Deglutition: Swallowing Process PDF

Section 4: Deglutition I. Deglutition (Swallowing) Oropharynx = Common opening to trachea and esophagus Must swallow wiothout Highly complex reflex: Requires 6 cranial nerves. aspirating food 5,7,9,10,11,12 Three phases of deglutition: A. Oral phase...

Section 4: Deglutition I. Deglutition (Swallowing) Oropharynx = Common opening to trachea and esophagus Must swallow wiothout Highly complex reflex: Requires 6 cranial nerves. aspirating food 5,7,9,10,11,12 Three phases of deglutition: A. Oral phase (Voluntary) chewing, then: 1. Food bolus is formed 2. Positioned on back of tongue 3. The rostral portion of tongue is pressed against hard palate, jaw closes and bolus is forced into the oropharynx. cant swallow without touching tongue roof mouth B. Pharyngeal phase (Involuntary) 1. Function is to move the bolus from the oropharynx into the esophagus. 2. Stimulus: Bolus pushed back by tongue into the oropharynx. 3. Afferent impulses from pressure receptors in oropharynx go to the Swallowing Center in medulla/pons that coordinates response. 4. Efferent actions: 3 actions if pharyngeal phase a. Soft palate: Constrictor muscles close off nasopharynx -prevents reflux of food into the nasopharynx b. Airway opening is closed and respiration inhibited to prevent entry of food bolus into the trachea: 1) Contraction of pharyngeal and laryngeal mm. Larynx raised Glottis closes to close off the glottis and prevent aspiration of food Epiglottis covers glottis 2) Respiratory center (CNS) is inhibited momentarily (~1 sec). occurs centrally (CNS) c. Upper esophageal sphincter (UES) relaxes briefly and contraction of pharyngeal muscles propels bolus through UES: Breathing resumes Bolus entry into upper esophagus initiates the involutary esophageal phase espophageal peristalisis next stage 26 Supplemental Figure 10 afferent signal comes from pharyngeal phase forcing bolus through the junction of the prpharynx and UES CNS swallow center Reprinted from The Physiology and Pathophysiology of Digestion, Morgan & Claypool., 2018 27  C. Esophageal phase (Involuntary) 1. Primary esophageal peristalsis: Afferent impulse originates from the junction between oropharynx and UES Reflex is mediated through Swallowing Center Efferent action: Vagal outflow!! a. Traveling ring of muscle contraction behind bolus and muscle relaxation in front of bolus. peristalsis b. Relaxation of lower esophageal sphinter (LES) after bolus enters body of esophagus. The LES remains open until ring of contraction passes. c. LES closes completely to prevent gastric reflux. the esophagus at and aboral to blous is also relaxed after contraction ring passes Fig. 16. LES before swallow Primary Esophageal Peristalsis: The primary peristaltic wave in the esophagus is a continuation of the Peristaltic wave originating in pharynx. First, there is relaxation of the esophagus ahead of the bolus, most easily demonstrated in the lower esophageal sphincter (LES). The peristaltic wave passess from the pharynx into the striated muscle portion of the esophagus and then through the smooth muscle portion without any hesitation due to the centrally mediated vagal neural outflow. LES relaxation is maintained until the peristaltic wave enters the sphincteric segment, after which the sphincter pressure returns to its previous resting tone. Modified from Mittal. Am. J. Physiol.doi.org/10.1152/ajpgi.00182.2016 2. Secondary esophageal peristalsis: Action: If bolus not cleared with primary wave, or if gastric content or gas is refluxed, then distension of esophagus initiates a secondary esophageal wave. 28  Afferent impulse originates from esophageal body (not UES) at the bolus Reflex is mediated through Swallowing Center Efferent action: Same as primary esophageal wave initiated at bolus Can have multiple secondary esophageal waves summary: Fig. 17. Secondary esophageal peristalsis has a similar sequence as primary esophageal peristalsis EXCEPT the afferent impulse arises from distension of the esophagus. Typically, this is caused by an ineffectual primary peristatltic wave or by refluxed gas or gastric content. Secondary esophageal peristaltic waves continue until the bolus passes. 