During a day a normal person swallows approximately 600 times: 350 of these are during the day, 200 when eating, and 50 times when asleep . Here we will look at all the phases of swallowing and how the signals flow from mouth – to brain – to pharynx, esophagus, diaphragm and stomach.
Simply described, the swallowing process starts when we transfer food from the plate to our mouths (the “pre oral phase”).
After this we close our lips, chew, reduce the food to manageable pieces and mix it with saliva. And then - immediately before the swallowing reflex is triggered - we press our lips together creating a low pressure in our mouths (the ’oral phase’ or ’mouth phase’), this activity takes up to 10 seconds. [2, 3]
The chewing is carried out with the tongue’s upwards and backwards movements towards the pharynx. At the same time the floor of the mouth is raised. The decrease in pressure in the mouth eases the transport of the food mixture from the mouth to the pharynx. [2, 3]
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: Nucleus Ambiguus and 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.
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 CN (cranial nerve) Trigeminus, Facialis, Glossopharyngeus and Vagus.
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.
If the combination of the information received from these two sources is interpreted as that something is to be swallowed, for example a piece of food, this instruction is sent to the second swallowing centre.
The second swallowing centre transmits signals to the muscles via the motor nerves – the downward-transmitting efferent nerve pathways. Here there is a pre-programmed ‘go/no-go’ response: ‘swallow’ or ‘don’t swallow’ - a so-called stereotypical muscle reponse. When something is to be swallowed the command is sent first to the Nucleus Ambiguus (NA) an efferent nucleus which, in its turn, sends the instruction to swallow to the major components of the swallowing musculature via the motor, downward-transmitting, efferent nerve pathways. Concurrently, the impulses are also sent to the third swallowing centre.
The five motor nerves that are important for swallowing are: CN Trigeminus, Facialis, Glossopharyngeus, Vagus and Hypoglossus. The first four are both sensory (afferent) and motor (efferent) nerve pathways which send information both to and from the brain.
The third swallowing centre transmits information to the Nucleus Dorsalis Nervi Vagi (NDNV) an efferent nucleus, and then onwards to the esophagus’ musculature to complete the swallowing action and to transport the food portion downwards to the stomach.
The nerves can be thought of as cables that contain various fibres, motor neurons, that conduct signals to the muscles and glands. In the brain stem there are three different kinds of motor neurons that are important in the act of swallowing:
The GSE transmits signals onwards to the tongue’s and the eyes’ voluntary musculature, (skeletal striated muscles).
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.
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:
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 . The pharynx phase takes between 0,5 and 1 second.
The coordination between breathing and swallowing is essential to avoid food ‘going down the wrong pipe’.
Breathing and swallowing are guided by different centres in the brain stem. However, all the muscles that are active in these two functions are controlled from the same concentrated grouping of specialised nerve cells, nucleus, in the brain stem. This means that for a couple of breaths during the act of swallowing, the swallowing centre takes control.
To prevent food being ingested into the air pathway, the body ‘holds its breath’ for two seconds, thus twice as long as it takes for the swallow to pass the pharynx (see the Pharynx phase, as described above). When the oral phase signals that swallowing is underway, the body breathes in. During the subsequent exhalation, the food portion is driven to the back of the tongue and the exhalation stops and the body holds his or her breath for two seconds. The swallow then passes the pharynx and breathing is resumed with a continued exhalation. This exhalation, which is controlled by one of the brain’s swallowing centres, drives aromas up into the nose and the olfactory nerves, which is crucial for the smell and taste experience. Only now, after this has happened passed, does the person breathe in again.
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 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. 
Dr. Mary Hägg, Doctor of Medicine, Post-doctoral researcher at Uppsala University specialising in orofacial medicine, Head of Department at the Speech and Swallowing Centre: Ear, Nose and Throat Clinic at Hudiksvall Hospital, Sweden. Registered Dentist. In cooperation with MYoroface.
(Translation from the original Swedish text by T.W.Morris, MYoroface.)
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