Pain and analgesia Flashcards
What is pain?
PAIN definition
“An unpleasant sensory and emotional experience associated
with actual or potential tissue damage, or described in
terms of such damage”
International association of the study of pain
It is an important, protective mechanism that
alerts us to actual or potential tissue damaging
stimuli
Acute pain is a warning device essential to survival
* Normally only elicited by noxious, potentially tissue
damaging stimuli
What are the descriptors of pain?
- Duration : acute/chronic
- Pathology : inflammmatory, traumatic, neuropathic
- Triggering : evoked, sponatenous
- Origin :
- somatic -skin, muscle, bone
- visceral - heartm kidneysm gastrointestinal
- Characteristics : shooting, cutting, electric, burning, freezing, cramping, aching
What are noxious stimuli?
- Thermal = hot/cold
- Mechanical - stubbed toe, pin prick, sound?)
- Chemical (Acid, wasp sting, capsaicin)
- Itch
- Electrical
- Normally stops on the removal f the stimuli
What is the transduction of noxious stimuli?
- Transduced by nociceptors (sensory receptors)
- Free branching nerve endings
- Selective activation of nociceptors can lead to the conscious experience of pain
- Majority of nociceptors respond to mechanical, thermal and chemical stimuli = polymodal nociceptors
- Some show selectivity:
- Mechanical nociceptors
- Thermal nociceptors
- Chemical nociceptors
Where are nociceptors present?
- mostly in body tissues, but are notablu absent in the brain itself
What is the difference between nociception and pain?
- Nociception = the sensory process that provides signals that trigger pain
- Pain is the unpleasant sensory and emotional experience associated with this
- You can have nociception without pain e.g. spinal cord injury
- vice versa
- Nociception cna be pleasurable
What is hyperplasia and inflammation?
-enhanced pain sensation
- May activate nociceptors themselves or sensitise nocieptors (lower their threshold) e.g. Prostaglandins sensitise nociceptors thus NSAIDS = useful analgesics
- Primary hyperalgesia occurs at the site of the injury
- Secondary hyperalgesia occurs at the site of injury
- Secondary hyperalgesia in surrounding tissue
What are the types of nociceptors?
- A delta fibres (light myelinated) = Fast, short, sharp first pain
- C fibres (unmyelinated) = Slower, duller, longer, second pain
What are spinal mechanisms?
A delta and C fibres enter the dorsal horn
Branch into ascending & descending collaterals
Travel in Lissauder’s tract (a few spinal segments)
Synapse with second order neurones in the dorsal horn substantia
gelatinosa (Rexed’s lamiae 1-111)
Release excitatory neurotransmitters glutamate and substance P
Explain the perception of pain
-Subjective
- Variable - depemds on the behavioural context (same level of nociceptor activity can induce more/less pain depending on the situation)
- Multi-dimensional
- Sensory
- Affective (emotional)
- Cognitive/motivational
We have endogenous analgesic systems that work to modulate the pain signal
What is the gate control theory of pain?
- Simultaneous activity in low-threshold mechanoreceptors (A beta fibres) can inhibit the onward passage of the nociceptive signal
- Theory : that large diameter mechanoreceptors and nociceptors activate the smae projection neurone. Simultaneous activity of both these afferents will suppress/ dilute the nociceptive signak
What is transcutaneous electrical stimulation (TENS)?
- Gate control theory = thought to be the rationale for TENS for the relief od pain
- May interrupt pain signals travelling up the spinal cord
- May also stimulate descending control systems
What is the descending control of pain?
- People may sustain terrible injuries and not feel the pain
- Due to us havinf endogenous analgesic systems built into the nervous system
- They regulate the gain of the pain system
- Fulfils a useful survival role
What are central analgesic systems?
- Electrical stimulation evoked analgesia from midbrain and pontine sites
- Via inhibition of spinal sensory neurones
- Chemical activation or disinhibition of specific areas
- Revealed regional specificity e.g. Periaqueductal grey
- Deep brain stimulation (DBS) in man :
- Rare : specidic refractory pains
- Aim for PAG/PVG
- Can produce selective analgesia
- There are circuits present in the brain that can alter nociception/ pain
What are nodes identified in the descending control systems?
- Hypothalamus
- Peri-aqueductal grey (PAG- midbrain)
- Rostral ventromedial Medulla (RVM)
- Dorsolateral pons
- Descending pathways inhibit projection neurones in the dorsal horn via direct and indirect (interneurone) pathways
- Act at doral horn to inhibit the onward transfer of the nociceptive signal
