Cranial Nerve Nuclei

=Objectives=

Functional categories

 * Purely sensory
 * CN 1, 2, 8


 * Purely motor
 * CN 3, 4, 6, 11, 12


 * Mixed sensory and motor
 * CN 5, 7, 9, 10


 * Gustatory
 * CN 7, 9, 10


 * Associated with somatosensory sensations
 * CN 5, 7, 9, 10

Describe the pathways from the facial motor nucleus

 * 1) The facial motor nucleus resides in the pons.
 * 2) The fiber tract moves dorsomedially to wrap around the medial longitudinal fasciculus and the abducens nucleus
 * 3) The fiber changes directions to move ventrolaterally and exit the pons.

Describe the cranial nerve reflexes

 * Jaw Jerk
 * When the mouth is partially opened and the muscles relaxed, tapping the chin causes the jaw to close. Monosynaptic reflex, in CN V, out CN V. The afferent limb of CN V3 innervating the masseter muscle relays to the mesencephalic nucleus of V.  The sensory neuron synapses onto the trigeminal motor nucleus which causes the masseter muscle to constrict via CN V.


 * Corneal Reflex
 * Touching the cornea causes blinking of the eyelids (afferent CN V1; efferent CN VII). Disynaptic reflex.  The nasocilliary nerve senses touch and relays onto the spinal trigeminal nucleus.  An interneuron bilaterally innervates the facial motor nucleus.  CN VII innervates the obicularis oculi muscle, which constricts.


 * Salivation reflex
 * The tongue relays gustatory sensation through CN 7, 9, or 10 to the rostral solitary nucleus (NTS). The secondary neurons then innervate the salvatory nuclei.  Parasympathetic nerves then innervate the pteygopalantine, submandibular, and otic ganglion (via CN 7, 9), which ultimately innervate the salivary glands.


 * Gag reflex
 * CN IX sensation in oropharnyx --> Spinal trigeminal nucleus --> interneuron to reticular formation (abdominal gag) and ambiguus nucleus (pharnyx, larnyx) --> CN X innervates pharneal plexus muscles


 * Cough reflex
 * Vagus nerve in respiratory tree --> solitary nucleus (caudal) --> "cough center" --> Trigeminal motor nucleus & Ambiguus nucleus & hypoglossal nucleus & sinal motoro cord neurons.

Differentiate between facial paralysis caused by an upper motor lesion and a facial nerve palsy

 * The upper motor neurons corresponding to the upper face (ie frontalis muscle) innervate the facial nerve nuclei bilaterally. The upper motor neurons corresponding to the lower face (ie obicularis oris) innervate contralaterally only.
 * Thus, a unilateral stroke in the precentral gyrus will result in a contralateral palsy of the lower face. The upper face will have (near) normal strength.
 * The facial nerve innervates facial muscles on the ipsilateral face.
 * Any lesion of the facial nerve will result in an ipsilateral palsy of both the upper and lower face.

Differentiate between a CN III palsy caused by compression from ischemia

 * CN III is composed of branchial motor and parasympathetic fibers. The motor fibers are grouped on the inside of the nerve (e.g. the white inner circle); the parasympathetic fibers are grouped in the outside of the nerve (e.g. the yellow outer circle above).
 * An ischemic injury will preferentially effect the interior fibers, results in motor defects but normal pupil responses
 * A compression injury will preferentially effect exterior fibers --> normal occulomotor movements but pupil dilation

Describe the causes and demographics of trigeminal neuralgia

 * Trigeminal Neuralgia: a brief, severe shooting neuropathic pain in the distribution of CN V.
 * Idiopathic (micro infarcts of the nerve and demyelination): occurs in elderly
 * Compression by vein or superior cerebellal artery: elderly
 * Multiple sclerosis: may be first symptom; women 20 - 40.
 * Post-herpetic: any age, but more common in the elderly

Differentiate between ptosis caused by Horner's syndrome versus a CN III palsy

 * Interruption of the sympathetic fibers results in miosis and ptosis (because of paralysis of the pupillary dilator muscle and of Müller's muscle, respectively). The lesion may be central, between the hypothalamus and the points of exit of sympathetic fibers from the spinal cord (C8 to T3, mainly T2), or peripheral, in the cervical sympathetic chain, superior cervical ganglion, or along the carotid artery.
 * CN III palsy results in a exotropia (down and out/lateral strabismus) and paralysis of the medial rectus, superior rectus, inferior rectus and the levator palpebrae superioris muscles.
 * Horner's syndrome is accompanied by a loss of sweating on the same side as the ptosis. Occulomotor functions are still in tact.

=References=

=Links= Multiple Cranial Nerve Palsy a nice discussion in Harrison's Internal Medicine that integrates concepts.