410 Neuroanatomy: Draw It to Know It
Know-It Points
Final Common Pathway
■ Th e abducens nucleus of cranial nerve 6 comprises
pools of motoneurons and interneurons.
■ Th e motoneurons innervate the ipsilateral lateral
rectus muscle, which drives the attached eye laterally.
■ Th e interneurons project fi bers across midline that
ascend the medial longitudinal fasciculus and synapse
in the oculomotor nucleus.
■ Th e oculomotor nucleus innervates the ipsilateral
medial rectus muscle, which drives the attached eye
medially.
■ Th rough the fi nal common pathway, the abducens
nucleus and the contralateral oculomotor nucleus act
in tandem.
■ In a complete abducens nuclear injury, there is loss of
gaze to the side of the lesion.
■ In an internuclear ophthalmoplegia (or MLF
syndrome), the ipsilateral eye is unable to adduct and
the opposite eye has horizontal nystagmus when it
a b d u c t s.Horizontal Saccade Circuitry
■ Th e superior colliculus is commonly divided into a
dorsal, “visuosensory” division and a ventral, “motor”
division.
■ Th e frontal eye fi elds directly excite contralateral
excitatory burst neurons.
■ Th e frontal eye fi elds also send an indirect excitatory
pathway through the superior colliculus, which
also excites the contralateral excitatory burst
neurons.
■ Th e excitatory burst neurons (of the paramedian
pontine reticular formation) excite the ipsilateral
abducens nucleus.
■ Th e excitatory burst neurons also excite the ipsilateral
inhibitory burst neurons (of the medullary reticular
formation), which inhibit the contralateral abducens
nucleus.
■ Th e omnipause cells tonically inhibit the excitatory
burst neurons and also the inhibitory burst neurons.
■ Th e excitatory burst neurons inhibit the omnipause
neurons and inactivate their tonic suppression of the
burst neurons.
■ Th e neural integrator receives fi bers from and projects
fi bers to a wide array of nuclei, including the excitatory
and inhibitory burst neurons, to sustain gaze.Vertical Saccades ( Advanced )
■ Th e rostral interstitial nuclei of the medial
longitudinal fasciculus (riMLF) comprise the
excitatory burst neurons for vertical and torsional
saccades.
■ Th e interstitial nuclei of Cajal (INC) are the neural
integrator for vertical and torsional saccades.
■ Th e riMLF produces bilateral projections for upward
gaze but only ipsilateral projections for downward gaze.
■ From the subject’s point of view, the right riMLF
produces clockwise torsional rotation of the eyes, and
the left riMLF produces counterclockwise torsional
rotation of the eyes.