3. “Tertiary esophageal peristalsis” -Smooth muscle esophagus shows evidence of reflex contractions mediated locally via ganglia of sm. muscle esophageal enteric nervous system striated (skeletal) muscle esohagus orally in domestic animals short piece of smooth muscle esophasgus at aboral end in dogs and cats stretch of smooth muscle esophagus with blous will initiate basic peristalsis -Placement of orogastric feeding tube proximal to LES Clinical: tube feeding incapacitated dog or cats best if you can place feeding tube oral to LES: distends smooth muscle esophagus (3 wave) LES closes properly to prevent reflux 29 Dog, cat, pig: - oral > 90% striated muscle - aboral < 10% smooth muscle Dominance of striated muscle allows eating with head down - ie against gravity II. Esophageal musclature and neural control A. Species differences in musclature. Two types of esophageal musculature exist: striated and smooth muscle. Distribution of Fig. 18. these types varies with the species. avian: - esophagus all smooth muscle - gravity affectws swallowing Primates, equine, ruminant - oral 2/3 striated muscles - aboral 1/3 smooth muscle (ruminants = esophageal groove) Peristaltic movement is highly coordinated between 2 muscle types swallowing center acts via the vagus nerve B. Neural control of esophageal peristalsis motility Extrinsic innervation (vagi) dominates control of esophageal peristalsis. However, vagal innervation differs between the two muscle types: 1. Striated muscle esophagus: Fig. 19. Swallow center Special Visceral Efferent (SVE) nerve (vagal motor n.). Striated muscle PS nicotinic transmission to striated muscle -Coordinated serial contraction fine motor nerve for each striated myocyte which are arranged in 'herringbone' pattern -Vagotomy or striated muscle disease amotile esophagus Myasthenia gravis : autoimunity to motor end plate VIP, NO, ATP > difficulty swallowing 30  2. Smooth muscle esophagus/LES: Swallow center Vagal PS n. Intrinsic plexi Effector neurons Smooth muscle effect neurons driven do induce peristalsis in smooth muscle esophagus using inhibitory and cholineric muscarinic nerves (like basic peristalsis) -Coordination of ENS for serial contraction coordinates seamlessly with wave of striated esophagus -Vagotomy if transect esophagus without damaging vagi, esophageal peristalsis wave continues past cut esophagus C. Lower esophageal sphincter: 1. Main function is to prevent gastric reflux into esophagus but relaxes during swallowing reflex. controlled by swallowing center a. Relaxation during swallow: Inhibitory effectors (relax clasp fibers) - ON relaxes circular muscle Reduced cholinergic tone (relax sling fibers) – OFF relaxed longitudinal muscle b. Sustained contraction, at rest: Circular smooth muscle (clasp fibers) – Inhibitory OFF contracts Longitudinal smooth muscle (sling fibers) – Cholinergic ON contracts c. Increased tone after meal: Plasma gastrin adds to contraction During interdigestive period: MMC (migrating motor complexes) activity 31 2. Anatomical contributions to LES a. Flutter valve of intraabdominal esophagus intra-abdominal esophagus collapses due to higher ressure in abdomen than thorax b. Diaphragm pinching diaphragm narrows esophagus as it passes through c. Esophagus enters stomach at an angle no direct pressure toward cardia d. Redundancy of gastric mucosa (horse) stratified squamous cardia excessive overlaps cardia opening with back pressure Failure of vomition in horses: 1. more acute esophageal angle into stomach 2. redundant cardia mucosa 3. high LES tone with ore smooth muscle esophagus achalasia = failure of LES to open due to damaged afferent nerves from GERD (gastric acid damage) Megaesophagus 32 Objectives: 1. Know the three phases of deglutition: a. Voluntary and involuntary events b. Purpose for each sequence of muscle contractions c. Neural pathways involved in the swallowing reflex. 2. Discuss the functions of the upper and lower esophageal sphincters. 3. Explain the difference between primary, secondary and tertiary esophageal peristalsis. 4. Describe the differences in esophageal musclature and the innervation of each type. 5. Explain how gastric reflux is prevented at the lower esophageal sphincter. Related Questions: 1. If you transect the striated muscle esophagus without injuring the vagus nerves, what will happen to a primary esophageal wave? 2. What skeletal muscle disease can cause megaesophagus? 33