Chewing & Swallowing (2018 Version) PDF

CHEWING & SWALLOWING  CHEWING & SWALLOWING Swallowing is primarily a motility function Chewing, Movement of ingested material through the pharynx, oesophagus Relaxation of stomach to receive the ingested material  OBJECTIVES ❖ List the functions of “mastication” ❖ Describe...

CHEWING & SWALLOWING  CHEWING & SWALLOWING Swallowing is primarily a motility function Chewing, Movement of ingested material through the pharynx, oesophagus Relaxation of stomach to receive the ingested material  OBJECTIVES ❖ List the functions of “mastication” ❖ Describe the reflex action of chewing ❖ Define deglutition ❖ Describe the stages of swallowing ❖ Name the regulatory pathways involved in the swallowing reflex ❖ Classify causes of dysphagia  Mastication or CHEWING Food is mechanically broken up prior to being swallowed Reflex action accomplished by the action of the teeth (incisors cut, molars grind) and jaw muscles (involves higher centres of the brain) Edentulous patients restricted to soft diet  Functions of Mastication Aids the digestive process Mixes food with saliva and lubricates it Allows digestive enzymes to act on larger surface areas of food Helps to prevent excoriation or damage to the gut wall Increases the ease of emptying & movement of food through the gut  Regulation of Chewing Control centre includes nuclei in: Brain stem Cerebral cortex Hypothalamus Amygdala  Regulation of Chewing Motor branch of the 5th cranial nerve (trigeminal nerve) innervates the muscles of chewing  Chewing Reflex 1. Initiated by presence of food in the mouth 2. ⇒ Reflex inhibition of the muscles of mastication 3. Lower jaw drops 4. ⇒ Stretch reflex of the jaw muscles 5. Rebound contraction to raise the lower jaw 6. ⇒ Closure of the teeth 7. Food is compressed against the linings of the mouth 8. ⇒ Reflex inhibition of the muscles of mastication  Deglutition or Swallowing Reflex action which allows the delivery of air, liquids and solids from the mouth to the stomach  Regulatory Pathways Swallowing Centre Brain Stem Reticular formation n. ambiguus & tractus solitarius,, Dorsal motor nucleus of Vagus Afferents Afferents from the Pharynx & nuclei of Trigeminal nerve Efferents (NV), NVII & Hypoglossal nerves NV, NVII, NX Tongue Hypoglossal n. Pharynx, Oesopahgus Nerves of the Orad stomach N. Ambiguus Effectors  Vagal Integrative Centre for Nervous Control of the Upper GI Tract Dorsal motor n. of Vagus + nuclei of ambiguus + n. tractus solitarius + area postrema form: Dorsal Vagal Complex Dorsal vagal complex resides in medulla oblongata Central vagal integrative control of oesophagus, stomach, duodenum, pancreas, gall-bladder Adapted from Rhoades & Bell, 2009 Mylohyoideus Temporalis Masseter Pteryhyoideus lat. & med. Tensor palatini Digastricus Stylohyoideus Facial muscles Digastricus post. Cricothyroideus Cricoarytenoideus Interarytenoideus Palatopharyngeus Palatoglossus Levator palatini Salpingopharyngeus Constrictors (sup.med., inf.) Esophagus Geniohyoideus Thyrohyoideus Post intrinsic tongue muscles Hypoglossus Genioglossus Styloglossus  Cineradiographic Frames of Swallowing in Man (Davenport, 1977)  Mechanism of Swallowing Voluntary Stage Involuntary Stage ✔ Occurs in the mouth 2 phases: ✔ Food is squeezed & rolled 1. Pharyngeal phase posteriorly by tongue (< 1 sec) ✔ Tongue presses the bolus upward & backward against hard palate, 2. Oesophageal phase towards the soft palate, (8-10 sec) into the oropharynx ✔ Time is indeterminate  A. Buccal Stage B. Beginning of the Pharyngeal phase C. End of Pharyngeal phase D. Beginning of the Oesophageal phase  Pharyngeal Phase Pharynx is the gateway to the respiratory system