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Last updated: Mon, Mar 3, 2025
The primary descending output of the periaqueductal gray, PAG, goes to the rostral ventromedial medulla or RVM. In turn, the PAG is the major source of input to the RVMV. The RVM also receives input from the basal ganglia, the hypothalamus, the preoptic area, the amygdala, and the limbic areas of the brain. The RVM receives little information directly from the spine, but has access to some spinal information through the PAG and other brain stem locations.
The RVM sends its primary output down into the spinal cord, where its axons make contacts primarily primarily in laminae I, II, and V. These are the same laminae of the spinal gray matter which receive nociceptive input from the body, and from which the ascending projection neurons originate.
The descending RVM axons are known to cause a number of effects, although the details of the connectivity of RVM neurons in the dorsal horn are yet to be fully understood.1 They can both inhibit and excite nociceptive neurons in laminae I, II, and V. They directly inhibit ascending projection neurons; they inhibit or excite interneurons; and they inhibit sensory neurons pre-synaptically.
A 2009 study concluded that axons descending from the RVM can affect wide-dynamic-range spinal neurons in different ways, depending upon whether the WDR neurons receive much C-fiber input. Those without C-fiber input (i.e., non-nociceptive) were facilitated, while those with C-fiber input were inhibited. On the other hand, descending inhibition and facilitation have been found to have different time courses, and are caused by different populations of RVM cells.2 This implies that descending facilitation and inhibition are at least partially independent.