Development and normal swallowing function

The newborn infant is reflex bound and automatically makes certain oral motor movements. For example, the newborn's automatic-phasic bite

Newborn Phases Swallowing

Figure 15.1 (a) The adult oropharynx. The phases of swallowing are labeled. 1, preparatory phase; 2, oral phase; 3, pharyngeal phase; 4, esophageal phase. (b) The infant oropharynx. Note that the infant oral cavity providing little space for manipulation of the food bolus. The larynx is elevated so that the epiglottis almost touches the soft palate. The tongue is entirely within the oral cavity, with no oral pharynx. Reproduced with permission from reference 15.

Figure 15.1 (a) The adult oropharynx. The phases of swallowing are labeled. 1, preparatory phase; 2, oral phase; 3, pharyngeal phase; 4, esophageal phase. (b) The infant oropharynx. Note that the infant oral cavity providing little space for manipulation of the food bolus. The larynx is elevated so that the epiglottis almost touches the soft palate. The tongue is entirely within the oral cavity, with no oral pharynx. Reproduced with permission from reference 15.

pattern, although it may look like chewing, actually starts liquid flowing into the mouth with its pump-like action. The rooting reflex elicited by stroking the side of the mouth and resulting in the head turning towards the source of stimulation, is a food-seeking response. Over time, these reflexive movements of the newborn are gradually refined and incorporated into more voluntary feeding patterns.14

Therefore, by increasing the intraoral space, the infant begins to suppress reflexive suckling patterns and starts to use voluntary sucking patterns. In contrast to suckling, true sucking involves a raising and lowering of the body of the tongue with increased use of intrinsic musculature. Most infants complete the gradual transition from suckling to true sucking by 9 months of age. This is considered a critical step in the development of oral skills that will permit handling of thicker textures and spoon-feeding.16

As with sucking, chewing patterns emerge gradually during infancy. Between birth and 5 months of age, a phasic bite-release pattern develops. Jaw opening and closing begins as a reflex and evolves into a volitionally controlled bite. True chewing develops as activity of the tongue, cheeks and jaws co-ordinate to participate in the breakdown of solid food. The eruption of the deciduous teeth between the ages of 6 and 24 months provides a chewing surface and increased sensory input to facilitate the development of chewing.16

The concept of a 'critical period' is relevant to feeding development. A critical period is a fairly well-delineated period of the time during which a specific stimulus must be applied in order to produce a particular action. After such a critical period a particular behavior pattern can no longer be learned. The 'sensitive period' is the optimal time for the application of a stimulus. After the sensitive period, it is more difficult to learn a specific pattern of behavior.17

Current knowledge of the swallowing mechanism is derived mainly from radiographic studies, which have been in use since the early 1900s. Plain films of the pharynx were replaced in the 1930s by cineradiography, which, in the 1970s, was subsequently replaced by videofluoroscopy. Videofluoroscopy permits instant analysis of bolus transport, aspiration and pharyngeal function.18 Using this descriptive method deglutition can be divided into four phases: the oral preparatory phase, oral voluntary phase, pharyngeal phase, and esophageal phase (Table 15.2).19

The oral preparatory phase occurs after food is placed into the mouth. The food is prepared for pharyngeal delivery by mastication and mixing with saliva. This is a highly co-ordinated activity

Table 15.2 Phases of normal deglutition

Phase

Activities

Time

Pre-oral phase

food introduced into the oral cavity

varies; depends on substance

(voluntary)

Oral phase

bolus formation and passage to

Less than 1 s

(voluntary/involuntary)

the pharynx

Pharyngeal phase (involuntary)

respiration ceases; pharyngeal peristalsis; epiglottis closes; larynx closes, elevates and draws forward. UES relaxes

1 s or less

Esophageal phase (involuntary)

esophageal peristalsis; opening of lower esophageal sphincter

8-20s

UES, upper esophageal sphincter

Development and normal swallowing function 237

that is rhythmic and controlled to prevent injury to the tongue. The tongue is elevated towards the palate by the combined actions of the digastric, genioglossus, geniohyoid and mylohyoid muscles. Intrinsic tongue muscles produce both the initial depression in the dorsum that receives the food and the spreading action that distributes the food throughout the oral cavity. The buccinator muscles hold food between the teeth in dentulous infants and help to generate suction in neonates. In this phase, the soft palate is against the tongue base secondary to contraction of the palatoglossus muscles, which allows nasal breathing to continue.2,20

During, the oral propulsive phase, the bolus is propelled into the oropharynx. The oral phase is characterized by elevation of the tongue and a posterior sweeping or stripping action produced mainly by the action of styloglossus muscles. This propels the bolus into the pharynx and triggers the 'reflex swallow'. The receptors for this reflex are thought to be at the base of the anterior pillars, but there is evidence that others exist in the tongue base, epiglottis and pyriform fossae. Sensory impulses for the reflex are conducted through the afferent limbs of cranial nerves V, IX and X to the swallowing center. Oral transit time is less than 1 s.2,21

