PHYL 1400
January 15 & 18, 2010
Sensory Physiology
I. General principles
II. Specific sensory systems
A. Visual system
B. Auditory system
C. Vestibular system
D. Somatosensory system
Literature:
Vander’s Human Physiology, 11th Ed.
pp. 192-229
Please note:
This lecture does not cover gustation
and olfaction (no exam questions
on these topics)
Instructor:
Dr. Stefan Krueger
Dept. of Physiology & Biophysics
Tupper Building, Room 5-F
stefan.krueger@dal.ca
PHYL 1400 -- SENSORY
LECTURE TITLE
PHYSIOLOGY
-- SECTION TITLE
A. General principles
A. Section
of sensory
title
physiology
Subsection
Sensory receptors
title 1
•
•
•
•
Slide
What 1are sensory receptors?
Slide
2 sensory receptors translate a stimulus
How do
Slide
3
into neuronal
activity?
What are sensory unit and receptive field?
Primary sensory
Subsection
title 2coding
sensory
information sent to the brain, what
Slide
4
•In the
for:
Slide 5
•is characteristic
stimulus
• Slide
6 type?
• stimulus intensity?
• stimulus duration?
• stimulus location?
Neural pathways of sensory information
• Ascending sensory pathways (1): Convergence and divergence
• Ascending sensory pathways (2): Specific vs. nonspecific pathways
• Do descending pathways have any influence descending pathways on
the transduction of sensory information to the brain?
• Where does sensory information end up in the cortex?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Sensory receptors
• convert stimulus into neuronal activity
• are either
a. endings of afferent neurons or
b. specialized cells adjacent to an afferent
neuron
Next: How do sensory receptors translate a stimulus into neuronal activity?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Stimulus transduction
1. Stimulus --> opening (or closing) of ion
channels in specialized receptor membrane
2. Current flow through ion channels -->
receptor potential (= graded change in
membrane potential).
3. Receptor potential spreads within afferent
neuron until it reaches region with high density
of voltage-dependent sodium channels. If
receptor potential reaches threshold for their
activation, action potentials are generated.
4. Action potentials propagate within afferent
neuron and cause release of neurotransmitter
Next: What are sensory unit and receptive field ?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Sensory unit and receptive field
• Sensory unit: Primary sensory neuron with all
receptor endings or associated sensory
receptor cells
(Q: How does sensory unit relate to sensory
receptor ?)
• Receptive field: Area of the body surface in
which a stimulus leads to activation of the
sensory unit
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Information on stimulus type
• Each sensory receptor is particularly sensitive to one stimulus type or modality
Table 1: Sensory receptor classes and their preferred stimulus type
Sensory receptor
Stimulus modality
Location
Mechanoreceptors
Stretch and pressure
Skin, muscle and tendons,
blood vessels
Thermoreceptors
Cold, warmth
Skin
Photoreceptors
Light
Retina
Chemoreceptors
Certain chemicals
Tongue, nose
Nociceptors
Stimuli causing tissue
damage
Throughout body
• All receptors of a sensory unit respond to same stimulus modality
• Receptive fields of sensory units responding to different modalities often overlap
Next: How is information on stimulus intensity encoded in the sensory signal?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Information on stimulus intensity
Next: How is information on stimulus duration encoded in the sensory signal?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Information on stimulus duration
Some primary sensory neurons fire action potentials as long
as stimulus is present, others do not:
Adaptation = Sensory receptors decrease in sensitivity to
stimulus of constant strength
action potential frequency decreases
Sensory receptors can be
• Rapidly adapting:
Receptors signal changes in stimulus intensity
• Slowly adapting:
Receptors signal the continued presence of the stimulus
Next: How is information on stimulus location encoded in the sensory signal?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Information on location
The acuity (= precision of stimulus location)
depends mainly on the receptive field size and
density of sensory units
Lateral inhibition can further enhance
sensory acuity
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Convergence and divergence of ascending pathways
The ascending neuronal pathways to cortex are
polyneuronal: Primary sensory neurons synapse
onto higher-order neurons
• Divergence: One primary sensory neuron
synapses onto many higher-order neurons.
• Convergence: Higher-order sensory neuron
may receive input from more than one primary
sensory neuron (information processed rather
than just relayed).
