11.3 Axial Muscles of the Head, Neck, and Back

Facial and Eye Muscles

Facial and eye muscles handle two things you rely on constantly: nonverbal communication and vision. The facial expression muscles let you smile, frown, squint, and produce the subtle movements that convey emotion. The extrinsic eye muscles control precise eyeball movements so you can track objects, shift your gaze, and focus at different distances.

All muscles of facial expression share one key feature: they're innervated by the facial nerve (CN VII). Damage to this nerve (as in Bell's palsy) can paralyze one side of the face, which highlights just how dependent these expressions are on a single cranial nerve.

Muscles of Facial Expression

Around the eyes and forehead:

• Orbicularis oculi encircles the eye and closes the eyelids. It's responsible for blinking, winking, and squinting in bright light.
• Frontalis raises the eyebrows and wrinkles the forehead, producing surprised or worried expressions.
• Corrugator supercilii draws the eyebrows medially and downward, creating the furrowed look of frowning or deep concentration.

Around the mouth:

• Orbicularis oris encircles the mouth and closes or protrudes the lips. You use it when whistling, kissing, or rounding your lips to say an "o" sound.
• Buccinator compresses the cheeks against the teeth. It keeps food between your teeth during chewing and helps push air through pursed lips when whistling. Think of a trumpet player's cheeks.
• Zygomaticus major and minor elevate the corners of the mouth to produce a smile.
• Risorius pulls the corner of the mouth laterally, contributing to a grin or smirk.
• Levator labii superioris elevates the upper lip, producing a snarl or expression of disgust.
• Depressor labii inferioris depresses the lower lip, contributing to a pout.
• Depressor anguli oris depresses the corners of the mouth, creating a frown.

A helpful way to study these: group them by whether they elevate structures (zygomaticus, levator labii, frontalis) or depress them (depressor labii, depressor anguli oris). The orbicularis muscles are sphincters that close openings.

Extrinsic Eye Muscles

Six muscles control eyeball movement. They attach to the bony orbit (origin) and insert on the surface of the eyeball. They're organized into two groups:

Four rectus muscles (named for the direction they face):

1. Superior rectus elevates the eye and slightly adducts it (looking up and slightly toward the nose)
2. Inferior rectus depresses the eye and slightly adducts it (looking down and slightly toward the nose)
3. Medial rectus adducts the eye (looking toward the nose)
4. Lateral rectus abducts the eye (looking away from the nose)

Two oblique muscles (these pull at an angle, so their actions are less intuitive):

1. Superior oblique depresses, abducts, and intorts the eye (looks down and out; the top of the eye rotates medially)
2. Inferior oblique elevates, abducts, and extorts the eye (looks up and out; the top of the eye rotates laterally)

The oblique muscles are the tricky ones on exams. Remember that the superior oblique actually looks down, and the inferior oblique looks up. Their names refer to their position on the eyeball, not the direction of gaze.

These six muscles coordinate to produce three types of eye movement:

• Saccades: rapid jumps between fixation points (like when your eyes dart across a line of text)
• Smooth pursuit: steady tracking of a moving object (following a ball in flight)
• Vergence: both eyes rotating inward or outward to focus at different distances (converging to read a book, diverging to look at a distant sign)

Suprahyoid and Infrahyoid Muscles

The hyoid bone is a small, U-shaped bone in the anterior neck that doesn't articulate with any other bone. It serves as an anchor point for muscles involved in swallowing and speech. Two groups of muscles attach to it: suprahyoid muscles above and infrahyoid muscles below.

Suprahyoid Muscles

Located superior to the hyoid bone, these muscles elevate the hyoid and larynx during swallowing. This upward movement helps close off the trachea so food and liquid don't enter the airway (aspiration). They also assist with opening the mouth by depressing the mandible.

The four suprahyoid muscles:

• Digastric (has two bellies connected by a tendon)
• Stylohyoid
• Mylohyoid
• Geniohyoid

Infrahyoid Muscles

Located inferior to the hyoid bone, these muscles depress the hyoid and larynx after swallowing, returning them to their resting position. They also stabilize the hyoid during speech, providing a steady platform for precise vocal cord movements.

The four infrahyoid muscles:

• Sternohyoid
• Sternothyroid
• Thyrohyoid
• Omohyoid

How They Work Together in Swallowing and Speech

During swallowing:

1. Suprahyoid muscles contract to elevate the hyoid bone and larynx.
2. This elevation tips the epiglottis over the tracheal opening, protecting the airway.
3. After the food bolus passes, infrahyoid muscles contract to pull the hyoid and larynx back down to resting position.

During speech:

• Both groups contract in a balanced way to stabilize the hyoid bone.
• This stable base allows the larynx and vocal cords to make the fine adjustments needed for phonation (producing voice) and articulation (shaping sounds into words).

Muscle Structure and Function Review

A few structural concepts tie these axial muscles together:

• Fascicles are bundles of muscle fibers within a muscle. The arrangement of fascicles (parallel, circular, convergent, etc.) affects how a muscle generates force and the range of motion it produces.
• Origin is the muscle's attachment to the less movable bone; insertion is the attachment to the more movable bone. During contraction, the insertion is pulled toward the origin.
• Antagonistic muscle pairs work in opposition. When one contracts, the other relaxes. The suprahyoid and infrahyoid groups are a clear example: one elevates the hyoid while the other depresses it.