Structure of the Small Intestine

The small intestine is the main site of digestion and absorption within the gastrointestinal tract. It is a hollow tube greater than 6 m in length with a lume-nal diameter of approximately 4 cm. The first 20 cm distal from the pylorus is the duodenum, the next 2.5 m is the jejunum, and the final half is the ileum. There are no anatomically distinguishing characteristics along the small intestine; any alterations in architecture are gradual.

All segments of the small intestine possess a mucosa with the same sophisticated structural pattern along its length. The mucosal lining is surrounded by two muscle layers. The first innermost layer consists of circular smooth muscle sheets orientated radially around the lumen. The second thinner layer of longitudinal sheets is surrounded by a thin serosal layer. The regulation of muscular movement (motility) is achieved by the enteric nervous system, which consists of two matrices of interconnecting neurones. The first (outermost) matrix is the myenteric plexus situated between the two muscle layers. Underneath the mucosa and above the circular muscle layer is the submucosal plexus, from which extend sensory neurones into the mucosa. Interneurons connect the two plexi, which in turn receive postganglionic parasympathetic nerve fibers.

The enteric nervous system is a highly complex network exhibiting a high degree of autonomy over gut function and employing as many as 20 neuro-transmitters and neuromodulators as well as the classical neurotransmitter mechanisms of the auto-nomic nervous system. The enteric nervous system is only surpassed by the brain and spinal chord in its capacity for information processing.

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