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Neurophysiology of Swallowing

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1 Neurophysiology of Swallowing
Stages of Swallowing

2 Stages of Swallowing The act of eating, from the moment food is brought to the mouth and transported to the beginning of the digestive tract, is a complex and coordinated neurological process. The function of the swallowing apparatus is to transport materials from the oral cavity to the stomach without allowing entry of substances into the airway. Classically, the act of deglutition is described in four phases: (1) The oral preparatory phase, when food is manipulated in the mouth and masticated, if necessary, to reduce it to a consistency ready for swallow;

3 Stages of Swallowing (2) The oral phase of swallow, when the tongue propels food posteriorly until the swallow response is triggered; (3) The pharyngeal phase, when the swallow response is triggered and the bolus is moved through the pharynx; and (4) The esophageal phase, when esophageal peristalsis carries the bolus through the cervical and thoracic esophagus and into the stomach. There are many types of normal swallows depending on the type and volume of food being swallowed and the voluntary control exerted over it.

4 Oral Preparatory Phase
 In the unimpaired individual, the oral cavity functions as a sensory and motor organ affecting changes in the physical properties of the food bolus to make it swallow- safe. Movement patterns in the oral preparatory phase of the swallow vary depending on the viscosity of the material to be swallowed and the amount of oral manipulation the individual uses in savoring a particular food. This is a voluntary phase of variable length depending upon the texture of the food.

5 Oral Preparatory Phase
From the time material is placed in the mouth, labial seal is maintained to ensure that no food or liquid falls from the mouth. The oral cavity is moist, the nostrils are open, the jaws are closed, and the lips are together but relaxed. The larynx and pharynx are at rest and nasal breathing continues. If mastication is required, the tongue positions food on the teeth. The upper and lower teeth meet and crush the material.

6 Oral Preparatory Phase
The food falls medially toward the tongue, which moves the material back onto the teeth as the mandible opens. The cycle is repeated numerous times before forming a bolus. During active chewing, the soft palate is not pulled down and forward and premature spillage is common and entirely normal. In addition to the cyclic movement during mastication, the tongue mixes the food with saliva. Tension in the buccal musculature closes off the lateral sulcus and prevents food particles from falling laterally into the sulcus.

7 Oral Preparatory Phase
A great deal of sensory information is processed from sensory receptors throughout the oral cavity, including the tongue. It is likely that information on bolus volume comes from the shape of the tongue as it surrounds the bolus prior to swallow. After chewing, the tongue pulls the food into a semi- cohesive bolus. If larger volumes of thicker foods are placed in the mouth, the tongue will subdivide the food after chewing, forming only part of it into a bolus to be swallowed at one time.

8 Oral Preparatory Phase
Subsequent “portions” are sequestered on the side of the mouth for later swallows. When sufficient chewing occurs and the bolus as been shaped, one of two normal hold positions occur: In the "tipper" hold position, the bolus is held between the midline of the tongue and the hard palate with the tongue tip elevated and contacting the anterior alveolar ridge. In the "dipper" hold position, the bolus is held on the floor of the mouth in front of the tongue. The function of the oral preparatory phase is to reduce food to consistency for swallow and provide pleasure.

9 Oral Preparatory Phase: Summary
Functional behaviors of the oral prep phase include:  Labial closure  Facial tone  Lateral and rotary jaw movement  Lateral and rotary tongue movement  Lingua-velar seal Innervation is provided by:  Cranial nerve V - mandibular movement  Cranial nerve VII - lip shape & facial tone  Cranial nerve V, X, XI - velar movement  Cranial nerve XII - tongue shape and position

10 Oral Phase The oral phase of swallow is initiated when the tongue begins posterior movement of the bolus. If the bolus is held in the dipper position (approx 20%) of population), the tongue tip moves forward and lifts the bolus onto the tongue and into the tipper position. In what has been described as a smooth stripping action, the midline of the tongue sequentially squeezes the bolus posteriorly against the hard palate. The midline of the tongue moves the bolus in an anterior to posterior rolling action with tongue elevation progressing sequentially more posteriorly to push the bolus backward.

11 Oral Phase The sides and tip of the tongue remain firmly anchored against the alveolar ridge. A central groove is formed on the tongue, acting as a ramp or chute for food to pass through as it moves posteriorly. The bolus is kept centered on the tongue by the flattening of the cheeks against the lateral tongue borders and the narrowing of the faucial pillars as the soft palate is pulled down and forward to seal off the oral cavity from the pharynx. Thicker foods require more pressure to propel them cleanly and efficiently through the oral cavity.

