Anatomy and Physiology of Swallowing
Original Editor - Srishti Banerjee
Top Contributors - Ewa Jaraczewska and Jess Bell
Introduction[edit | edit source]
Deglutition, or swallowing, refers to the movement of liquids or solids from the mouth to the stomach via the pharynx and oesophagus. Swallowing starts to develop in utero from around 15 weeks gestational age. It continues to develop after birth, ultimately resulting in the conscious control of swallowing.[1] Multiple organ systems are involved in swallowing, including the musculoskeletal, neuromuscular and respiratory systems. Normal swallowing requires a coordinated effort of over thirty muscles,[2] the central nervous system, and five cranial nerves.[3] Other key anatomical structures involved in this process are the nose, nasal cavity, oral cavity, and pharynx. This article explores the relevant anatomy and physiology of normal swallowing.
Developmental Anatomy and Swallowing[edit | edit source]
There are a number of anatomical changes that occur from infancy to childhood.
- teeth have not erupted
- the hard palate is flatter
- larynx and hyoid bone are located high in the neck
- the epiglottis touches the back of the soft palate
- the airway is separated from the oral cavity by a soft tissue barrier
- jaw, oral cavity and larynx are small
- tongue occupies a significant portion of the mouth
- uses tongue, sucking pads, and sulci to suck
- sucking pads provide stability
- lack of a distinct oropharynx
- the base of the tongue is close to the larynx
- the mouth is bigger, so the tongue lies at a lower position (floor of the mouth) and sits behind the teeth
- the mandible extends down and forward
- the oral cavity expands
- the neck gets longer, so the larynx descends to a lower position in the neck
- the contact between the epiglottis and soft palate is gradually lost
- the pharynx lengthens vertically
- the hyoid descends
- the sucking pads degenerate
Note: these anatomical changes are essential for speech, but they make humans more prone to aspiration.[5]
Adult Anatomy and Swallowing[edit | edit source]
Muscles[edit | edit source]
Table 1 lists the major muscles of swallowing and their function. Some muscles are listed more than once in the table to highlight their relationships with different muscles and structures.
Muscles | Function | |
---|---|---|
Tongue muscles[5] | Intrinsic tongue muscles:
|
|
Masticatory muscles[5] | Mylohyoid
Medial and lateral pterygoid muscles |
|
Neck muscles[5] | Anterior belly of digastrics (ABD)
Posterior belly of digastrics (PBD) |
|
Soft palate and pharynx[5] | Tensor veli palatini (TVP)
Levator veli palatini (LVP) Palatopharyngeous (PPh) Salpingopharyngeous (SaPh) Stylopharyngeus (SPh) Intrinsic laryngeal muscles (IL) |
|
Upper oesophageal sphincter[9] | Extrinsic muscles
Intrinsic muscles:
|
Extrinsic muscles:
Intrinsic muscles:
|
Cranial nerves[edit | edit source]
Cranial nerves modulate the execution of swallowing.[11] Table 2 summarises the function and the clinical relevance of the six cranial nerves involved in swallowing.
Cranial nerves (CN) | Function | Clinical Relevance |
---|---|---|
Trigeminal nerve (TN) CN V |
|
|
Facial nerve (FN) CN VII |
|
|
Glossopharyngeal nerve (GN) CN IX |
|
|
Vagus nerve (VN) CN X |
|
|
Hypoglossal nerve (HN) CN XII |
|
|
Accessory nerve CN XI |
|
|
Anatomical Structures[edit | edit source]
"Feeding and breathing share the same anatomy."[13]
Nose and nasal cavity: we breathe through our nose while eating solids since our is mouth processing the food, and our lips are sealed to prevent food from escaping anteriorly. Nasal air pressure oscillates with masticatory jaw movement. It becomes positive relative to atmospheric pressure during jaw closure and negative during jaw opening.[13]
Oral cavity: tongue movement corresponds with cyclic jaw movement when food is in the mouth. The tongue repositions food laterally and the cheeks reposition food medially.[13]
Pharynx: a breathing, mastication and swallowing route. The pharynx dilates to maintain the airway for breathing and constricts to push the food bolus to the oesophagus for swallowing.[13]
Larynx and vocal folds: the posterior aspect of the larynx forms the anterior wall of the upper oesophageal sphincter.
