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PNS – Afferent Division
Sensory Physiology
Part 2
Special Senses – External Stimuli
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Vision
Hearing
Taste
Smell
Equilibrium
Figure 10-4: Sensory pathways
Cross-Section of the Eye
Organization of the Retina
Figure 17.6b, c
Vision
• Light enters the eye through the pupil, diameter of pupil
modulates light
• Shape of lens focuses the light on the retina
• Retinal rods and cones are photoreceptors
• Reflected light translated into mental image
Figure 10-36: Photoreceptors in the fovea
Pupils
• Bright light they constrict to ~ 1.5 mm
• Dark they dilate to ~ 8 mm.
• Controlled by the autonomic nervous system,
pupillary reflex
Image Projection
•The image projected onto the retina is inverted or
upside down. Visual processing in the brain reverses
the image
Image Projection
• Convex structures of eye produce
convergence of diverging light rays that reach
eye
Refraction of Light
Figure 10-30a
Optics
Figure 10-31a
Optics
Figure 10-31b
Mechanism of Accommodation
• Accommodation is the process by which the eye adjusts the
shape of the lens to keep objects in focus
Figure 10-32a
Mechanism of Accommodation
Figure 10-32b
Common Visual Defects
Figure 10-33a
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Retina
Photoreceptors - rods and cones detect light stimulus
Bipolar - generate APs
Amacrine & Horizontal cells – local integration of APs
Ganglion cells converge form optic nerve
Retina
Photoreceptors
Neurons
Amacrine
cell
Optic
nerve Ganglion
fibers cell
Cone Rod
Horizontal
cell
Bipolar
cell
Pigmented
epithelium
Photoreceptors
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Rods - light-sensitive but don’t distinguish colors; monochromatic, night vision
Cones - Three types; red, green, & blue, distinguish colors but are not as
sensitive, high acuity day vision
Photo-transduction
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Each rod or cone contains visual pigments consisting of a lightabsorbing molecule called retinal bonded to a protein called opsin
Rod
Outer
segment
Disks
Cell body
Inside
of disk
cis isomer
Light
Enzymes
Synaptic
terminal
Cytosol
Retinal
Rhodopsin
Opsin
trans isomer
Phototransduction
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Rods contain the pigment rhodopsin, which changes shape when
absorbing light
Retinal Changes Shape
Retinal restored
Opsin inactivated
Photo-transduction
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Photons "bleach" opsin, retinal changes shape and released, transduction cascade, decreased
cGMP, Na+ channel closes, K+ opens , hyperpolarization reduces NT release
(a) In darkness, rhodopsin is
inactive, cGMP is high, and
ion channels are open.
(b) Light bleaches rhodopsin. Opsin
decreases cGMP, closes Na+
channels, and hyperpolarizes the cell.
Activated
retinal
Pigment epithelium cell
Opsin (bleached
pigment)
(c) In the recovery phase, retinal
recombines with opsin.
Activates
transducin
Retinal converted to
inactive form
Disk
Transducin
(G protein)
Inactive
rhodopsin
(opsin and retinal)
Cascade
Decreased
cGMP
cGMP
levels high
Na+
Na+ channel
closes
Na+
K+
K+
Membrane
hyperpolarizes
to -70 mV.
Membrane potential
in dark = -40mV
Light
Tonic release of neurotransmitter
onto bipolar neurons
Neurotransmitter decreases in proportion
to amount of light.
Retinal recombines
with opsin to
form rhodopsin.
Photo-transduction
Light
Active
rhodopsin
INSIDE OF DISK
EXTRACELLULAR
FLUID
PDE
Membrane
potential (mV)
Plasma
membrane
0
Dark Light
Inactive
rhodopsin
Transducin
Disk
membrane
cGMP
–40
GMP
Na+
Hyperpolarization
–70
Time
CYTOSOL
Na+
Photo-transduction
• In the dark, rods and cones
release the neurotransmitter
glutamate into synapses
with neurons called bipolar
cells
• Bipolar cells are
hyperpolarized
• In the light, rods and cones
hyperpolarize, shutting off
release of glutamate
• The bipolar cells are then
depolarized
Dark Responses
Light Responses
Rhodopsin inactive
Rhodopsin active
Na+ channels open
Na+ channels closed
Rod depolarized
Rod hyperpolarized
Glutamate
released
No glutamate
released
Bipolar cell
hyperpolarized
Bipolar cell
depolarized
Convergence and Ganglion Cell Function
Figure 17.18
The Retina & Visual Acuity
Light adapted eye has greatest
visual acuity at the fovea Photopic vision (cones)
Dark adapted eye has least
visual acuity at the fovea but
has greater acuity in
the parafoveal region
Scotopic vision (rods)
Fovea
Visual Integration / Pathway
Retinal cells
2x binocular vision
plus accessory
structures
Optic disk - blood supply
optic nerve
Retina
Vision Integration / Pathway
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Optic nerve
Optic chiasm
Optic tract
Thalamus
Visual cortex
Figure 10-29b, c: Neural pathways for vision and the papillary reflex
The Ear / Auditory Physiology
External Ear Structures & Functions
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Pinna—Collects sound waves and channels them into the external
auditory canal.
External Auditory Canal—Directs the sound waves toward the
tympanic membrane.
Tympanic membrane—Receives the sound waves and transmits the
vibration to the ossicles of the middle ear.