12 Oral Phase Sensory receptors in the oropharynx and tongue itself are stimulated and send sensory information to the cortex and brainstem. When the leading edge of the bolus or "bolus head" passes any point between the anterior faucial arches and the point where the tongue base crosses the lower rim of the mandible, the oral phase of the swallow is terminated.

13 Oral Phase: Summary The function of the oral phase is movement of food/liquid bolus into the pharynx. During the oral phase, the tongue seal around bolus with tip anchored at alveolar ridge; The midline of tongue is forced upward by actions of the mylohyoid, geniohyoid, and digastric muscles; The root of tongue presses against the velum through action of the styloglossus and hyoglossus muscles; and The bolus is propelled upward and backward toward faucial arches . 

14 Oral Phase: Summary Innervation is provided by:
Cranial nerve XII - tongue movement Cranial nerves V & XII - floor of oral cavity movement This stage usually take less than 1 to 1.5 seconds to complete.

15 Pharyngeal Stage The most sensitive area for triggering the pharyngeal swallow, an involuntary process, is not the tonsillar fauces or that general area, but the leading edge of the epiglottis. The posterior movement of the bolus is not interrupted.

16 Pharyngeal Stage In younger individuals, the triggering of the pharyngeal swallow occurs at the anterior faucial arches. In older individuals, it occurs when the tongue base crosses the lower rim of the mandible. A number of physiological activities occur as a result of pharyngeal triggering including: (1) elevation and retraction of the velum and complete closure of the velopharyngeal port to prevent material from entering the nasal cavity. (2) elevation and anterior movement of the hyoid bone and larynx;

17 Pharyngeal Stage (3) closure of the larynx at all three sphincters, from bottom to top, the true folds, the laryngeal vestibule, and epiglottis; (4) opening of the cricopharyngeus to allow material to pass from the pharynx into the esophagus; (5) ramping of the base of the tongue to deliver the bolus to the pharynx followed by tongue base retraction and pharyngeal wall contraction to create positive pharyngeal pressure; and (6) progressive top to bottom contraction of the pharyngeal constrictors.

18 Pharyngeal Stage Let’s spend a little bit of time understanding some very important aspects of this pharyngeal phase: laryngeal elevation, anterior hyoid traction, UES relaxation, and UES opening. Laryngeal elevation is the vertical movement of the entire laryngeal complex above the critical height needed to achieve closure of the laryngeal vestibule (Cook & Kahrilas, 1999). Elevation of the larynx during swallowing plays an important role in protecting the laryngeal inlet (Fukushima, Shingai, Kitagawa, Takahashi, Taguchi, Noda, & Yamada, 2003).

19 Pharyngeal Stage The thyrohyoid (TH) muscle is the most important muscle for laryngeal elevation and it is thought to be innervated by the pharyngeal branch of the vagus nerve.

20 Pharyngeal Stage Because of different innervation, laryngeal elevation alone does NOT open the UES. It is possible to have elevation of the larynx, through contraction of the thyrohyoid muscle but NO movement of the hyoid, therefore NO opening of the UES. Opening of the UES can only be achieved through anterior movement of the hyoid bone.

21 Pharyngeal Stage Anterior hyoid traction is a forward pulling force exerted on the larynx by contraction of the suprahyoid musculature. Specifically, contraction of the anterior belly of the digastric muscle and the the geniohyoid muscle pull the hyoid) up and forward.

22 Pharyngeal Stage Because the only insertion of the cricopharyngeus is to the cricoid cartilage of the larynx, the sphincter muscle and larynx are obliged to move in unison in a forward direction. This action opens the UES.

23 Pharyngeal Stage Like any sphincter, the cricopharyngeus can only contract or not contract. It has to be OPENED by traction, pressure and gravity or a combination of all. During swallowing, in addition to traction, opening of the UES is highly dependent on bolus size and weight. Boluses of 1-5ml may be too small to produce effective opening of the UES. Patients may aspirate on thin small liquid boluses during videofluoroscopic swallow studies but not while drinking larger boluses.

24 Pharyngeal Stage Another aspect of the pharyngeal swallow stage is epiglottic movement. As the hyoid bone moves anteriorly, the lateral hyoepiglottic ligaments exert traction preferentially on the upper third of the epiglottis bringing it to a position below horizontal (Vandaele, Perlman, & Cassell, 1995).

25 Pharyngeal Stage UES relaxation and UES opening are separate and distinct events. The recurrent laryngeal nerve provides motor innervation of the cricopharyngeus, which also has innervation from the pharyngeal plexus. Damage to either or both will affect the relaxation of the UES as opposed to its opening. Clearly, if it does not relax, then the traction forces required to open it will either not work or will work poorly. UES relaxation occurs during swallowing-associated laryngeal elevation.

