Neuro Physiology - Pain

Question 1

What is pain/nociception/chronic pain?

Answer 1

An unpleasant sensory and emotional experience associated with potential or actual tissue damage.

Nociception is the detection of a noxious stimulus by the brain, with nociceptive pain being caused by stimulation of peripheral pain nerves called nociceptors.

Chronic pain continues after the cause has been removed, beyond the expected recovery time (some definitions suggest 3 months)

Question 2

What do nociceptors detect?

Answer 2

Chemical mediators
Histamine, serotonin, H+ ions, Potassium, Substance P

Mechanical Stress

Thermal damage

Question 3

How can nociceptive pain be classified?

Answer 3

Either by cause (such as inflammatory, traumatic, ischaemic) or by character

Somatic - well localised, can deep or superficial (Such as a fracture or abrasion|)
Visceral - poorly localised, often referred. Eg. Diaphragmatic irritation & shoulder tip pain.

For example, appendicitis often initially presents with visceral diffuse lower abdominal pain, localising to the RIF when somatic stimulus from the inflamed peritoneum is received.

Question 4

What is neuropathic pain?

Answer 4

Pain caused by dysfunction within the nervous system itself rather than an external stimulus.

Possible features:
Hyperalgaesia - disproportionate pain response to a normally moderately painful stimulus
Allodynia - sensation of pain to a normally painless stimulus (Eg. Trigeminal neuralgia)
Phantom pain - Pain from an absent part of the body (Eg. amputee)

Question 5

What types of pain fibres are there? How does the gate theory of pain affect them?

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Answer 5

Aδ Small myelinated fibres, diameter up to 5 μm, sharp pain
50m/s conduction velocity

C Smallest fibres, less than 2μm, dull pain
2m/s conduction velocity

Both Aδ and C enter the dorsal horn in the spinal cord, where they have an inhibitory effect on the inhibitory interneurones in the substantia gelatinosa. They also synapse directly with second order ascending pathways, and here, they are excitary.

Aβ light touch fibres also synapse here, exciting both the inhibitory interneurone, and the nociceptive centre. The inhibitory interneurone prevents depolarisation of the ascending pathways.

This explains why rubbing a painful stimulus numbs it, as it increases the stimulus on the inhibitory interneurone.

Question 6

Draw a diagram to demonstrate the pain pathway to the brain

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Answer 6

First order neurones
Aδ and C fibres enter the dorsal horn through the dorsal root ganglion (Aδ fibres travel in lamina I and III, and C fibres in lamina II + III). Here they act to inhibit interneurones (which also receive excitatory signals from descending modulator pathways and Aβ fibres). These interneurones onward transmission.

The Aδ and C fibres also synapse with second order neurones in the lateral spinothalamic tract, decussating at the level they synapse, ascending to the thalamus.

In the thalamus, these synapse with third order neurones, and continue to the post-central gyrus ad sylvian fissure.

https://teachmeanatomy.info/neuroanatomy/pathways/ascending-tracts-sensory/