The hypoglossal nerve is the twelfth paired cranial nerve, named from ancient Greek 'hypo' meaning under, and 'glossal' meaning tongue. It has a purely somatic motor function, responsible for innervating all the extrinsic and intrinsic muscles of the tongue (except the palatoglossus, innervated by the vagus nerve). In this article, we shall look at the anatomy of the hypoglossal nerve, its anatomical course, motor functions and clinical correlations.
The hypoglossal nerve arises from the hypoglossal nucleus in the medulla oblongata of the brainstem. It then passes laterally across the posterior cranial fossa, within the subarachnoid space. The nerve exits the cranium via the hypoglossal canal of the occipital bone. Now extracranial, the nerve receives a branch of the cervical plexus that conducts fibres from C1/C2 spinal nerve roots. These fibres do not combine with the hypoglossal nerve, they merely travel within its sheath. It then passes inferiorly to the angle of the mandible, crossing the internal and external carotid arteries, and in an anterior direction enters the tongue.
The hypoglossal nerve holds the primary responsibility for motor innervation of the vast majority of the muscles of the tongue (except the palatoglossus). These muscles can be divided into two groups, extrinsic and intrinsic:
Together, these muscles are responsible for providing the ability for the tongue to move in a variety of directions.
The C1/C2 roots that travel with the hypoglossal nerve have their own motor functions. These roots branch off in order to innervate the geniohyoid (responsible for elevating the hyoid bone) and thyrohyoid (responsible for depressing the hyoid bone) muscles. Additionally, another branch containing C1/C2 fibres descends to innervate the ansa cervicalis - a loop of nerves that is part of the cervical plexus. From the ansa cervicalis, nerves arise to innervate the omohyoid, sternohyoid and sternothyroid muscles, all of which act to depress the hyoid bone.
Cranial Nerve Examination: The hypoglossal nerve is typically examined by asking the patient to protrude their tongue. Other movements, such as asking the patient to push their tongue against their cheek and feeling for the pressure on the opposite side of the cheek, may also be used if damage is suspected.
Hypoglossal Nerve Palsy: Damage to the hypoglossal nerve is a relatively uncommon cranial nerve palsy. Possible causes include head & neck malignancy and penetrating traumatic injuries. If the symptoms are accompanied by acute pain, a possible cause may be dissection of the internal carotid artery. Patients will present with deviation of the tongue towards the damaged side on protrusion, as well as possible muscle wasting and fasciculations (twitching of isolated groups of muscle fibres) on the affected side.
The hyoid bone is an important anatomical structure located in the neck, and it is responsible for supporting the tongue, larynx, and other structures. The hyoid bone is moved by several muscles, and they all act to depress it. Knowing how the hyoid bone is moved and how it works is important for understanding some clinical conditions related to it.
When conducting cranial nerve examinations, the hypoglossal nerve is examined by asking the patient to protrude their tongue. In cases where damage to the nerve is suspected, the patient may also be asked to push their tongue against their cheek and feel for the pressure on the opposite side of the cheek.
Damage to the hypoglossal nerve is a relatively uncommon cranial nerve palsy. It may be caused by head and neck malignancies, penetrating traumatic injuries, or even dissection of the internal carotid artery, in cases where the damage is accompanied by acute pain. Those suffering from hypoglossal nerve palsy may present with a number of symptoms, including deviation of the tongue towards the damaged side on protrusion, muscle wasting, and fasciculations (twitching of isolated groups of muscle fibers).
Diagnosis of hypoglossal nerve palsy is typically done by a qualified medical professional. They will conduct an examination which includes observation of the patient's tongue to detect any deviation to the side of the nerve damage, as well as any muscle wasting. In some cases, imaging tests such as CT scans or MRIs may also be done to confirm the diagnosis.
The treatment of hypoglossal nerve palsy depends on the underlying cause. In cases of traumatic injury, the treatment may involve reconstructive surgery, while in cases of tumors or other malignancies, more aggressive treatments may be necessary. In cases of arterial dissection, it may be necessary to surgically repair the artery. In cases where the cause is unknown, the treatment may involve physical therapy to help improve the patient's ability to move their tongue.
The depression of the hyoid bone is a very important physiological process that is achieved through the movement of several muscles. Damage to the hypoglossal nerve can cause a relatively rare cranial nerve palsy, and the treatment of this condition will depend on the underlying cause. Proper diagnosis is essential for proper treatment, and imaging tests can help confirm the diagnosis. With the right treatment, patients can potentially regain control of their tongue.
