Back To CourseBiology 105: Anatomy & Physiology
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When chewing on a juicy steak, there's a lot more to what goes on than meets the eye. Besides the obvious preparation of the food, there are four major nerves involved in helping you taste, chew, and swallow the delicious meat. In addition, the way you relay your satisfaction with the food to others through facial expressions and words has to do with these four nerves as well.
Assuming you're not a vegetarian, the smell of a nice steak is going to bring a smile to your face. You use your smile to nonverbally communicate your pleasure. The most important nerve controlling muscles of facial expression, including those involved in a smile, is unsurprisingly called the facial nerve, also known as cranial nerve VII.
In addition to helping you show your satisfaction at the thought of eating a nicely cooked dinner, the facial nerve has other critical functions involved in eating that delicious meal. As you put a piece of the steak into your mouth, you're going to experience a pleasant taste. This is mainly thanks in part to the fact that the facial nerve also conveys the sensation of taste from the anterior two-thirds of your tongue.
And, even before you put the steak in your mouth, there is saliva inside of your mouth that will help in the process of digestion. Two out of the three of the most important salivary glands are innervated by the facial nerve. These glands are known as the submandibular and sublingual salivary glands. Combined, these two glands produce the vast majority of the saliva in your mouth.
Finally, you may enjoy and love your meal so much that you'll begin to cry tears of joy. This is thanks to the fact that the facial nerve also innervates the lacrimal gland, the gland that produces tears.
As you cry tears of joy at the awesome taste in your mouth, all thanks to the facial nerve, you'll want to wipe those tears away from your eyes and cheek. As you wipe away the tears, you can sense the cloth against your skin. This is all thanks to the fact that the most important nerve involved in facial sensation is called the trigeminal nerve, also known as cranial nerve V.
Hence, while CN VII provides motor movement for the muscles of facial expression, CN V is the most important nerve in facial sensation. In addition, CN V is the nerve that innervates the muscles that help move your jaw up and down in order to chew that savory steak. Hence, CN V innervates the muscles of mastication.
In between chewing your food and enjoying its sensation, you'll probably want to express to whomever cooked the meal how much you enjoyed it. It did make you cry, after all; that's how good it was. One of the most important structures involved in speech is your tongue. Try holding your tongue with your clean fingers and tell me how well you can speak out loud. You're not going to do too well.
In order to get the muscles of your tongue to move about and help you express your sincere appreciation of the food, you're going to have to have a nerve responsible for the innervation of those muscles. Hence, the nerve that innervates the muscles of the tongue is called the hypoglossal nerve or cranial nerve XII.
The tongue isn't just for speech. It helps you chew your food and helps to move food toward the back of the tongue and mouth so it can be swallowed.
As the tongue helps to move the food you've chewed to the back of the tongue, the molecules of food that give rise to taste will activate taste sensors located in the back one-third of your tongue. If you recall, CN VII is responsible for taste sensation in the anterior two-thirds of the tongue. Well, it's the glossopharyngeal nerve, also known as cranial nerve IX, that is the nerve responsible for the sensation of taste in the posterior one-third of the tongue.
In addition, cranial nerve IX is a nerve that innervates one of the three major salivary glands, called the parotid gland. The other two salivary glands, if you remember, were innervated by the facial nerve. Finally, CN IX is like CN XII, as it also innervates the muscles involved in swallowing your food.
To help you remember what each of these nerves is responsible for, here is a little story.
The facial nerve is also known as cranial nerve number VII. Think of rolling a lucky 7 on a die at a casino! What would happen if you rolled a lucky 7? You would smile, cry tears of joy, salivate at the thought of all of that money, and taste the sweet victory on the tip of your tongue. Therefore, cranial nerve VII is responsible for facial expressions such as smiling, hence the reason why it's called the facial nerve! It also innervates the lacrimal glands for crying, the salivary glands for salivation, and the taste buds at the front of your tongue for tasting that sweet victory!
The trigeminal nerve is known as cranial nerve number V. Think of the five fingers on your hand. Now that you've won all of that money, you can buy three really big gems to put on your fingers. Visualize using the five fingers of your hand, three of them with really big diamond rings, putting finger food into your mouth so it can be chewed. In addition, your face can feel your five fingers pressing against your lips as they touch your lips.
Your face's ability to feel the five fingers and chew the food your five fingers put into your mouth is all thanks to the fifth cranial nerve that innervates muscles of mastication and gives you the ability of facial sensation. The three (tri) gems on your five fingers should tell you the trigeminal nerve is also known as cranial nerve number V.
After getting rich, you become a powerful businessman who has put on a few pounds after eating a bit of that five-finger food. Hence, people begin to stereotypically call you a 'fat cat.' You also get a fat cat as a pet. I'm sure you know that cats have nine lives, and our ninth cranial nerve is called the glossopharyngeal nerve. The 'glosso' in 'glossopharyngeal' refers to the tongue and 'pharyngeal' refers to the pharynx, which helps you swallow.
Your poor fat cat has a lot health issues due to his weight. Therefore, you need to give your fat cat, who has nine lives like all cats, some pills to keep him healthy and help him keep all of his nine lives. In order to give him his medicine, you need to put the pill at the back of his tongue, where he will hate the taste of the pill, so that he can swallow it. Hence the ninth cranial nerve is involved in taste sensation at the back of the tongue, 'glosso,' and in swallowing, thanks to the pharynx.
Since your poor cat had a bitter pill to swallow, you decide to buy him a dozen - that's right, twelve - doughnuts to spoil him. Now I know exactly why he's so fat. Since the twelve doughnuts are a little bit too big for the cat to swallow, he decides to lick the frosting off of the top of the twelve doughnuts with his tongue. The twelfth cranial nerve innervates the tongue used to lick the twelve doughnuts. It's called the hypoglossal nerve - again, 'glossal' referring to the tongue.
P.S. As a veterinarian, I do not recommend you give your cat doughnuts for a wide variety of reasons.
Let's review what the cranial nerves do one more time.
The facial nerve, also known as cranial nerve VII, is the most important nerve controlling muscles of facial expression. It is responsible for the sensation of taste in the anterior two-thirds of your mouth. It also innervates two salivary glands and the lacrimal gland.
The trigeminal nerve, also known as cranial nerve V, is the most important nerve involved in facial sensation and innervates the muscles of mastication.
Your tongue also helps you chew, or masticate, your food. It is innervated by the nerve that innervates the muscles of the tongue, called the hypoglossal nerve or cranial nerve XII. Don't forget that this nerve also helps you speak, as it's kind of hard to do so without your tongue.
Finally, the glossopharyngeal nerve, also known as cranial nerve IX, is a nerve that is responsible for the sensation of taste in the posterior one-third of the tongue and, like the hypoglossal nerve, innervates muscles involved in swallowing that juicy piece of steak.
You will be able to describe the functions of the facial nerve, trigeminal nerve, hypoglossal nerve and glossopharyngeal nerve after watching this video lesson.
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Back To CourseBiology 105: Anatomy & Physiology
16 chapters | 179 lessons | 15 flashcard sets