4.Function (physiology)

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Explanation

Swallowing is a complicated sequence of voluntary and reflex movements involving various organs and tissues. This module describes the functional (physiological) aspect of swallowing.

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Explanation

Eating behavior refers to the general act of taking in food. The 5-stage model of swallowing1) facilitates conceptual understanding of the overall swallowing process. As described in detail in the following section, during the preparatory and oral stages, the phases (bolus transfer) and stages (physiological motion events) vary depending on the form of food, in other words, whether drinking liquid (liquid swallowing) or eating solid food (mastication and swallowing)2).

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Explanation

During the anticipatory stage, we decide what and how to eat and carry food to the mouth. When eating, a large amount of information is processed concurrently and hierarchically in the brain before (or in parallel with) taking in food through the mouth. After the visual information of food enters the occipital lobe, we process visual form information (eg, what this is) in the temporal lobe, check it against our memory, and recognize it as "meat", with additional information available on our preferences. At the same time, spatial information (eg, where the table and a plate are) and other information (eg, how to use cutlery) are processed in the parietal lobe. In the meantime, biological information such as blood sugar and hormones circulating the body stimulate the feeding center in the hypothalamus to generate appetite, an instinctive desire, and strengthen the intention to eat. These pieces of information are integrated in the prefrontal cortex to form motion programs such as the order in which to eat, the direction in which to reach out, using cutlery, how wide to open the mouth, and how strongly to chew. These motion programs are then transmitted from the motor cortex to the brainstem and spinal cord. At the same time, in order to execute these actions efficiently, it is important that we are awake, or are conscious, and can both spatially and temporally maintain, allocate, select, and direct our attention to eating behavior and the object to eat and cutlery to use. Thus, many areas of the brain are activated during the anticipatory stage. In addition to those organs directly involved in eating and swallowing, sensory organs of vision, hearing, and smell and the motor functions of the upper limbs, trunk, and other parts of the body also play an important role in eating behavior.

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Explanation

During the (oral) preparatory stage, food is putting into mouth, chewed into small pieces, and mixed with saliva to form a bolus. Putting of food, in which the lips and teeth play a crucial role, is achieved by opening and closing the lips and jaws. The mode of food capturing varies depending on the form of food and the type of cutlery used. The movements of the lips are controlled by mimetic muscles. These muscles function to hold food from falling out of the mouth during mastication. The buccinator muscle functions to keep food from falling into the oral vestibule.

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Explanation

A large piece of solid food is crushed with the upper and lower incisors and canines. The dental arch is composed of 4 groups of teeth. Alignment of the upper and lower dental arches is referred to as occlusion. The molar teeth are for crushing and grinding food. Between the teeth and the alveoli is the periodontium, which serves as a baroreceptor. Tooth loss and resulting loss of occlusal pressure result in alveolar resorption, collapse of the oral cavity, and reduced masticatory efficiency.

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Explanation

Mandibular movements for mouth opening and closing cause the upper and lower dental arches to approach to each other to compress and crush food. During mouth closing, the mandible also moves laterally to grind food (sliding movement). Masticatory muscles are innervated primarily by the trigeminal nerve. This movement is initially controlled voluntarily by the cerebral cortex, and then becomes a periodic spontaneous movement stimulated by signals from a rhythm generator located in the mastication center of the brainstem3). Mastication requires not only masticatory muscles, but also coordination of various organs including the tongue, cheeks, and lips.

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Explanation

Salivation is regulated reflexly by the autonomic nerve. Sympathetic stimulation induces the secretion of a small amount of mucous saliva, while parasympathetic stimulation induces the secretion of a large amount of serous saliva. Both the sympathetic and parasympathetic nerves promote salivation without antagonism between the two. Salivation is also promoted by stimulation of the oral, esophageal, and gastric mucosae, as well as taste and olfactory sensations and coordinated reflexes, and is regulated by the lower centers in the brainstem and the upper centers in the cerebrum4). Saliva not only helps mastication and swallowing, but also has various other functions including cleaning, development of taste, and digestion.

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Explanation

The tongue has 2 functions during swallowing: transferring food to the molar region for crushing and grinding (ie, stage 1 transport in the process model5)) and transporting the food bolus from the oral cavity to the pharynx (stage 2 transport5). Tongue movements are driven by the intrinsic and extrinsic muscles of the tongue, which are innervated by the hypoglossal nerve.

