DR. Rajaa Ahmad
pharynxTime food spends in pharynx is about 1—2 seconds. The pharynx is a cavity behind the mouth from which the tubular esophagus leads to the stomach . The pharynx connects the nasal and oral cavities with the larynx and esophagus . It can be divided into three parts : a. nasopharynx b. oropharynx c. laryngeopharynx Normally , only the oropharynx and laryngophrynx transmit food. The pharynx and esophagus do not digest food .The pharyngeal muscles are skeletal muscles , they are under voluntary control ,only that swallowing can be voluntary initiated.
Swallowing ( Deglutition)Swallowing is a complex reflex initiated when pressure receptors in the walls of the pharynx are stimulated by a food or drink forced into the rear of the mouth by the tongue . These receptors send afferent impulses to the swallowing center in the brain stem medulla oblongata . This center then elicit s swallowing via efferent fibers to the muscles in the pharynx and esophagus as well as to the respiratory muscles .
In general , swallowing reflexes can be divided into three stages :
1. voluntary stage . 2. pharyngeal stage.
3. esophageal stage.
Voluntary stage :swallowing is initiated by voluntary action of collecting the oral contents( bolus) on the tongue and propelling them backward into the pharynx.
Pharyngeal stage : this stage begins as the bolus of food reaches the pharynx and stimulates sensory receptors ( swallowing receptors ) around the pharyngeal opening especially on the tonsillar pillars and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscles contraction as follows :
a. the soft palate raises preventing reflex of food into nasal cavity .b. impulses from swallowing center inhibit respiration raise the larynx and is close the glottis (around the vocal cords ) keeping the food from moving into the trachea .
As the tongue forces the food further back into the trachea , the epiglottis backward to cover the closed glottis . destruction of vocal cords or of the muscles that approximate them can cause strangulation
c. once the larynx is raised the upper esophageal sphincter (UES) also called pharyngoesophageal sphincter relaxes thus allowing food to move easily and freely into the upper esophagus . Between swallowing the UES remains strongly contracted thereby preventing air from going into the esophagus during respiration .
The entire pharyngeal stage of swallowing occur in less than six seconds .
pharyngeal stage of swallowing is principally a reflex action . It is initiated by voluntary movement of the food into back of the mouth (voluntary stage ) which in turn elicits involuntary pharyngeal sensory receptors to elicit the swallowing reflex .
Nervous initiation of the pharyngeal stage :The most sensitive tactile areas of the posterior mouth and pharynx for initiating the pharyngeal stage of the swallowing lie in a ring Around the pharyngeal opening with greatest sensitivity on the tonsillar pillars . Impulses are transmitted from these areas through sensory trigeminal and glossopharyngeal nerves to swallowing center in the medulla oblongata either into or closely associated with the tract solitarsus which receives essentially all sensory impulses from the mouth . the motor impulses from swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted by 5th (trigeminal ) , 9th (glossopharyngeal) 10th ( vagus) and 12th ( hypoglossal) cranial nerves and few of superior cervical nerves
Swallowing is difficult when the mouth is open as anyone who has spent time in the dentist's chair feeling saliva collect in the throat is well aware.
A normal adult swallows frequently while eating , but swallowing also continues between meals . The total number of swallowing per day is 600, 200 while eating and drinking , 350 while awake without food and 50 while sleeping .
The swallowing center specifically inhibits the respiratory center of the medulla during this time. Yet, even while a person is talking, swallowing interrupts respiration for such a short time that it is hardly noticeable.
3. Esophageal Stage : During this stage peristalsis transport the food from esophagus into the stomach . Skeletal muscles surround the esophagus just below the pharynx forming upper esophageal sphincter ( UES ) , whereas the stomach muscles in the last portion of the esophagus form the lower esophageal sphincter ( LES ) . The esophagus normally exhibit two types of peristaltic movements :
a. primary peristalsis
b. secondary peristalsis
Primary Peristalsis :is contributed of the peristalsis wave that begins in pharynx and spread into the esophagus during the pharyngeal stage of swallowing . This wave passes all the way from the pharynx to the stomach in about 8-10 seconds .
Food swallowed by a person in an upright position is transmitted to the lower end of esophagus more rapidly than peristaltic wave itself, in about 5-8 seconds because of the effect of gravity pulling the food downward .
Swallowing can occur even while a person in upside down since it is not gravity but the peristaltic wave that moves the food to the stomach .
The esophageal phase of swallowing begins with the relaxation of the upper esophageal sphincter ( UES ) . Immediately after the food has passed , the sphincter closes , the glottis opens and breathing resumes.
Secondary peristaltic wave which move all the food that has entered the esophagus into the stomach . These waves results from distension of the esophagus itself by food , these waves continue until all the food has emptied into the stomach . Secondary waves are initiated by myenteric nervous system and partly by vagal afferent to the medulla and back again to esophagus through glossopharyngeal and vagal nerve fibers .
When the vagus nerves to the esophagus are cut , the myenteric nerve plexus of the esophagus becomes excitable after several days to cause strong secondary peristaltic waves . Therefore even after paralysis of the brain stem swallowing reflex ,food fed by the tube or in some other way into the esophagus still passes readily into the stomach .
When the esophageal peristaltic wave approaches towards stomach a wave of relaxation called "receptive relaxation" transmitted through the myenteric inhibitory neuron which release VIP precedes the peristalsis . Furthermore the entire stomach , duodenum become relaxed as these waves reach the lower esophageal sphincter ( LES ) which allows easy propulsive of the swallowing food into the stomach .
EsophagusIt is a straight , collapsible tube about 25cm long . It provides a passageway for food from the pharynx to the stomach .
skeletal muscles surrounding the esophagus just behind pharynx forming upper esophageal sphincter ( UES ), whereas the smooth muscles in the last portion of the esophagus form the lower esophageal sphincter ( LES ) or gastro esophageal sphincter or cardiac sphincter.
Pharyngoesophageal sphincter or UES which is relax reflexly upon swallowing . also Gastroesopharngeal sphincter or LES is tonically active but relaxes upon swallowing . The tonic activity of the LES between meals prevents reflux of the gastric contents into the esophagus
Mucous glands are scattered throughout submucous of the esophagus . their secretions moisten and lubricates the inner lining of the tube . In the upper esophagus mucus prevents mucosal excoriation by newly entering food , whereas at the LES mucus protect the esophageal wall from digestion of acidic gastric juices that reflex from stomach into the lower esophagus .
Despite this protection , a peptic ulcer at times can still occur at the gastric end of the esophagus .