmit them to CNS (conscious) or to motor
  neurons (subconscious reflex arc)
• Motor
• Take Aps to an effector muscle or gland
• Muscle contracts or gland releases product

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Synapse

• Aps must get from sensory --gt relay --gt motor
  neurons
• The neurons are not joined/continuous
• Synapses connect two neurons
• Chemical signals (neurotransmitters) carry the AP
  from the pre-synaptic neuron to the post-synaptic
  neuron

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Pain withdrawal reflex

• E.1.3, p. 278

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Pain Receptors

• In humans, pain receptors, or nociceptors, are a
  class of naked dendrites in the epidermis
• They respond to excess heat, pressure, or
  chemicals released from damaged or inflamed
  tissues

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Parts of a reflex

• Pain receptor
• Receives pain stimulus
• AP starts
• Sensory neuron
• Transmits pain stimulus to relay neuron
• Relay neuron
• in the spinal cord
• Motor neuron
• Motor end plate
• Effector muscle
• Muscle contracts to withdraw hand from pain source

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Sensory receptors

• E.2.1, p. 279

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Sensory receptors

• Chemoreceptors
• Electroreceptors
• Mechanoreceptors
• Photoreceptors
• Thermoreceptors

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Chemoreceptors

• Receptors that have special membrane proteins
• The membrane proteins bind to specific chemicals
• The membrane proteins start the depolarization
  process of an AP

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Chemoreceptor Examples

• Smell, taste, blood pH
• Taste buds on the tongue
• Certain receptors detect the presence of sugar
• Membrane proteins come in contact with sugar
• An AP is stimulated when sugar is present
• The chemoreceptor send the message --gt sensory
  neuron --gt brain
• Our brain interprets this signal as a sweet
  taste. This is our experience

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Electroreceptors

• Detect electric fields
• Electric fields are generated every time a muscle
  contracts (EKG)
• Example Sharks
• Prey moves, their muscles generate an electric
  field
• Electric field is quickly conducted through water
• Electroreceptors on the shark detect the prey
  (neurons depolarized, etc.)
• Shark eats prey

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Mechanoreceptors

• Sense changes in movement
• Movement generate Aps, which to go the brain
• Example Fish
• Lateral line system detects vibrations in the
  water

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Mechanoreceptors Inner ear

• Semicircular canals (3) are filled with fluid
• Tiny hairs float/wave in the fluid
• Each hair is connected to a mechanoreceptor
• When the head changes position, the fluid in the
  canals moves
• The fluid pushes the hairs
• The mechanoreceptors sense the hair movement and
  send APs to the brain

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Photoreceptors

• Rods and cones in the eye
• Photopigments are broken down when exposed to
  light
• Aps sent to the brain
• The brain interprets these signals as colors,
  shapes, etc.

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Thermoreceptors

• Cold receptors (near the skin surface) send Aps
  to one part of the hypothalamus
• Warm receptors (deeper under the skin) send Aps
  to a slightly different part of the hypothalamus
• The hypothalamus also monitors blood temp

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Eye diagram

• E.2.2, p. 280

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Eye anatomy

• Sclera
• Choroid
• Retina
• Vitreous humor
• Fovea
• Optic nerve
• Blind spot

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Eye anatomy

• Eyelid
• Cornea
• Conjunctiva
• Aqueous humor
• Iris
• Colored area around pupil
• Lens
• Focuses light
• Pupil
• Dark circle in the center of the eye
• Changes diameter based on light conditions

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Fig. 50-18
Choroid
Sclera
Retina
Ciliary body
Suspensoryligament
Fovea (centerof visual field)
Cornea
Iris
Opticnerve
Pupil
Aqueoushumor
Lens
Central artery andvein of the retina
Vitreous humor
Optic disk(blind spot)
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Fig. 50-23
Retina
Choroid
Photoreceptors
Neurons
Retina
Cone
Rod
Light
Tobrain
Optic nerve
Light
Ganglioncell
Amacrinecell
Horizontalcell
Opticnerveaxons
Bipolarcell
Pigmentedepithelium
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Retina

• E.2.3, p. 280

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Retina

• Layers of the retina
• Optic nerve
• Ganglion cells
• Bipolar neurons
• Rod and cone cells
• Pigmented epithelium
• Rod and cone cells detect light
• These are the photoreceptors

