Overview
Movement
of our tongue is important for speech and swallowing. Our tongue is a large
muscle, and is affected in a number of diseases that affect the hypoglossal
nerve. The hypoglossal nerve is the 12th and final cranial nerve. It
is the only cranial nerve to emerge medial to the olive, and innervates the
intrinsic muscles of the tongue.
Gross Anatomy
The
hypoglossal nerve is also known as cranial nerve 12 and is a purely somatic
motor nerve. It arises from the hypoglossal nucleus of the brainstem, the ventromedial
medulla to be precise. It is the only cranial nerve to emerge medial to the
olive (an ovaloid structure responsible for pathways of motor learning and
hearing) and emerges just medial to the pyramids (the descending corticospinal
tract runs in the pyramids).
The nerve
runs downward in the subarachnoid space, and runs behind the accessory and
vagus nerves and leaves the skull. It runs between the internal carotid artery
and internal jugular vein, and lies on the carotid sheath through the hypoglossal
canal, an opening located within the depths of the foramen magnum. As the nerve
leaves the cranium, it combines with the ventral rami of C1-2 from the cervical
plexus, which do not merge with the hypoglossal nerve, but simply attach to it
its sheath and travel with it. The section of the C1 spinal nerve that descends
with the hypoglossal nerve before it separates and forms the superior root of
the ansa cervicalis is called the descendens hypoglossi.
The hypoglossal
nerve then passes deep to the posterior belly of digastric and runs to the
submandibular region, where it will run on the lateral side of the hyoglossus
muscle. It continues on its anterior course, running inferiorly to the lingual
nerve. The lingual nerve is a sensory branch of the mandibular division of the
trigeminal nerve, which also allows the chorda tympani branch of the facial
nerve (taste from the anterior two thirds of the tongue) to hitch a ride back
to the brainstem. The nerve will then pass inferior to the angle of the
mandible, and then pass anteriorly to move toward the tongue. The nerve
innervates three of the four extrinsic tongue muscles (genioglossus, hyoglossus
and styloglossus, with palatoglossus being innervated by the vagus nerve), and
all the intrinsic muscles of the tongue (inferior and superior longitudinal,
vertical and transverse). All of these muscles function to move the tongue.
Clinical Anatomy
12th nerve palsy- Palsy
of the nerve causes the tongue to deviate toward the affected side. It occurs
following thrombotic changes to the brainstem, or atherosclerotic disease of
the vertebrobasilar anastomosis. It may occur after neck surgery, as the nerve
descends in the cervical region to reach the mouth.
Bulbar palsy- This
disease refers to impairment of the 9th to 12th cranial
nerves due to a lower motor neurone lesion i.e. the nerves themselves. This
causes wasting and fasciculations of the tongue. A pseudobulbar palsy is damage
to the nuclei of the above nerves, resulting in upper motor neurone signs of
the tongue and palate (tremors, increased tone). As four cranial nerves are
affected the signs are wide ranging, from dysphagia (glossopharyngeal and
cranial accessory nerves), dysarthria and dysphonia (vagus and hypoglossal
nerves) to nasal regurgitation. Contrasting to myaesthenia gravis, the eyes are
spared in this disease.
Quick Anatomy
Key Facts
Developmental precursor- Telencephalon (secondary vesicle of the developing nervous
system)
Muscles- Extrinsic tongue muscles- genioglossus, hyoglossus
and styloglossus, with palatoglossus being innervated by the vagus nerve
Intrinsic tongue muscles- inferior
and superior longitudinal, vertical and transverse muscles
Aide-Memoire
Hypo means under, and glosso means tongue
Remember that the nerve does not innervate the palatoglossus
muscle
Summary
The hypoglossal
nerve is the 12th and final cranial nerve. It is the only cranial
nerve to emerge medial to the olive, and innervates the intrinsic muscles of
the tongue.
References
1.
Frank H.Netter MD: Atlas of Human Anatomy, 5th
Edition, Elsevier Saunders, Chapter 1 Head and Neck
2.
Chummy S.Sinnatamby: Last’s Anatomy Regional and
Applied, 12th Edition, Churchill Livingstone Elsevier
3.
Richard L. Drake, A. Wayne Vogl,
Adam. W.M. Mitchell:
Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone
Elsevier
4.
Elliiot L.Manchell: Gray's Clinical Neuroanatomy: The Anatomic Basis for
Clinical Neuroscience
5.
The Definitive Neurological Surgery Board Review
By Shawn P. Moore,
2005
6.
Human Neuroanatomy
By James R. Augustine,
2008