Pain Flashcards
Why do we need pain?
- Without it we would be unaware of minor injuries
- Minor injuries could become infected resulting in loss of finger etc
How does pain differ from classical senses?
It is both a discriminative sensation and a graded motivation
Specificity theory
Pain is a distinct sensation, detected by and transmitted by specific receptors and pathways to distinct pain ares of the brain
Convergence theory
Pain is an integrated plastic state represented by a pattern of convergent somatosensory activity within a distributed network
What is a nociceptor?
A sensory receptor for painful stimuli
How are nociceptors classified?
According to activating stimulus, fibre-type and conduction velocity
Nociceptors are a subset of?
Afferents with free nerve endings
Fast pain
First pain, sharp and intermediate
Can be mimicked by direct stimulation of A-delta fibre nociceptors
Slow pain
Second, more delayed, diffuse and longer-lasting
Mimicked by stimulation of C fibre nociceptors
Hyperalgesia
Increased response to a painful stimulus
Allodynia
Painful response to a normally innocuous stimulus
Peripheral effects of inflammatory response
- Tissue damage releases inflammatory substances which affect nerve function
- Prostaglandins lower threshold for action potential
- Lowered nociceptor threshold heightens pain in hyperalgesia
Central sensitisation
- Local release of prostaglandins from nociceptive dorsal horn neurons
- Lower threshold for action potential - hyperalgesia
- Neurons also become sensitive to non-nociceptive inputs
- Normally innocuous stimuli perceived as painful - allodynia
Hyperpathia
- Variant of hyperalgesia and allodynia, underlying causes different
- Results when there is fibre/axonal loss (central or peripheral)
- Raising of detection threshold (greater stimulation before detected)
- When detection threshold is exceeded, subsequent excitability is much greater - patients report explosive pain
Phantom limb pain
- After amputation, patients have illusion that limb is still present
- Central representation of the body persists in the absence of peripheral input
- Attempts to block pain pathways usually fail, this pain may also be centrally represented
- Pain may be a representation of what we expect pain to be
Why may children born without limbs experience phantoms?
Central maps may be partly pre-formed in the womb
Referred pain
- Pain due to damage in the viscera is often perceived as coming from specific locations according to what organ is affected
- Heart attack often left shoulder/arm pain
- Thought to reflect convergence of visceral afferents onto similar pathways as cutaneous afferents in the CNS
- Useful in aiding clinical diagnosis
Perception of pain varies according to its context
- WWII soldiers with severe wounds experienced little/no pain
- Indian fakirs demonstrate mind over matter when walking on fire
- Suggestion that pain will be relieved results in pain relief - placebo effect
- Mechanisms exist to overcome even severe pain
Physiological basis of pain modulation
- Stimulation of periaqueductal grey activated brainstem nuclei that modulate activity of dorsal horn neurons
- Descending inputs in dorsal horn activate enkephalin-releasing interneurons which presynaptically inhibit nociceptive fibres
- Enkephalins are members of endogenous opioid peptides that includes endorphins and dynorphins
Modulation may appear locally
- Rubbing an injury often relieves pain
- Due to local inhibition by mechanoreceptors (AB fibres) of nociceptive (C fibre) inputs in the spinal cord