Next: Specific and nonspecific ascending pathways
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Specific and nonspecific ascending pathways
• Specific ascending neuronal pathways carry
information on one stimulus modality.
• Nonspecific (or polymodal) pathways carry
convergent information from several stimulus
modalities.
Next: Influence of descending pathways on the transduction of sensory information
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Influence of descending pathways
Descending pathways can inhibit the transduction
of sensory information to the brain.
Not all sensory information reaches
consciousness.
Next: Where does sensory information end up in the cortex?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- GENERAL PRINCIPLES
Sensory processing in the cortex
Nonspecific pathways
Terminate in regions important in controlling arousal
and alertness (cortex and in brainstem)
Specific ascending pathways
• Terminate in specific primary sensory areas
• Further processing in associational cortical areas
(also input from brain regions involved in attention,
memory)
Perception (= understanding of sensations)
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Back to Sensory Physiology
PHYL 1400 -- SENSORY PHYSIOLOGY
B. Visual system
Optics of vision
• Structures of the eye and their function
• Optical portion of the eye: Function
• Optical portion of the eye: Pathologies
The retina and phototransduction
• Sensory receptors and higher-order sensory
neurons in the retina
• Phototransduction
Retinal efferents
• Cortical efferents
• Other ascending pathways
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Eye structures and their function
Regulation of light amount entering the eye
• Iris
• Pupil
Light refraction
• Cornea
• Lens
• Ciliary muscles
Light detection
• Retina (fovea, optic disc)
Support structures
• Choroid
• Sclera
• Aqueous humor
• Vitreous humor
Next: Function of the optical portion of the eye
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Optical portion of the eye: Function
Refraction: Lens (25%) and cornea (75%) focus
images on retina
Accommodation: Adjustment of lens convexity
by ciliary muscles. Images of nearby as well as
distant objects can be focused onto retina.
Next: Pathologies of the optical portion of the eye
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Optical portion of the eye: Pathologies
1) Cataract (opaque lens)
2) Refractive errors
a) Myopia (shortsightedness; eye too long to
allow lens to focus distant objects on retina)
b) Hyperopia (farsightedness; eye too short to
allow lens to focus near objects on retina)
c) Presbyopia (loss of lens elasticity with age)
d) Astigmatism (irregular curvature of cornea or
lens)
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Cellular elements of the retina
Photoreceptors
• Cones: Color vision
• Rods: Vision under low illumination levels
• Cones and rods depolarized in darkness, continuously
release neurotransmitter.
Light elicits hyperpolarization and attenuation of
neurotransmitter release.
Higher-order sensory neurons
• Bipolar cells: Excited or inhibited by either rods or cones,
graded potentials and NT release
• Ganglion cells: Excitatory input from bipolar cells, generate
action potentials. Only neurons to project beyond retina. Die in
glaucoma and macular degeneration.
• Inhibitory interneurons: Horizontal cells between
photoreceptors, amacrine cells between bipolar and ganglion
cells: Lateral inhibition and further processing.
Next: Phototransduction
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Phototransduction
• Discs (membrane stacks) contain
photopigment rhodopsin (rods) or
opsins (cones)
• Darkness:
cGMP constantly generated
⇒ cGMP-gated cation channels open
⇒ persistent depolarization
⇒ continuous neurotransmitter release
• Light:
Conformational change of retinal
(photopigment) in opsin or rhodopsin
⇒ degradation of cGMP
⇒ closure of cGMP-gated cation channels
⇒ membrane hyperpolarization
⇒ reduction of neurotransmitter release
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VISUAL SYSTEM
Retinal efferents
To the cortex
• Retinal ganglion cell axons form optic nerve & tract →
neurons in thalamus → visual cortex
• Crossing of axons from ganglion cells in nasal half of retinas at
optic chiasm
⇒ Right half of visual field represented in left visual cortex
To other targets
• To brainstem: Control of changes in pupil size in response
to illumination
• To brainstem: Gaze fixation
• To hypothalamus: Control of biological clock by light
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PHYL 1400 -- SENSORY PHYSIOLOGY
C. Auditory system
Sound transmission
• Sound transmission in the outer and middle ear
• Detection of sound waves in the inner ear
• Signal transduction in hair cells
Auditory pathway
• Auditory pathway
• How are sound pitch, loudness and direction
encoded in the efferent auditory information?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- AUDITORY SYSTEM
Sound transmission (1): Outer and middle ear
What is sound?
