Pain Flashcards
What is the receptive field ?
An area that a particular nerve responds to a stimulus in.
What is a stimulus turned into ?
Depolarisation - Receptor potential - Generator potential
What determines acuity ?
- Area of receptive field
- Density of neurones in the area
What are Ab fibres ?
Large myelinated axons that carry information on touch, vibration and pressure
What are Aℑ fibres ?
Small myelinated axons that carry information on fast , sharp pain and cold
What are C fibres ?
They are unmyelinated axons that carry information on dull, slow pain and warmth
Which fibres mediate proprioception ?
Aa and Ab
Where do Aa get signals from ?
Extrafusal muscle spindles and GTO’s
Where do Ab get signals from ?
Secondary receptors of muscle spindles
What 3 things do sensory pathways do ?
- Converge
- Adapt
- Laterally inhibit
Definition of nociception
The detection of noxious harmful stimuli by Aℑ and C fibres that have the ability to cause tissue damage.
What is the gate control theory ?
Aa and Ab fibres activate inhibitory interneurones by releasing endorphins that inhibit transmitter release in Aℑ and C fibres. Descenind inhibition from PAG and NRM also activate these inhibitory interneurones and hence close the gate.
Which is the name of the neurotransmitter release ?
Substance P
How do NSAID’S work ?
They stop COX from converting arachidonic acid into prostaglandins. Less prostaglandins less sensitisation to bradykinin, less depolarisation and so less pain.
How do local aesthetics work ?
Block all Na+ - axonal transmission is stopped
How does TENS (Transcutaneous Electrical Nerve Stimulation) work ?
Some method as closing the gate. Aa and Ab fibres are activated and they activate inhibitory interneurones which prevent signals passing up through Aℑ and C fibres.
How do opiates work ?
- Reduce the amount of neurotransmitter release
- Activate the descending inhibitory pathways
- Reduce the sensitivity of nociceptors
What is Neuropathic pain ?
Pain caused by nerve damage. Usually a burning or prickling sensation. There is no obvious tissue damage, usually resistant to medical treatment.
Why does referred pain occur ?
The visceral and somatic nerve fibres enter the spinal cord at the same level. This can sometimes lead to misinterpretation of incoming signals by higher brain centres.
Where does the LST terminate ?
In the ventroposteior thalamic nuclei
Where does the AST terminate
In the medial thalamic nuclei
Where are signals sent from the thalamus ?
The cortex, brainstem and limbic system
Where does pain perception occur ?
In the somatosensory cortex
What is involved in the descending control of pain
The medial thalamus, hippocampus and amygdala are involved and they relay this to the brainstem.
Where do descending pathways go ?
Via the PAG
Definition of Hyperalgesia
Increased perception of pain
Allodynia
Decreased threshold of pain response
What happens during peripheral sensitisation ?
The nociceptive receptors have an increased sensitivity to stimuli
What is central sensitisation ?
Increases the response of the 2nd order neurone in the CNS to normal input of both noxious and noxious stimuli.
What can central sensitisation lead to ?
The development and maintenance of chronic pain.
What are the 3 stages of central sensitisation ?
- Wind up
- Classical
- Long-term potentiation
What is wind up ?
Occurs only during the stimulus, and causes an increase in the number of AP’s that are fired due to a stimulus. Substance P and CGRP are involved.
What is classical ?
Classical involves opening up new synapses. Starts with the stimulus and outlasts it. It results in secondary hyperalgesia.
What is secondary hyperalgesia ?
Secondary hyperalgesia is defined as an increase in pain sensitivity when a noxious stimulus is delivered to a region surrounding, but not including, the zone of injury.
What is long-term potentiation ?
Involved both AMPA and NMDA receptors activated by glutamate. Synapses are strengthened.
Features of acute pain
- < 1 month
- Tissue damage
- Has a protective function
- Pain resolves upon healing
- Usually nociceptive
- Responds to conventional analgesics
Features of chronic pain
- > 3 months
- Pain beyond the expected timeframe of healing
- No protective function
- Degrades health and function
- Usually neuropathic
- Doesn’t respond well to conventional analgesics
What is neuropathic pain ?
Damage or disease affecting the somatosensory NS, it causes pain.
What can block transduction ?
- Ice
- Rest
- NSAID’s
What can block transmission ?
- Nerve block
- Opiates
- Cordotomy
What can help with reducing pain perception ?
- CBT
- Relaxation
- Distraction
What helps control descending modulation ?
- Opioids
- Anti-depressants
