The early neurogenic phase of POI
Skin, muscle and peritoneal incision during laparotomy causes a neurological reflex arc via splanchnic afferent nerves that synapse in the spinal cord with adrenergic neurons. This signal activates efferent nerves in the direction of the digestive tract resulting in paralysis of the entire digestive tract(Fox and Powley, 1985; Barquist et al., 1996; Vergnolle and Cirillo, 2018). In the second neurological phase, when the bowel is manipulated and stimulated more intensely, additional pathways are activated by the brainstem(Fox and Powley, 1985). These pathways relay to hypothalamic and pontine-medullary nuclei such as the nucleus tractus solitarius and the paraventricular and supraoptic nuclei of the hypothalamus. It should be noted that these neural relays are also adrenergic in nature(De Winter et al., 1997a). Corticotrophin releasing factor (CRF) plays a central role in this activation pathway, leading to activation of the vagal nerve (Boeckxstaens et al., 1999; Browning et al., 2017). Intense stimulation of the splanchnic nerves activates another inhibition pathway of the digestive motor system via nitrergic (NO) and vipergic (VIP) synapses (Barquist et al., 1996; Boeckxstaens et al., 1999).
This neurological phase reaches its peak during the surgical procedure and in the immediate postoperative period. Once the abdomen is closed and stimulation by intestinal manipulation, traction of the abdomen for laparotomy and distension for laparoscopy have been completed, activation of these pathways will cease. An inflammatory cascade secondary to the tissue damage and local inflammation generated by the surgical procedure will then begin, which explains the potentially prolonged nature of the POI.