Cranial muscles are skeletal muscles located in the head and neck. They attach to the skull, including bones and fascia, and uniquely, often insert directly into the skin. These muscles are essential for various functions, enabling interaction with the environment and supporting basic physiological processes like senses, communication, and food consumption.
Major Groups of Cranial Muscles and Their Roles
Muscles of Facial Expression
The muscles of facial expression, also known as mimetic muscles, are located just beneath the skin. They produce a wide range of facial movements, such as smiling, frowning, and raising eyebrows. Examples include the orbicularis oculi, which surrounds the eye for blinking and eyelid closure, and the buccinator muscle in the cheek, which prevents food accumulation during chewing. The zygomaticus major and minor are involved in smiling, and the mentalis muscle controls lower lip movements. These muscles are innervated by the facial nerve (cranial nerve VII).
Muscles of Mastication
Muscles of mastication are four paired muscles that facilitate jaw movements at the temporomandibular joint (TMJ). They include the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles. The masseter and temporalis muscles elevate the mandible, closing the mouth and enabling biting. The medial pterygoid also aids in jaw elevation and protrusion, while the lateral pterygoid is the primary muscle for jaw depression and opening the mouth. These muscles are innervated by the mandibular division of the trigeminal nerve (cranial nerve V3).
Extrinsic Eye Muscles
Extrinsic eye muscles, also called extraocular muscles, are six muscles attached to each eyeball and one that controls the upper eyelid. They allow for precise eye movements in all directions, including side-to-side, up and down, and rotational movements. The four rectus muscles (superior, inferior, medial, lateral) move the eye up, down, inward, and outward. The superior and inferior oblique muscles contribute to rotation, while the levator palpebrae superioris elevates the upper eyelid. These muscles receive innervation from the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) cranial nerves.
The Tongue
The tongue comprises both intrinsic and extrinsic muscles, which manipulate food and produce speech. Intrinsic muscles originate and insert within the tongue, changing its shape. Extrinsic muscles originate from bones outside the tongue and insert into it, altering its position for protrusion, retraction, and side-to-side movements. The hypoglossal nerve (cranial nerve XII) innervates most tongue muscles, with the palatoglossus receiving innervation from the vagus nerve.
Muscles of the Pharynx and Larynx
Muscles of the pharynx and larynx are involved in swallowing and voice production. The pharynx, a muscular tube connecting the nasal cavities to the larynx and esophagus, uses constrictor muscles to propel food down during swallowing. Longitudinal muscles of the pharynx help shorten and widen the pharynx and elevate the larynx. The larynx, housing the vocal folds, contains intrinsic muscles that control sound production by adjusting vocal cord tension, length, and position. Both pharyngeal and laryngeal muscles are innervated by branches of the vagus nerve (cranial nerve X) and glossopharyngeal nerve (cranial nerve IX).
Coordinated Actions in Everyday Life
The various groups of cranial muscles work in concert to facilitate complex daily activities.
Eating
Eating involves a synchronized sequence of muscle actions. The muscles of mastication rhythmically elevate, depress, and move the jaw side-to-side to grind food. The buccinator and orbicularis oris muscles keep food contained within the oral cavity. The tongue’s intrinsic and extrinsic muscles manipulate the food bolus, positioning it for chewing and guiding it towards the pharynx for swallowing.
Speaking
Speaking requires intricate coordination between respiratory muscles, laryngeal muscles, and the muscles of the tongue and facial expression. The intrinsic laryngeal muscles adjust the vocal folds to produce different pitches and volumes. The tongue’s muscles then articulate these sounds into distinct words by altering the shape of the oral cavity. Facial muscles also contribute to speech by shaping the lips for sounds and adding visual cues that enhance communication.
Expressing Emotions
Expressing emotions relies on the coordinated movements of the facial expression muscles. Contractions of muscles like the zygomaticus major for a smile or the corrugator supercilii for a frown create a universal language of emotion. These expressions are rapid and involve multiple muscle groups acting together to convey a specific feeling. The ability to produce these varied expressions allows for rich nonverbal communication.
Vision
Vision, particularly tracking moving objects, relies on the coordination of the extrinsic eye muscles. The six muscles attached to each eye work in synchronized pairs to direct both eyeballs precisely and in unison. This alignment is necessary for depth perception and three-dimensional vision, allowing the brain to merge the two images. Blinking, controlled by the orbicularis oculi, protects the eye and distributes tear film, maintaining clear vision.
Common Issues Affecting Cranial Muscles
Temporomandibular Joint (TMJ) Disorders
Temporomandibular Joint (TMJ) disorders affect the jaw joints and the muscles of mastication. Symptoms include pain or tenderness in the jaw, facial pain, and difficulty opening or closing the mouth. These disorders can arise from factors like teeth clenching (bruxism) or trauma to the jaw or head. Dysfunction occurs when the jaw muscles, bones, and joints lose their harmonious movement.
Bell’s Palsy
Bell’s Palsy is a neurological disorder characterized by sudden weakness or paralysis of the muscles on one side of the face. It results from damage to the facial nerve (cranial nerve VII), which controls facial expressions. Symptoms include a drooping eyebrow and mouth, difficulty closing one eyelid, and altered taste sensation. The exact cause is often unknown, but it is linked to inflammation or viral infections affecting the facial nerve.
Strabismus
Strabismus, commonly known as crossed eyes, is a condition where both eyes do not look in the same direction. This misalignment stems from problems with the neuromuscular control of eye movement, involving the eye muscles, their nerves, or the brain’s control centers. Untreated strabismus, especially in children, can lead to reduced vision in the misaligned eye.
Dysphagia
Dysphagia refers to difficulty swallowing, a symptom resulting from various medical conditions impacting the muscles or nerves involved in swallowing. This impairment can affect the oral phase, where food is manipulated in the mouth, or the pharyngeal phase, where the food bolus is propelled down the throat. Conditions affecting the nervous system, such as stroke, or muscle disorders can disrupt the coordinated action of the cranial muscles involved in swallowing. Untreated dysphagia can lead to serious complications like aspiration, where food or liquid enters the airway, potentially causing lung infections.