Upper oesophageal sphincter: a fibromuscular structure located behind the larynx. It is bordered posterolaterally by the pharyngoesophageal muscles and superiorly by the pharynx. Inferiorly, it continues into the oesophagus.[9]
Neural Coordination of Swallowing[edit | edit source]
Swallowing requires a coordinated contraction of the muscles in the mouth, pharynx, upper oesophageal sphincter, and upper oesophagus via central control. Swallowing centres activate the voluntary motor centres and inhibit the respiratory centres - this prevents food from entering the trachea. There is also activation of the:
- reflex centres
- nuclei of the cranial nerves that are involved in the movement of the tongue, larynx and pharynx
Please see the swallowing flow chart in Figure 2 for more information.
Physiology of Swallowing[edit | edit source]
Swallowing is often described as having four phases: (1) oral preparatory phase, (2) oral propulsive phase, (3) pharyngeal phase, and (4) oesophageal phase. The oral phases change depending on whether we are drinking liquids or eating solids.
Swallowing Liquids[edit | edit source]
Oral Preparatory Phase[edit | edit source]
The oral preparatory phase starts as soon as you take a sip. It is a voluntary phase of swallowing,[14] and includes the following steps:[5]
- formation of a liquid bolus in the mouth
- holding the bolus in the anterior part of the mouth, known as the anterior floor of the mouth, or on the surface of the tongue against the hard palate
- the tongue and the soft palate seal the oral cavity posteriorly to prevent leakage of the liquid into the oropharynx
Oral Propulsive Phase[edit | edit source]
The oral propulsive phase immediately follows the oral preparatory phase:
- the tip of the tongue rises to touch the hard palate
- as the tip of the tongue rises, the oral cavity opens
- the tongue-to-palate contact area gradually widens in an anterior-posterior direction
- the bolus is pushed backwards along the pharynx
To prevent liquid aspiration, "the bolus is commonly held in the oral cavity until just before the initiation of the pharyngeal swallow."[13] With solids, the food is propelled to the oropharynx, accumulated, and bolus is formed before swallowing.
Swallowing Solids[edit | edit source]
The Process Model of Feeding describes the oral stage of eating solid food.
Oral Preparatory Phase[edit | edit source]
Stage I Transport[5]
- after food enters the mouth, the bolus is taken to the post-canine dentition, where it is rotated laterally
Food Processing[5]
- mastication (i.e. chewing) reduces the food particle size
- saliva softens the food, making it an appropriate consistency for swallowing
- there are cyclical movements of the jaw, with movements of the tongue, cheek, soft palate, and the hyoid bone
- unlike with liquids, the posterior cavity isn't sealed - the cyclical movement of the jaw and soft palate allows air to be pumped into the nasal cavity through the pharynx; this delivers the aroma of the food to the chemoreceptors in the nose
- movements associated with chewing include:
- early jaw opening: the lips are sealed, and the tongue and jaw move forward and downward
- late jaw opening: the lips are open, and the tongue curls (this prevents biting of the tongue)
- the tongue moves mediolaterally and rotates along its antero-posterior axis
- the hyoid bone has an important role in controlling jaw movements, acting as a stabiliser for the jaw and the tongue
Clinical relevance: Older adults have increased chewing cycles and activity of the jaw adductor muscles during mastication, which increases mastication time. However, despite this increase, the bolus size at the time of swallow onset is larger in older adults than in younger people because of a decrease in masticatory function.[13]
Oral Propulsive Phase[edit | edit source]
Stage II Transport[5]
- when the bolus of food is ready to be swallowed, it is placed on the surface of the tongue and is propelled backwards to the oropharynx
- chewing continues while food remains in the oral cavity[15]
Clinical relevance: "The initial consistency of food affects the duration of oropharyngeal bolus aggregation before the swallow and the number of chewing cycles.[16] There are more chewing cycles, and the oropharyngeal bolus aggregation time is longer for hard foods than for soft foods.[16]
Pharyngeal Phase[edit | edit source]
During the pharyngeal phase, there are two key events:
- food passage: the bolus is propelled from the pharynx to the upper oesophageal sphincter to the oesophagus
- the tongue elevates and pushes the bolus against the wall of the pharynx
- tongue elevation blocks the oral cavity
- the pharyngeal constrictor muscles contract, creating a peristaltic wave
- the bolus is pushed in the downward direction
- airway protection: the epiglottis, arytenoids, and vocal folds are the three gatekeepers to prevent aspiration:
- the soft palate elevates and comes into contact with the lateral and posterior walls of the pharynx, which closes the nasopharynx
- the elevation of the soft palate prevents the regurgitation of food
- the vocal cords close and seal the epiglottis
- the transverse arytenoid muscles tilt forward to contract the epiglottic base and seal the epiglottis
- the suprahyoid and thyrohyoid muscles contract and pull the hyoid bone up
- as the larynx is attached to the hyoid bone, it moves up and forward following this muscle contraction
- as a result, the backward tilting of the epiglottis seals the laryngeal vestibule
All the above activities must occur before the upper oesophageal sphincter opens because once it does open, the bolus will pass down to the oesophagus.