Closure of the Nasopharynx Respiration inhibited simultaneously ~3/10s  Closure of the nasopharnyx: ❖ contraction of the tensor palatini & levator palatini ⇒ elevate the soft palate ❖ Breathing stops for ~3/10 sec Contraction of Superior constrictor muscle forms the Ridge of Passavant ⇒ tight seal ⇒ initiates the pharyngeal peristaltic wave ⇒ propels the bolus through the pharynx Approximation of Palatopharyngeal folds on either side of the pharynx to form a saggital slit ⇒ allows properly masticated food to pass to the posterior pharynx  Closure & displacement of the Larynx to prevent food getting into the trachea ⇒ Hyoid bone is raised upward and forward, ⇒ due to contraction of the geniohyoid & mylohyoid muscles ⇒ raises & moves the larynx forward, ⇒ brings it under the base of the tongue Vocal cords are also approximated Epiglottis swings backward over the superior opening of the larynx, forming a ridge ⇒ prevents food getting to the vocal cords to cause strangulation  Oesophageal Phase Serves to leisurely conduct the bolus from the pharynx to the stomach Oesophagus is closed at its upper end by the UOS (thickening of circular muscle layer) Cricopharyngeal or Pharyngoesophageal sphincter (striated)  Oesophageal Phase Central and peripheral mechanisms coordinate the contractions Provide sequential impulses distally that result in peristalsis Body is innervated primarily by Vagus Contains somatic motor fibres from n. ambiguus – innervate striated muscle Contains autonomic nerve fibres that synapse with the myenteric plexus  Opening of the UOS is achieved by : ✔ Relaxation of the cricopharyngeus ✔ Pharyngeal peristaltic wave ✔ Elevation of the hyoid bone or displacement of the larynx ✔ Suppression of nerve impulses from Swallowing centre (n.ambiguus)  Entry of the bolus into the oesophagus stretches the myenteric plexus ⇒ starts the primary peristaltic wave (OPW) ⇒ initiated in the pharnyx Bolus is propelled down the oesophagus at 4 cm/s in 8-10 s Aided by gravity Liquids reach the LOS ahead of the primary OPW High tone of the LOS is inhibited prior to the arrival of the OPW Retained food causes distension of the myenteric plexus Gives rise to secondary OPW  Tertiary and other waves in the oesophagus are usually non-propulsive May indicate pathology LOWER OESOPHAGEAL SPHINCTER Physiological sphincter formed by: Smooth muscle thickens at the junction of stomach, Crural portion of the diaphragm surrounds the esophagus & exerts pinchcock action, Oblique fibres from the stomach form a flap valve, This closes esophagogastric junction when intragastric pressure is high  High resting tone of the UOS and LOS Barrier to entry of air & gastric contents Flaccid body during rest Sequential contractions of body to propel bolus distally Contractions coordinated both centrally and peripherally  LOS contraction due to myogenic, neural (ACh), humoral factors Smooth muscles contract when stretched LOS relaxation is neurally mediated Vagal efferent fibres stimulate enteric inhibitory neurons (EIN) EIN cause relaxation of the smooth muscle Release VIP & NO  Somatic & visceral motor nerves in the Vagus coordinate contractions of oesophagus Vago-vagal reflexes are initiated by stretching of the body of oesophagus Receptive relaxation of stomach allows the bolus to enter and complete swallowing  Disorders of Chewing & Swallowing Diseases of the sphincters Achalasia, Incompetent LOS (“heart-burn”) Damage to the nerve supply NV, IX, X by poliomyelitis, encephalitis Defective muscle function Myasthenia gravis, Muscle dystrophy Diffuse oesophageal spasm Glottis remains open Drug-induced e.g. anaesthesia Nasopharynx does not close (cp. new-born babies)