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.