In the next step of the swallowing process, the upper oesophageal sphincter opens, and the bolus is transported forward. For the upper oesophageal sphincter to open, the following events must occur:
- relaxation of the cricopharyngeus muscle
- contraction of suprahyoid and thyrohyoid muscles which pulls the hyrolaryngeal complex forward, leading to the opening of the upper oesophageal sphincter
- pressure created by the descent of the bolus helps to open the upper oesophageal sphincter
Oesophageal Phase[edit | edit source]
The oesophagus is a tube-like structure that has three anatomical segments: cervical, thoracic and abdominal. It extends from the upper oesophageal sphincter to the lower oesophageal sphincter. The top third of the oesophagus is largely made up of striated muscles, while the bottom two-thirds is smooth muscle.[17]
The oesophageal phase includes the following activities:
- food passes through the upper oesophageal sphincter to the oesophagus
- a peristaltic wave develops, which transports the bolus through the oesophagus
- during swallowing, the lower oesophageal sphincter relaxes to allow the bolus to enter the stomach - it is contracted at rest to prevent regurgitation of food from the stomach
This optional video discusses the anatomy and physiology of swallowing:
Swallowing and Respiration[edit | edit source]
Swallowing and breathing are coordinated actions. Swallowing usually starts during the expiratory phase of breathing.[13] [19] During swallowing, breathing briefly pauses because the airway is physically closed by the elevation of the soft palate and tilting of the epiglottis. There is also neural suppression of respiration in the brainstem.[5] This pause continues for 0.5-1.5 seconds during swallowing, before respiration resumes with expiration after swallowing.
This sequence of exhale-swallow-exhale prevents the inhalation of any food that remains in the pharynx.[5][20] Other patterns of swallowing and respiration include (1) “inhale-swallow-exhale”, (2) “exhale-swallow-inhale” and (3)“inhale-swallow-inhale”. These patterns occur rarely in healthy adults.[13][21]
Note: "When performing sequential swallows while drinking from a cup, respiration can resume with inspiration."[22] During mastication, the respiratory cycle duration decreases and then increases with swallowing following the sequence of exhale-swallow-exhale.[16]
Resources[edit | edit source]
- Hao N, Sasa A, Kulvanich S, Nakajima Y, Nagoya K, Magara J, Tsujimura T, Inoue M. Coordination of Respiration, Swallowing, and Chewing in Healthy Young Adults. Front Physiol. 2021 Jul 13;12:696071.
- Matsuo K, Palmer JB. Coordination of Mastication, Swallowing and Breathing. Jpn Dent Sci Rev. 2009 May 1;45(1):31-40.
- Feeding and Swallowing Development in Children
References[edit | edit source]
- ↑ Panara K, Ramezanpour Ahangar E, Padalia D. Physiology, Swallowing. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541071/ [last access 23.05.2024]
- ↑ Umay E, Akaltun MS, Uz C. Association between swallowing muscle mass and dysphagia in older adults: A case-control study. J Oral Rehabil. 2023 Jun;50(6):429-439.
