Cutaneous Testing Flashcards
where are the receptors for touch sensation localised ?
in the dermis and epidermis
when is our ability to discriminate between 2 points greatest ?
when there is a low level of convergence 1st order sensory neurons onto 2nd order neurons in the spinal cord
what is used to assess touch thresholds ?
using von frey aesthesiometer
what fibres are activated for touch sensation in human skin ?
reliant exclusively on activation of myelinated sensory afferent fibres
what channels can capsaicin activate ?
activate ion channels that are linked to thermal heat perception
what is the Trpv1 channel permeable to ?
calcium ion s
what is the threshold for thermal pain sensitivity ?
> 40 degrees
what is the Trpv2 channel activated by ?
activated by nociceptive heat stimuli
what pathway carries innocuous mechanical sensations to the brain and how does it ascend to the brain ?
dorsal column-medial lemniscus pathway
ascends to the brain in the ipsilateral spinal cord
where does the somatosensory cortex receive inputs from ?
ventro-basal complex within the thalamus
what does the 2-pt discrimination test tell us ?
it tells us that the 2-pt discrimination thresholds are different at different body surface locations
what factors influence human 2 point discrimination ?
- density of mechanoreceptors innervating an area of skin = if an area is more densely innervated by mechanoreceptors then the discrimination i smaller
- size of receptive field = area of skin contains many receptors with small receptive fields then the 2 point discrimination is smaller because the receptors are able to pack together more closely but still activate 2 different CNS neurons 2 enable 2 different points to be distinguished
- amount of cortical tissue deciphering the info= greater cortical representation means greater processing of mechanical activity
what is the firing adaptation, receptive field and physiological role of merkels disks?
- slow adaptation firing rates
- small receptive field
- detects form, shape, texture, rough edges of objects
what is the firing adaptation, receptive field and physiological role of ruffini’s endings ?
- slow adaptation firing rates
- large receptive field
- detects lateral movements of the skin, indentation and stretching of the skin
what are the firing adaptations, receptive field and physiological role of meissners corpuscles ?
- fast adaptation firing rates
- small receptive field
- detects low frequency states ranging from 30-50Hz and rough textures and movements
what are the firing adaptations, receptive field and physiological role of pacinians corpuscles ?
- fast adaptation firing rates
- large receptive field
- detects high frequency vibrations within the range of 250-350Hz and fine textures and movements
explain the pathway involved in ascending non-painful touch to the brain ?
dorsal column medial lemniscus pathway
- primary afferents enter into the spinal cord via the dorsal roots and ascend to the medulla on the ipsilateral side
- 2 dorsal column pathways= fasiculus gracilis and fasiculus cuneatus
- in the medulla the first order neurons synapse with the 2nd order neurons which then decussate onto the contralateral side to ascend to the thalamus
- at the thalamus the 2nd order neurons synapse with the 3rd order neurons which then ascend to various areas in the cerebral cortex
what primary afferents are involved in innocuous touch sensation pathways ?
A alpha
A beta
A delta
what information does the fasiculus gracilis transmit ?
information caudal to T6
what information does fasiculus cuneatus transmit ?
information rostral to T6
if a correlation coefficient is close to 1 what does this tell you ?
indicates strong positive linear correlation
because it is a positive value it indicates a direct relationship whereas if it was close to -1 it would indicate an indirect relationship
why is the representation of mechanical stimuli greater than thermal stimuli ?
because there is a greater density of mechanoreceptors in the somatic nervous system and also the density of thermal receptors throughout the skin is not evenly spread out
what is the main receptor for hot thermal stimuli ?
Trpv1
what is the main receptor for thermal cold stimuli ?
Trpm8
what receptor is activated by capsaicin ?
the vanilloid receptor Trpv1
what happens to the sensitivity to thermal hot stimuli after capsaicin application to the skin and why ?
human sensitivity to thermal hot stimuli is more sensitive after capsaicin application because it inccreases the sensitivity of the VR1 receptors
- it causes increased sensitivity by causing the Trpv1 receptors to open at lower temperatures
what endogenous substances can increase the sensitivity of Trpv1 receptors and why does this occur ?
e.g. substance P and bradykinin
they increase the sensitivity of nociceptors by reducing the pain threshold and this is known as primary hyperalgesia
what should happen to the von frey tests after capsaicin application ?
it should cause the pain threshold to occur using a small diameter von frey hair because the Trpv1 receptors are thermal nociceptors and therefore it would be expected that they would become more sensitive to mechanical stimuli as nociceptors can be polymodal permitting their activation by mechanical stimulation
define magnification factor:
it is the amount by which a point on the skin is expanded in a central representation
what does an area containing cels with small receptive fields need ?
it needs a large number of cels to represent a given surface area
what does an area with cells that have large receptive fields need?
it requires only a few cells to represent the given area
what does an area of the body with smaller receptive fields occupy ?
occupies a larger cortical area and provides greater acuity
