Chewing and swallowing

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.

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.

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 that blows by when we speak or whistle.

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.

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.