Lec 4 23 April 20 17
Dr.baybeen AlselevanyUpper Gastrointestinal Tract (mouth, pharynx, esophagus and stomach)
Mouth, Pharynx &EsophagusObjectives:
Ingestion: mechanics of ingestion
a.mastication (chewing)).
b.swallowing (deglutition).
Saliva and Salivary glands
The mouth or oral cavity is a cavity like structure that houses the teeth, tongue, hard palate and soft palate. Its the site of ingestion of food, beginning of mechanical (mastication) and chemical (action of saliva) digestive processes. The mouth chews and lubricates food, starts its digestion .In the mouth, food is mixed with saliva and propelled into the esophagus.
Ingestion of food: intake, in this case in the form of food and fluids. Mechanics of food ingestion: a- Mastication (chewing). b- Swallowing (deglutition).
Mastication (Chewing)
Chewing mixes the ingested food with salivary enzymes and lubricates it to facilitate swallowing. The teeth are admirably designed for chewing, the anterior teeth (incisors) providing a strong cutting action and the posterior teeth (molars), a grinding action. Chewing (mastication) breaks up large food particles by teeth and mixes the food with the secretions of the salivary glands. Large food particles can be digested, but they cause strong and often painful contractions of the esophageal musculature. Chewing is important for digestion of all foods especially fruits and raw vegetables because these have indigestible cellulose membrane around their nutrient portions that must be broken before the food can be digested.
Chewing is a voluntary process .Most of the muscles of chewing is innerved by the somatic motor branch of the fifth cranial nerve to the skeletal muscles of the mouth and jaw. Chewing process is controlled by nuclei in the brain stem. Much of the chewing process is caused by a chewing reflex, which may be explained as follows: The presence of a bolus of food in the mouth at first initiates reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. The drop in turn initiates a stretch reflex of the jaw muscles that leads to rebound contraction. This automatically raises the jaw to cause closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time; this is repeated again and again .
Saliva and Salivary Glands
Saliva is released into the oral cavity by three pairs of glands called:.
1.Parotid glands: are the largest of the major salivary glands which lies below the ear. These glands secrete a clear watery serous fluid rich in α- amylase are innervated by glossopharyngeal cranial nerve (IX). 2. Submandibular glands are located in the floor of the mouth on the inside surface of the lower jaw, are innervated by Facial cranial nerve (VII). 3. Sublingual glands: are the smallest of the major salivary glands. There are found on the floor of the mouth under the tongue, are innervated by Facial cranial nerve (VII) supply.
Composition of Saliva
Daily secretion of saliva normally ranges between 800 1500 ml .
Saliva has a PH between 6 7 which help to neutralize gastric acid
Saliva is composed primarily of water, digestive enzymes such as salivary α-amylase (for partial digesting starches) and lingual lipase.
Ionic composition of saliva. Under normal conditions saliva contains large quantities of potassium and HCO3- greater than plasma and small concentrations of both Na+ and chloride several times less than in plasma.
Functions of Saliva
Protection: saliva dilutes buffers, cleanses and helps prevent dental caries.lysozyme and IgA are antimicrobial.2.Taste: water moistens and dissolve food molecule. 3. Lubrication: mucus eases passage of food.4. Digestion: alpha amylase for partial digesting starches.. 5.Aid speech by facilitating movements of the lips and tongue. 6. Facilitate swallowing.
Control of Salivary Secretion
Salivary glands are controlled mainly by parasympathetic nervous signals from the superior and inferior salivary nuclei in the brain stem. Salivary nuclei are located approximately at the juncture of medulla and pons. Stimulation of the parasympathetic nerve supply causes profuse secretion of watery saliva. Atropine and other cholinergic blocking agents reduce salivary secretion.
1. Taste and tactile stimuli from tongue and other areas of mouth and pharynx. Many taste stimuli especially sore taste such as acidic fruit juices which are the strong stimulus, elicit copious secretion of saliva .this reflex response is initiated by chemoreceptors and pressure receptors in the wall of the mouth and on the tongue. Also certain tactile stimuli such as the presence of smooth objects in the mouth (e.g. pebble ) cause marked salivation , whereas rough objects cause less salivation .
2. Food in the mouth causes reflex secretion of saliva, and so does stimulation of the vagal afferent fibers at the gastric end of the esophagus. 3. Salivation can be stimulated or inhibited by nervous signals arriving in the salivary nuclei from higher centers of the CNS. for instance , when a person smell or eats favorite foods salivation is greater than when disliked food is smelled or eaten . Also the sight and even thought of food causes salivary secretion (makes the mouth water).Fear, sleep, fatigue and dehydration all inhibit salivation.
4. Salivation also occurs in response to reflexes originating in the stomach and upper small intestinesparticularly when irritating foods are swallowed or when a person is nauseated because of some gastrointestinal abnormality.
Sympathetic stimulation can also increases salivation to less extent than does parasympathetic stimulation.
