Humans make many different facial expressions and they are an extremely important way that we communicate. They let people know when you are happy, like after getting a good mark on an anatomy test, or curious like when you learn something new about the human body. Let’s learn about the muscles responsible for our facial expressions!
First, let’s start with the scalp which is a layer of skin and subcutaneous tissue that covers the cranium. It extends from the supraorbital margins on the frontal bone to the superior nuchal line on the occipital bone.
Laterally, it covers the temporal fascia and extends to the zygomatic arches on each side. The scalp is made of five layers, which can be remembered easily as the first letter of each spells out the word SCALP.
Going in the order from superficial to deep: S stands for skin, C stands for connective tissue, A stands for aponeurosis or epicranial aponeurosis, L stands for loose connective tissue, and P stands for pericranium, which is the periosteum on the external surface of the cranium.
The first three layers - skin, connective tissue, and the aponeurosis - are connected tightly together, forming a single unit called the scalp proper.
Muscles of facial expression, simply known as the facial muscles are found deep to the skin of the scalp, face, and neck.
Most facial muscles are attached to bones or fascia on one end, and skin on the other, so that when they contract they create facial expressions.
All facial muscles originate from the mesoderm of the second pharyngeal arch, along with the facial nerve. During embryological development, a muscular sheet forms and begins to spread over the scalp, face and neck which later forms the facial muscles.
This spreading carries the branches of the nerve of the second arch with it, which is the facial nerve, or cranial nerve seven.
So, all facial muscles are innervated by the branches of the facial nerve which are the posterior auricular, temporal, zygomatic, buccal, marginal mandibular, and cervical nerves.
A way to remember these last five branches that emerge from the parotid gland is by using the mnemonic “To Zanzibar By Motor Car!”
Now, let’s look at each of the facial muscles! First up, the occipitofrontalis muscle which has two bellies. The two bellies are connected by the epicranial aponeurosis, which is the aponeurosis that makes the third layer of the scalp.
The frontal belly originates on the epicranial aponeurosis and inserts on the skin superior to the eyes and nose. It is innervated by the temporal nerve.
The occipital belly originates on the superior nuchal line of the occipital bone and inserts on the epicranial aponeurosis. The occipital belly is innervated by the posterior auricular nerve.
Now, when the frontal belly contracts, it protracts the scalp and when the occipital belly contracts it draws the scalp back. When they work together they elevate the eyebrows and wrinkle the forehead, making you look surprised.
Next up, is the orbicularis oculi muscle, which has two main parts: the palpebral part and the orbital part. The palpebral part is found mostly within the eyelid, and it originates on the medial palpebral ligament and inserts on the lateral palpebral ligament.
The orbital part originates from the medial orbital margin and medial palpebral ligament and inserts on the skin around the orbit, forming a continuous ring that surrounds the orbital orifice.
The superior part of the orbicularis oculi muscle is innervated by the temporal nerve, while the zygomatic nerve innervates the inferior part of the muscle.
The function of the orbicularis oculi muscle is to close the eyelids. The palpebral part closes the eyelids gently, like when you blink or sleep, while the orbital part closes the eyelids tightly, like when you wink or squeeze the eyes shut.
Ok, let’s move on to the muscles of the eyebrows and the nose. The muscle of the eyebrows is called the corrugator supercilii, which sounds like a spell you’d learn at Hogwarts, but is actually a small muscle found deep in the eyebrows and is innervated by the temporal nerve.
Corrugator supercilii originates on the medial part of the superciliary arch and extends laterally to insert onto the skin of the medial half of the eyebrow.
When it contracts, the corrugator supercilii pulls eyebrows medially and inferiorly, making vertical wrinkles at the root of the nose, which occurs when frowning.
Now let’s move on to the nose. There are three muscles of the nose: the procerus, the nasalis, and the levator labii superioris alaeque nasi.
All three muscles of the nose are innervated by the zygomatic nerve. The procerus originates on the nasal bone and the lateral nasal cartilage and inserts onto the skin in the lower part of the forehead between the eyebrows.
When it contracts, it depresses the medial ends of the eyebrows, and creates transverse wrinkles on the dorsum of the nose. The largest muscle of the nose is the nasalis, which is made of two parts: a transverse part and an alar part.
The transverse part originates on the maxilla on either side of the nose, and extends towards the dorsum of the nose meeting in the midline to insert on an aponeurosis.
The alar part also originates on the maxilla, over the lateral incisor, and inserts on the alar cartilage. The transverse part narrows the nostrils and the alar part expands - or flares - them.
The levator labii superioris alaeque nasi originates on the frontal process of the maxilla and inserts on the upper lip and the alar cartilage of the nose. It flares the nostrils and raises the upper lip.
Ok, now let’s take a short break and try to remember which facial muscle closes the eyelids.…. Right, it’s the orbicularis oculi muscle!
Now let’s tackle some muscles in the oral region! First up, is the orbicularis oris muscle, which encircles the mouth. It originates on the maxilla, the mandible and on the angle of the mouth, called the modiolus.
All of the fibers insert on the mucous membrane of the lips. The orbicularis oris muscle is innervated by the buccal nerve and its function is to close the mouth, by bringing the lips together, and protrudes the lips, like when kissing or whistling. So, next time you are whistling a happy tune, remember it’s the orbicularis oris that does the work.
As for the muscles of the lips, we’ll divide them into the muscles of the upper lip and the muscles of the lower lip.