Sunday, July 10, 2011

Lecture Notes for Muscles of the Head and Neck

Humans have well developed facial musculature that allows for a variety of complex facial expressions needed for non-verbal communications. Since we are social animals this is a necessity. Animals that are not social have less need for facial expressions and do not have well developed facial muscles.
Muscles of Facial Expression
Muscles of Facial Expression the muscles of facial expression are located in a superficial position on the scalp, face and neck. These muscles all originate on the bones if the skull or in the fascia and insert into the skin. They are innervated by the facial cranial nerve (Cranial nerve VII) . The locations and points of attachment are such that when  contracted, they cause move-ments of the skin around the eyes, nose or mouth. Muscles of the face move skin rather than a joint when they contract. The  nerves of the face are located right under the skin  and are vulnerable to trauma. The  pattern of enervation of the facial muscles becomes apparent in  stroke victims and in a condition known as Bell’s Palsy, which is the paralysis of the facial cranial nerve.

There are 2 types of muscles of the face which function in opposing ways. Sphincter muscles are circular shaped muscles which close orifices of the body. Dilators have the opposite effect- they open orifices.The muscles of the face can be divided into several groups. These are muscles of the scalp, muscles of the eye, nose and mouth and muscles of mastication (chewing).

Muscles of the scalp
The frontalis muscle located on the forehead wrinkles the forehead and raises the eyebrows wrinkling the skin horizontally. It draws the scalp anteriorly.
The ocipitalis draws the posterior of the scalp. The ocipitalis and frontalis are connected by a  sheet of fascia called the galea aponeurotica. They can be considered separate muscles or part of a larger group known as the Ocipitofrontalis muscle. In conjunction with the temporoparietalis muscle above the ear the ocipitofrontalis is referred to as the Epicranius muscle group.
The temporalis muscle elevates and retracts the mandible.
The auricular muscles (anterior, posterior and superior) sit above the temporalis and encircle the external ear (pinna). They are vestigial,  since we are no longer able to rotate our ears as other mammals do. If you can wiggle your ears, the auricular muscles are responsible for that feat.

Muscles of the eye
Around the eye, the Obicularis oculi closes the eye and the levator palpebrae superioris opens the eye. The corrugator muscle above the eyebrow is responsible for frowning (vertical furrows above the glabella). Frowning is a result of the eyebrows coming together.

Muscles of the mouth
Around the mouth, the obicularis oris closes  the mouth and several muscles (zygomaticus major, levator labii superioris, depressor labii inferioris, mentalis and risorius ) radiate out from the lips to open the mouth. At the corners of the mouth there is a small area of muscle attachment ( a fibromuscular condensation) known as the Modiolus. Six muscles of the mouth attach to the modiolus (Zygomaticus major, levator angulis oris, risorius, depressor anguli oris, buccinator and obicularis oris). The cheek is made up of the buccinator muscle which functions in whistling, blowing, sucking and chewing. The  risorius muscle is responsible for drawing the angle of the mouth laterally.The muscles that radiate upward from the mouth (levator labii superiorus, levator anguli oris) are responsible for elevating the upper lips.
Zygomaticus major and minor are responsible for retracting the corners of the mouth in a smile or grimace.
Risorius which radiates from the mouth laterally, pulls the corners of the mouth laterally.
Obicularis oris closes and purses  the mouth.
Depressor anguli oris depresses the corners of the mouth.
Depressor  labii inferioris  depresses the lower lip.
Buccinator compresses the checks.
Mentalis protrudes the lower lip.

Muscles of the nose
The nasalis muscle which sits over the nose is responsible for flaring and compressing the nostrils another action that is somewhat subdued in humans.The procerus muscle located above the bridge of the nose (glabella) is resposible for  creating horizontal wrinkles at the bridge of  the nose.

