Physiology of Pain Flashcards
why is pain difficult to assess?
very subjective as pain perception and tolerance varies between different people
define chronic pain?
lasting over 3 months
how does chronic pain prevalence change with age?
more common in elderly
what is pain?
unpleasant sensory and emotional experience associated with actual tissue damage or described in terms of such damage
(not all pain related to actual damage)
what are the 4 processes in the physiology of pain?
transduction
transmission
modulation
perception
what is transduction?
translation of noxious/damaging stimulus into electrical activity at the peripheral nociceptor (receptor)
how body conveys stimulus to CNS
what is transmission?
propagation of pain signal as a nerve impulse through the nervous system
what is modulation?
modification/hindering of pain transmission in the nervous system (e.g by inhibitory neurotransmitters like endogenous opioids)
what is perception?
conscious experience of pain
causes physiological and behavioural responses
what are nociceptors?
free nerve endings
primary sensory afferent neurons
normally activated by intense noxious stimuli (Mechanical, thermal or chemical)
- must be severe stimuli
what do nociceptors do?
first order neurons that relay information to second order neurons in the CNS by chemical synaptic transmission
describe what happens in nociceptors
free nerve ending (e.g in skin, or organ etc) depolarizes due to noxious stimuli which elicits AP that propagate to the CNS
central terminal is in CNS and releases neurotramsmitters that excite second order neurones
where is the cell body (soma) in nociceptors?
within dorsal root ganglion or trigeminal ganglion
pseudounipolar - one side = free nerve endings, other side = central terminal
what are the neurotransmitters in the nociceptive pathway?
glutamate and peptides (substance P, neurokin A)
through what system do the second order neurones ascend the spinal cord?
anterolateral system terminating in the thalamus
what are the 2 parts of the anterolateral system used by second order neurons?
spinothalmic tract (STT) spinoreticular tract (SRT)
what is the spinothalmic tract?
involved in pain perception
goes to thalamus
what is the spinoreticular tract?
involved in autonomic response to pain, arousal, emotional responses, fear of pain
also goes to thalamus
where does the pain go after the thalamus?
sensory information is relayed (third order neurones) to the primary sensory cortex
what are the 2 subtypes of nociceptors?
A delta fibres
C fibres
what are A gamma fibres?
mechanical/thermal nociceptos
thinly myelinated so very fast
respond to noxious mechanical and thermal stimuli
mediate first/fast pain (stabbing pain)
what are C fibres?
nociceptors that are unmyelinated (slow conduction)
respond to all noxious stimuli (polymodal)
mediate second/slow pain (burning, throbbing pain)
how can pain be classified?
mechanisms (e.g nociceptive, inflammatory, neuropathic, dysfunctional etc)
Time course (acute/chronic etc)
Severity
Source of origin (somatic vs visceral)
what is nociceptive pain?
normal response to injury of tissue by noxious stimuli
what causes nociceptive pain?
intense stimulation of nociceptors by noxious stimuli
adaptive
acts as warning to protect body
what causes inflammatory pain?
activation of the immune system by tissue injury/infection
pain activated by mediators released at site of inflammation by leukocytes, vascular epithelium and mast cells
what does inflammatory pain cause?
heightened pain sensitivity to noxious stimuli (hyperalgesia) and pain sensitivity to innocuous (harmless) stimuli (allodynia)
why can inflammatory pain be useful?
discourages physical contact and movement of the area while it heals
is inflammatory pain adaptive?
yes
name 2 types of neuropathic pain
neuropathic
dysfunctional
what is neuropathic pain?
pain caused by damage to neural tissue
can be burning, shooting, numbness, pins and needles
can be less localised
give some examples of neuropathic pain
compression neuropathy peripheral neuropathy central pain (after stroke/spinal injury) postherpatic neuralgia trigeminal neuralgia phantom limb
is neuropathic pain protective?
no
what is dysfunctional pain?
no identifiable damage or inflammation
not protective and maladaptive
give some examples of dysfunctional pain
fibromyalgia
irritable bowel
tension headache
how is pathological pain treated?
simple analgesics usually not effective
sometimes treated with non pain relieving drugs (anti-depressants or anti-epileptics)
what is referred pain?
pain developed in one part of the body is felt in another part
which types of pain is usually referred?
deep pain
superficial pain usually not referred
what causes referred pain?
convergence of nociceptive visceral and skin afferents upon the same spinothalmic neurons at same spinal level
give some examples of sites of referred pain?
heart = left arm diaphragm = left shoulder/neck liver = right side of neck stomach/pancreas = epigastric appendix = middle abdomen gall bladder = right shoulder/neck
