whiskers, barrels and active sensing Flashcards
why study whiskers?
Common throughout mammals
Mice have whiskers, so we can study somatosensation in mice
There are very important experiments based on the genetic manipulability of mice can use powerful experimental techniques
somatotopic map of whiskers in rats
Vibrissae (whiskers) occupy roughly a quarter of the somatosensory cortex
High magnification factor, high neural resources, must be behaviourally important
layout of vibrissae in mice
Arranges in a very regular layout, rows and column : barrels
highly stereotyped over many animals
vibrissae layout compared to layer 4 cortex
Precise correspondence between barrels in primary somatosensory cortex and whispers on the snout
one to one relationship of whisker to barrel
Spatial layout of whiskers on the snout is reflected in the primary somatosensory cortex
what is layer 4 mice cortex stained with to view the barrels?
cytochrome oxidase staining
multi-electrode recording in layer 4 cortex mice when whiskers are deflected
100 electrodes, inserted into the somatosensory cortex
There is at least one microelectrode in every barrel
Deflected one of the whiskers of the anethitised rats and measured the activity in each of the microelectrodes
Spot of activity at a certain position of the microelectrode array, exactly as you’d expect If a barrel had a one to one relationship with whiskers
neurons in each barrel primarily sensitive to:
deflection of one whisker
what are barrels?
areas where dense cell bodies form a ring around less dense cell bodies
at what layer can you see the patchy labelling (barrels)
layer 4
no real signs at the superficial or deepest layers
why do we see barrels?
cell bodies of neurons in the ring, but their processes are oriented to the interior of the barrel
what axon terminals are in the barrel cortex?
thalamo-cortical
what do barrels receive their input from?
the VPN
get primary ascending drive from the thalamus
why are we able to use cytochrome oxidase for staining?
Cytochrome oxidase is a metabolic enzyme, see it in parts of tissues that need a lot of energy, presynaptic terminals are full of mitochondria, mitochondria is full of cytochrome oxidase
what are the three ascending whisker pathways?
lemniscal
paralemniscal
extralemniscal
where do mechanoreceptors of the follicle sinus complex have their cell bodies?
in the trigeminal ganglion (in the brainstem) which is the analogue of the dorsal root ganglion
lemniscal pathway
primary afferents project to the thalamus
neurons in the thalamus project to the somatosensory cortex
what is the follicle sinus complex innervated by?
5th (trigeminal) cranial nerve
nerve endings in follicle sinus complex
large diversity e.g.,
merkel endings
lanceolate endings
how do animals detect touch with their whiskers?
Whiskers are a touch organ, mechanical energy, bending of whiskers
When the moving whiskers touch an object it moves, out of line with their other whiskers
what is whisking?
when mice’s whiskers are under mechanical control
used to explore the space around its head
electrophysiology experiment of mechanotransduction in the whisker follicle
Whole cell electrophysiological recording from Merkel cells
Mechanical stimulation elicits membrane current
Merkel cells express piezo2
Mechanically activated current inhibited by piezo2 blocker
what is in the trigeminal nuclei?
the principal nucleus (Pr5 = PrV)
barrelettes are found
where do nerves from the barrelettes project to?
the ventral posterior medial thalamus
where are barreloids found?
the ventroposterior medial nucleus in the thalamus
where are barrel like structures found in the lemniscus pathway?
every level of the system except the trigeminal nucleus itself
how was the C2 barrel column identified
intrinsic optical imaging
Used an imaging technique to identify the barrel column while the brain was intact
Dropped a tiny crystal of a fluorescent dye onto C2 barrel
Then they sliced up the brain, because they had the dye they could find the part of the brain that correlated to the C2 whisker column
Used intracellular recording to record the membrane potential of different neurones in different parts of whisker C2 barrel column
Able to record from up to 6 neurons simultaneously
Then tested for synaptic connectivity by passing current through pipette number 1 to elicit an action potential in neuron number 1, can record what happens in cell number 2. If they are connected there should be a post synaptic potential in cell number 2 very soon afterwards
Can test connections between layers, build up neural circuitry
what did Lefort et al (2007) discover
Confirms the classical circuit
Something not from classical circuit: highest connection between layer 4 to layer 4
Next most strong is connections within layer 3 and within layer 5
Dominant connectivity is feedback connections within a layer: feedback is a strong characteristic of cortical circuits
difference of patterns of whiskers
Pattern of whiskers from animal to animal is very stereotyped
There are different strains of mice, some of these mice have a few extra whiskers
these mice also have a corresponding extra barrel
what is the causal engine?
Do whiskers drive the development?
Barrels/ organisation of cortex send descending information that influences the development of the periphery?
periphery plays instructional role
What happens when C row whiskers are lesioned on the day of birth?
C row barrels fail to develop
pole localisation task
Pole could be towards ear or more toward nose
Mice to learn they should lick when pole is in more posterior location, if they do then they are rewarded with a drop of water, should refrain from licking in anterior position
Mouse is very good at this task
what happens in the pole detection task when you inhibit the barrel cortex using muscimol
performance drops
cortex is necessary
what happens in the pole detection task when you trim the whiskers?
performance drops
when the whiskers grow performance picks up again
whiskers are necessary
pole detection task
all whiskers except one row
electrophysiological recording from the corresponding barrel columns
some neurons were very selective to the two types of trial
responded much more on Go trials than No Go trials
how does single neuron activity correlate with decision
Decoding: Correlate animal’s response (go or nogo) with neuronal response
Best 10% of cortical neurons correlate very well with mouse’s decision
