Central Nervous System Pain Pathways

Abstract

Nociceptive primary afferents detect stimuli that are normally perceived as painful, and these afferents form synapses in the dorsal horn of the spinal cord and the spinal trigeminal nucleus. Here they are involved in highly complex neuronal circuits involving projection neurons belonging to the anterolateral tract (ALT) and interneurons, which modulate the incoming sensory information. The ALT neurons convey somatosensory information to a variety of brain regions that are involved in the various aspects of the pain experience. A spinothalamic-cortical pathway provides input to several regions of the cerebral cortex, including the first and second somatosensory areas (S1, S2), the insula and the cingluate cortex. These regions are thought be responsible for the sensory-discriminative aspects of pain (S1), pain-related learning (S2), the autonomic and motivational responses (insula), and the negative affect (cingulate). Another ascending system, The spinoparabrachial-limbic pathway targets a variety of brain regions, including the amygdala, and is likely involved in the affective component of pain. A descending system that includes the limbic system, the periaqueductal gray matter of the midbrain, the locus coeruleus, and the rostral ventral medulla, can suppress pain, and this operates partly through the monoamine transmitters noradrenaline and serotonin which are released in the spinal and trigeminal dorsal horn.