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Lecture 13

PSL300H1 Lecture Notes - Lecture 13: Kinocilium, Piriform Cortex, Medical Optical Imaging


Department
Physiology
Course Code
PSL300H1
Professor
Michelle French
Lecture
13

Page:
of 5
PSL300H1F L13, Wed Oct. 12, 2011
Vestibular, Taste, and Olfaction
Vestibular apparatus
Mechanoreceptors in 2 inner ear structures:
semicircular canals; detect angular acceleration
otolith organs; detect linear forces or accelerations on
head(remember F = ma)
Semicircular canals
Really are complete circles, not semicircles
Set of 3 on each side of head
oriented orthogonally (right angles) to one another so
cover all 3 dimensions in space
not same as hair follicle receptors in skin
hair cell receptors located within enlargement called
“ampulla”
hair cell stereocilia embedded in the cupula
(gelatinous) which forms a flexible barrier across the
ampulla
horizontal not rly horizontal, at angle
anterior is largest
fluid goes thru each circle
at ampulla, have block of gelatinous substance, flap
hair cell stereocilia embedded in gelatinous cupula
Stimulus
Rotation of head causes walls of semicircular canals
to rotate with head
endolymph inside canals stays put (inertia) exerts
pressure on cupula in direction opposite to the head
rotation – cupula fixed in walls of canal bulge in
cupula of stereocilia
only canals oriented within or close to plane of head
rotation are stimulated
Net pressure bending stereocilia
receptor potential
Hair cell
Stereocilia polarize the hair cell by a gradient in
length and ‘kinocilium’ (longest) – long on one side,
short on other
stereocilia are joined by tip links
bending cilia towards longest one stretches tip links
yank on prots in membrane open up ion (Na, K)
channels depolarization, EPSPs and other receptor
potentials thru mechanoreceptors
bending cilia towards shortest one relaxes tip links
closes ion channels hyperpolarization
since in normal upright stage – no motion; there
is tension so there is some receptor potential
(slightly depolarizing) due to background activity,
so can signal both directions
rly the cupula that will bend the stereocilia
Bilateral effects
For a given canal, hair cell response can be either
similar or opposite on the 2 sides of the head
rotating head to right side depolarizes horizontal
canal hair cells on right side (max spiking),
hyperpolarizes these afferents on left side of head (no
discharge)
balanced out
some planes w symmetrical response
usually opposite of pairs have opposite response
anterior on one side paired w posterior on other side
horizontals pair up on both sides
could theoretically lose one horizontal canal but
need both for centering
Adaptation
Hair cells are slowly adapting
BUT: constant rotational velocity gradually elicits
endolymph movement thru friction net movement
of fluid
net force on cupula decays over time no hair cell
deflection after 30 s no more signal of rotational
velocity so signals acceleration
get opposite effect
since pushing on
cupula in other
direction
hyperpolarization
mechanical effect
from fluid
Otolith organs
Receptors are also hair cells
cilia embedded in matrix containing calcium
carbonate crystals
effect of net motion of crystals
cilia deflected by linear accelerations (forces)
adapt to constant linear velocities
otolith organs near utricle and saccule
difference is orientation
gelatinous matrix replaced by calcium carbonate
pick up linear forces bend stereocilia
pick up direction/influence of gravity
gave other orientations present within the saccule
different sets of
hair cells
saccule picks up
vertical
orientations/force
s
utricle picks up
horizontal/sidewa
ys
orientations/force
s
fracture line
called striola – get
flipping of
orientations,
respond to same
plane but
opposite
directions (like for
directional cells in
V1)
all info proprioceptive in nature of what head is doing
at one time
info goes to Area 3a of S1 – vestibular info, stretching
muscle spindles head mapping of proprioceptive
system
Olfactory receptors
2 Chemoreceptor systems we talk about
Unique – primordial sensory system developed in
mammals
over 1,000 dif receptor genes; ~350 rec prots
expressed in humans
compared to 3 in eyes for colour
clustered together
each olfactory cell contains one receptor type
olfactory cells have their own axon which projects to
olfactory bulb in the brain
so if lose a cell (ex. Cold), regenerate all the time,
so regenerate the axons all the time so recreate
the whole pathway
Sensory transduction
Resting potential of potential of -45 mV
odorant response is slow depolarization
receptor is G-protein coupledactivates adenylyl
cyclase [cAMP] increases cAMP-gated cation
channels opened depolarization action potentials
at initial segment of axon
rmbr when going thru g-prot couple receptors – need
only 1 odorant to bind generate lots of cAMP (2nd
msger) open many ion channels Ca comes in
depolarization train of spikes in axon
so very sensitive, why dogs have good sense of
smell
Olfactory bulb
Axons of olfactory receptors project thru bottom of
skull (cribiform bone), right on top of nose, right
underneath frontal lobe
all axons of same receptor type terminate in a
common ‘glomerulus’ of mitral cell dendrites, go thru
many holes in skull along way
same receptor prots so go to same mitral cell
glomerulus (interface on dendrites of mitral cell and
axonal termini of incoming axons) – big clump
make knob-like formation
sits right on top of nasal cavity,
axons terminate
not all in one nerve, many
nerves going thru holes in skull
all axons of same type of
receptor cell, so congregate on
same glomerulus
lateral inhibition by neighbouring neurons
strongest odour dominates
project to several parts of brain (some
autonomic, all limbic – edge of nervous system –
emotion experience + motivation)
project straight to cortex without going thru thalamus
whereas all other sensory systems go thru thalamus
Olfactory Sensory Neurons
Any single odorant molecule typically activates a
small set of OSNs, rarely just one. Response pattern
across population of sensory neurons codes specific
odorants.
Indiv receptor can all respond to same stimulus – so a
stimulus can have dif receptors
So coding of any given odourant relies on pattern
of activity across of population of receptors
Rare for only 1 receptor to be activated
Population code for odor
Specific odor usually activates a number of different
olfactory receptors
set of responsive olfactory cells activates a specific
set of glomeruli (and hence mitral cells)
mitral cells project to ‘limbic’ cortex
spatial – any given olfactant/smell/set of chemical will
activate mitral cells within a spatial territory
pentanoic acid – parts respond to phenols
so some areas respond to both phenols and
pentanoic acid – but dif patterns