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Major muscles of facial expression and mastication, and neck muscles.
Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be looking at the main muscles of the head and neck. The head which you can see in the frontal view without the skin on the right is the most superior structure of the human body. It is attached to the trunk of the body by the neck and contains the brain, the ears, the eyes, the nose and the mouth. In this tutorial, we will look at some of the main muscles that are found on the head and the neck including the muscles of mastication, the muscles of facial expression, the suprahyoid and infrahyoid muscles and the muscles of the lateral neck.
Let's begin by taking a look at the muscles of mastication.
The first muscle of mastication that we'll look at is the temporal muscle. This muscle is also known by its Latin name of temporalis. Flat and fan-shaped, the temporal muscle is located on the lateral side of the skull and you can see its shape on the right. Its origin is located in the temporal fossa which is a large shallow depression on the lateral aspect of the skull in which you can see highlighted in green on the right. The fibers of this muscle then descend through the space between the zygomatic arch and the skull and insert via a thick tendon on the coronoid process of the mandible.
The temporal muscle is innervated by the deep temporal nerves which are branches of the mandibular nerve – the third division of the fifth cranial nerve, the trigeminal nerve. The temporal muscle can be divided into two parts according to function. The anterior part of the muscle moves the mandible forward or facilitates the protrusion of the mandible and the posterior part of the muscle on the other hand facilitates elevation and retraction of the mandible. In other words, it pulls the mandible upwards and backwards.
Activation of both the anterior and posterior parts of the muscle elevates the mandible leading to the strong closure of the jaw. It should be noted that the temporal muscle is the most powerful muscle of the temporomandibular joint. Because of its size, this muscle can be palpated relatively easily especially when opening and closing the mouth.
The next muscle of mastication we will look at is the masseter muscle. Although the temporalis is the most powerful muscle of the temporomandibular joint, the masseter is the most powerful muscle of mastication which is another term for chewing. This thick rectangular-shaped muscle is comprised of both the superficial and deep part. The two parts of this muscle both originate from the zygomatic arch. However, the superficial part of the muscle inserts on the masseteric tuberosity on the outer surface of the angle of the mandible while the deep part of the muscle inserts onto the outer surface of the ramus of the mandible. The entire muscle is innervated by the masseteric nerve – a branch of the mandible nerve which is the third division of the fifth cranial nerve, the trigeminal nerve.
The function of the masseter muscle is to elevate the mandible causing a powerful closure of the jaw. When the superficial part of the muscle contracts, it causes the protrusion of the mandible by moving it forward. In addition to closing the jaw, the masseter also helps stabilize tension of the articular capsule of the temporomandibular joint. This muscle can be palpated from the oral cavity at the insertion point of its deep fibers.
The third muscle of mastication we will look at is the lateral pterygoid muscle which is one of two pterygoid muscles of the jaw. This muscle lies deep to the masseter muscle and superficial to the medial pterygoid muscle which we'll look at later. The lateral pterygoid muscle is a two-headed muscle and its two heads lie horizontal to each other. The superior head of the muscle has its origins in the infratemporal crest of the greater wing of the sphenoid bone and inserts into the articular disc of the temporomandibular joint.
The inferior head of the lateral pterygoid muscle originates from the pterygoid process of the sphenoid bone and inserts on the condylar process of the mandible. The entire muscle is innervated by the lateral pterygoid nerve – a branch of the mandibular nerve. Unlike the muscles of the mastication, the lateral pterygoid muscle functions to open the jaw. The bilateral contraction of the muscle also results in protrusion of the jaw whereas unilateral contraction moves the mandible laterally. This movement is otherwise known as the laterotrusion.
The final muscle of mastication we'll look at is the medial pterygoid muscle. As you can see, it is located inferior to the lateral pterygoid. The medial pterygoid muscle also has two heads – a deep head and a superficial head. The fibers of the medial pterygoid muscle originate from the pterygoid fossa and the pterygoid process of the sphenoid bone and insert distally on the pterygoid tuberosity of the inner surface of the mandible. It is innervated by the medial pterygoid nerve which is also a branch of the mandibular nerve. Contraction of this muscle elevates the jaw closing it and protruding the mandible.
