Syllabus (Fourth Edition, 2023)
Topics
i. Describe the physiology of pain, including peripheral nociception, conduction, receptors, mediators and pathways, spinal cord modulation and central processing of pain.
Topics not covered in previous SAQs
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Learning Objectives for the First Part Examination in Intensive Care Medicine
- This will ensure that trainees, tutors, and examiners can work from a common base.
- All examination questions are based around this Syllabus.
- These learning objectives are designed to outline the minimum level of understanding required for each topic.
- The accompanying texts are recommended on the basis that the material contained within them provides sufficient information for trainees to meet the learning objectives.
- Trainees are strongly encouraged to explore the existing and evolving body of knowledge of the Basic Sciences as they apply to Intensive Care Medicine by reading widely.
- For all sections of the syllabus an understanding of normal physiology and physiology at extremes of age, obesity, pregnancy (including foetal) and disease (particularly critical illness) is expected.
- Similarly, for pharmacology, trainees are expected to understand a drug’s pharmacology in these contexts.
- An understanding of potential toxicity and relevant antidotes is also expected.
Definitions
Throughout the document specific wording has been used under the required abilities to indicate the level of knowledge and understanding expected and a glossary of these terms is provided.
Definitions
| Calculate | Work out or estimate using mathematical principles. |
| Classify | Divide into categories; organise, arrange. |
| Compare and contrast | Examine similarities and differences. |
| Define | Give the precise meaning. |
| Describe | Give a detailed account of. |
| Explain | Make plain. |
| Interpret | Explain the meaning or significance. |
| Outline | Provide a summary of the important points. |
| Relate | Show a connection between. |
| Understand | Appreciate the details of; comprehend. |
SAQs
i. Describe the physiology of pain, including peripheral nociception, conduction, receptors, mediators and pathways, spinal cord modulation and central processing of pain.
2023A 09
Define pain (10% of Marks). Describe how pain is detected and modulated in response to a peripheral noxious stimulus? (90% of Marks)
2019B 12
Define pain. Outline the processes by which pain is detected in response to a peripheral noxious stimulus.
2013B 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)
CICMWrecks Answer
Pain
Pain is an unpleasant sensory and emotional experience, associated with, or resembling that associated with, actual or potential tissue damage. (IASP, Revised 2020)
Anatomical and physiological components of the response to pain
- Primary afferent nociceptors
- Pseudo-Unipolar
- Cell body in dorsal root ganglion
- Free nerve endings or specialized terminal structures
- Pacinian corpuscles
- Location
- Skin
- Connective tissue
- Muscle
- Blood vessels
- Viscera
- Responds to simuli
- Mechanical
- Chemical
- Thermal
- Pseudo-Unipolar
Nociception
- Transient Receptor Potential (TRP) Ion Channels
- Capsaicin
- Heat
- Acid
- Inflammation
- Ischaemia
- Acid-Sensing Ion Channel (ASIC)
- 5-HT3 receptors
- On activation of nociceptors
- Na channels open
- Depolarization from < -60mV to -40mV
- Rapid membrane depolarization
- Voltage Dependent Calcium Channels (VDCC) open and increase intracellular Ca
- Substance P and Calcitonin Gene-Related Peptide (CGRP), Neurokinin A released in response to Ca
- Mediate inflammation → macrophages, neutrophils, mast cells activated
- Increase excitability of sensory and sympathetic fibres
- Vasodilation, extravasation of plasma proteins
- Inflammatory cells release substances
- Histamine
- Bradykinin
- Serotonin
- Nitric Oxide
- Peripheral sensitization → Primary hyperalgesia
- to dampen pain response
- Silent nociceptors – Do not respond to external stimulus, however activated by inflammatory mediators
- Voltage-gated Potassium Channels stabilize the membrane potential
Signal Pathway
- Signal transduced via C fibres (0.5~2m/sec) or Aδ fibres (6~30m/sec) to dorsal horn
- Aδ activation → short, pricking pain
- C activation → burning, aching pain
- Primary afferent enters spinal cord at the spinal level, or above or below the spinal level via lissauer tracts and synapse with interneurons
- Via C fibres or Aδ fibres
- Signal relay via spinal interneurons
- Signalling via Glutamate and NMDA or AMPA receptors
- Nociceptor-specific
- Respond only to pain
- Wide Dynamic Range (WDR)
- Respond to nociceptors and other stimulus
- Central sensitization → allodynia
- Inhibitory interneurons and descending pathways modulate pain
- “Gate control theory of pain” → Aβ fibres carrying signals from benign stimuli inhibit noxious interneuron transmission
- Signal transduction to thalamus via anterolateral spinothalamic tract
- Small but significant proportion of signal transduction also occurs ipsilaterally
- Signal distributed to cerebral cortex via thalamus
- Putamen
- Hypothalamus
- Amygdala
- Periaquaductal grey matter
- Hippocampus
- Cerebellum
- Somatosensory cortex
Descending inhibitory pathways
- Noradrenaline
- Serotonin
- Substance P
- CCK
- GABA
Sakurai 2016
Examiner Comments
2023A 09: 26% of candidates passed this question.
