Question

how does the autonomic nervous system control digestion? where is the enteric nervous system?

Answer 1

Autonomic system is the extrinsic system of nerves supplying the gut. It consists of parasympathetic and sympathetic innervation.

Parasympathetic innervation: it is made up of cranial and secral divisions:-

Cranial parasympathetic fibres originate in medula, come through Vagus and supply the esophagus, stomach, small intestine, pancreas and first half of large intestine. They also make synaptic connections with intramural plexus.

Sacral parasympathetic fibres originate in sacral spinal cord, pass through pelvic nerves to hypogastric ganglion as a post ganglionic fibre, supply the distal half of large intestine and rectum. The sigmoid, rectal and anal regions have an especially rich supply of parasympathetic fibres which help in defecation reflex. Functions:-

• Excitation of holding musculature of gut except the sphincters.
• Increase in gastrointestinal motility and secretory activity.

Sympathetic innervation:- the sympathetic fibres to gut arise from T8 to L2. The sympathetic innervate all portions of GITrather than being more extensively supplied to portions near the oral cavity and anus. Functions:-

• Vasoconstriction
• Excitation of ileocaecal and internal anal sphincters, and smooth muscles of muscularis mucosae.
• Inhibition of motility.

Enteric Nervous System consists of two plexus-

• Myenteric plexus (Auerbach's plexus). It ispresent between circular and longitudinal muscle fibres of muscular coat of GIT. It causes increase in tone of gut wall, intensity of rhythmical contractions of gut wall, rate of contraction and velocity of contraction.
• Meissner's plexus (Submucosal plexus). It is present in the submucosal layer. It controls the secretory activity and blood flow to the gut. It is mainly concerned with controlling function within the inner wall of each minute segment of the intestine. It helps to control local intestinal secretion, local absorption and local contraction of the submucosal muscle.