Which descending pathway originates from the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) and exerts inhibitory control over nociceptive transmission in the dorsal horn via release of norepinephrine and serotonin?

Explanation

## Descending Pain Inhibitory Pathways **Key Point:** The dorsolateral funiculus (DLF) contains the descending monoaminergic pathways (noradrenergic and serotonergic) that mediate endogenous pain inhibition from the brainstem. ### Anatomy and Function The PAG and RVM send descending projections through the dorsolateral funiculus to the dorsal horn, where they release: - **Norepinephrine** — activates α~2~-adrenergic receptors on dorsal horn neurons, inhibiting nociceptive transmission - **Serotonin (5-HT)** — activates 5-HT~1A~ and 5-HT~7~ receptors, suppressing pain relay ### Pathway Hierarchy ```mermaid flowchart TD A[Cortex, Hypothalamus, Amygdala]:::outcome --> B[Periaqueductal Gray PAG]:::action B --> C[Rostral Ventromedial Medulla RVM]:::action C --> D[Dorsolateral Funiculus DLF]:::action D --> E[Dorsal Horn Laminae I-II]:::action E --> F[Inhibition of Nociceptive Neurons]:::outcome G[Norepinephrine + Serotonin]:::action --> E ``` ### Clinical Correlates **High-Yield:** Tricyclic antidepressants (amitriptyline, nortriptyline) and SNRIs (venlafaxine) enhance descending inhibition by increasing norepinephrine and serotonin in the dorsal horn — hence their use in chronic pain and neuropathic pain. **Clinical Pearl:** Lesions of the dorsolateral funiculus (e.g., spinal cord injury) impair descending inhibition, predisposing to central pain and neuropathic pain syndromes. **Mnemonic:** **DLF = Descending Locus of Facilitation** (of pain inhibition) — remember that the DLF is the major inhibitory pathway. [cite:Guyton and Hall 14e Ch 48]