2008A14 Describe the pathways whereby myocardial ischaemia may be experienced
 as pain in the throat or arm regions.

Cause

·   Myocardial O₂ demand exceeds supply

·   Anaerobic metabolism

·   ↑H⁺, K⁺, CO₂, adenosine; ↓pO₂

·   Activation of chemical nociceptive receptors

Significance

·   Frequently referred. Perception from ear superiorly to umbilicus inferiorly

·   Frequently misdiagnosed

Definition

·   Where pain is perceived distant to the site of insult

Purpose

·   ? Accident of evolution

·   ? Allows somatotopic localization of phylogenetically ancient pathways (my thought)

Mechanisms

·   Convergence-projection: most accepted

o Visceral 1° afferents onto nociceptive specific somatic 2° afferents

o Due to embryological origin of visceral and somatic structures

·   Convergence-facilitation

·   Axon-reflex theory

·   Hyperexcitability theory

·   Thalamic convergence theory

Nociceptive receptor

·   Free nerve endings of post-ganglionic neurons, in myocardium

·   Ionotropic e.g. ASIC (faster)

·   Metabotropic: e.g. bradykinin (slower)

1° afferent (nociceptor)

·   Unmyelinated C fibre
(hence slow onset, dull, vague; responsive to opioids)

·   Piggybacks onto autonomic efferents

·   With SNS:

o With post-synaptic fibres to paravertebral ganglia

o With pre-synaptic fibres via white ramus communicans to spinal cord T1-4
(hence predominantly chest)

o Entry at dorsal horn (not intermediolateral)

o Ascent or descent 1-2 layers via Lissauer’s tract
(hence also upper limbs, abdomen)

o Direct synapse with wide dynamic range 2° afferents in deeper layers

o Convergence onto somatic nociceptive specific 2° afferents in superficial layers
(hence referred)

·   With PSNS:

o With post-synaptic nerves to local ganglia

o With pre-synaptic cranial nerves (CNIX, CNX): to nucleus tractus solitarius
(hence also neck, jaw, ear)

2° afferent

(visceral)

·   Wide dynamic range: multimodality, high threshold, only conveys pain if sensitized

·   Nociceptive-specific: low threshold, pain only

·   Decussation via anterior commissure

o Incomplete for older pathways
(hence bilateral with L > R)

·   Ascent via spinothalamic tracts (STT)

o Neo-STT -> thalamus (VPL nucleus)

o Paleo-STT/archi-STT -> mostly brainstem

3° afferent

·   VPL thalamus -> 1° somatosensory cortex (localisation)

·   Brainstem -> medial thalamus, hypothalamus, amygdala (affective, autonomic)