The Normal Enteric Nervous System

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The mature ENS is absolutely unique and different from any other region of the peripheral nervous system (PNS). First, the ENS is independent and can function in the absence of input from the brain or spinal cord [3, 4]. Second, in contrast to the remainder of the PNS, the ENS can mediate reflexes, even when it is isolated from the central nervous system (CNS). This ability of the ENS is often overlooked, even though it has long been known to be true. As the 19th Century turned to the 20th, Bayl-iss and Starling reported that enteric reflexes could be mediated by "the local nervous mechanism" of the gut [14, 15]. These investigators described what they called the "law of the intestine" (now known as the peristaltic reflex) in extrinsically denervated loops of dog intestine. This is a reflex, evoked by increased intraluminal pressure, that consists of a wave of oral excitation and anal relaxation that descends in the bowel and is propulsive.

Essentially the same reflex can also be elicited in vitro in preparations of guinea pig intestine [16]. The fact that reflex activity can be manifested by segments of gut in vitro, which have clearly lost all connection to dorsal root or cranial nerve ganglia, the brain and the spinal cord, indicates that every neural element of the peristaltic reflex arc (sensory receptors, primary interneurons, motor neurons, and effectors) must be intrinsic components of the wall of the gut.

These observations were taken into account by Lang-ley in his seminal work on the autonomic nervous system [17]. Together with Langley's idea that most enteric neurons receive no direct input from the CNS, the independence of the ENS caused Langley to classify the ENS as a third component of the autonomic nervous system. The sympathetic division was defined as that with a thoracic and lumbar outflow of preganglionic axons from the CNS, while the parasympathetic was the division with a cranial and sacral outflow. The ENS, which mainly lacks either outflow had to be classified as a separate division, since it met the criteria of neither of the other two. Anatomical observations have more recently confirmed the distinct nature of the enteric innervation. The internal ultrastructure of the ENS is more similar to that of the CNS than to any other region of the PNS [3, 18-21]. The ENS lacks internal collagen and its neurons receive support from enteric glia, which resemble astrocytes, and not from Schwann cells. Phenotypic diversity of peripheral neurons peaks in the ENS, and every class of neurotransmit-ter known to be present in the CNS is also represented in the ENS [3, 4]. Intrinsic neuronal reflexes evoke secretion as well as motility [22]; furthermore, most enteric neurons not only lack connection to the CNS, but some actually project centripetally, beyond the confines of the gut, to innervate extra-enteric targets. These outside-the-bowel projections of enteric neurons make it possible for the ENS to affect directly the function of prevertebral sympathetic ganglia [23-25], the gallbladder [26], and the endocrine and exocrine pancreas [27, 28].

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