Raphespinal tract

The raphespinal tract is an unmyelinated descending serotonergic tract involved in pain modulation. It is a descending pain-inhibiting pathway; it is a component of the reticulospinal tract.

It originates in the raphe nuclei (particularly the nucleus raphe magnus) of the reticular formation of the medulla oblongata (as well as serotonergic neurons of the gigantocellular reticular nucleus ), and projects to the spinal trigeminal nucleus, and posterior grey column (especially the substantia gelatinosa (lamina II)) of spinal cord.

It consists of two components:


 * Anterior raphespinal tract - descends in the anterior funiculus of the spinal cord.
 * Lateral raphespinal tract - descends in the lateral funiculus of the spinal cord.

Pathways
Pain pathways converging upon the raphe nuclei to modulate pain via the raphespinal tract include:
 * Laminae I and V of spinal cord→ spinomesencephalic tract → periaqueductal gray → nucleus raphe magnus →
 * Laminae I and V of spinal cord → spinomesencephalic tract → mesencephalon raphe nuclei →
 * Nociceptive group C first-order nerve fiber → interneurons of lamina II (substantia gelatinosa) and lamina III of the posterior grey column of the spinal cord → second-order neurons of laminae V-VIII of spinal cord → spinoreticular tract → nucleus raphe magnus and gigantocellular raphe nucleus →
 * Raphespinal tract → spinal trigeminal nucleus and posterior grey column of the spinal cord → activating serotonergic synapses with inhibitory enkephalinergic interneurons → inhibitory enkephalinergic synapses with nociceptive first-order nerve fibers.

Function
Electrical stimulation of either the periaqueductal gray or (its downstream target) nucleus raphe magnus induces profund analgesia; this effect can be abolished both by transection of the raphespinal tract as well as by opioid receptor antagonists (evidencing one of the mechanisms by which opioid bring about pain relief). An electrical stimulator implant of the periaqueductal gray can be used clinically for pain management, evoking instantaneous pain relief upon activation.

The raphespinal tract appears to also be involved in modulating motor activity as serotonin increases the excitability of motor neurons - serotonin-blocking medications can alleviate spasticity caused by damage to the motor pathways.