The Hypoglossal Nerve
The hypoglossal nerve is the twelfth (CN XII) of the twelve cranial nerves. The fibers consist of efferent motoric and afferent sensory fibers.
- The word afferent implies toward the central nervous system (CNS), as in from a peripheral area e.g. a limb, to the brain.
- The word efferent is the opposite of afferent, meaning away from the central nervous system, and towards the periphery i.e. when the stimulus is carried away from the brain from a peripheral area.
Efferent (motor) pathway
The pathway for the efferent motoric fibers of the hypoglossal nerve is the following (from the initiation in the brain to the termination in the periphery):
- The nucleus of the hypoglossal nerve is situated in the brainstem at the level of the hypoglossal trigone.
- The hypoglossal nerve exits the skull via the hypoglossal canal.
- It continues downwards when it accumulates fibers from the cervical spinal nerves C1 and C2.
- It then gives a large branch as it laterally passes the internal carotid artery, known as the superior root of the ansa cervicalis.
- This branch descends and innervates the anterior part of the omohyoid muscle, the sternothyroid muscle and the sternohyoid muscle before it loops backwards (via the ansa Cervicalis) and innervates the posterior part of the omohyoid muscle. (The loop on the posterior side is known as the inferior or posterior root of the ansa cervicalis and stems from the cervical spinal nerves C2 and C3.)
- Meanwhile the main branch of the hypoglossal nerve continues and innervates the thyrohyoid muscle with fibers from the first cervical spinal nerve C1.
- It then continues to give off muscular branches to the styloglossus muscle, hyoglossus muscle, geniohyoid muscle (with fibers from C1), and the genioglossus muscle . These muscles in addition to the palatoglossus muscle (which is not innervated by the hypoglossus nerve) are known as the extrinsic muscles of the tongue.
- In addition, the hypoglossal nerve also innervates the superior and inferior longitudinal muscles of the tongue, the transverse muscle of the tongue and the longitudinal muscle of the tongue (which are the intrinsic muscles of the tongue).
Afferent (sensory) pathway
The pathway for the afferent sensory fibers of the hypoglossal nerve is the following (from the initiation in the periphery to the termination in the brain):
- Afferent fibers come via the meningeal branches of the hypoglossal, which cover the diploë of the occipital bone, the dural walls of the anterior portion of the posterior cranial fossa as well as those of the occipital and inferior petrosal sinuses. These afferent fibres merge with the hypoglossal nerve close to the hypoglossal canal., crossing over the efferent motoric fibers and run parallel with them until the merger of the cervical spinal branch C1.
- At the fork of C1 and the hypoglossal nerve, the afferent sensory fibers wrap around the C1 fibers coming in and run back up the C1 spinal nerve, synapsing upon one of the spinal ganglia that are wedged between the cervical vertebrae.
- They then proceed postganglionically into the very top of the spinal cord just under the brainstem and into an ascending spinal tract.
- The hypoglossal nerve is the twelfth cranial nerve.
- It carries both sensory afferent fibres, which carries sensory information to the brain, as well as efferent motor fibres which carries motor impulses from the brain to target muscles.
Major branches of the hypoglossal nerve:
- Meningeal branches, which carry sensory afferent fibres from the diploë of the occipital bone, the dural walls of the anterior portion of the posterior cranial fossa as well as those of the occipital and inferior petrosal sinuses.
- Superior root of ansa cervicalis, which innervates the anterior part of the omohyoid muscle, the sternothyroid muscle, and the sternohyoid muscle.
- a muscular branch known as the nerve to thyrohyoid muscle
- further muscular branches which innervate the styloglossus, hyoglossus, geniohyoid and genioglossus muscles (extrinsic muscles of the tongue)
- finally, the hypoglossal nerve also gives off muscular branches which innervate the intrinsic muscles of the tongue.