whiskers, barrels and active sensing

Question 1

why study whiskers?

Answer 1

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

Question 2

somatotopic map of whiskers in rats

Answer 2

Vibrissae (whiskers) occupy roughly a quarter of the somatosensory cortex

High magnification factor, high neural resources, must be behaviourally important

Question 3

layout of vibrissae in mice

Answer 3

Arranges in a very regular layout, rows and column : barrels
highly stereotyped over many animals

Question 4

vibrissae layout compared to layer 4 cortex

Answer 4

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

Question 5

what is layer 4 mice cortex stained with to view the barrels?

Answer 5

cytochrome oxidase staining

Question 6

multi-electrode recording in layer 4 cortex mice when whiskers are deflected

Answer 6

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

Question 7

neurons in each barrel primarily sensitive to:

Answer 7

deflection of one whisker

Question 8

what are barrels?

Answer 8

areas where dense cell bodies form a ring around less dense cell bodies

Question 9

at what layer can you see the patchy labelling (barrels)

Answer 9

layer 4
no real signs at the superficial or deepest layers

Question 10

why do we see barrels?

Answer 10

cell bodies of neurons in the ring, but their processes are oriented to the interior of the barrel

Question 11

what axon terminals are in the barrel cortex?

Answer 11

thalamo-cortical

Question 12

what do barrels receive their input from?

Answer 12

the VPN
get primary ascending drive from the thalamus

Question 13

why are we able to use cytochrome oxidase for staining?

Answer 13

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

Question 14

what are the three ascending whisker pathways?

Answer 14

lemniscal
paralemniscal
extralemniscal

Question 15

where do mechanoreceptors of the follicle sinus complex have their cell bodies?

Answer 15

in the trigeminal ganglion (in the brainstem) which is the analogue of the dorsal root ganglion

Question 16

lemniscal pathway

Answer 16

primary afferents project to the thalamus
neurons in the thalamus project to the somatosensory cortex

Question 17

what is the follicle sinus complex innervated by?

Answer 17

5th (trigeminal) cranial nerve

Question 18

nerve endings in follicle sinus complex

Answer 18

large diversity e.g.,
merkel endings
lanceolate endings

Question 19

how do animals detect touch with their whiskers?

Answer 19

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

Question 20

what is whisking?

Answer 20

when mice’s whiskers are under mechanical control

used to explore the space around its head

Question 21

electrophysiology experiment of mechanotransduction in the whisker follicle

Answer 21

Whole cell electrophysiological recording from Merkel cells

Mechanical stimulation elicits membrane current

Merkel cells express piezo2

Mechanically activated current inhibited by piezo2 blocker

Question 22

what is in the trigeminal nuclei?

Answer 22

the principal nucleus (Pr5 = PrV)
barrelettes are found

Question 23

where do nerves from the barrelettes project to?

Answer 23

the ventral posterior medial thalamus

Question 24

where are barreloids found?

Answer 24

the ventroposterior medial nucleus in the thalamus

Question 25

where are barrel like structures found in the lemniscus pathway?

Answer 25

every level of the system except the trigeminal nucleus itself

Question 26

how was the C2 barrel column identified

Answer 26

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

Question 27

what did Lefort et al (2007) discover

Answer 27

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

Question 28

difference of patterns of whiskers

Answer 28

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

Question 29

what is the causal engine?
Do whiskers drive the development?
Barrels/ organisation of cortex send descending information that influences the development of the periphery?

Answer 29

periphery plays instructional role

Question 30

What happens when C row whiskers are lesioned on the day of birth?

Answer 30

C row barrels fail to develop

Question 31

pole localisation task

Answer 31

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

Question 32

what happens in the pole detection task when you inhibit the barrel cortex using muscimol

Answer 32

performance drops
cortex is necessary

Question 33

what happens in the pole detection task when you trim the whiskers?

Answer 33

performance drops
when the whiskers grow performance picks up again
whiskers are necessary

Question 34

pole detection task
all whiskers except one row
electrophysiological recording from the corresponding barrel columns

Answer 34

some neurons were very selective to the two types of trial
responded much more on Go trials than No Go trials

Question 35

how does single neuron activity correlate with decision

Answer 35

Decoding: Correlate animal’s response (go or nogo) with neuronal response
Best 10% of cortical neurons correlate very well with mouse’s decision