Myenteric plexus (Auerbach plexus)
Imagine if you had to consciously control your intestines each time you needed to digest food? Luckily, we don’t need to do that. Instead, we have our autonomic nervous system which takes care of all the involuntary processes necessary to keep our bodies functioning, and so is the case with the intestines. In order to function properly and continuously, the gastrointestinal (digestive) tract needs to have partial autonomic control over its functions for the sake of unhindered food processing, digestion and absorption. This is why the gastrointestinal tract has its own nervous system called the enteric nervous system. This system is embedded in the walls of the digestive tract and includes two plexuses (myenteric and submucosal) as well as their associated ganglia.
The myenteric plexus, also known as the Auerbach plexus, is a network of autonomic motor nerve fibers located in the muscular layers of digestive organs. Its main function is to supply motor innervation to the muscular coat (muscularis externa) of hollow organs of the alimentary canal in order to maintain movements of the gastrointestinal tract (peristalsis). Its function is partly controlled by the vagus nerve, which is a connection between the central and enteric nervous systems. The function of the myenteric plexus is mediated by numerous neurotransmitters and neuromodulators, mainly acetylcholine and nitric oxide.
This article will discuss the anatomy and function of the myenteric plexus.
|Location||Between two layers of muscle in muscular layers of digestive organs from the esophagus to the anal canal|
|Structure||Myenteric ganglia and motor nerve fibers|
|Function||Control of the motility and movement of the digestive tract|
The myenteric plexus (Auerbach plexus) is one of two principal components of the enteric nervous system. It is embedded in the walls of the digestive tract from the esophagus and all the way to the rectum. More specifically, it is sandwiched between the inner circular and outer longitudinal muscle layers of the muscular layer of hollow digestive organs.
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The histology of the Auerbach’s plexus is still not entirely known. The plexus comprises myenteric ganglia and the nerves emanating from them.
The myenteric neurons can be divided into activating (cholinergic) and inhibitory (nitrogenergic) neurons.
The activating neurons use acetylcholine to stimulate the smooth muscles of the hollow organs, while the inhibitory neurons use nitric oxide to inhibit muscular activity.
The principal function of the myenteric plexus is to produce the peristaltic activity of the intestine. The peristalsis is a series of wave-like muscle contractions that move the food through the digestive tract. The peristalsis is enabled by the reflex activity of the hollow organs known as the peristaltic reflex. The peristaltic reflex (myenteric reflex) is induced by mechanical stimuli or physical distention of the organ’s wall by food, but the reflex is limited and controlled by the myenteric plexus. The damage of this plexus results in disorders of peristaltic movement, which can be a life-threatening condition (e.g. achalasia, Hirschsprung disease, gastroparesis, etc).
The activity of the intestinal smooth muscle is mediated via neurotransmitters that can produce either relaxation or contraction of these smooth muscles. There are a number of neurotransmitters involved in the process of motor smooth muscle control. The main neurotransmitters that stimulate contraction (excitation) are acetylcholine and tachykinin. In contrast, the main neurotransmitter for muscle relaxation (inhibition) is nitric oxide (NO). Apart from NO, there are also vasoactive intestinal peptides (VIP), pituitary adenylate cyclase-activating peptides and purine.
The myenteric plexus can function autonomously, as it is mainly controlled locally by the interstitial cells of Cajal, which are the electrical pacemakers of the gut. These cells connect both with the smooth muscle of the alimentary canal and myenteric neurons and regulate and modulate their activity in order to achieve continuous and rhythmic peristalsis. However, even though being able to function autonomously, the myenteric plexus does receive the central parasympathetic inputs from the vagus nerve, allowing the physiological state of the body to influence the activity of the gut (e.g. in the “rest and digest” periods, the parasympathetic activity is increased).
Achalasia is a chronic, progressive disease that occurs due to the degeneration of ganglion cells of the myenteric plexus of the esophagus. The exact etiology of this disease is still under discussion. However, it is known that the inflammatory and degenerative processes target primarily inhibitory (nitrogenergic) neurons of the plexus. In contrast, the activating (cholinergic) neurons are largely spared. This predominance of activation over inhibition leads to the inability of esophageal muscles to relax, with increasing muscle tone particularly in the lower parts of the organ. The clinical presentation includes problems with swallowing (dysphagia), regurgitation of unprocessed food, heartburns, etc. The diagnosis is easily made by standard contrast X-ray, usually confirmed by esophageal manometry. Treatment of achalasia focuses on relaxing or stretching open the lower esophageal sphincter so that food and liquid can move more easily through your digestive tract. This can be achieved by non-surgical or surgical procedures.
Hirschsprung disease (also known as congenital megacolon) is a birth defect characterized by the absence of ganglionic cells in the myenteric plexus in a segment of the bowel. This results in loss of peristalsis and subsequently to the obstruction of bowels. Clinically, Hirschsprung disease is presented usually with abdominal distension, bilious emesis, and failure to pass meconium. If left untreated, this condition can lead to toxic megacolon, a life-threatening complication.
Myenteric plexus (Auerbach plexus): want to learn more about it?
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