The hypoglossal nerve (or the 12th cranial nerve) is exclusively motor nerve that innervates almost all the muscles of the tongue, as well as the one suprahyoid muscle, the geniohyoid muscle.
The hypoglossal nerve originates from its motor nucleus, called simply the nucleus of the hypoglossal nerve.
This thin and long nucleus lies in the ventral portion of the brainstem, near the midline. Its rostral portion belongs to the field called the triangle of the hypoglossal nerve.
The neurons of this nucleus are multipolar and they send the efferent fibers that extend forward and laterally. They pass between the medial lemniscus and the complex of the olivary nuclei, and in a form of a dozen fibers leave the brainstem through the anterolateral sulcus.
This nerve innervates:
- All of the intrinsic muscles of the tongue
- All the extrinsic muscles of the tongue except the palatoglossus muscle
It is interesting that the fibers of the corticonuclear tract connect the motor cortex with the nucleus of the hypoglossal nerve. The corticonuclear fibers intended for this connection decussate, so their damage manifests as the paralysis of the muscles of the contralateral side of the tongue.
The nucleus of the hypoglossal nerve also gets a portion of the sensory fibers through the many of its synaptic relations with the solitary nucleus and the sensory nuclei of the trigeminal nerve, and in that way it is included in the reflex movement of the tongue such as swallowing or chewing.
Course and relations
The hypoglossal nerve leaves the brainstem through the anterior lateral sulcus. The dozen of its roots merges into the unique trunk of the nerve in the hypoglossal canal. After the nerve leaves the cranium, the branches of the cervical plexus join the hypoglossal nerve. These branches carry general somatic motor efferent fibers from C1 and C2 spinal nerves and general afferent fibers from the spinal ganglion of C2. These spinal nerves basically “use” the hypoglossal nerve to reach the hyoid muscle.
When the nerve exits the cranium, it enters the retrostyloid space, which is a part of the lateral pharyngeal space. In this space, the nerve is closely related to all of its elements, such as:
- The internal carotid artery
- The internal jugular vein
- The glossopharyngeal nerve
- The vagus nerve
- The accessory nerve
The nerve descends vertically through the retrostyloid space. At first, it is placed laterally to the internal carotid artery, but after that, it crosses the artery and positions itself medially to it.
The hypoglossal nerve holds this relation until it reaches the beginning of the occipital artery. There, the nerve turns horizontally forward, towards the tongue. It first crosses over the carotid triangle, and the over the submandibular triangle of the neck.
The horizontal portion of the nerve first crosses the lateral side of the external carotid artery. After that, the nerve crosses the deep side of the posterior belly of the digastric muscle and the medial surface of the stylohyoid muscle.
After this, the hypoglossal nerve passes over the superficial side of the hyoglossus muscle, and passes above the greater horn of the hyoid bone. This segment of the pathway, from the hyoglossus muscle and the greater horn of the hyoid bone, is the part of the nerve that forms the superior edge of the anterior lingual triangle (Pirogoff triangle) and it is covered with the submandibular gland.
Finally, the hypoglossal nerve together with the secretory duct of the submandibular gland passes through the intermuscular crack between the anterior edge of the hyoglossus muscle and the posterior edge of the mylohyoid muscle and enters the sublingual area where it splits to its terminal branches. At this final part of the pathway, the lingual nerve is positioned superiorly to the hypoglossal nerve.
The hypoglossal nerve has its lateral and terminal branches.
The lateral branches are:
- The meningeal branch that carries the fibers from the sensory spinal ganglion of the spinal nerve C2. This branch returns to the skull through the hypoglossal canal and innervates:
- The dura mater on the floor
- The posterior wall of the posterior cranial fossa.
- The superior root of the ansa cervicalis that supplies the infrahyoid muscles:
- The sternohyoid muscle
- The sternothyroid muscle
- The omohyoid muscle
This branch actually carries the fibers from the cervical plexus that joined the nerve outside the cranium
When it comes to the terminal branches, the nerve literally diverges to these branches after it passes through the mentioned crack between the hyoglossus and the mylohyoid muscle. The branches of the hypoglossal muscle - the lingual nerves, that are exclusively motor, innervate the styloglossus, hyoglossus, genioglossus, and the intrinsic muscles of the tongue.
Penetrating injuries to the neck, and various lesions of the skull base may also affect the nucleus of the hypoglossal nerve. The injury of the nerve manifests as the flaccid paralysis and the atrophy of the ipsilateral muscles of the tongue with the following speech impairment.
To test the function of the hypoglossal nerve, the physician should ask the patient to protrude their tongue. The tongue then should be palpated to check the tone of the muscles and as well as the ability of the sensation.
Normally, the simultaneous contraction of the paired genioglossus muscles causes the tongue to protrude straightforward. During examination of the patient, it is important to remember that a unilateral lesion of the hypoglossal nerve will cause the tongue to deviate towards the side of the lesion (impaired side) since the functional genioglossus on the intact side is unopposed by the paralyzed, inactive genioglossus muscle on the lesion side.