Now that we've finished with the muscles of mastication, let's look at the muscles of facial expression. Before we go on to talk about the individual muscles, I just want to point out that we won't be covering all the muscles of facial expression within this tutorial, however, the muscles that we will be looking at can be divided into three groups – the orbital muscles, the nasal muscles and the oral muscles.
Let's begin by looking at the orbital muscles. The first muscle of facial expression I want to talk about is the corrugator supercilii seen here highlighted in green. This muscle has its origin in the superciliary arch. It inserts on the skin of the eyebrow and is innervated by the seventh cranial nerve – the facial nerve. This muscle draws the eyebrows medially and downwards, for example, when we are frowning. It is therefore also known as the frowning muscle.
Another muscle of facial expression that belongs to the orbital group is the orbicularis oculi muscle. This muscle encircles the orbit and extends into the eyelid. It is comprised of three parts – an orbital part, a palpebral part and a lacrimal part. The entire muscle is innervated by the temporal and zygomatic branches of the facial nerve. The orbicularis oculi muscle has several functions. For instance, the palpebral part facilitates the gentle closure of the eyelid while the orbital part facilitates the forcible closure of the eyelid. In addition, the muscle also helps with the distribution of tear film and the drainage of tears by aiding in the emptying of the lacrimal sac.
Now that we're finished looking at the orbital group of the muscles of facial expression, let's look at the nasal group. The procerus muscle has its origins in the nasal bridge and inserts into the skin between the eyebrows. It is innervated by the facial nerve and activation or contraction of this muscle draws the eyebrows downwards resulting in the formation of transverse wrinkles over the nose.
Also belonging to the nasal group of muscles of facial expression is the nasalis muscle. This muscle has its origin in the maxilla inserting into the aponeurosis on the dorsum of the nose as well as the nasal cartilages. It is innervated by the buccal branch of the facial nerve. The nasalis muscle also known as the naris muscle can be divided into two parts – each of which perform a different function. The transverse part of the muscle also known as compressor nasalis compresses the nares whereas the alar part of the muscle also known as dilator nasalis opens the nares.
Let's now move on to the oral group of the muscles of facial expression. The first muscle in the oral group that we'll look at is the zygomaticus minor muscle. It has its origin in the body of the zygomatic bone and inserts in the skin of the upper lip at the angle of the mouth, adjacent to the insertion of the levator labii superioris muscle. The zygomaticus minor muscle is innervated by the facial nerve. Contraction of this muscle elevates the upper lip. As such, it assists in smiling.
The zygomaticus major muscle also belongs to the oral group of muscles and is situated lateral to the zygomaticus minor. It has its origin in the zygomatic bone lateral to the origin of the zygomaticus minor muscle and inserts into the skin at the angle of the mouth in and around the orbicularis oris muscle. The function of the zygomaticus major muscle is to raise the angle of the mouth superolaterally thereby assisting in smiling. It is also innervated by the facial nerve.
The buccinator muscle which lies deep to the other muscles of facial expression forms the lateral wall of the buccal region in the oral cavity. It has its origins in the maxilla and mandible and inserts at the angle of the mouth. Like all the muscles of facial expression, the buccinator is innervated by the facial nerve. Contraction of this muscle pulls the cheeks inwards. It assists in mastication by keeping food on the occlusal surface of the posterior teeth during chewing.
The next muscle – the risorius muscle – is also commonly referred to as the laughing muscle. This thin muscle has its origins in the fascia of the masseter and inserts into the skin at the angle of the mouth. It is also innervated by the facial nerve. The reason the risorius is also known as the laughing muscle is because it stretches the lips laterally retracting the labial commissure when it contracts and effectively widening the mouth such as when smiling.
