Somatosensory system Flashcards
What are the three main types of sensory receptor and what is each responsible for?
Mechanoreceptors –> touch, pressure, vibration, proprioception, muscle length, muscle tension
Thermoceptors –> temperature
Nociceptors –> pain
What are the three main types of sensory fibre?
A-beta –> large, myelinated, fast
A-delta –> myelinated, less fast than A-beta
C –> unmyelinated, slow, free nerve endings
What are the three types involved in?
AB –> skin mechanoreceptors
Ad –> pain, temp
C –> pain, temp, itch
What are sensory receptors?
transducers that convert energy from environment into neuronal APs
What is the absolute threshold?
level of stimulus that produces a positive response of detection 50% of the time
What are TPR channels?
transient receptor ion channels
family of heat channels activated by different temperatures
What are the types of TPR channel?
4x heat activated –> TRPV1-4
2x cold activated –> TRPM8, TRPA1
What are the features of tonic receptors?
= slow adapting receptors
continually transmit impulses
eg. Merkel cells
What are the features of phasic receptors?
= fast adapting receptors
only transmit impulse at start + end of stimulus
eg. Pacinian corpuscle
What is two point discrimination?
min distance at which two points are perceived as seperate
What type of pain do the following fibres mediate?
a) A-d
b) C
A-d –> sharp, intense, first pain
C –> dull, persistent, second pain
What is special about C fibres?
polymodal –> respond to all modalities (only one type)
What is the role of lateral inhibition?
prevents overlap of receptive fields
What fibres are involved in the dorsal column?
A-beta fibres
*see notes on pathways
Which part of the spinothalamic tract do the following ascend in:
a) pain
b) temp
c) crude touch
pain + temp = lateral
crude touch = anterior
Where do the first order neurons of the following terminate?
a) dorsal column
b) spinothalamic tract
a) medulla
b) dorsal horn
What is the spinoreticular tract?
emotional component
What does a blocked anterior spinal artery cause?
ischaemic damage to anterior part of SC
What does spinothalamic tract damage cause?
pain + temp loss BELOW level of lesion
retained light touch + vibration sensation due to intact DC
What can be used to establish the level of a lesion?
EPT (=electric perceptual thresholds)
What is pain?
an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are the types of pain?
Acute eg. skin cut
Muscle eg. lactic acidosis, fibromyalgia
Somatic –> well localised eg. inflammation, infection
Visceral –> deep, poorly localised eg. stomach, colon, IBS
Referred –> from internal organ/structure eg. angina
Neuropathic –> dysfunction of NS
What is allodynia?
pain due to a stimulus that does not normally provoke pain
What is hyperalgesia?
increased pain from a stimulus that normally provokes pain
How can synaptic plasticity lead to chronic pain?
synaptic plasticity –> decreased inhibitory influences on DH –> chronic pain
What happens in normal pain sensation?
chemical/heat stimulus –> nociceptor nerve ends –> DH –> pain –> inhibitory input
What are the two pathways involved in descending pain modulation + what are their neurotransmitters?
Monoamines released from brain stem –> inhibit SC excitability
two types:
1) PAG-RVM axis –> seretonin
2) locus cereleus –> NA
How do endogenous opioids affect descending inhibition of pain?
increase inhibition
What is the underlying mechanism of hyperalgesia?
loss of inhibitory input of pain
What is the underlying mechanism of allodynia?
rewiring in the DH
