somatosensory function Flashcards
somatosensory mechanisms: define the terms: receptor, stimulus threshold, stimulus intensity, adaptation, receptive field and lateral inhibition
what type of receptors are used for touch and proprioception
mechanoreceptors
how do mechanoreceptors work
transduce mechanical stimulus (deformation) into electrical signals
what are mechanoreceptors anatomically
modified terminals of peripheral axons of primary sensory neurones
what 3 things does mechanoreceptor function depend on
degree of specialisation, location, physiological properties
mechanoreceptor function: range of degrees of specialisation
from free nerve endings to elaborate accessory structures
mechanoreceptor function: examples of locations
various layers of skin, around hair shaft, in muscles, in tendons
mechanoreceptor function physiological properties: what determines sensitivity
activation threshold (all touch and proprioception receptors are low threshold); may be slow or fast adapting
where are sensory neurone cell bodies
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)
what do type 1 axons innervate
muscle spindles and tendon organs
what type of axon are most other mechanoreceptors innervated by
type II/A B axons
are axons innervating sensory neurones slow or fast conducting
fast conducting
define receptive field
number of receptors innervated by one sensory neuron
receptive field vs resolution
the larger the receptive field, the lower the resolution (less precise perception; receptive field density varies over body)
what is intensity of stimulus coded by
frequency of firing of neuron
effect on amplitude of action potential of intensity of stimulus
no change
how does firing frequency usually relate to stimulus intensity
firing frequency = log (stimulus intensity), i.e. if stimulus increases 10 fold, firing frequency doubles
thermoreceptor fibre types
Ad, C fibres with free nerve endings
thermoreceptor channels
transient receptor potential (TRP) ion channels
4 heat activated TRP channels
TRPV1-4; capsaicin causes TRP to remain on for a long period of time as long binding time
2 cold activated TRP channels
TRPM8, TRPA1
5 types of mechanoreceptors
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)
define stimulus threshold
point of intensity at which person can just detect presence of a stimulus 50% if time (absolute threshold)
define stimulus intensity
encoded by how quickly neurone fires, so increased strength or duration of stimulus, increased neurotransmitter release and so greater intensity
tonic receptor adaptation
detect continuous stimulus strength (do not adapt or adapt very slowly); continue to transmit impulses to brain as long as stimulus present (not habituated)
Merkel cells as example of tonic receptor
slowly adapt following superficial pressure and fine touch to be perceived
phasic receptor adaptation
detect a change in stimulus strength and adapt quickly; transmit impulse at start and end of stimulus (when change occurs)
example of phasic receptor
Pacinian corpsucle: sudden pressure excites receptor, transmits signal again when pressure released
