Gastrointestinal system sympathetic/parasympathetic

Both the parasympathetic and sympathetic nervous systems provide extrinsic gastrointestinal innervation. Parasympathetic stimulation increases muscle contraction of the gut, while sympathetic stimulation inhibits contractions. Stimulation of either a- or 13-adrenoceptors will result in inhibition of contractions. The intramural nervous system consists of a myenteric (Auerbach s) plexus between the circular and longitudinal muscle areas and a submucosal (Meissner s) plexus between the muscularis mucosa and the circular muscle layers. These two plexuses contain stimulatory cholinergic neurons. 

The autonomic nervous system is divided into the sympathetic and parasympathetic components, which typically exert opposing effects. The sympathetic system is involved in the fight or flight reaction (increased blood pressure and heart rate, and accommodation for increased vision, for example) that prepares the organism for stressful situations. The parasympathetic system conversely establishes a more relaxed situation, for instance, the rest period after a meal. The autonomic nervous system that is responsible for the independent control of the mechanical and secretory functions of the gastrointestinal tract is sometimes called the enteric system. 

The enteric nervous system (see Chapter 6 Introduction to Autonomic Pharmacology) is composed of interconnected networks of ganglion cells and nerve fibers mainly located in the submucosa (submucosal plexus) and between the circular and longitudinal muscle layers (myenteric plexus). These networks give rise to nerve fibers that connect with the mucosa and deep muscle. Although extrinsic sympathetic and parasympathetic nerves project onto the submucosal and myenteric plexuses, the enteric nervous system can independently regulate gastrointestinal motility and secretion. Afferent fibers present in the mucosa and muscularis connect to cell bodies in the plexuses that mediate local reflexes. 

BZ is a competitive inhibitor of muscarinic receptors associated with the parasympathetic nervous system that innervate the eyes, heart, respiratory system, skin, gastrointestinal tract, and bladder. The sweat glands, innervated by the sympathetic nervous system, are also modulated by muscarinic receptors. By any route of exposure, the onset of action is approximately 1 h, with peak effects occurring 8h postexposure. Signs and symptoms gradually subside over 2-4 days. Most of the absorbed BZ is excreted via the kidney. 

The ANS is the major involuntary portion of the nervous system and is responsible for automatic, unconscious bodily functions, such as control of heart rate and blood pressure and both gastrointestinal and genitourinary functions. The ANS is divided into two major subcategories the parasympathetic autonomic nervous system (PANS) and the sympathetic autonomic nervous system (SANS). 

The acetylcholine is accumulated in the parasympathetic nerve endin that supply the smooth muscle action to the iris, ciliary body of the eye, tl bronchial tree, blood vessels, gastrointestinal tract, and urinary bladder. Tl secretory glands of the respiratory tract are similarly inhibited, as are the sympathetic nerve endings to the sweat glands. Voluntary muscles are paralyzed through accumulations of acetylcholine in the motor nerve endings. The central nervous system is likewise affected. 

A. Anatomic Aspects of the ANS The motor (efferent) portion of the ANS is the major pathway for information transmission from the central nervous system (CNS) to the involuntary effector tissues (smooth muscle, cardiac muscle, and exocrine glands Figure 6-1). The enteric nervous system (ENS) is a semiautonomous part of the ANS, with specific functions for the control of the gastrointestinal tract. The ENS consists of the myenteric plexus (plexus of Auerbach) and the submucous plexus (plexus of Meissner) and includes inputs from the parasympathetic and sympathetic nervous systems. 

The autonomic system is responsible for the moment-to-moment modification of practically all functions of the body. Although there are intrinsic plexi that promote normal activity, the sympathetic and parasympathetic systems enhance or retard function. Auerbach s and Meissner s plexi become activated by intraluminal distension and initiate peristalsis in an anterograde fashion. Increased parasympathetic activity, primarily from the vagus nerve in the cranial axis, increases gastrointestinal motility. The same can occur from microbial and chemical irritation of the inner walls. The effects are decreased transit time and decreased water absorption, among other effects. The person may also have other symptoms including discomfort, nausea, oating, and irritation. 

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