Question 22

Define pain. (10% of marks) Describe the anatomical and immediate physiological components of the response to pain arising from the insertion of an arterial line. (90% of marks)

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College Answer

The pain pathways following arterial line insertion involve sensing the stimulus and transmitting the sensation to the central nervous system. It was expected candidates could provide some detail about the major features along this pathway including a description of sensors, nerve types, spinal cord input and decussation with subsequent projection to higher centres. Better answers provided additional details about modulation and descending pathways. The question also required a description of the physiology with some discussion of the mediators involved and explanation of how a stimulus or tissue may result in the perception of pain. Common omissions included insufficient detail of the pain pathway and limited or no discussion of the physiological components.


  • Pain is "an unpleasant sensory or emotional experience associated with actual or potential tissue damage, or described in terms of such damage".
  • Nociceptor transduces a painful stimulus into an action potential:
    • Mechanical stimulus (tissue distortion)
    • Chemical stimulus (inflammatory mediators released through tissue damage)
  • From the nociceptor, the action potential travels 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)


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.

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.


Kendroud, Sarah, et al. "Physiology, nociceptive pathways." StatPearls [Internet] (2020).

Cohen, Milton, John Quintner, and Simon van Rysewyk. "Reconsidering the International Association for the Study of Pain definition of pain." Pain reports 3.2 (2018).