Swallowing (Deglutition)
The pharynx subserves respiration as well as swallowing. Pharynx is a passageway for food moving from the posterior oral cavity to the esophagus .The pharynx and esophagus do not digest food, but both are important passageways and their muscles walls function in swallowing. The pharyngeal muscles are skeletal muscles which are under voluntary control. Pharynx is divided into 3 parte: nasopharynx is a part of respiratory system and has no role in food passage; oropharynx and laryngopharynx are both common passageways for air and food.The pharynx is converted for only a few seconds at a time into a tract for propulsion of food. It is especially important that respiration not be compromised because of swallowing. In general, swallowing can be divided into (1) a voluntary stage, which initiates the swallowing process; (2) a pharyngeal stage, which is involuntary and constitutes passage of food through the pharynx into the esophagus; and (3) an esophageal stage, another involuntary phase that transports food from the pharynx to the stomach
Stages of swallowing: 1. Voluntary oral stage. 2. Involuntary pharyngeal stage. 3. Involuntary esophageal stage.
1. Voluntary oral stage: swallowing is initiated by voluntary action of collecting the bolus on the tongue and propelling them backward into the pharynx. Swallowing becomes entirely automatic and ordinarily cannot be stopped and food enters the upper pharynx.
2. Involuntary Pharyngeal stage: occurs involuntarily and the food pass from the pharynx to esophagus. This stage begins as the bolus of food reaches the pharynx; it stimulates sensory receptors (swallowing receptors) around the pharyngeal opening especially on the tonsillar pillars. 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. The Impulses are transmitted from these areas through sensory cranial nerves trigeminal (5th) and glossopharyngeal nerves (9th) to swallowing center into medulla oblongata 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 Trigeminal, glossopharyngeal, vagal and hypoglossal cranial nerves and initiate a series of automatic pharyngeal muscle contractions as follows:
a. the soft palate raises upward preventing reflex of food into nasal cavity
b.the swallowing center during pharyngeal inhibits the respiratory center in the medulla.
c. there is a closure of glottis (around the vocal cords) keeping the food from moving into the trachea . Destruction of vocal cords or of the muscles that approximate them can cause strangulation. Even while a person is talking, swallowing interrupts respiration for such short time that it is hardly noticeable.
d. opening of esophagus relaxation of upper esophagus sphincter (UES) and a fast peristaltic wave (primary peristalsis) initiated by the nervous system of the pharynx forces the bolus of food to upper esophagus.. Between swallowing the UES remains strongly contracted thereby preventing air from going into the esophagus during respiration.
The musculature of the pharyngeal wall and upper third of the esophagus is striated muscle. Therefore, the peristaltic waves in these regions are controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves.
2. Involuntary esophageal stage: the esophagus functions to conduct food rapidly from the pharynx to the stomach. Skeletal muscles surrounding the esophagus just behind pharynx forming upper esophageal sphincter (UES), which is relax reflexly upon swallowing, whereas the smooth muscles in the last portion of the esophagus form the lower esophageal sphincter (LES) or gastroesophageal sphincter..
The esophageal phase of swallowing begins with the relaxation of the UES which is closed immediately after the food has passed, the glottis opens and breathing resumes. The esophagus normally exhibit two types of peristaltic movements:
1. Primary Peristalsis: is continuation 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 to 10 seconds. When humans are in an upright position, liquids and semisolid foods generally fall by gravity to the lower esophagus e more rapidly than the peristaltic wave itself, in about 5 to 8 seconds.
2. Secondary peristaltic wave. Secondary peristaltic waves result from the distension of esophagus itself by the retained food; these waves continue until all the food has emptied into the stomach. The 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 efferent nerve fibers.. In the lower two thirds of the esophagus, the musculature is smooth muscle, but this portion of the esophagus is also strongly controlled by the vagus nerves acting through connections with the esophageal myenteric nervous system. 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 ,this sphincter normally remains tonically constricted except during swallowing which prevents reflux of gastric contents into the esophagus.
Esophageal Secretion: mucous glands are scattered throughout submucous of the esophagus. Their secretion moistens 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 by acidic gastric juices that reflux from stomach into the lower esophagus .Despite this protection , a peptic ulcer at times can still occur at the gastric end of the esophagus.
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.
Home Work
Q1: When you chew bread for a few minutes, it tastes sweet .explain how this happens?
Answer: salivary amylase digests the starch in the bread, breaking the starch into maltose and isomaltose.These sugars are responsible for the sweet taste.
Q2: You and your anatomy and physiology instructors are lost in the desert without water. Your instructor suggests that you place "a pebble" in your mouth. What would you do and why?
Answer: tactile stimulation in the mouth increases saliva production, making the mouth feel less dry.
Q3.what happens if you try to swallow and speak at the same time?
Answer: usually if a person takes to swallow and speak at the same time the epiglottis is elevated, the laryngeal muscles closing the opening to the larynx are mostly relaxed, and food or liquid could enter the larynx causing the person to choke.