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Explanation

The sensory functions of the tongue include somatic senses (tactile sense, pressure sense, temperature sense, pain sense, and deep senses including position and motor senses) and taste. Different parts of the tongue are innervated by different afferent nerves; the anterior two-thirds is innervated by the lingual nerve (trigeminal nerve) for somatic senses and the chorda tympani nerve (facial nerve) for taste; the posterior one-third is innervated by the glossopharyngeal nerve for both senses. Taste sensation is mediated by chemoreceptor cells in the taste buds, which detect 4 basic taste types: salty, sour, sweet, and bitter. This sensory information is transmitted to the brainstem, thalamus, and cerebral cortex for the adjustment of swallowing movement.

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Explanation

Swallowing is a series of movements mediated by the swallowing reflex. It involves shutting off the airway (stopping breathing) and allowing the food bolus to pass through the pharynx into the esophagus. People swallow about 600 times per day6).

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Explanation

The major pharyngolaryngeal muscles involved in swallowing include the muscles that lift the hyoid bone superoanteriorly, those that lift the larynx, those that close the larynx, and those that contract the pharynx. The cricopharyngeal muscle relaxes during the pharyngeal stage.

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Explanation

The major motor nerves involved in swallowing include, among the cranial nerves, the trigeminal, facial, glossopharyngeal, vagus, and hypoglossal nerves.

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Explanation

The major sensory nerves involved in swallowing include the trigeminal, glossopharyngeal, and vagus nerves.

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Explanation

The reflex mechanism of swallowing is triggered by stimulation of sensory receptors in the oral and pharyngeal mucosa, and this information is transmitted through afferent nerves, including the glossopharyngeal and vagus nerves, to the swallowing center in the medulla oblongata of the brainstem, reflexly causing movements of swallowing-related muscles via efferent nerves. Substance P is increasingly being recognized as a key afferent neurotransmitter mediating swallowing and cough reflexes7). The information from the sensory receptors is also transmitted to the thalamus, cerebral limbic system, and cerebral cortex. The upper centers in the cerebral cortex and limbic system are considered to control the activity of the swallowing reflex centers. The areas of the cerebral cortex known to be involved in swallowing include the orofacial, pharyngeal, and laryngeal regions of the primary motor and sensory cortexes, the insular cortex, and the anterior cingulate cortex8).

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Explanation

Swallowing movements include reflex and voluntary movements, the former being mediated by the brainstem and the latter controlled by the cerebral cortex. The brainstem contains a masticatory rhythm generator and also presumably contains a central pattern generator (CPG) for swallowing movement. The activity of these reflex centers appears to be regulated by higher centers and modified by afferent inputs from peripheral sensory receptors8).

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References

  1. Leopold NA, Kagel MC. Swallowing, ingestion and dysphagia: a reappraisal. Arch Phys Med Rehabil. 64:371-373. 1983.
  2. Baba M. Process Model (Swallowing). Journal of Clinical Rehabilitation, 18(1): 49, 2009.
  3. Nakamura Y. Physiology of Mastication: Ishiyaku Pub. Inc., 1998.
  4. Matsuo R. The central control mechanism of salivation: Folia Pharmacologica Japonica, 127(4): 261-266, 2006
  5. Palmer JB, Integration of oral and pharyngeal bolus propulsion: a new model for the physiology of swallowing. Dysphagia Rehabilitation, 1: 15-30, 1997.
  6. Lear CS, Flanagan JB Jr, Moorrees CF. The frequency of deglutition in man. Arch Oral Biol. 10:83-100, 1965.
  7. Matsumoto S, Shimodozono M, Kawahira K. Biomarkers for dysphagia: Substance P. Geriatric Medicine 45(10): 1331-1335, 2007.
  8. Yamada Y. Neurophysiology of swallowing: Dysphagia Rehabilitation, 10(1): 3-11, 2006.

Recommended readings

  1. Saitoh E and Mukai M. Dysphagia Rehabilitation, 2nd Edition: Ishiyaku Pub. Inc.
  2. Fujishima I. Dysphagia in Stroke, 2nd Edition: Ishiyaku Pub. Inc.
  3. J. A. Logemann/translated by Michi K and Michiwaki Y. Evaluation and treatment of swallowing disorders: Ishiyaku Pub. Inc.
  4. M. A. Cray, M. E. Groher/translated by Fujiyama I. Introduction to Dysphagia: Ishiyaku Pub. Inc.
  5. M. E. Groher/translated by Fujiyama I. Dysphagia, 3rd Edition: Pathophysiology and Rehabilitation: Ishiyaku Pub. Inc.
  6. Kim C.L, Julie M.L, Kellie L. S./translated by Kaneko Y. Update on Eating/Swallowing Mechanism: Ishiyaku Pub. Inc.
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