Download - pure motor nerve (somatic) - “fixator muscles” of tongue

#12 cranial nerve XII
= hypoglossal nerve
- pure motor nerve (somatic)
- “fixator muscles” of tongue
- styloglossus (retracts tongue)
- hyoglossus (depresses tongue)
- genioglossus (protracts tongue)
- intrinsic muscles of the tongue
- hypoglossal nerve: anatomic course
- hypoglossal motor nucleus is located in the
central gray matter of the myelencephalon
(medial to parasympathetic nucleus of CN X)
- axons pass ventrally, medial to olivary nucleus
- roots exit brainstem lateral to pyramidal tracts,
superior to, and in line with, the spinal nerve
motor roots exiting anterior cervical spinal cord
- nerve exits posterior fossa via hypoglossal canal,
through the occipital condyle, exiting canal to
descend lateral to internal carotid artery and
medial to internal jugular vein, passing forward
lateral to CN X and external carotid artery, then
medial to digastric and stylohyoid muscles
- gives off motor nerves to “fixators”: styloglossus, hyoglossus, and genioglossus muscles
- enters the tongue to innervate the intrinsic muscles of the tongue
- unlike most cranial nerves which have bilateral corticobulbar innervation, receiving upper
motor neuron (cortical) innervation from both cerebral hemispheres, tongue (CN XII) has
only contralateral (crossed) corticobulbar motor innervation
- therefore, if there is a lesion of the pre-central gyrus (upper motor neuron lesion),
there will be weakness and atrophy of the contralateral tongue muscles
- with UMN lesion, on protraction, tongue deviates towards the side of the lesion ©