The natural swallowing process – from mouth to stomach

The oral-phase is consciously controlled (voluntary) and is managed by the brain’s cortex region. But, as soon as the mixture has passed the anterior palatal arch towards the pharynx (in to the pharynx phase or pharyngeal phase) the swallow reflex takes over and this is controlled by the brain stem – no longer consciously controlled. Both phases however – the oral phase and the pharynx phase – are dependent upon the so-called sensory motor reflex circuit.

The perceptions of pressure, texture, taste and temperature are transmitted up to the brain (via the afferent nerve pathways), and down to the relevant muscles (via the efferent nerve pathways).

The oral cavity’s sensory nerves send these signals to the brain stem and it is based on these that the brain controls the musculature of the face, mouth, esophagus, diaphragm, down to the stomach and the upper and lower intestines.

This important superfast, ‘broadband’ connection is thus crucial in allowing the brain to control the entire swallowing process. Before we go further through the next swallowing phases, we will describe in more detail how this broadband connection to and from the brain actually works.

In the brain stem we find the Nucleus Tractus Solitarius (NTS), the afferent nucleus. The NTS is the core that gathers all incoming sensory signals from the oral cavity and pharynx via the afferent nerve pathways, and transmits them either to the brain’s cortex or directly to the network-like system in the brain stem called the Formatio Reticularis (FR).

In the FR the afferent signals from the NTS and the cortex are first interpreted and then passed through various distribution nodes on to the efferent nuclei:

1. Nucleus Ambiguus
2. Nucleus Dorsalis Nervi Vagi.

These efferent nuclei, in their turn, send impulses to the appropriate muscles and glands – the effect organs – that are to be activated. How these function during swallowing we will explain in more detail below.

In the FR there are three of these distribution nodes (swallowing centres) that are key to the swallowing process; and also a number of other centres that control, amongst other functions: breathing, chewing, coughing, vomiting, evacuation of the bowels and bladder, and those muscles that control the body’s posture.

1. Brain stem: controls non-voluntary “unconscious” automatic functions such as breathing, blood pressure, heart rhythm, the reflex swallowing phases; and also functions as a communication node between the cerebrum, the cerebellum, the spinal cord and the peripheral nervous system.
2. Cortex: controls our voluntary “conscious” most advanced functions such as language, thinking, fine-motor skills and the voluntary swallowing phases.
3. Cerebellum: co-ordinates our movements, our balance and our ability to act in response to our immediate surroundings.
4. Corpus callosum: connects the two brain hemispheres’ cortex areas with each other. It consists of some 200 to 800 million nerves that co-ordinate the activities of the brain’s two hemispheres

These afferent nerves transmit this stimulation to the brain. The food or the drink’s structure, taste and temperature, and its contact with the lips, cheeks and pharynx; as well as the tongue’s contact with the anterior palatal arch and the palate, stimulate the sensory nerves in the mouth and they send information to the brain stem via the sensory, upward-transmitting afferent nerve pathways. The four sensory nerves are:

The NTS in the brain stem, which gathers all incoming signals from the oral cavity and the pharynx, sends a part of this information directly to the first of the three swallowing centres in the brain stem, whilst the remainder of the information continues upwards to the cortex to be processed before being switched back to the first swallowing centre.

The nerves can be thought of as cables that contain various fibres, motor neurons, that conduct signals to the muscles and glands. ^[6] In the brain stem there are three different kinds of motor neurons that are important in the act of swallowing:

1. General Somatic Efferent (GSE)
   The GSE transmits signals onwards to the tongue’s and the eyes’ voluntary musculature, (skeletal striated muscles).
2. Special Visceral Efferent (SVE)
   The SVE transmits signals to the voluntary musculature in the mouth, chewing muscles, facial musculature, pharynx, larynx, esophagus and diaphragm. Note however that the neurological connection to the diaphragm is somewhat more complex.
3. General Visceral Efferent (GVE)
   The GVE transmits signals to the glands, blood vessels and smooth muscles in the pharynx, stomach and rectum.