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Retina

• Light passes through the nerves/ganglions
• Rod and cone cells absorb light
• AP sent to the brain
• Any light not absorbed by R and Cs is absorbed by
  the pigmented epithelium
• Prevents reflections

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Rods and Cones

• E.2.4, p. 281

32

• The human retina contains two types of
  photoreceptors rods and cones
• Rods are light-sensitive but dont distinguish
  colors
• Cones distinguish colors but are not as sensitive
  to light
• In humans, cones are concentrated in the fovea,
  the center of the visual field, and rods are more
  concentrated around the periphery of the retina

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Fig. 50-22
Light Responses
Dark Responses
Rhodopsin inactive
Rhodopsin active
Na channels open
Na channels closed
Rod depolarized
Rod hyperpolarized
No glutamatereleased
Glutamatereleased
Bipolar cell eitherdepolarized orhyperpolarized
Bipolar cell eitherhyperpolarized ordepolarized
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Rod cells

• Night vision, black/white vision
• Not in the fovea
• Use the pigment rhodopsin
• Several rods link to one bipolar neuron
• More sensitive
• Less accurate
• Aps are passed to bipolar neurons
• If Aps reach threshold, signal sent to CNS/optic
  nerve

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Cone cells

• Day vision, color vision
• Mostly in fovea
• Contain 3 pigments sensitive to different colors
  of light
• 1 cone --gt 1 bipolar neuron
• Less sensitive (more cone stimulation is needed
  for the BP neuron to fire)
• More accurate
• Table p. 281

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Processing of visual stimuli

• E.2.5, p. 281

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Receptive areas
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Receptive areas

• Groups of rods
• Center rods are excitatory
• Send an impulse to the ganglion
• Outside rods are inhibitory
• Stop the ganglion from sending AP to the brain

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Response of receptive areas

• Light --gt center
• Increased impulses to the brain
• Light --gt edge
• Decreased impulses to the brain
• Light --gt both center and edge
• No change (same as dark)

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Reason/effect

• Called edge enhancement
• Enhanced contrast (different between light and
  dark) at the edge of an object
• The object seems darker
• The area around it seems lighter

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Hermann grid illusion, p. 282

• There are false grey spots at each white
  intersection
• When looking directly at any single intersection,
  the grey dot disappears

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Looking at A

• White intersection stimulates excitatory rods in
  the center of the RA
• But, white lines around the edges of A stimulate
  the inhibitory rods of the RA
• The intersections appear to have less contrast
• They appear grey

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Looking at B

• White stimulates excitatory rods in the center of
  the RA
• The edges around B are mostly black, so the
  inhibitory rods on the edges of the RA dont fire
• The lines appear to have more contrast
• They appear white

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Looking at the grey dots

• Looking directly at any single grey dot causes it
  to disappear
• Direct focus uses the fovea, which is more
  accurate/sensitive
• Has smaller receptive fields

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Contra lateral processing

• p.282

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C-L processing

• Some nerve fibers of the optic nerve cross before
  reaching the brain
• Some nerves from the left eye are processed by
  the right side of the brain
• Some nerves from the right eye are processed by
  the left side of the brain
• NOT all nerves cross!

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• The optic nerves meet at the optic chiasm near
  the cerebral cortex
• Here, axons from the left visual field (from both
  the left and right eye) converge and travel to
  the right side of the brain
• Likewise, axons from the right visual field
  travel to the left side of the brain

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Overall effect

• The left visual field is processed by the right
  brain
• Uses nerves from both eyes
• The right visual field is processed by the left
  brain
• Uses nerves from both eyes
• Used for determining distances and sizes (depth
  perception)

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Fig. 50-24
Rightvisualfield
Opticchiasm
Righteye
Lefteye
Leftvisualfield
Optic nerve
Primaryvisual cortex
Lateralgeniculatenucleus
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Hearing

• E.2.7, p. 283

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Sound

• Sound is vibrating waves of air molecules
• The ear detects these vibrations
• Pinna focuses the vibrations to the ear drum
• Ear drum vibrates the Ossicles
• Ear drum --gt hammer --gt anvil --gt stirrup

