In the spinal cord, C fibres and A-delta fibres have their first central synapse with the second-order sensory neurone that connects to the brain in lamina I to III (but predominantly lamina II, the substantia gelatinosa) of the dorsal horn. Non-nociceptive A-beta fibres terminate predominantly in lamina III and IV of the dorsal horn and have virtually no connections to the ‘pain’ lamina II. The arrangement of these fibres is summarised in Figure 1.1. The first central synapse is an important site of modulation of nociceptive input into the central nervous system.
From the dorsal horn, the second-order neural pathways for nociception ascend predominantly in the contralateral spinothalamic tracts. There are also extensive synaptic connections at or near the level of the dorsal root entry into the spinal cord before progression up the spinothalamic tract of the spinal cord that modify transmission or elicit local ‘reflex’ responses (eg nonspecific withdrawal of a limb).
The ascending pathways synapse throughout the brain stem and midbrain, including to visceral and autonomic control centres, the nucleus raphe magnus, the periaqueductal grey matter and the rostral ventral medulla. On reaching the ventral posterior medial and ventral posterior lateral thalamic nuclei, signals follow complex integrating and radiating pathways that project to the cerebral cortex as well as other brain stem nuclei. The cortical projections determine the conscious perception of pain, precise localisation (parietal cortex), complex behavioural responses, and learning and emotional responses (frontal cortex).
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