The enteric nervous system in health and disease

Abstract

The enteric nervous system (ENS is a vast collection of at least 100 million neurons residing in the gastrointestinal tract. This is the largest collection of neurons outside of the spinal cord and brain. The cells are organized into 2 networks composed of: (1) clusters of nerves called ganglions and (2) interneurons connecting them. These plexuses lie in the submucosa (submucosal plexus) and between the circular and longitudinal layers of the gut (myenteric plexus) (Figure 1). They create a mesh of neurons that wraps the gastrointestinal tract in 2 layers. These neurons are responsible for coordinating the primary activities of the intestines: motility and secretion. The ENS, or "little brain" of the gut, is automatic and can control gastrointestinal functions on its own. In fact, the guinea pig small intestine, denervated from any outside nerve inputs by the vagus or sympathetic nerves, moves a pea with orderly peristalsis from one end of the bowel to the other. In addition to local ENS reflexes, loops of reflexes involve the major visceral ganglia (celiac and mesenteric) and the prevertebral ganglia. Stimulation of these pathways can stimulate or inhibit the intestine at the same or different areas. The parasympathetic nervous system sends inputs from the brain to the gut that generally promote motility and secretion and originate in the vagus and sacral plexus. The sympathetic nervous system carries primarily inhibitory signals to the gut via the thoracolumbar splanchnic nerves (Figure 2). These 2 systems modulate intestinal events based on brain inputs that tip the balance of power toward sympathetic influence in periods of "flight or fight," or toward parasympathetic influences in periods of feeding.