TACTILE SENSATION Flashcards
Describe the tactile sensation pathway from periphery to CNS.
touches reaches CNS via dorsal root afferent neurons
What are primary afferent neurons?
What are the 2 branches?
cell bodies in dorsal root ganglia (body) or cranial ganglia (head)
- peripheral axon branch: to the skin (where it has specialized mechanosensory endings)
- central axon branch: to the spinal cord and brainstem
How might touch afferent neurons encode different somatosensory stimuli (poke feels different than pinch or itch)?
different receptive fields for different neurons
- SA vs. RA
different receptor types for different touch sensations
- some receptors are specifically tuned for vibration, light touch, etc.
- low vs. high threshold
What force-transducing molecules can sense mechanical force?
all have mechanically-gated ions – can turn force into energy very quickly
- ENaC family (important in worms, some evidence for roles in flies and mammals)
- TREK1 (K+ channel, not likely very important in touch)
- TRP family (part of mechanosensitive channels in flies and worms, no direct role discovered yet in mammals)
- Piezo family (large protein with many transmembrane segments, evolutionarily conserved across animals)
What is dermatome?
area of the body sending inputs to a specific segment of the spinal cord
Is the body necessary for the feeling of being touched?
no – feeling of touch on the body is ultimately generated in the brain (and that activity is sufficient to generate the percept)
How is the representation of a particular sense in the cortex often organized?
according to some continuous parameter of the stimuli
What do tactile circuits help preserve?
spatial information, as it travels from periphery to cortex
Is touch detected by multiple receptor types?
yes – detected by a variety of receptor types, which each respond to a particular type of touch
What do different receptors produce?
different types of afferent responses (ie. slowly adapting vs. rapidly adapting)
Why is it useful to have differing spatial acuity between body areas (not just have high spatial acuity everywhere)?
–
Describe the tactile specialization of etruscan shrews.
- accurately identify prey based only on tactile information from whiskers – have no eyes
- can tell prey is a cricket, and can also find exactly where to bite it (just below head) for maximum impact
Describe the tactile specialization of rodent whiskers.
- somatotopic organization principle – anatomy of primary somatosensory cortex that encodes the whiskers has great layout
- each barrel has neurons that encode different properties of that whisker – direction, position, texture, etc.
Describe the tactile specialization of star-nosed moles.
- 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 rays to actively probe the environment to find food
- rays (nose) are over-represented in S1
What distinguishes the perception of a gentle poke from a hard pinch?
from a peripheral standpoint:
- different receptor types that encode different things (ie. light touch, pain), each with different thresholds
from a ‘coding in the brain’ standpoint:
- differences in the pathways that transmit the response from periphery to brain
Temperature detection of thermoreceptors vs. nociceptors.
thermoreceptors have lower detection thresholds than nociceptors
Paper 1 Epilogue: What did the same researchers discover about rats and tickling in a different paper?
they used tickling to train rats to play hide and seek and determined that rats seek out tickling, but will disengage if there’s too much tickling
Paper 1 Epilogue: Self-tickling
allo-touch and tickling both elicit USVs and trunk S1 firing
- self-touch suppresses USVs and trunk S1 firing
- this effect is mediated by GABA
- self-touch suppressed USVs even when elicited by electrical stimulation of layer 5 trunk somatosensory cortex
rats were trained to nose-poke a button for a tickle reward
- after training, rats would poke the button and then freeze (fear response)
- sometimes they would also emit alarm calls
- such ambivalence (“Nervenkitzel”) resembles tickle behaviours in children
What is a Merkel cell?
very close to skin surface, good at tactile sensation (ie. braille)
What is a raster plot?
shows action potentials over time
What is the structure of the Piezo family?
Piezo family genes encode channel subunits with many transmembrane domains
What does area 1 of S1 encode?
mostly touch
What does area 2 of S1 encode?
both touch and proprioception
What does area 3a of S1 encode?
mostly proprioception
What does area 3b of S1 encode?
mostly touch
What is needed for effective object localization/recognition?
need to both touch object, and know where arm is (proprioception) for
What is the von Frey test?
uses increasingly stiff wires to poke foot and elicit withdrawal (mechanical force)
What is tail pressure?
nociceptive mechanical force
What is a hot plate?
thermal nociceptive test
