Sensory systems Flashcards
(8 cards)
Describe the nociceptor
Free nerve endings
Activated by noxioius stimuli
Generates AP intensity encoded through firing rate, propagate centrally
Mechanical- (Adelta fibres- fast, first pain) activated by strong shearing force in skin like a cut or a blow- sharp pain
Polymodal- (C fibres- slow, second pain) respond to sharp blow and damaging heat, chemicals (H, histamine, prostaglandins, bradykinin)- dull burning pain
Describe the dorsal horn
Pain neurones have cell bodies in the dorsal root ganglion, synapse with second order neurons in the dorsal horn
C fibres can synapse with a projection neuron or an interneuron
Majority of interneurons are inhibitory and modulate the activity of projectory neurons
Pain must overcome inhibition to be sent to the brain- gate theory of pain
- gatekeeper interneuron is tomically active and found in the substantia gelatinosa, preventing accidental firing of ascending pain to brain.
Describe facilitated pain
Persistent/chronic pain increases the sensitivity to pain
Process of peripheral and central sensitisation, modification of neurotransmission
Plasticity
Allodynia- painful response to a normally non-painful stimulus due to lowered pain threshold.
Hyperalgesia- enhanced painful response to a normally painful stimulus
Describe neurogenic inflammation
Noxious stimulation causes the release of various factors, like substance P, that act on the surrounding vasculature to cause plasma extravasation and immune cell migration which produce proinflammatory substances that act on the nociceptor to lower threshold.
What is secondary hyperalgesia
Increase in pain sensitivity distant from the site of injury
Antidromic action potentials propagate along fibre branches that innervate injured and neighbouring, uninjured, tissue that results in the sensitisation of these neurones - Neurogenic inflammation occurs.
In dorsal horn:
Glutamate stimulating AMPA and substance P (released in response to high requency APs) stimulating NK-1 causes sufficient depolarisation to remove the Mg block in NMDA receptors for Ca influx➡ secondary messenger cascades.
- increasing responsiveness and amplification of rate of AP firing along ascending nociceptor pathway.
Non-noxious and noxious signals
Non-noxious signals are sent from A beta mechanoreceptors with both interneuron and ascending fibre. The inhibitory interneuron is being stimulated to be inhibitory.
Arrival of noxious AP via A delta or C fibre inhibit the inhibitory interneuron directly or indirectly. This allows passage of pain stimuli to ascend to brain.
TENS
The gate can be closed by rubbing the injured area
- Transcutaneous Electrical Nerve Stimulation
It involves overstimulating the non-noxious fibres, causing inhibition of noxious fibres and creating an analgesic effect.
Inhibiting pain signal
The brain stem nuclei is rich in opioids. Release of 5-HT, NAd. and enkephalin causes an inhibitory effect to close the gate, preventing signal ascending.
Opioid drugs do the same, as well as the Battlefield Effect
