Neuroscience/Objectives/Lecture 21

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Cranial nerve motor systems I: Somatic motor nuclei (GSE)

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Identify the location in the brainstem of the somatic motor nuclei for cranial nerves III, IV, VI, and XII.

Cranial nerve Motor nucleus Location
CN III Oculomotor nucleus Rostral midbrain; ventral to the Edinger-Westphal nucleus; rostral to the trochlear nucleus; at the periphery of the periaqueductal gray.
CN IV Trochlear nucleus Rostral midbrain; caudal to the oculomotor nucleus; at the periphery of the periaqueductal gray.
CN VI Abducens nucleus Deep to facial colliculus in caudal pons; dorsal and medial to the facial nucleus.
CN XII Hypoglossal nucleus Deep to hypoglossal trigone in mid- to caudal medulla; anterior and medial to dorsal motor nucleus of the vagus.
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Describe the muscle groups in the periphery innervated by the cranial nerves associated with these somatic motor nuclei.

Cranial nerve Muscle Primary action
CN III Superior rectus Elevation
Inferior rectus Depression
Medial rectus Adduction
Inferior oblique Elevation
CN IV Superior oblique Intorsion, depression
CN VI Lateral rectus Abduction
CN XII Genioglossus Tongue protrusion
Styloglossus Retracts the posterior tongue
Hyoglossus Depression of the tongue
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Discuss the visceral motor component of cranial nerve III and its function in the pupillary light reflex and accommodation.

The visceral component of CN III is carried in GVE fibers that originate in the Edinger-Westphal nucleus. These preganglionic parasympathetic neurons synapse with neurons in the ciliary ganglion and send postganglionic fibers that innervate the ipsilateral iris and ciliary muscles.

The circuit employed by the pupillary light reflex consists of:

  • optic nerve afferents synapsing on neurons of the ipsilateral pretectal nucleus of the superior colliculus
  • these neurons send bilateral efferents to the right and left Edinger-Westphal nuclei
  • the E-W nucleus innervates the ipsilateral iris and ciliary muscles
  • because of the bilateral distribution of E-W innervation, light perceived in a single eye will result in the constriction of the pupils in both eyes

During the accommodation reflex, E-W neurons are activated, resulting in pupillary constriction (contraction of iris) and thickening of the lens (accommodation) to adjust for brightness and focus, respectively.

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Discuss what brainstem nuclei control vertical and horizontal eye movements.

Center Nucleus Location
Horizontal gaze center Paramedian pontine reticular formation Near abducens nucleus in caudal pons
Vertical gaze center Rostral interstitial nucleus of the medial longitudinal fasciculus Rostral midbrain, ventral to the oculomotor complex

The two centers are connected by the medial longitudinal fasciculus.

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Describe what occurs during convergent eye movements.

During convergent eye movements, both eyes adduct in order to view a nearby object. The tone increases in the medial recti, while the tone of the lateral recti decreases. Convergence (i.e. the accommodation reflex) is mediated by neurons in the midbrain reticular formation (rostral interstitial MLF, riMLF).

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Indicate the role of the medial longitudinal fasciculus in coordinating eye movements.

The medial longitudinal fasciculus allows communication between interneurons in the abducens nuclei and motor neurons in the oculomotor nuclei. In particular, the PPRF stimulates excitatory interneurons in the ipsilateral abducens nucleus, which (via the MLF) contact the contralateral oculomotor nucleus, stimulating the contralateral medial rectus to contract. Similarly, the PPRF inhibits excitatory interneurons in the contralateral abducens nucleus, which project (via the MLF) to the contralateral oculomotor nucleus; by releasing the excitation on these motor neurons, the contralateral medial rectus tone decreases.

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Describe the clinical effects of damage to the peripheral nerve fibers or the nuclei of cranial nerves III, IV, VI, and XII.

All lesions of extraocular muscles or their nerves results in diplopia.

Lesion Deficits
Abducens nerve
  • Loss of horizontal abduction beyond midline
  • Unopposed medial rectus → affected eye is diverted medially on forward gaze
  • Horizontal diplopia
Trochlear nerve
  • Skewed deviation of the eyes with torsional deficit
  • Unopposed inferior oblique → extorsion of affected eye, weak downward gaze
  • Vertical diplopia, which increases when looking down
Oculomotor nerve
  • Loss of horizontal adduction → horizontal diplopia
  • Unopposed lateral rectus → abduction of affected eye
  • Loss of innervation to levator palpebrae → incomplete ptosis
    • Ptosis is incomplete since sympathetic innervation to eyelid is preserved
  • Lack of parasympathetic innervation to iris → mydriasis, loss of accommodation
Sympathetic lesion
  • Ptosis (drooping eyelid)
  • Miosis (increased pupillary constriction) due to unopposed parasympathetic action
Hypoglossal nerve
  • Ipsilateral paralysis of the tongue, with ipsilateral atrophy
  • Ipsilateral fasciculations (tiny, spontaneous contractions)
  • Deviation of the tongue towards the side of the lesion when it is protruded
    • The bilateral genioglossus muscles protrude the tongue by pulling it forward and medially
    • Loss of ipsilateral innervation result in unopposed action of the contralateral genioglossus, resulting in the tongue deviating towards the lesioned side
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Understand the distinction between upper and lower motor neuron lesons involving these cranial nerves.

Lesioning the MLF disrupts the communication between the abducens nucleus and contralateral oculomotor nucleus, resulting in internuclear ophthalmoplegia, a loss of adduction in the affected eye. Vergence and vertical gaze are intact since the oculomotor neurons are unaffected and these two functions are under the control of the riMLF.

Bilateral MLF lesions result in loss of adduction in both eyes during horizontal gaze. Again, vergence and vertical gaze are still intact.

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