Lecture 4: Touch Specialists and Pain

Question 1

Describe the tactile specialization of the etruscan shrews.

Answer 1

accurately identify prey based only on tactile information from whiskers – have no eyes

not only can they tell it is a cricket, they can also find exactly where to bite the cricket (just below the head) for maximum impact

Question 2

Describe the tactile specialization of rodent whiskers.

Answer 2

great example of the somatotopic organization principle – anatomy of primary somatosensory cortex that encodes the whiskers has an incredible layout

as expected, each ‘barrel’ has neurons that encode different properties of that whisker – direction, position, texture, etc.

Question 3

Describe the tactile specialization of the star-nosed mole.

Answer 3

completely blind, but has the highest concentration of tactile receptors and the fastest tactile identification behaviour ever observed

have extremely high density of touch receptors on the rays of their stars – use star rays to actively prove the environment to find food

are over-represented in S1

Question 4

How does the etruscan shrew identify the cockroach so accurately only using touch?

Answer 4

use whisker system to determine various features of an organism to determine if it’s actually a prey species

Question 5

Name two features you would expect from the neuroanatomy of the star-nosed mole.

Answer 5

• expanded ‘nose’ representation in S1
• large number of mechanoreceptors (tactile receptors) in the star

Question 6

Describe the hierarchical organization of S1.

Answer 6

• spatial map of response type (RA vs. SA) is layered on top of somatotopic map
• within the somatotopic map, specific cortical areas (or ‘columns’) encode each place on the body
• within each column there is spatial segregation of SA and RA responses
• this creates a hierarchical organization of body position → tactile vs. proprioception → SA vs. RA

Question 7

Do different fields of S1 encode different properties?

Answer 7

yes

Question 8

What does area 1 of S1 encode?

Answer 8

mostly encodes touch

Question 9

What does area 2 of S1 encode?

Answer 9

touch and proprioception

Question 10

What does area 3a of S1 encode?

Answer 10

encodes mostly proprioception

Question 11

What does area 3b of S1 encode?

Answer 11

mostly encodes touch

Question 12

What are all the areas of S1 organized in?

Answer 12

a somatotopic map

Question 13

How does proprioception help in object recognition?

Answer 13

need to touch the object and know where your arm is – 2 pieces of information together – for effective object localization

Question 14

What distinguishes the perception of a gentle poke from a hard pinch?

Answer 14

from a peripheral standpoint: different receptor types
- some encode light touch, some encode pain
- each type has a different threshold

from a ‘coding in the brain’ standpoint: differences in the pathways that transmit the response from the periphery to the brain

Question 15

Considering temperature detection, do thermoreceptors or nociceptors have a lower detection threshold?

Answer 15

thermoreceptors

Question 16

What are dynamic ranges?

Answer 16

range of stimuli that produce a detectably different response in the neuron

ie. what is the temperature range in which a thermoreceptor will tell you that there is a change in temperature

Question 17

Do thermoreceptors and nociceptors have the same dynamic ranges?

Answer 17

no

Question 18

What is TRPV1 (VR1) required for?

Answer 18

for thermal, but not mechanical nociception

Question 19

What is the von Frey test?

Answer 19

uses increasingly stiff wires to poke the mouse’s foot and elicit withdrawal (mechanical force)

Question 20

What force is tail pressure an example of?

Answer 20

nociceptive mechanical force

Question 21

What type of test is a hot plate?

Answer 21

thermal nociceptive test (withdrawal of foot from hot plate at increasing temperatures)

Question 22

What is TACAN?

Answer 22

ion channel that responds to mechanical currents, and mediates mechanical nociception

Question 23

Where is TACAN expressed?

Answer 23

in non-peptidergic nociceptive neurons (marked by P2X3)

Question 24

What do TACAN mutants show at nociceptive levels of force?

Answer 24

at nociceptive levels of force (2, 4g), TACAN mutants (red line, TAM+) show withdrawal rather than nocifensive responses

Question 25

How was it determined that TACAN contributes to mechanosensitive currents?

Answer 25

sufficiency experiment

Question 26

In a TACAN necessity experiment (loss-of-function), pain can still be detected in the absence of TACAN. How?

Answer 26

• incomplete knockout
• other receptors

Question 27

What are the two phases of pain perception?

Answer 27

• first pain: sharp initial pain sensation
• second pain: duller, burning, longer-lasting pain

Question 28

What fibres are involved in first pain?

Answer 28

Aδ fibre – myelinated (increases conduction speed)

Question 29

What fibres are involved in second pain?

Answer 29

C fibre – unmyelinated

Question 30

Why are the two phases of pain perception temporally distinct?

Answer 30

different pain fibres conduct signals at different speeds

Question 31

What route in the CNS do tactile afferents follow?

Answer 31

dorsal medial pathway

Question 32

Where does information cross the midline (decussate) in the dorsal medial pathway?

Answer 32

brainstem

Question 33

What route in the CNS do pain afferents follow?

Answer 33

anterolateral pathway

Question 34

Where does information cross the midline (decussate) in the anterolateral pathway?

Answer 34

in spinal cord

Question 35

What nuclei is the DCN composed of?

Answer 35

gracile nucleus + cuneate nucleus

Question 36

Where is touch in the right hand mostly encoded?

Answer 36

in left hemisphere of the somatosensory cortex

Question 37

Pain and tactile afferents follow distinct routes in the CNS. What is this useful for?

Answer 37

to try to localize spinal lesions and determine where they are

Question 38

A patient comes into a neurology clinic after a bad skiing accident. The doctor discovers reduced pain sensitivity in their left leg, but the two-point discrimination test is normal on that leg. The opposite is true for the right leg – reduced two-point discrimination but normal pain sensitivity. What kind of injury do you suspect?

Answer 38

lesion on right side of spinal cord

Question 39

What are the two mechanisms for modulating pain?

Answer 39

• placebo effect
• gate theory of pain

Question 40

What can inhibit pain sensations?

Answer 40

• descending inputs
• touch sensation

Question 41

What is the placebo effect?

Answer 41

taking medication that is thought to reduce pain can lead to relief, even if there is no pharmacological effect

Question 42

Does context matter for pain?

Answer 42

yes

ie. soldiers relieved from battle with injuries perceive less pain

Question 43

What can descending modulation involve?

Answer 43

endogenous opioids – ie. enkephalins

Question 44

What can reduce the placebo effect?

Answer 44

opioid blockers – ie. naloxone

Question 45

What is the gate theory of pain?

Answer 45

local interneurons in the dorsal horn are activated by mechanosensory afferents and inhibit anterolateral system

ie. mechanosensory inputs (non-painful simulation) from rubbing painful part of skin can inhibit the pain responses you’re experiencing