Physiology of Pain Flashcards
What does the IASP define pain as?
An unpleasant sensory and emotional experience, associated with active tissue damage or described in terms of such damage
What are the four processes in the physiology of pain?
Transduction, transmission, modulation, perception
What is transduction of pain?
Translation of noxious stimulus into electrical activity at the peripheral nociceptor
What is transmission of pain?
Propagation of pain signal as nerve impulses through the nervous system
What is pain modulation?
Modification/hindering of pain transmission in the nervous system (e.g by inhibiting neurotransmitters like endogenous opioids)
What is perception of pain?
Conscious experience of pain, causes physiological and behavioural responses
What does pain begin with?
The activation of nociceptors
What are nociceptors?
Specific primary sensory afferent neurons normally activated by intense noxious stimuli
What does the central terminal release?
Neurotransmitters that excise second order neurons
What does depolarisation caused by noxious stimuli elicits?
Action potentials that propagate to the CNS
What kind of neurons are nociceptors?
First order neurons that relay information to second order neurons in the CNS by chemical synaptic transmission
Where do second order neurons ascend?
The spinal cord in the anterolateral system to terminate in the thalamus
What makes up the anterolateral tract of the spinal cord?
The spinothalamic tract (STT) and the spinoreticular tract (SRT)
What is the spinothalamic tract involved in?
Pain perception (location, intensity)
What is the spinoreticular tract involved in?
Autonomic responses to pain arousal, emotional responses and fear of pain
Where is sensory information relayed from the thalamus?
To the primary sensory cortex (by third order neurons)
What kind of tissue do nociceptors innervate, and what activates them?
Innervate peripheral tissues; activated only be mechanical, thermal or chemical stimuli that are noxious
What are A-delta fibres?
Mechanical/thermal nociceptors that are thinly myelinated (conduction velocity 6-30m/s), respond to noxious mechanical/thermal stimuli, mediate “first” pain
What are C-fibres?
Unmyelinated nociceptors (conduction velocity 0.5-2 m/s), collectively respond to all noxious stimuli (are polymodal), mediate “second” pain
What are some ways to classify pain?
Mechanisms = nociceptive, inflammatory, pathological
Time course = acute, chronic, breakthrough pain
Severity = mild, moderate, severe
Source of origin = somatic, visceral
What does nociceptive pain represent?
Normal response to injury of tissues by noxious stimuli
What is the only way nociceptive pin can be provoked?
By intense stimulation of nociceptors by noxious stimuli
What is the function of nociceptive pain?
Adaptive = functions as early warning physiological protective system to detect and avoid noxious stimuli
What activates inflammatory pain?
The immune system by tissue injury or infection
What releases mediators at the site of inflammation to cause inflammatory pain?
Leucocytes, vascular endothelium, tissue resident mast cells (causes hyperalgesia and allodynia)
What is hyperalgesia?
Heightened pain sensitivity to noxious stimuli
What is allodynia?
Pain sensitivity to innocuous stimuli
How is inflammatory pain adaptive?
It promotes repair until healing occurs by discouraging physical contact with the affected part and also discourages movement
What are the two categories of pathological pain?
Neuropathic and dysfunctional
What cause neuropathic pain?
Damage to neural tissue = compression neuropathies, peripheral neuropathies, central pain (following stroke), postherpetic neuralgia, trigeminal neuralgia, phantom limb
How can neurological pain be perceived?
Burning, shooting, numbness, pins and needles, may be less localised
What are some features of dysfunctional pain?
No identifiable damage or inflammation, is maladaptive
What are some examples of dysfunctional pain?
Fibromyalgia, IBS, tension headache, TMJ disease, interstitial cystitis
How can dysfunctional pain be treated?
Simple analgesics not effective, sometimes treated with drugs not originally mean for pain (antidepressants or anti-epileptics)
What is referred pain?
Pain developed in one part of the body felt in another structure away from the place of its development
What kind of pain is usually referred?
Deep or visceral pain (pain from superficial structures isn’t normally referred)
What causes referred pain?
Convergence of nociceptive visceral and skin afferents upon the same spinothalamic neurons at the same spinal level