- ↑ Arvedson J, Lefton-Greif M, Reigstad D, Brodsky L. Clinical swallowing and feeding assessment. San Diego, CA: Plural Publishing; 2020.
- ↑ 4.0 4.1 Kaiser L, Park T. Feeding and Swallowing Development in Children. Graduate Independent Studies - Communication Sciences and Disorders 2020; 27.
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 Banerjee S. Anatomy and Physiology Significant to Dysphagia. Plus Course 2024
- ↑ Toth J, Lappin SL. Anatomy, Head and Neck, Mylohyoid Muscle. [Updated 2023 Jun 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan- Available from https://www.ncbi.nlm.nih.gov/books/NBK545293/#:~:text=Introduction,inserts%20on%20the%20hyoid%20bone. [last access 12.06.2024]
- ↑ Pterygoid muscles. Available from https://www.kenhub.com/en/library/anatomy/pterygoid-muscles [last access 12.06.2024]
- ↑ Tranchito EN, Bordoni B. Anatomy, Head and Neck, Digastric Muscle. [Updated 2024 Jan 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK544352/
- ↑ 9.0 9.1 9.2 Ramaswamy AT, Martell P, Azevedo R, Belafsky P. The upper oesophagal sphincter: anatomy and physiology. Ann Esophagus 2022;5:30
- ↑ Rathee M, Jain P. Anatomy, Head and Neck, Stylohyoid Muscle. [Updated 2023 Jan 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK547653/
- ↑ 11.0 11.1 Costa MMB. Neural Control of Swallowing. Arq Gastroenterol. 2018 Nov;55Suppl 1(Suppl 1):61-75.
- ↑ Florie MGMH, Pilz W, Dijkman RH, Kremer B, Wiersma A, Winkens B, Baijens LWJ. The Effect of Cranial Nerve Stimulation on Swallowing: A Systematic Review. Dysphagia. 2021 Apr;36(2):216-230.
- ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 Matsuo K, Palmer JB. Coordination of Mastication, Swallowing and Breathing. Jpn Dent Sci Rev. 2009 May 1;45(1):31-40.
- ↑ Mélotte E, Maudoux A, Panda R, Kaux JF, Lagier A, Herr R, Belorgeot M, Laureys S, Gosseries O. Links Between Swallowing and Consciousness: A Narrative Review. Dysphagia. 2023 Feb;38(1):42-64.
- ↑ Palmer JB. Bolus aggregation in the oropharynx does not depend on gravity. Arch Phys Med Rehabil. 1998 Jun;79(6):691-6.
- ↑ 16.0 16.1 16.2 Matsuo K, Hiiemae KM, Gonzalez-Fernandez M, Palmer JB. Respiration during feeding on solid food: alterations in breathing during mastication, pharyngeal bolus aggregation, and swallowing. J Appl Physiol (1985). 2008 Mar;104(3):674-81.
- ↑ Chaudhry SR, Bordoni B. Anatomy, Thorax, Esophagus. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482513/
- ↑ Fauquier ENT. The 4 Stages of Swallowing: Biomechanics & Bolus Movement. Available from:https://www.youtube.com/watch?v=HIaW3FUVM3k [last accessed 27/5/2024]
- ↑ Hao N, Sasa A, Kulvanich S, Nakajima Y, Nagoya K, Magara J, Tsujimura T, Inoue M. Coordination of Respiration, Swallowing, and Chewing in Healthy Young Adults. Front Physiol. 2021 Jul 13;12:696071.
- ↑ McFarland DH, Lund JP. Modification of mastication and respiration during swallowing in the adult human. J Neurophysiol 1995;74(4):1509–17
- ↑ Selley WG, Flack FC, Ellis RE, Brooks WA. Respiratory patterns associated with swallowing: Part 1. The normal adult pattern changes with age. Age Ageing. 1989 May;18(3):168-72.
- ↑ Matsuo K, Palmer JB. Anatomy and physiology of feeding and swallowing: normal and abnormal. Phys Med Rehabil Clin N Am. 2008 Nov;19(4):691-707, vii.