The Hypoglossal Nerve
The Hypoglossal Nerve is the twelfth of the Twelve Cranial Nerves. The fibers consist of Efferent Motoric and Afferent Sensory fibers. The word Afferent means toward the centre, as in from a peripheral area of a limb to the central nervous system. The word Efferent is the opposite of Afferent, meaning away from the centre and toward the periphery; when the stimulus is carried back to the brain from a peripheral area.
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 Radix 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 (the ANSA Cervicalis – Loop) and innervates the posterior part of the Omohyoid muscle.
- The loop on the posterior side is known as the inferior or posterior Radix 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 and gives branches to Styloglossus Muscle, the Hyoglossus Muscle, the Geniohyoid Muscle (with fibers from C1), the Genioglossus Muscle, the Superior and Inferior Longitudinal muscles of the tongue, the Transverse muscle of the tongue and the Longitudinal muscle of the tongue.
- The Extrinsic muscles of the tongue include the Genioglossus muscle, the Hyoglossus muscle, the Palatoglossus muscle and the Styloglossus muscle.
- The Intrinsic muscles of the tongue include the Superior and Inferior Longitudinal muscles, the Transverse muscle and the Vertical muscle.
- The Afferent fibers start just inside the Hypoglossal Canal in the meninges and as they exit the skull as a small side branch that merges with the Hypoglossal Nerve, 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 cervical 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.