Chewing and swallowing

Definitions & Key takeaways

Chewing and swallowing are two important processes that allow us to eat and drink. Chewing breaks down food into small pieces that can be safely swallowed.

Swallowing is a complex process that involves many muscles. The tongue voluntarily pushes food into the back of the throat, called the pharynx. A flap of tissue called the epiglottis blocks the airway so that food doesn't go up the nose. Muscles in the pharynx involuntarily push food down into the esophagus, which carries food to the stomach.

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Introduction0:00–0:25

Some people eat to live, others live to eat. For both groups - the first step of digestion is chewing, or mastication.
This is where food gets moistened by saliva, and is broken down into smaller bits that are easy to swallow and pass through the esophagus.
It’s also the step that helps smear the food out over the tongue so that it can be fully tasted. The journey of food starts in the oral cavity, which is the first part of the digestive tract.

Oral cavity0:25–1:03

The oral cavity is like an empty room, there’s the roof, which is formed by the hard and soft palate, the floor, which is formed by the tongue and the mylohyoid muscles, the lateral walls formed by the inside of the cheeks, and there’s the front which gets sealed off by the lips and teeth.
A layer of epithelial cells line the inside of the mouth and form the first line of defense against pathogens. The surface of the epithelial cells is kept moist by mucus secreted by salivary glands.

Salivary glands1:03–2:15

The major salivary glands are actually located outside the oral cavity. These are the parotid glands, found in front of each ear, the submandibular or submaxillary glands, found under the mandible and the sublingual glands that sit beneath the tongue, under the floor of the mouth.
Now, even though the salivary glands are not located inside the mouth, they have ducts that travel to the oral cavity so they can secrete saliva into it.
The secretion of saliva is mainly controlled by the parasympathetic nervous system through cranial nerves. The parotid glands are innervated by the glossopharyngeal nerve, or cranial nerve nine, while the submandibular and sublingual glands are innervated by the facial nerve, or cranial nerve seven.
Saliva is mostly made of water, but it has other important components like salivary amylase, which is a digestive enzyme that breaks down starch, and mineral salts like sodium bicarbonate that help maintain a pH of 6.5 to 7.5 inside the mouth.
It also contains mucus which protects the oral mucosa from drying out from the air the blows by when we speak or whistle.

Mastication2:15–3:17

Ok, now when you take that first delicious bite of spaghetti with pesto sauce, you first have to chew the food so it’s easy to swallow.
This is called mastication and it involves moving the jaw, at the temporomandibular joint. This is coordinated by numerous muscles, including: the temporalis muscle, which is fan-shaped and on both sides of the cranium; the masseter muscle, which connects to the mandible and the zygomatic arch of the temporal bone; the medial pterygoid muscle, which connects to the mandible and medial pterygoid plate; and the lateral pterygoid muscle, located at the condylar process.
These muscles are innervated by branches of the trigeminal nerve, or cranial nerve five. And they work together to move the jaw up and down, allowing the teeth to grind and mechanically break down the food.
The tongue also comes in to help. It moves from side to side repositioning food and pushing it between the teeth for extra grinding.

Salivation3:17–4:24

Now, as we chew, the food gets detected by mechanoreceptors near the periodontal ligaments around the roots of each tooth, as well as by taste receptors on the tongue.
These receptors send information through the trigeminal, facial, and glossopharyngeal nerves to a group of salivatory nuclei in the brain stem.
The salivatory nuclei then send efferent information through the facial and glossopharyngeal nerves to the salivary glands, which secrete more saliva.
Now, it turns out that just the sight or smell of that spaghetti with pesto sauce can also cause the secretion of saliva, and it’s based on learned behavior.
In fact, this goes back to observations from the famous researcher Ivan Pavlov, who observed that his dog would start drooling at the sight of food.
Pavlov then trained his dog to associate food with a bell, and found that eventually the dog would drool whenever it heard the bell.
This is called classical conditioning. And now that you’ve seen this - you might crave spaghetti with pesto sauce when you hear this sound.
Ok, now as saliva mixes with food inside the mouth, the digestive enzyme amylase begins breaking down carbohydrates found in foods like potatoes and rice.

Swallowing4:24–5:22

And when the food is chewed and mixed with saliva, it creates a soft, mushy mass called a bolus, which is ready to be swallowed.
The saliva also lubricates the surfaces of the mouth and pharynx making it easier for the food to go down. So eventually the spaghetti with pesto sauce gets swallowed - which is also called deglutition.
And the food bolus moves from the mouth to the oropharynx, or the throat. Now, the pharynx is divided into three parts; the nasopharynx, oropharynx, and laryngopharynx.
And swallowing is coordinated by muscles under both voluntary and involuntary control, and it can be broken into three phases; the oral phase, the pharyngeal phase, and the esophageal phase.

Oral Phase5:22–5:44

The oral phase begins with the voluntary movement of the tongue. So, when the food is ready to be swallowed, the tip of the tongue lifts up and the base of the tongue moves backwards, pushing the bolus to the back of the mouth and oropharynx.
This initiates the involuntary pharyngeal phase, where the food bolus passes through the pharynx and into the esophagus.

Pharyngeal Phase5:44–6:55

During this phase, mechanoreceptors, thermoreceptors, and taste receptors in the pharynx and upper esophagus detect the food.
These receptors send afferent, sensory, information, through the trigeminal, glossopharyngeal, and vagus nerves to the medulla, which is where a region called the swallowing center is located that controls the involuntary phases of swallowing.
The swallowing center sends efferent, motor, information, mainly through the glossopharyngeal and vagus nerves to the muscles of the pharynx and upper esophagus.
The pharyngeal phase of swallowing begins when this efferent information causes the soft palate and uvula to move upwards, closing the path to the nasopharynx so food cannot reflux up into the nasal cavity.
At the same time, it makes the epiglottis, which is a flap-like structure, move downwards and cover the larynx to prevent food from entering the trachea and respiratory tract.
Finally, the upper esophageal sphincter, which is a muscular ring that guards the entrance to the esophagus, relaxes, allowing the food bolus to slip through.
The epiglottis then moves back to its original position to restore breathing. In the last phase, the esophageal phase, the food bolus passes into the esophagus and the upper esophageal sphincter closes to prevent food from refluxing back into the pharynx.

Esophageal Phase6:55–7:43

Then, the swallowing center, sends a message through the vagus nerve, to stimulate an involuntary muscular movement of the esophagus, called peristalsis.
Peristalsis is a wave like motion of that passes through the esophagus and squeezes the bolus downwards, to the lower esophageal sphincter which is the entrance to the stomach.
Finally, the lower esophageal sphincter relaxes and allows the food bolus to enter inside the stomach. With solid food, swallowing takes about 10 seconds to pass from the mouth to the stomach, whereas with non-solid food, like soup, it only takes about 1 second.

Review7:43–8:19

So to recap - When we eat, the first step is to break food down by chewing and amylase in the saliva so it’s easy to swallow.
Then, the step of swallowing begins which includes the voluntary oral phase, where the tongue pushes the food bolus to the pharynx.
This triggers the involuntary pharyngeal and esophageal phases, coordinated by the swallowing center in the medulla, which prevent food from refluxing back to the nasal cavity or entering the trachea and lungs.
Finally, a peristaltic contraction of the esophagus carries the food bolus downwards and into the stomach.