• Sound = Waves of compressed and
expanded air
• Loudness determined by wave amplitude
• Pitch determined by the frequency of the
wave.
Outer and middle ear
amplify sound
1. Sound waves funneled by external
auditory canal onto tympanic membrane,
which starts to vibrate
2. The tympanic membrane vibration causes
movement of middle ear bones
3. Middle ear bones couple vibrations to oval window (= membrane separating middle ear and inner
ear). Leverage of middle ear bones causes additional amplification of sound (=vibration amplitude)
Next: Sound transmission in the cochlea
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PHYL 1400 -- SENSORY PHYSIOLOGY -- AUDITORY SYSTEM
Sound transmission (2): Cochlea
4. Vibrations of the oval window cause pressure
waves in fluid-filled cochlear duct.
5. Pressure waves in the cochlear duct cause
vibrations of basilar membrane (different regions
depending on sound pitch).
6. Hair cells (= sensory receptors) in Organ of Corti
on top of the basilar membrane move and their
stereocilia (hair-like protrusions) are bent.
Next: Sensory transduction in hair cells
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PHYL 1400 -- SENSORY PHYSIOLOGY -- AUDITORY SYSTEM
Sound transmission (3): Sensory transduction in hair cells
4. Bending of stereocilia
⇒ stretch of tip links
⇒ opening of mechanically gated
cation channels
5. Channel opening
⇒ depolarization of hair cells
⇒ neurotransmitter release
6. NT release from hair cells
⇒ primary sensory neurons depolarized
⇒ fire action potentials
Next: Auditory pathway; how sound pitch, loudness, and direction are encoded
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PHYL 1400 -- SENSORY PHYSIOLOGY -- AUDITORY SYSTEM
Neuronal pathway of the auditory system
Hair cells → primary sensory neurons → cochlear nuclei (brainstem) →→ thalamus → auditory cortex
Encoding of sound pitch, loudness, and direction
• Sound pitch: Each hair cell in cochlea responds only to limited range of sound frequencies,
depending on location on basilar membrane. Efferents from neighboring regions in the cochlea end up
in neighboring regions of auditory cortex (tonotopic organization).
• Loudness: The louder sound, the higher the action potential frequency in sensory neuron
• Sound localization performed in brainstem by neurons receiving input from both ears: Comparison
of time and intensity of the two inputs
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PHYL 1400 -- SENSORY PHYSIOLOGY
D. Vestibular system
The vestibular apparatus
• Structure of the vestibular apparatus
• Function of the semicircular canals
• Function of the otolith organs
Vestibular pathways
• Vestibular efferents and their functions
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VESTIBULAR SYSTEM
The vestibular apparatus
Vestibular apparatus: Series of fluid-filled tubes
in the inner ear. Together with cochlea often also
called labyrinth.
Consists of semicircular canals and otolith
organs, utricle and saccule
Function: Detection of rotational and linear
accelerations of the head
Next: Function of semicircular canals
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VESTIBULAR SYSTEM
Semicircular canals
• Detect rotational acceleration along three
perpendicular axes.
• Have hair cells in ampullae (= bulges) of each
canal with stereocilia in cupula (=gelatinous
mass)
Head rotation:
Endolymphatic fluid exerts pressure on cupula,
causing bending of stereocilia
⇒ Cation channels are either opened or closed
(depending on direction of rotation)
⇒ De- or hyperpolarization of hair cells
⇒ In- or decrease neurotransmitter release
⇒ NT release causes in- or decrease in firing
frequency of primary sensory neurons
Next: Function of otolith organs
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VESTIBULAR SYSTEM
Otolith organs
• Detect linear accelerations of head in horizontal or
vertical directions
• Utricle: Respond to accelerations in the horizontal plane
• Saccule: Respond to accelerations in the vertical plane
• Stereocilia of hair cells in utricle and saccule are
ensheathed in gelatinous substance containing otoliths.
