somatosensory function

Question 1

what type of receptors are used for touch and proprioception

Answer 1

mechanoreceptors

Question 2

how do mechanoreceptors work

Answer 2

transduce mechanical stimulus (deformation) into electrical signals

Question 3

what are mechanoreceptors anatomically

Answer 3

modified terminals of peripheral axons of primary sensory neurones

Question 4

what 3 things does mechanoreceptor function depend on

Answer 4

degree of specialisation, location, physiological properties

Question 5

mechanoreceptor function: range of degrees of specialisation

Answer 5

from free nerve endings to elaborate accessory structures

Question 6

mechanoreceptor function: examples of locations

Answer 6

various layers of skin, around hair shaft, in muscles, in tendons

Question 7

mechanoreceptor function physiological properties: what determines sensitivity

Answer 7

activation threshold (all touch and proprioception receptors are low threshold); may be slow or fast adapting

Question 8

where are sensory neurone cell bodies

Answer 8

peripheral nervous system (dorsal root ganglion for body or trigeminal ganglia for face); dorsal horn neurones can have axons that project to brain (projection neurones), or have axons that remain in spinal cord (interneurones)

Question 9

what do type 1 axons innervate

Answer 9

muscle spindles and tendon organs

Question 10

what type of axon are most other mechanoreceptors innervated by

Answer 10

type II/A B axons

Question 11

are axons innervating sensory neurones slow or fast conducting

Answer 11

fast conducting

Question 12

define receptive field

Answer 12

number of receptors innervated by one sensory neuron

Question 13

receptive field vs resolution

Answer 13

the larger the receptive field, the lower the resolution (less precise perception; receptive field density varies over body)

Question 14

what is intensity of stimulus coded by

Answer 14

frequency of firing of neuron

Question 15

effect on amplitude of action potential of intensity of stimulus

Answer 15

no change

Question 16

how does firing frequency usually relate to stimulus intensity

Answer 16

firing frequency = log (stimulus intensity), i.e. if stimulus increases 10 fold, firing frequency doubles

Question 17

thermoreceptor fibre types

Answer 17

Ad, C fibres with free nerve endings

Question 18

thermoreceptor channels

Answer 18

transient receptor potential (TRP) ion channels

Question 19

4 heat activated TRP channels

Answer 19

TRPV1-4; capsaicin causes TRP to remain on for a long period of time as long binding time

Question 20

2 cold activated TRP channels

Answer 20

TRPM8, TRPA1

Question 21

5 types of mechanoreceptors

Answer 21

hair follicle, Meissner’s corpuscle (fine discriminative touch, low frequency vibration), Merkel cells (light touch and superficial pressure), Pacinian corpuscle (deep pressure, high frequency vibration and tickling), Ruffini endings (continuous pressure or touch and stretch)

Question 22

define stimulus threshold

Answer 22

point of intensity at which person can just detect presence of a stimulus 50% if time (absolute threshold)

Question 23

define stimulus intensity

Answer 23

encoded by how quickly neurone fires, so increased strength or duration of stimulus, increased neurotransmitter release and so greater intensity

Question 24

tonic receptor adaptation

Answer 24

detect continuous stimulus strength (do not adapt or adapt very slowly); continue to transmit impulses to brain as long as stimulus present (not habituated)

Question 25

Merkel cells as example of tonic receptor

Answer 25

slowly adapt following superficial pressure and fine touch to be perceived

Question 26

phasic receptor adaptation

Answer 26

detect a change in stimulus strength and adapt quickly; transmit impulse at start and end of stimulus (when change occurs)

Question 27

example of phasic receptor

Answer 27

Pacinian corpsucle: sudden pressure excites receptor, transmits signal again when pressure released