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.
2019B 12: 33% of candidates passed this question.
Starting with the WHO definition of pain, followed by a brief description of the nature of noxious stimuli (thermal, mechanical, chemical) then proceeding to mention the nature of the cutaneous receptors would have been a very good start to this question. Following this, a description of the various substances involved in pain (K, prostaglandins, bradykinin, serotonin, substance P) and outlining the types of nerve fibres involved in pain transmission and how they synapse in the spinal cord and cortex was expected. The presence and nature of the descending inhibitory pathways was mentioned by very few.
2013B 22: 11 candidates passed (40.8%).
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.
2014A 01 – 2011B 15
Outline the motor and sensory pathways involved in withdrawing the lower limb from a painful stimulus.
CICMWrecks Answer
Withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli.
The upper centres are not involved.
Afferent
- Nociceptors detect the painful stimulus
- Conveyed to the dorsal horns of spinal cord by Aδ (fast) fibres and C (slow) fibres
- Through the cauda equina
- Synapse at the same spinal level, or 1-2 levels above via Lissauer’s tract
Reflex arc
- (fast response to painful stimulus)
- (outside of conscious control, but can be modulated by the cerebral cortex)
- Interneurons:
- Travel to the ipsilateral anterior horn, synapse with motor neurons
- Cross the midline to the contralateral anterior horn
- Efferent
- Motor neurons in the ipsilateral limb cause contraction of flexors, and relaxation of extensors
- Contralateral limb motor neurons cause relaxation of flexors, and contraction of extensors
- Causes withdrawal of painful limb, and shift In centre of gravity to the opposite limb
Conscious pathways
(slow response to painful stimulus)
Interneurons:
- Cross the midline via the anterior white commissure
- Ascend in the spinothalamic tract to the thalamus
- Synapse in the thalamus
- Travel to the post-central gyrus of the cerebral cortex
Efferent:
- Originate in the motor cortex (pre-central gyrus)
- Project to medulla
- 90% decussate at pyramids to innervate limbs
- Travel down the lateral corticospinal tract
- 10% continue down ipsilateral anterior corticospinal tract to innervate axial muscles
- Decussate at the target spinal level
- Synapse at the anterior horn to lower motor neurons
Mooney / Sakurai 2016
Examiner Comments
2014A 01: 26% of candidates passed this question.
It was expected that candidates would outline both motor and sensory pathways and mention
a reflex arc and conscious pathways.
2011B 15: 5 (20%) of candidates passed this question.
A good answer required a description of the sensory pathway(s) (eg nociceptors, Aδ and C fibres, spinal dorsal horn, spinothalamic, thalamic and cortical pathways), reflex arc (nocioceptive sensory fibres synapse with spinal inter-neurons that in turn synapse with peripheral motor neurons supplying the lower limb as well as inter-neurones that also synapse with motor neurons on the contra-lateral lower limb producing a crossed extensor response), central integration and the motor pathways (fibers from the contra-lateral motor (and pre-motor) cortex pass through the posterior internal capsule forming the lateral and ventral cortico-spinal tracts, the cortico-spinal tracts pass through the anterior brainstem, the lateral tract decussating in the caudal medulla, continue to synapse with spinal motor neurons in the ipsilateral lumbosacral anterior horn cells, passage via peripheral nerves and flexor muscle stimulated, extensors inhibited, resulting in withdrawal of the limb) Common mistakes included errors in naming nerve pathways and receptors. Another error was to confuse the polysynaptic pain response pathways with the monosynaptic stretch reflex. Very few candidates mentioned feedback regulation via cerebellar input and at spinal level from muscle spindles and Golgi tendon organs
Syllabus: G12b
Recommended sources: Ganong Review of Medical Physiology Chps 11 & 16
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