The orbicularis oris muscle encircles the mouth as well as shapes and controls the size of the opening of the mouth. Its fibers insert into the skin and the mucous membranes of the lips. The function of this muscle is to move the lips. For example, pursing the lips, parting the lips, grimacing and closing the lips. It is innervated by the facial nerve.
The depressor anguli oris muscle also belongs to the oral group of muscles of facial expression. This triangular-shaped muscle has its origin on the inferior border of the mandible. It inserts into the skin at the angle of the mouth. As the name suggests, the contraction of this muscle depresses the angle of the mouth. For example, when frowning. It is also innervated by the facial nerve.
The final muscle of facial expression that we will cover in this tutorial is the mentalis muscle. This muscle has its origin in the mandible near the midline and inserts into the skin of the chin. It also forms the furrow between the lower lip and the chin. Contraction of this muscle raises the chin protruding the lower lip and narrowing the oral vestibule. It is also innervated by the facial nerve.
Now that we've finished looking at the muscles of facial expression, let's look at the suprahyoid and infrahyoid muscles beginning with the suprahyoid muscles. The suprahyoid muscles assist in both mastication and swallowing. There are four suprahyoid muscles, however, we'll only cover three of them in this tutorial with the stylohyoid muscle being covered as a separate tutorial. Let's begin by looking at the digastric muscle.
The digastric muscle is comprised of two parts – an anterior belly and a posterior belly – which are separated by an intermediate tendon. The anterior belly of the digastric muscle originates in the digastric fossa of the mandible. Because the anterior belly of the digastric muscle arises from the first pharyngeal arch embryologically, it is innervated by the mylohyoid nerve which is a branch of the mandible nerve. On the other hand, the posterior belly has its origin on the mastoid notch of the temporal bone. Because the posterior belly of the digastric arises from the second pharyngeal arch, it is therefore innervated by the facial nerve. The fibers of both bellies insert onto the intermediate tendon. Functionally, the digastric muscle elevates the hyoid bone and depresses the mandible.
The mylohyoid muscle is situated deep to the digastric muscle. It has its origin in the mylohyoid line of the mandible. In this image, you can see its fibers arising from the mylohyoid raphe at the midline. It inserts anteriorly on the body of the hyoid bone. The mylohyoid muscle is innervated by the mylohyoid nerve, a branch of the mandibular nerve. The mylohyoid muscle forms the floor of the mouth and functions to elevate the hyoid bone and depress the mandible.
The third suprahyoid muscle we'll be looking at is the geniohyoid muscle seen here. This muscle has its origin in the mental spine on the inner surface of the mandible. It then radiates posteriorly and inferiorly to insert onto the body of the hyoid bone. It is innervated by branches of the cervical plexus as well as the cervical nerves C1 and C2 which are accompanied by the twelfth cranial nerve – the hypoglossal nerve. The function of the geniohyoid muscle is to elevate the hyoid bone and the floor of the mouth.
Now let's have a look at the infrahyoid muscles. Amongst the infrahyoid muscles, there's only one muscle we'll have a look at today – the omohyoid muscle. The omohyoid muscle can be divided into two parts – a superior belly and an inferior belly. The superior belly of the omohyoid arises from this intermediate tendon and goes on to insert on the lateral border of the hyoid bone. The inferior belly of the omohyoid muscle originates in the superior border of the scapula. From here, it ascends craniomedially crossing the internal jugular vein and emerging into an intermediate tendon. Like all the infrahyoid muscles, the omohyoid muscle is innervated by the deep cervical ansa which arises from the cervical plexus. Contraction of this muscle depresses the hyoid bone.
Now that we've finished looking at the suprahyoid and infrahyoid muscles, let's look at our last group of muscles of the head and neck – the muscles of the lateral neck. The sternocleidomastoid muscle has two muscle heads – a sternal head and a clavicular head. The sternal head of this muscle has its origin in the manubrium of the sternum while its clavicular head has its origin on the medial part of the clavicle. The entire muscle inserts superiorly on the mastoid process of the temporal bone and on the superior nuchal line of the occipital bone. It is innervated by the eleventh cranial nerve – the accessory nerve – and by direct branches of the cervical plexus. Bilateral contraction of this muscle extends the head leading to dorsal extension of the head whereas unilateral contraction of the muscle flexes the cervical spine laterally to the same side and rotates the head to the opposite side. It is also an accessory muscle of inspiration when the head is in a fixed position.
