The fourth and last layer is the highly-fibrous adventitia, which connects and binds the esophagus to structures in the immediate vicinity. Loose connective tissue, small vasculature, a lymphatic network, and nerve fibers compose this layer. The esophagus lacks a serosa, which is normally present in the rest of the GI tract and wraps around the entire tube.
The function of the esophagus is for the controlled downward transport of food and liquid substances further into the stomach and GI tract for continued digestion; and conversely, for the controlled upward elimination of toxic, upsetting, gaseous, or harmful substances back through the mouth. Phases of swallowing dictate the progression of substance into the stomach. The esophageal phase of swallowing is preceded by the oral and pharyngeal phases. The sequence may also be thought of as a process of preparation, transfer, and transport of the bolus. The oral phase begins after the chewing of food that mixes saliva for the beginning of the digestive process. Chewing reduces the substance to a safe size for swallowing and begins sensory information relaying to the brain stem. Towards the end of the oral phase, a chewed bolus of food is involuntarily pushed towards the pharynx via the muscular oscillations of the tongue. The involuntary motion of food may be overridden by voluntary control, such as when chewing gum. When the bolus reaches the pharynx, somatosensory receptors are activated and the involuntary swallowing reflex is initiated, leading to the pharyngeal phase.
The pharyngeal phase functions for propelling the bolus from the mouth through the pharynx and into the esophagus. A sophisticated series of reflex patterns coordinated by the swallowing center in the brainstem relies on afferent sensory neurons to get activate. The sequence ensures that the bolus continues onto the correct anatomical trajectory as seen in Fig.3 :
• the soft palate elevates to create a narrow passage for food to move into the pharynx and to eliminate the chance of reflux into the nasopharynx above
• the opening of the larynx is covered by the epiglottis moving downward and the larynx moving upward, which seals off the trachea
• the upper esophageal sphincter (UES) relaxes
• the peristaltic wave of contraction begins, propelling the food through the UES at a velocity of 40 cm a second
An important note: the UES also functions to prevent air from traveling into the esophagus instead of the trachea during breathing. The reflex is self-modulating according to the volume of the bolus and can affect either the time that the UES is open or the amplitude of the pharyngeal contractions.
The esophageal phase picks up with the bolus passing the open UES and the cervical esophagus relaxing to accept the bolus. The swallowing reflex now closes the UES and prevents reflux back into the pharynx. Primary peristaltic contraction, which is initially caused by the swallowing reflex during the pharyngeal phase, pushes the bolus in a wave-like sequence with constant high-pressure directly behind the bolus. Primary peristalsis is voluntarily initiated about once a minute, as we naturally swallow minor amounts of saliva throughout the noneating periods of the day. The reflex is programmed to follow a series of excitation signals after an initial inhibition signal to the myenteric plexus in descending fashion.
The first inhibitory signal halts all ongoing activity and primes the rest of the esophagus for peristalsis. The signal lengthens as it descends down the esophagus and establishes a latency gradient. Inhibitory neurons generate nitric oxide while excitatory neurons are cholinergic and rely on acetylcholine. When nitric oxide synthase is inhibited, it nullifies the latency gradient and produces muscle contraction. When the inner circular layer of the muscularis propria contracts, it decreases the diameter of the esophagus; and when the outer longitudinal layer contracts, it shortens that segment of the esophagus. The movement of the bolus is usually accelerated by gravity depending on standing or sitting, yet the power of contraction is strong enough to push the bolus into the stomach even if the esophagus is inverted and the body is upside down. The bolus travels at a velocity between three and four centimeters a second, taking about five to six seconds to travel the length of the esophagus. The enteric nervous system is in complete control of the smooth muscle tissue during peristalsis.
The lower esophageal sphincter (LES) is relaxed upon the initial primary peristaltic reflex. In this case, relaxed does not mean open. It is normally under a constant tonic muscle contraction, keeping the distal esophagus environment separate from the fundus of the stomach. It is normally about two to four centimeters in length at the gastroesophageal junction. Vasoactive intestinal polypeptide (VIP) and nitric oxide are the neurotransmitters released by peptidergic fibers of the vagus nerve that inhibit and relax the smooth muscle of the LES, which can last between six to ten seconds. Normally, the LES is held in resting pressure by a combination of myogenic and neurogenic inputs. The pressure of the descending bolus causes the LES to open to an appropriate diameter and allow passage. The LES then closes immediately and returns to its normal tonic contractive state. If the esophagus still contains food and is distended after the primary peristalsis, then a secondary peristalsis event will ensue at the point of distention to clear out the remaining substance. This secondary contraction is monitored for and initiated by the enteric nervous system. Another factor that causes the LES to open is during gastric distension, leading to transient LES relaxation (TLESR), such as when belching to release trapped gases. Despite a highly regulated reflex system, the esophagus and the LES can be prone to detrimental dysfunction that can have serious effects on the health of the individual.
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