Pain - physiology to pharmacology
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What is pain?
Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage
What factors can affect the emotional components of pain?
- Past experiences
- Suggestion eg placebo
- Emotional state (particularly fear and anxiety)
- Simultaneous activation of other sensory modalities
What is the difference between physiological and pathological pain?
Physiological pain is proportional to the intensity of noxious stimulus. It protects against further injury. Pathological pain is pain greater than apparent noxious stimulus and is detrimental e.g. phantom limb pain.
List the components of the pain pathway
- Brain - conscious experience of pain
- Spinal cord - processing occurs at the level of the dorsal horn (modulation)
- Afferent fibre - conducts action potential to CNS (transmission)
- Nociceptor - Generates an action potential (transduction)
Describe the reflex response to pain
Afferent signal comes up into the dorsal horn --> interneurone synapses with motor nerve ---> moves limb away. (Complex signals are also sent out to stabilise the uninjured side of the body).
What is noiciception?
The detection of noxious stimuli that actually or potentially cause damage to an organism
True or false: nociception does not occur when an animal is unconscious?
What type of nerve fibres are involved in nociception?
C fibres and A delta fibres
In descending inhibtion, descending pathways from periaqueductal grey (PAG) and rostral ventromedial medulla (RVM) contain endogenous ... pathways? At what level of the spinal cord do these pathways project to reduce the transmission of pain?
How does Gate control reduce transmission up the pain pathway?
Activation of AB fibres with tactical, non-noxious stimuli activates inhibitory neurones in the dorsal horn i.e. rubbing alleviates pain
What are the two types of sensitisation?
Peripheral and central
What is hyperalgesia?
An increased response to a stimulus which is normally painful
What are the two types of hyperalgesia?
- Primary - site of injury
- Secondary - area immediately around the site of injury
What is allodynia?
Pain due to a stimulus which does not normally provoke pain
Describe the difference between peripheral and central sensitisation
- In peripheral sensitisation nociceptors become increasingly reactive with continuing or repeated stimulation. This causes sensitisation of peripheral nociceptors (C and Adela) so they transmit an amplified signal to the spinal relay centre.
- Central sensitisation is when over activity of second order neurones in the dorsal horn leads to enhanced pain transmission.
What is analgesia?
An absence of pain or a reduction in intensity of pain perceived
List examples of analgesic drugs
- Local anaesthetics
- Non-steroidal anti-inflammatory drugs
- Alpha2-adrenoceptor agonists
- Nitrous oxide
List examples of non-pharmacological analgesic methods
- Nursing care
- Transcutaneous electrical nerve stimulation (TENS)
Where do different analgesic drugs act along in the pain pathway?
- Brain - opioids
- Spinal cord - opioids and alpha2-agonists
- Afferent fibre - LAs
- Nociceptor - NSAIDs
What is preventative analgesia? Why is this used?
The administration of analgesic drugs throughout the peri-operative period. It is used to prevent or limit the development of sensitisation induced by pre-, intra- or post-operative noxious stimulation
What is multimodal analgesia? Why is it used?
The use of a combination of drugs that act at different points in the nociceptive pathway. It provides more effective analgesia with fewer adverse effects (as you are not using a high dose of one drug)
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