0520 - Biological basis of pain - EG

Question 1

What is pain?

Answer 1

Pain is not a sensation (like vision/taste/touch etc) but an unpleasant sensory and emotional experience with sensory components (hot, sharp, dull, aching) and emotional components (distressing, exhausting). The perception of pain is a function of the brain’s processing of sensory input and motivates a change in behaviour (like thirst and hunger). It acts as an early warning system to protect against tissue injury

Question 2

What is the difference between nociception and pain?

Answer 2

Nociception is the sensory process that carries signals that mediate the pain experience, but nociceptor activation does not always result in the perception of pain, which can result without nociceptor activation.

Question 3

What is hyperalgesia? How is it different to allodynia?

Answer 3

an increased sensitivity to moderate nociceptor activation, can be centrally or peripherally mediated. Can be primary due to tissue damage or secondary - reactive to primary damage. Allodynia is pain produced by a normally innocuous stimuli, not by nociceptor sensitisation, but rather from central mechanisms.

Question 4

Do somatosensation receptors perceive pain?

Answer 4

Somatosensation is due to mechanoreceptors that respond to tactile stimuli and elicit either a fast-adaptive or slow-adaptive response, but of which is never perceived as painful, as pain and touch use separate pathways.

Question 5

What type of nerve fibres carry nociceptive pain? What type of pain do each carry?

Answer 5

Adelta (lightly myelinated, carry sharp localised pain, mechanical and thermal nociception, fast) and C fibres (non-myelinated, carry dull, diffuse burning pain, chemical, C-polymodal, mechano heat-insensitive nociceptors, slow)

Question 6

List the various types of nociceptors (5).

Answer 6

(1) Mechanical nociceptors respond to strong pinch or sharp objects that penetrate, squeeze or pinch the skin. (2) Thermal nociceptors that respond to noxious heat (above 45) or noxious cold (below 5). (3) Chemical nociceptors selective for irritants such as histamine, capsaicin, mustard. (4) C-polymodal nociceptors selective for various noxious mechanical, temperature, and irritant chemicals. (5) mechano heat-insensitive afferent.

Question 7

What chemical mediators increase nociceptor sensitivity?

Answer 7

bradykinin, prostaglandins, substance P, histamine.

Question 8

How do tactile afferents from the skin travel up the spinal cord from their entry into the dorsal horn to destination in the thalamus? Pain afferents from the skin?

Answer 8

tactile afferents carrying fine touch, vibration and proprioception ascend in the dorsal column nuclei; pain afferents enter into the zone of Lissauer and synapse in substantia geletinosa, cross to the contralateral side and enter the spinothalamic tract. NB head and face (V) synapse in spinal trigeminal nucleus then cross to travel to thalamus.

Question 9

What is referred pain?

Answer 9

Believed to be cross-talk between nociceptive inputs from viscera and cutaneous afferents which both travel in the spinothalamic tract, resulting in pain perceived in those dermatomes.

Question 10

Which spinal pain pathway is responsible for arousal?

Answer 10

spinoreticular tract

Question 11

List the various spinal pain pathways

Answer 11

(1) spinothalamic (most common, for pain location and intensity). (2) spinoreticular (arousal). (3) spinomesencephalic (behavioural emotional and autonomic components of pain)

Question 12

What is the periacqueductal grey (PAG)?

Answer 12

An important part of the spinomesencephalic pain pathway which coordinates behavioural and motivational responses, projecting to the amygdala for emotional responses to pain and the hypothalamus for ANS responses to pain, and the medullary raphe for analgesia.

Question 13

List the various levels (4) at which pain modulation can occur.

Answer 13

(1) peripheral with sensitisation of nociceptors, (2) spinal cord with the gate control theory where non-nociceptive mechanoreceptors inhibit nociceptive ones via interneurons (3) descending modulation from PAG-raphe-spinal pathway which inhibits incoming spinothalamic tract neurons in the dorsal horn for analgesia, (4) opioid mediated, endogenous opioid epetides such as enkephalin which reduces release of substance P and produces IPSPs

Question 14

What is the effect of naloxone?

Answer 14

Inhibition of enkephalin

Question 15

Contrast neuropathic pain and neurogenic pain.

Answer 15

neuropathic pain occurs in the absence of nociceptor stimulation, such as phantom limb, whereas neurogenic pain occurs due to primary damage of the nervous system, such as carpel tunnel