Question 9

College Answer

Candidates were expected to give a reasonable definition of pain incorporating the experience and tissue damage. Most candidates only partially incorporated both the actual or perceived harm and the sensory and emotional experience that is included in its formal definition. This question was then best answered by breaking down pain transmission and modulation into; peripheral, spinal cord, cortex, and central downregulation pathways. Good answers provided detailed and specific descriptions of the sensors, neural pathways, synapses, receptors, and neurotransmitters. Whilst pain transmission at the level of the spinal cord is complex, breaking this down into 1st and 2nd order neurons, main neurotransmitters and accessory neurotransmitters from interneurons and descending pathways was helpful. Whilst many covered some of this conceptually most answers did not provide sufficient detail to be considered a pass level answer. Many candidates described the withdrawal reflex to pain in detail which was not asked for and therefore did not  attract marks

Discussion

• Pain is "an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms of such damage".
• Pain is detected by nociceptors:
  • Nociceptors are nerve endings that transduce noxious stimuli into action potentials
  • Noxious stimuli sensed by these endings include:
    • Mechanical deformation
    • Acidity (pH <5.0)
    • Temperature (heat as well as cold)
    • Inflammatory mediators (eg. prostaglandins, bradykinin)
    • Contents spilled from damaged cells (eg. ATP)
• Pain transmission:
  • From the nociceptor, along a pain fibre
    • 20 m/s along a myelinated Aδ fibre
    • 2m/s along an unmyelinated C fibre
  • Past the body of the neuron, which lies in the dorsal root ganglion
  • Into the spinal cord via the dorsal root
  • Up (or down) the tract of Lissauer
  • Into the dorsal horn grey matter
  • Here, the primary afferents synapse with dorsal horn neurons, which:
    • Are arranged into discrete laminae which correspond to different spatial and functional groups
    • Are influenced by excitatory and inhibitory interneurons the activity of which is regulated by descending projections from the brain
  • Dorsal horn neurons then send projections across midline to the contralateral spinal cord, where their fibres form the ascending spinothalamic tract
  • The spinothalamic tract fibres synapse with tertiary neurons in the thalamus
    • The thalmus is responsible for sensory mapping and cortical representation of the geographical position of the pain information, and it also regulates some of the affective response to pain
  • They also project to multiple other higher centres:
    • Nucleus of the solitary tract and caudal ventrolateral medulla (cardiovascular response to pain)
    • Periaqueductal grey matter (descending regulation of pain signals)
    • Lateral parabrachial area (emotional and affective response to pain)

Hudspith, Michael J., Philip J. Siddall, and Rajesh Munglani. "Physiology of pain." Foundations of anesthesia (2006): 267-285.

Steeds, Charlotte E. "The anatomy and physiology of pain." Surgery (Oxford) 27.12 (2009): 507-511.

Renn, Cynthia L., and Susan G. Dorsey. "The physiology and processing of pain: a review." AACN Advanced Critical Care 16.3 (2005): 277-290.

Almeida, Tatiana F., Suely Roizenblatt, and Sergio Tufik. "Afferent pain pathways: a neuroanatomical review." Brain research 1000.1-2 (2004): 40-56.

Todd, Andrew J. "Neuronal circuitry for pain processing in the dorsal horn." Nature Reviews Neuroscience 11.12 (2010): 823-836.

Rexed, Bror. "The cytoarchitectonic organization of the spinal cord in the cat." Journal of Comparative Neurology 96.3 (1952): 415-495.

Snijdelaar, Dirk G., et al. "Substance P." European Journal of Pain 4.2 (2000): 121-135.

Harding, Erika K., Samuel Wanchi Fung, and Robert P. Bonin. "Insights into spinal dorsal horn circuit function and dysfunction using optical approaches." Frontiers in Neural Circuits 14 (2020): 31.