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Hearing

• Ossicles amplify and transfer vibrations to the
  cochlea
• Cochlea is fluid-filled and has hair
  mechanoreceptors
• Spiral shape
• Oval window vibrates, causing waves in the fluid

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• These vibrations create pressure waves in the
  fluid in the cochlea that travel through the
  vestibular canal
• Pressure waves in the canal cause the basilar
  membrane to vibrate, bending its hair cells
• This bending of hair cells depolarizes the
  membranes of mechanoreceptors and sends action
  potentials to the brain via the auditory nerve

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Fig. 50-8
Middleear
Outer ear
Inner ear
Stapes
Skullbone
Semicircularcanals
Incus
Malleus
Auditory nerveto brain
Bone
Cochlearduct
Auditorynerve
Vestibularcanal
Tympaniccanal
Cochlea
Ovalwindow
Eustachiantube
Auditorycanal
Pinna
Organ of Corti
Roundwindow
Tympanicmembrane
Tectorialmembrane
Hair cells
Hair cell bundle froma bullfrog the
longestcilia shown areabout 8 µm (SEM).
To auditorynerve
Axons ofsensory neurons
Basilarmembrane
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Fig. 50-8a
Middleear
Outer ear
Inner ear
Stapes
Skullbone
Semicircularcanals
Incus
Malleus
Auditory nerveto brain
Cochlea
Ovalwindow
Eustachiantube
Auditorycanal
Pinna
Roundwindow
Tympanicmembrane
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Fig. 50-8b
Cochlearduct
Bone
Auditorynerve
Vestibularcanal
Tympaniccanal
Organ of Corti
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Fig. 50-8c
Tectorialmembrane
Hair cells
To auditorynerve
Axons ofsensory neurons
Basilarmembrane
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Fig. 50-8d
Hair cell bundle froma bullfrog the
longestcilia shown areabout 8 µm (SEM).
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Fig. 50-9
Hairs ofhair cell
Neuro-trans-mitter atsynapse
Moreneuro-trans-mitter
Lessneuro-trans-mitter
Sensoryneuron
50
50
50
Receptor potential
70
70
70
Membranepotential (mV)
Membranepotential (mV)
Membranepotential (mV)
Action potentials
0
0
0
Signal
Signal
Signal
70
70
70
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
Time (sec)
Time (sec)
Time (sec)
(a) No bending of hairs
(b) Bending of hairs in one direction
(c) Bending of hairs in other direction
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Fig. 50-9a
Hairs ofhair cell
Neuro-trans-mitter atsynapse
Sensoryneuron
50
70
Membranepotential (mV)
Action potentials
0
Signal
70
Time (sec)
(a) No bending of hairs
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Fig. 50-9b
Moreneuro-trans-mitter
50
Receptor potential
70
Membranepotential (mV)
0
Signal
70
Time (sec)
(b) Bending of hairs in one direction
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Fig. 50-9c
Lessneuro-trans-mitter
50
70
Membranepotential (mV)
0
Signal
70
Time (sec)
(c) Bending of hairs in other direction
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• The fluid waves dissipate when they strike the
  round window at the end of the tympanic canal

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Fig. 50-10
500 Hz(low pitch)
Axons ofsensory neurons
1 kHz
Apex
Flexible end ofbasilar membrane
Ovalwindow
Vestibularcanal
Apex
2 kHz
Basilar membrane
Stapes

Vibration
4 kHz

Basilar membrane
8 kHz
Tympaniccanal
Base
Fluid(perilymph)
Base(stiff)
16 kHz(high pitch)
Roundwindow
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Fig. 50-10a
Axons ofsensory neurons
Apex
Ovalwindow
Vestibularcanal
Stapes
Vibration
Basilar membrane
Tympaniccanal
Base
Fluid(perilymph)
Roundwindow
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• The ear conveys information about
• Volume, the amplitude of the sound wave
• Pitch, the frequency of the sound wave
• The cochlea can distinguish pitch because the
  basilar membrane is not uniform along its length
• Each region vibrates most vigorously at a
  particular frequency and leads to excitation of a
  specific auditory area of the cerebral cortex

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Fig. 50-10b
500 Hz(low pitch)
1 kHz
Flexible end ofbasilar membrane
Apex
2 kHz
Basilar membrane
4 kHz
8 kHz
Base(stiff)
16 kHz(high pitch)