Q. Write Short Notes on Enteric Nervous System
The enteric nervous system, also called the intrinsic nervous system of the digestive tract, can be considered as the third division of the autonomic nervous system is located within the wall of the digestive tract, all the way from the esophagus to the anus.
Thus, unlike any other organ system, the digestive tract has its intramural (“within wall”) nervous system, which contains as many neurons as the spinal cord and endows the tract with a considerable degree of self-regulation.
It is comprised of two well-organized neural plexuses:
- The myenteric plexus is located between longitudinal and circular layers of muscle. It is involved in control of digestive tract motility.
- The submucosal plexus is located between the circular muscle and luminal mucosa. It senses the environment of the lumen and regulates gastrointestinal blood flow and secretion (epithelial cell function).
Indeed, the enteric nervous system controls all facets of digestive tract activity.
The enteric nervous system contains as many neurons as the spinal cord. It is sometimes referred to as the mini brain as it contains all the elements of a nervous system including sensory neurons, interneurons, and motor neurons.
It contains sensory neurons innervating receptors in the mucosa that respond to mechanical, thermal, osmotic and chemical stimuli. In other words, the sensory neurons possess receptors that respond to specific local stimuli in the digestive tract.
Motor neurons control motility, secretion, and absorption by acting on smooth muscle and secretory cells. Thus, motor neurons innervate the smooth muscle cells and exocrine and endocrine cells of the digestive tract to directly affect digestive tract motility, secretion of digestive juices, and secretion of gastrointestinal hormones. [As with the central nervous system, the input and output neurons of the enteric nervous system are lined by interneurons]. Some of the output neurons are excitatory, and some are inhibitory. For example, neurons that release acetylcholine as a neurotransmitter promote contraction of digestive tract smooth muscle, whereas the neurotransmitters nitric oxide and vasoactive intestinal peptide act in concert to cause its relaxation.
Interneurons integrate information from sensory neurons and feedback to the enteric motor neurons.
Parasympathetic and sympathetic nerves connect the central nervous system to the enteric nervous system or directly to the digestive tract. Although the enteric nervous system can function autonomously, normal digestive function often requires communication between the central nervous system and the enteric nervous system.
Finally, the enteric nervous system is primarily responsible for coordinating local activity within the digestive tract, To illustrate, if a large piece of food gets stuck in the esophagus, the enteric nervous system (myenteric nerve plexus in particular) coordinate local contractile responses to push the food forward.
It is important to note that the enteric nervous system can be influenced by the autonomic nervous system.