Sound and Hearing
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Sound waves travel toward tympanic membrane, which vibrates
Auditory ossicles conduct the vibration into the inner ear
Movement at the oval window applies pressure to the perilymph of the
cochlear duct
Pressure waves move through vestibular membrane through endolymph
to distort basilar membrane
Hair cells of the Organ of Corti are pushed against the tectorial
membrane
Figure 17.28a
Cochlea and Organ of Corti
Organ of Corti
• Ion channels open, depolarizing the hair cells,
releasing glutamate that stimulates a sensory
neuron.
• Greater displacement of basilar membrane, bending
of stereocilia; the greater the amount of NT released.
• Increases frequency of APs produced.
Signal Transduction in Hair Cells
• The apical hair cell is modified into stereocilia
Figure 10-21a
Pitch Discrimination
• Different frequencies of vibrations (compression
waves) in cochlea stimulate different areas of Organ
of Corti
• Displacement of basilar membrane results in pitch
discrimination.
Cochlea
(uncoiled)
Basilar
membrane
Apex
(wide and
flexible)
500 Hz (low pitch)
1 kHz
2 kHz
4 kHz
8 kHz
16 kHz
(high pitch)
Base
(narrow and stiff)
Frequency
producing
maximum vibration
Sensory Coding for Pitch
• Waves in basilar membrane reach a peak at different regions
depending upon pitch of sound.
• Sounds of higher frequency cause maximum vibrations of
basilar membrane.
Vestibular Apparatus
Vestibular apparatus provides
information about movement and
position in space
Figure 10-23a, b: ANATOMY SUMMARY: Vestibular Apparatus
Vestibular Apparatus
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Cristae are receptors within ampullae that detect rotational acceleration
Maculae are receptors within utricle and saccule that detect linear
acceleration and gravity
Vestibular Apparatus: Semicircular Canals
• Provide information about rotational
acceleration.
– Project in 3 different planes.
Figure 10-23b
Semicircular Canals
• At the base of the semicircular duct is the crista ampullaris, where
sensory hair cells are located.
– Hair cell processes are embedded in the cupula.
Semicircular Canals
• Endolymph provides inertia so that the sensory processes will
bend in direction opposite to the angular acceleration.
Rotational Forces in the Cristae
Figure 10-24
Vestibular Apparatus
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Cristae are receptors within ampullae that detect rotational acceleration
Maculae are receptors within utricle and saccule that detect linear
acceleration and gravity
Otolith Organs: Maculae
• The otolith organs sense linear acceleration and head position
Figure 10-25a
Otolith Organs
Figure 10-25a
Stereocilia and Kinocilium
• When stereocilia bend
toward kinocilium;
membrane depolarizes,
and releases NT
• When bends away from
kinocilium
hyperpolarization occurs
• Frequency of APs carries
information about
movement
Maculae of the Utricle and Saccule
• Utricle:
– More sensitive to horizontal acceleration.
• During forward acceleration, otolithic membrane lags
behind hair cells, so hairs pushed backward.
• Saccule:
– More sensitive to vertical acceleration.
• Hairs pushed upward when person descends.
Taste (Gustation)
• Taste Receptors - Clustered in taste buds
• Associated with lingual papillae
• Taste buds
– Contain basal cells which appear to be stem cells
– Gustatory cells extend taste hairs through a narrow taste pore
Taste (Gustation)
• Epithelial cell receptors
clustered in barrel-shaped
taste buds
• Each taste bud consists of
50-100 specialized
epithelial cells.
• Taste cells are not
neurons, but depolarize
upon stimulation and if
reach threshold, release
NT that stimulate sensory
neurons.
Taste
(continued)
• Each taste bud contains taste cells responsive to
each of the different taste categories.
• A given sensory neuron may be stimulated by
more than 1 taste cell in # of different taste buds
• One sensory fiber may not transmit information
specific for only 1 category of taste
• Brain interprets the pattern of stimulation with
the sense of smell; so that we perceive complex
tastes
Taste Receptor Distribution
• Salty:
– Na+ passes through
channels, activates
specific receptor
cells, depolarizing
the cells, and
releasing NT.
• Sour:
– Presence of H+ passes
through the channel,
opens Ca+ channels
Taste Receptor Distribution
• Sweet and
bitter:
– Mediated by
receptors
coupled to Gprotein
(gustducin).
(continued)
Summary of Taste Transduction
Figure 10-16
Smell (Olfaction)
• Olfactory epithelium with olfactory receptors, supporting cells,
basal cells
• Olfactory receptors are modified neurons
• Surfaces are coated with secretions from olfactory glands
• Olfactory reception involves detecting dissolved chemicals as
they interact with odorant binding proteins
Olfactory Receptors
• Bipolar sensory neurons located within olfactory epithelium
– Dendrite projects into nasal cavity, terminates in cilia
– Axon projects directly up into olfactory bulb of cerebrum
– Olfactory bulb projects to olfactory cortex, hippocampus, and
amygdaloid nuclei
Olfaction
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Neuronal glomerulus receives input from 1 type of olfactory receptor
Odorant molecules bind to receptors and act through G-proteins to
increase cAMP.
– Open membrane channels, and cause generator potential; which stimulate
the production of APs.
– Up to 50 G-proteins may be associated with a single receptor protein.
– G-proteins activate many G- subunits - amplifies response.