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ABSTRACT
The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intestine. The IPANs that have been physiologically characterized are in the intrinsic myenteric ganglia. They are numerous, about 650/mm length of small intestine in the guinea pig, and communicate with each other through slow excitatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the muscle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and with ascending and descending interneurons. The circular muscle contracts as an annulus, about 2-3 mm in minimum oral-to-anal extent in the guinea pig small intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how they are programmed to generate the motility patterns that are observed. KEY WORDS: afferent neurons, neural integration, synaptic transmission, sensory transduction, enteric reflexes
INTRODUCTION
The patterns of motility of the gastrointestinal tract include mixing and propulsive movements, which are more or less confined to regions, and organized patterns of movement such as swallowing and esophageal peristalsis, migrating complexes, vomiting, and defecation, which involve large sections of the digestive tract. In general, mixing and propulsive movements in the small and large intestines are generated and controlled locally through a restricted part of the nervous system, the enteric nervous system. The enteric nervous system is nevertheless subject to modification of its activity by signals emanating from the central nervous system and from other gut regions. In this review, the mechanisms by which the enteric nervous system determines the motility patterns of the small intestine are discussed.
Considerable progress has been made in unravelling the circuitry of the enteric nervous system and identifying the physiological properties of its major components in the small intestine. It is still a long step to explain how the activity of the enteric nervous system is itself integrated and how...