Otoliths respond to gravitational forces and cause
bending of hair cell stereocilia.
Next: Vestibular efferents and their function
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PHYL 1400 -- SENSORY PHYSIOLOGY -- VESTIBULAR SYSTEM
Vestibular pathways and their function
Pathway to
Function
Symptoms of
vestibular pathology
or trauma
Brainstem:
Fixation of gaze with
head movement
Nystagmus (jerky
back-and-forth
movement of eyes)
Spinal cord
Postural adjustment
of head and body
(usually compensation
through proprioceptive,
visual input)
Cortex (via
thalamus)
Perception of body
orientation and
acceleration
Vertigo
Neurons
controlling
eye muscles
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Mismatch of vestibular
and visual input:
Motion sickness
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PHYL 1400 -- SENSORY PHYSIOLOGY
D. Somatosensory system
Somatosensory receptors
• Somatosensory receptors on the body surface
• Free nerve endings: Stimulation
• Modulation of nociceptive information
Neural pathways
• Ascending somatosensory pathways
• Organization of the somatosensory cortex
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PHYL 1400 --SENSORY PHYSIOLOGY -- SOMATOSENSORY SYSTEM
Somatosensory receptors on the body surface
Receptor
type
Modality
Localization
Threshold
Adaptation
Meissner's
corpuscles
(A)
touch,
dynamic
pressure
glabrous skin
low
rapid
Merkel's
disks (B)
touch, static
pressure
associated
with hair
follicles
low
slow
Free nerve
endings (C)
pain,
temperature
skin and
viscera
high
slow
Pacinian
corpuscles
(D)
deep
pressure,
vibration
subcutis and
viscera
low
rapid
Ruffini's
corpuscle (E)
stretch,
torque
skin, along
stretch lines
low
slow
Next: Which stimuli activate free nerve endings in the skin?
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PHYL 1400 -- SENSORY PHYSIOLOGY -- SOMATOSENSORY SYSTEM
Free nerve endings: Thermoreceptors and nociceptors
Thermoreceptors
• Cold-sensing and warmth-sensing thermoreceptors
• Contain non-specific cation channels that open in
response to low or high temperatures
• Cold-sensing channels activated by menthol
• Warm-sensing channels activated by capsaicin
(chili peppers)
Nociceptors
• Fast-conducting nociceptors respond to intense
mechanical stimuli or excessive heat
• Slow (unmyelinated) nociceptors are polymodal:
Activated by temperature, intense mechanical stimuli,
chemicals (acids, compounds released by mast cells
and other cells of the immune system, ...)
Next: Afferent and efferent modulation of nociceptive information
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PHYL 1400 -- SENSORY PHYSIOLOGY -- SOMATOSENSORY SYSTEM
Modulation of nociceptive information
• Convergence of nociceptive afferents
⇒ Referred pain
• Plasticity of nociceptors and afferents after trauma
⇒ Hyperalgesia (increased sensitivity to pain) and
allodynia (pain sensation to touch and mild
temperature changes)
• Descending inhibition: Descending pathways inhibit
NT release from primary nociceptive neurons through
release of endogenous opioids
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PHYL 1400 -- SENSORY PHYSIOLOGY -- SOMATOSENSORY SYSTEM
Somatosensory pathways
Anterolateral system
• Nonspecific pathway for pain and
temperature
• Primary sensory neuron → spinal cord
→ thalamus → somatosensory cortex
• Axons of spinal cord interneurons cross
the midline
Dorsal column system
• Specific pathway for somatic
information from the body surface and
proprioception
• Primary sensory neuron → brainstem
(dorsal column nuclei) → thalamus
→ somatosensory cortex
• Axons of brainstem interneurons cross
the midline
Next: Organization of the somatosensory cortex
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PHYL 1400 -- SENSORY PHYSIOLOGY -- SOMATOSENSORY SYSTEM
Somatosensory cortex
• Somatosensory cortex is posterior to
motor cortex, in parietal lobe of
both cortical hemispheres
• Crossing afferents: left body
represented in right cortical
hemisphere and vice versa
• Somatotopic organization of
information from specific pathways
• Areas of body surface with high
receptor density occupy larger
areas in somatosensory cortex
• Spinothalamic afferents are not
topologically organized
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