Next let's have a look at the scalene muscles which are paired cervical muscles. There are three scalene muscles – the anterior scalene muscle, the middle scalene muscle and the posterior scalene muscle. Note that the image on the right shows the anterior scalene muscle. Also note that all three muscles are innervated by direct branches of the cervical and brachial plexuses.
Let's start with the anterior scalene muscle seen here. This muscle has its origins in the anterior tubercles of the transverse processes of the third to sixth cervical vertebrae and it inserts onto the first rib. The bilateral contraction of this muscle results in ventral flexion of the cervical spine while unilateral contraction results in lateral flexion of the cervical spine to the same side. In addition, this muscle also elevates the ribs thereby acting as an accessory muscle of inspiration.
The second of the scalene muscles is the middle scalene muscle which has its origins in the posterior tubercles of the transverse processes of the third to seventh cervical vertebrae. This muscle has its insertion on the first rib. In the same way as the anterior scalene muscle, the unilateral contraction of the middle scalene muscle results in lateral flexion of the cervical spine to the same side. It also elevates the ribs and is therefore an accessory muscle of inspiration.
The third scalene muscle – the posterior scalene muscle – has its origins in the posterior tubercles of the transverse processes of the fifth to seventh cervical vertebrae. The entire muscle inserts onto the second rib. In terms of its function, it raises the second rib, thereby, acting as an accessory muscle of inspiration. The function of this muscle also results in lateral flexion of the cervical spine to the same side.
The final muscle that we'll look at is this flat paired muscle of the superficial neck called the platysma. This superficial cutaneous muscle is directly connected to the skin and it extends from the skin of the face slightly above the insertions of the lower jaw to about the height of the second rib. This muscle is involved in changing facial expression but also involved in wrinkling the neck, depressing the corners of the mouth and depressing the mandible as well. It is innervated by the seventh cranial nerve – the facial nerve. As we age and the skin increasingly loses its elasticity, the inner faces of the platysma form two longitudinal wrinkles on the neck.
Now that we've discussed some of the main muscles of the head and neck, I'd like to discuss the medical condition that affects some of the facial muscles known as Bell's palsy. Bell's palsy is a condition defined by the paralysis, weakness or inability to move facial muscles. This condition is caused by damage to the facial nerve. It usually results in the inability to move one side of the face and affects speech, chewing and facial expression. It can be caused as a result of a number of factors including inflammation of the nerve due to herpes zoster infection, stroke tumors or as a result of fracture of the petrous bone.
Now that we've come to the end of this tutorial, let's quickly remind ourselves of the muscles we've looked at today. Within the muscles of mastication, we looked at the temporal muscle, the masseter muscle, the lateral pterygoid and the medial pterygoid. While within the muscles of facial expression, we looked at the orbital group which includes the corrugator supercilii and the orbicularis oculi. The nasal group which includes the procerus and the nasalis and the oral group which contains the zygomaticus minor, the zygomatic major, the buccinator, the risorius, the orbicularis oris, the depressor anguli oris and the mentalis. We also looked at the suprahyoid muscles including the digastric muscle, the mylohyoid, the geniohyoid, and remember that the stylohyoid although part of the suprahyoid muscle group was not discussed in this tutorial. The main infrahyoid muscle we looked at today was the omohyoid muscle. And finally in the last group of muscles the muscles of the lateral neck, we looked at the sternocleidomastoid muscle, the scalene muscles which included the anterior, middle and posterior scalene muscles as well as the platysma.
For our clinical note, we looked at Bell's palsy, a one-sided paralysis of the face.
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Now, good luck everyone, and I will see you next time.