26 Pharyngeal Stage The UES ceases contraction 0.1 seconds before it is pulled open by the movement of the hyoid and its attachments. This happens AFTER the larynx starts to rise, demonstrating that the two events, while related occur separately, i.e. laryngeal elevation can occur in the absence of UES opening. Clinically, this is a significant point in that impaired UES opening can result from either impaired traction on the sphincter or impaired sphincter relaxation. Instances of impaired traction can be felt in clinical assessment and/or evidenced fluoroscopically by diminished anterior hyoid displacement.

27 Pharyngeal Stage Impaired relaxation is only detectable manometrically. To summarize, laryngeal elevation is needed to move the entire larynx into a more protected position. After the larynx starts to rise, the cricopharyngeus stops contracting—it relaxes. With laryngeal excursion in an anterior direction, by anterior hyoid traction, the relaxed UES is pulled opened, and the epiglottis inverts. As the larynx lifts, approximately 2 cm in normal adults, and moves forward, the arytenoid cartilages are also being brought closer to the base of the tilting epiglottis facilitating closure of the airway entrance.

28 Pharyngeal Stage As the bolus enters the UES, the pressure of the bolus widens the opening. Larger boluses with larger intrabolus pressure result in wider UES opening (Coo, Dodos, Dantas, Massey, Kern, Lang, Brasseur & Hogan, 1989). Both the diameter and duration of deglutitive sphincter opening increase with increased swallow bolus volumes. This is one reason that the VFSS may often provide false positives. Apparent bolus pooling may result if the bolus size is not sufficient to open the UES as they do during real meals.

29 Pharyngeal Stage Timing
Finally, pharyngeal transit time, the time taken for the bolus to move from the point at which the pharyngeal swallow is triggered through the UES, is normally 1 second or less. The bolus moves smoothly and quickly over the base of the tongue through the pharynx and into the cervical esophagus. Bolus passage through the pharynx depends on gravity, pharyngeal shortening, and the propulsive forces of the pharynx.

30 Pharyngeal Stage Timing
Although the use of the term "peristalsis" is objectionable to some, this term most closely describes the rapid, orderly, sequential, moving front of contracting pressure wave that is generated during a normal pharyngeal swallow. Circumferential and vertical dynamics in pharyngeal swallowing are equally important (Salassa, 1997). The sequential circumferential pharyngeal forces are propulsive, but unequal. Anterior-posterior pressures exceed lateral pressures.

31 Pharyngeal Stage Timing
Vertical shortening, universal to "peristalsis" throughout the alimentary tract, is critical to normal effective bolus transport. The major muscles propelling the bolus include the palatopharyngeus and the stylopharyngeus which act to shorten the pharynx as the bolus arrives in the pharynx. Then, thyrohyoid shortening, laryngo-hyoid elevation, and the inferior movement of the tongue base contribute to further pharyngeal shortening. The three pharyngeal constrictors move the bolus in a sequentially downward direction toward the UES.

32 Pharyngeal Stage: Summary
The pharyngeal swallow response is triggered as the leading edge of the bolus passes the inferior rim of the mandible, although in younger persons the response may be triggered in the area of the faucial arches. During the pharyngeal phase, there is posterior movement of base of the tongue velopharyngeal closure; narrowing and shortening of the hypopharynx; elevation and anterior movement of the larynx; laryngeal closure; and relaxation and opening of UES.

33 Pharyngeal Stage: Summary
Innervation is provided by: CNs V, IX, X, XI - velar movement CNs V, VII, IX, X, XI, XII - pharyngeal, laryngeal movement CN IX - sensory input

34 Esophageal Phase The esophageal phase begins as the tail of the bolus passes through the UES and continues until it passes into the stomach at the LES. The larynx lowers, the glottis opens for resumption of respiration, and the UES contracts sealing off the cervical esophagus to prevent redirection of the bolus from the esophagus back into the pharynx. Food is transported through the esophagus by involuntary muscular movements called peristalsis. Normal esophageal transit time varies from 8 to 20 seconds.

35 Esophageal Phase In the section of the esophagus lying just above (7a) and around the top of the bolus (6), the circular muscle fibers contract, constricting the esophageal wall, squeezing the bolus downward.

36 Esophageal Phase Meanwhile, the longitudinal fibers lying around the bottom of and just below the bolus also contract shortening this lower section, pushing the walls outward so it can receive the bolus. These contractions are repeated in a wave that moves down the esophagus, pushing the food toward the stomach. Passage of the bolus is also facilitated by mucous secretory glands.

37 Esophageal Phase: Summary
The esophageal phase begins as the tail of the bolus passes through the UES. The UES closes and the airway opens. Peristalsis moves bolus down esophagus. LES opens allowing bolus to enter stomach. Cranial nerve X is involved in both striated and smooth muscle contraction.

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