Facial expressions
Joy: Smiling and laughing involves action of the zygomaticus, labii superioris, buccina-tor and risorius to produce a smile and the obicularis oculi which narrows or closes the eyes.
Sadness: also involves the obicularis oculi particularing if crying as well as  depressor muscles of the mouth and lip which tend to depress the corners of the mouth.
Anger:  can involve corrugator and procerus muscles (frowning). Obicularis oris, de-pressor labii in  tensing of the lips and mouth, masseter in clenching of the jaw. Curling of the upper lip (snarling) sometimes occurs. This is due to the action of the levator labii superiorus. Snarling reveals the canines which is also seen in other mammals.
Fear and surprise: both involve frontalis in raising of the eyebrows. Also involved  are the levator palpebrae superioris in the action of opening the eyes wide. The muscles of the mouth (obicularis oris, buccinator, risorius and mentalis) are involved in the opening of the mouth seen in both suprise and fear. In the case of fear, screaming may involve the muscles of the jaw as well.
Disgust:  involves muscles  surrounding  the mouth and chin in pulling the lips together and downward and also puckering and pursing the lips. Also involved are the obicularis oculi and the corrugator and procerus muscles in frowning and wrinkling the nose. The nasalis may be involved in that action as well.

Muscles of Mastication ( chewing)
The muscles  that are responsible for mastication (chewing) are the temporalis, the masseter and the pterygoids (lateral and medial).
The temporalis elevates and retracts the mandible. The masseter elevates the mandible as well. The medial pterygoid elevates the mandible and moves it laterally. The lateral pterygoid  protracts the mandible. These actions allow the teeth to grind food.

Muscles of the tongue
The tongue has both intrinsic and extrinsic muscles to control movement of the tongue in eating, speaking and swallowing. Intrinsic muscles are located within the tongue and are responsible for its mobility and changing shape. The extrinsic  muscles originate on structures outside the tongue and insert into it to cause large movements. The 4 paired extrinsic muscles are the genioglossus, styloglossus, hyoglossus and palatoglossus. These muscles are innervated by Cranial Nerve XII- the hypoglossal nerve.
Ocular muscles
Movement of the eyeball is controlled by 6 extrinsic ocular muscles. Four rectus muscles move the eyeball in the direction indicated by their names (superior, inferior, lateral and medial.) Two oblique muscles (superior & inferior) rotate the eyeball on its axis.
Muscles of the Neck
Muscles of the neck either support and move the head or are attached to structures  within the neck region such as  the hyoid bone and the larynx.
The posterior muscles of  the neck include the sternocleidomastoid, trapezius, splenius capitus, semispinalis capitus and longissismus capitus.
The sternocleidomastoid muscle:  originates on the sternum and clavicle and inserts on the mastoid process of the temporal bone. When contracted on one side it turns the head sideways in the other direction. When both sides contract, it pulls the head forward and down. The sternocleidomastoid is covered by a sheet like muscle called the platysma which  is a muscle of facial expression.
The  trapezius: extends over the neck but is considered a superficial muscle of the back.
The Splenius capitus: is a broad muscle deep to the trapezius. It originates in the ligamentum nuchae and the spinous processes of the 7th cervical and first 3 thoracic vertebrae. It inserts on the back of the skull below the superior nuchal line and on the mastoid process of the temporal bone. Contraction on one side causes the head t o rotate and extend to one side. Contraction  together causes extension of the head at the neck. Maximum contraction causes hyperextension of the neck and head.
The semipinalis capitus muscle: extends upward from the 7th cervical and the first 6 thoracic vertebrae to insert on the occipital bone. When the muscles con-tract together they extend the head at the neck ( along with the splenius capitus). If one of the muscles acts alone the head is rotated to the side.
The longissismus capitus: originates from the processes of the lower 4 cervical and upper 5 thoracic vertebrae to insert on the mastoid process of the temporal bone. This muscle extends the head at the neck, bends it to the side or rotates it slightly.

Suprahyoid muscles: These muscles are located above the hyoid bone.
The digastric  which is double bellied and has 2 origins acts to open the mouth and elevate the hyoid bone.
The myohyloid forms the floor of the mouth.  It aids in swallowing  by forcing food towards the back of the mouth.
The stylohyoid extends from the styloid process of the skull to the hyoid bone, which it elevates as it contracts. This acts to elevate the base of the tongue.
Infrahyoid muscles: These are located below the hyoid bone. They are named on the basis of the origin and insertion.
Sternohyoid-depresses the hyoid bone
Sternothyroid- pulls the larynx downward
Thyrohyoid- elevates the larynx and lowers the hyoid.
Omohyoid- depresses the hyoid bone

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