The pharyngeal phase of deglutition is the most complex and critical. The major component of the pharyngeal phase is the reflex swallow. This results from motor activity stimulated by cranial nerves IX and X. The reflex swallow may be triggered by a voluntary oral phase component or any stimulation of the afferent receptor in and around the anterior pillar.2 Bolus passage through the pharynx is accompanied by soft palate elevation, lingual thrust, laryngeal elevation and descent upper esophageal sphincter (UES) relaxation and pharyngeal constrictor peristalsis. The pharyngeal phase commences as the bolus head is propelled past the tongue pillars and finishes as the bolus tail passes into the esophagus.21 Once it begins, the pharyngeal phase is very quick, (1 s or less).2 It is characterized biomechanically by the operation of three valves and several propulsive mechanisms. The larynx closes and the palate elevates to disconnect the respiratory tract. The UES opens to expose the esophagus. At the completion of the pharyngeal phase, the airway valves (larynx, palate) open, and the UES closes so that respiration can resume.21

Pharyngeal bolus transit occurs in two phases: an initial thrust phase and a mucosal clearance phase.22 Bolus thrust, which propels most of the bolus into the esophagus, is provided by lingual propulsion, laryngeal elevation and gravity. The tongue has been linked to a piston, pumping the bolus though the pharynx.23 Patients with tongue impairment cannot generate large bolus driving forces despite an intact pharyngeal constrictor mechanism.24 Laryngeal elevation creates a negative post-crycoid pressure to suck the oncoming bolus towards the esophagus, and the elevated larynx holds the pharyngeal lumen open to minimize pharyngeal resistance.23

As the bolus enters the pharynx and is stripped inferiorly by the combined effects of gravity, the negative pressure mentioned above and the sequential contractions of the pharyngeal constrictors, the soft palate moves against the posterior pharyngeal wall to close off the nasopharyngeal port. The bolus divides around the epiglottis, combines and passes through the crycopharyngeal muscle, or upper esophageal sphincter.2

The UES is the high-pressure zone located between the pharynx and the cervical esophagus. The physiological role of this sphincter is to protect against reflux of food into the airways as well as to prevent entry of air into the digestive tract.25 Posteriorly and laterally the cricopharyn-geus muscle is a definitive component of the UES. The crycopharyngeus has many unique characteristics: it is tonically active, has a high degree of elasticity, does not develop maximal tension at basal length and is composed of a mixture of slow-and fast-twitch fibers, with the former predominating. These features enable the crycopharyngeus to maintain a resting tone and yet be able to stretch open by distracting forces, such as a swallowed bolus and hyoid and laryngeal excursion. The crycopharyngeus muscle, however, constitutes only the lower one-third of the entire high-pressure zone. The thyropharyngeus muscle accounts for the remaining upper two-thirds of the UES.

The UES function is controlled by a variety of reflexes that involve afferent inputs to the motorneurons innervating the sphincter.25 Based, on functional studies, it is believed that the major motor nerve of the crycopharyngeus muscle is the pharyngoesophageal nerve. Vagal efferents probably reach the muscle by the pharyngeal plexus, using the pharyngeal branch of the vagus.26 The superior laryngeal nerve may also contribute to motor control of the crycopharyngeus muscle.1 Sensory information from the UES is probably provided by the glossopharyngeal nerve and the sympathetic nervous system. There is probably little or no contribution by the sympathetic nervous system to crycopharyngeal control.26

The relaxation phase begins as the genioglossus and suspensory muscles pulls the larynx anteriorly and superiorly. The bolus is carried into the esophagus by a series of contraction waves that are a continuation of the pharyngeal stripping action.2 Proposed functions of the UES include prevention of esophageal distension during normal breathing and protection of the airway against aspiration following an episode of acid reflux.1,26 Qualitative abnormalities of the UES have been documented in infants with reflux disease.27

The esophageal phase occurs as the bolus is pushed through the esophagus to the stomach by esophageal peristalsis. Esophageal transit time varies from 8 to 20 s.14

'suck, swallow, breathe' pattern of early oral intake, resulting in diminished endurance during oral feeds. Apraxia of the oral swallow as well as reduction of oral sensation are also common. Other deficits include reduced bolus formation and transport, abnormal holding patterns, incomplete tongue-to-palate contact and repetitive lingual pumping.14

Oropharyngeal dysphagia results from either oropharyngeal swallowing dysfunction or perceived difficulty in the process of swallowing. Major categories of dysfunction are: an inability or excessive delay in initiation of pharyngeal swallowing; aspiration of ingestate; nasopharyngeal regurgitation; and residue of ingestate within the pharyngeal cavity after swallowing. Each of these categories of dysfunction can be mechanically subcategorized using fluoroscopic and/or mano-metric data.18

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Responses

  • mario
    Is lingual pumping in oral or pharyngeal phase?
    4 years ago
  • yrjö
    What muscle closes the nasopharyngeal port?
    4 years ago

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