The signal pathways from the above-named motor neurons are:

• CN (V) Trigeminus – signals via the SVE
• CN (VII) Facialis – signals via the SVE and the GVE
• CN (IX) Glossopharyngeus – signals via the SVE and the GVE
• CN (X) Vagus – signals via the SVE
• CN (XII) Hypoglossus – signals via the GSE

The sum of all the above signals to the brain stem set off a pre-programmed cooperation between all 148 muscles that are involved in the transport of each food bite from the mouth down to the stomach.

The food portion reaches the anterior palate arch and the reflex part of the process starts. The tongue’s movement backwards and upwards transports the food onwards towards the pharynx.

When the portion reaches the anterior palate arch, the swallow reflex takes over causing the larynx to rise and, at the same time, as the tongue begins its return movement forward, the epiglottis seals the airway and the food passes into the pharynx ^[2]. The pharynx phase takes between 0,5 and 1 second.

The esophagus’ longitudinal musculature is now activated, forming a stiff pipe and allowing the entrance to the upper esophagus – esophageal sphincter or the Posterior Esophageal Sphincter (PES) – to relax and open to allow the passage of the food portion into the esophagus.

At the same time the entrance to the lower esophagus – Lower Esophagus Sphincter (LES) or the upper stomach mouth – opens to complete the transfer of the food to the stomach. ^[2,4] This phase takes around 7 seconds to complete.

As well as the outer longitudinal layer of muscles, the esophagus has also an inner layer of circular musculature. To transport the food down to the stomach, the circular muscles produce coordinated wave motions – peristaltic waves. This explains why we can swallow even when we are lying down.

The muscle function and the downwards transport of the food mass is controlled by the Vagus nerve and a branch of the Glossopharyngeal nerve. Together these nerve pathways build a local network in the esophagus Plexus Pharyngeus. There are two different types of muscle in the esophagus: voluntary skeletal striated muscles and involuntary smooth muscles:

• The voluntary musculature is the same type as we have in our arms and legs: so-called skeletal striated muscles which are attached to the skeleton or tissue, and that are voluntarily controlled. They can be trained for increased strength and performance and can react quickly, but have relatively short term endurance.
• The smooth musculature has better long term endurance, but works relatively slowly. They cannot be controlled voluntarily but are instead controlled by the autonomic nervous system: functioning unconsciously and involuntarily. These muscles are stimulated via one of the three motor neurons in the brain stem which send signals to the involuntary musculature. Read more about this under the heading above: “The signals down to the muscles and glands are conducted via motor neurons in the brain stem”.^[7]
   

The esophagus’ upper third consists of skeletal striated muscles, the middle third is a mixture of skeletal striated muscles and smooth musculature, and the bottom third is solely smooth muscle. ^[6]

1. Svensson, P. (2008) I Hartelius, L., Nettelbladt, U. & Hammarberg, B. (red.). Logopedi. Lund: Studentlitteratur.
   (English translation, “Speech therapy”)
    
2. Logemann, J.A. (ProEd) (1998). Evaluation and treatment of swallowing disorders. (2^nd) Austin, Texas: San Diego College
    
3. O’Neill, P.A. (2000). Swallowing and prevention of complications. British medical bulletin, 56(2), 457-65.
    
4. Tibbling-Grahn L, broschyr 03 Svenska Dysfagiförbundet, Stoppar maten upp i bröstet när du äter? Mellangärdesbråck – En vanlig matstrupssjukdom i alla åldrar., (PDF).
   (English translation, “Does food get stuck in your chest when you eat? Hiatus hernia – a common medical condition in people of all ages”)
    
5. Kjellén G, Tibbling L. Manometric oesophageal function, acid perfusion test and symptomatology in a 55-year-old general population. Clinical Physiology. 1981; 1:405-15
    
6. Ekberg O, (2011), Röntgendiagnostiska avdelningen, Universitetssjukhuset MAS, Malmö, Normal sväljning inclusive anatomi och fysiologi, (PDF). Downloaded 2015-12-05
   http://media1.dysfagi.se/2011/06/svaljningssvarigheter.pdf 
   (English translation, ”Normal swallowing including anatomy and physiology”)
    
7. 1177.se, (2016), Muskler och senor, Downloaded 2016-09-20
   https://www.1177.se/gavleborg/liv–halsa/sa-fungerar-kroppen/muskler-och-senor/
   (English translation, ”Muscles and sinews”)