I ANATOMY                


II PHYSIOLOGY                               







Neurons in the nucleus prepositus hypoglossi and adjacent medial vestibular nucleus have an important role in generating the commands to hold eccentric positions of gaze, especially horizontal gaze.

Figure 1. Diagram of the ponto-medullary region, showing the structures involved in horizontal gaze (6th nerve N, and PPRF), and in gaze holding (N Prepositus Hypoglossi and Medial vestibular nucleus).



The paramedian pontine reticular formation (PPRF) contains excitatory burst neurons, which are inhibited by omnipause neurons; when the PPRF is activated by the contralateral frontal eye field, in turn it activates the adjacent abducens nucleus via the excitatory burst neurones: horizontal gaze is then mediated by abducens motor neurones which innervate the ipsilateral lateral rectus muscle and, via excitatory abducens interneurones crossing and then ascending in the MLF, the medial rectus subnucleus of the contralateral oculomotor nucleus.

Lesions of the PPRF, abducens nucleus and the MLF

1. The PPRF contains neurons that project to the abducens nucleus on the same side and is important for horizontal saccade generation.

Video 1. Saccadic palsy following cardiac surgery
-With the head fixed, volitional saccades are very slow, but eventually reach the target. Downward saccades are slightly faster.
-Pursuit is normal, and full in range both horizontally and vertically, even at higher frequencies.  Visually enhanced vestibulo-ocular reflex is normal.
-When the patient views a horizontally rotating optokinetic drum, her eyes became fixed laterally in the orbits without any corrective quick phases.
-Torsional head rolling produced normal ocular counter-rolling but without any torsional quick phases.


From: Eggers SD, Horn AK, Roeber S, et al. Saccadic palsy following cardiac surgery: a review and new hypothesis. Ann N Y Acad Sci. 2015;1343(1):113-119. doi:10.1111/nyas.12666

2. Lesions of the abducens nucleus may cause a loss of horizontal gaze toward the same side, since the nucleus contains internuclear neurons with projections through the opposite MLF to the opposite medial rectus subnucleus of the oculomotor nucleus. Lesions restricted to the abducens nucleus are rare since djacent structures, such as the PPRF, MLF, and the seventh cranial nerve, are commonly also affected.  Lesions of the abducens nucleus do not affect vertical and vergence eye movements1


Video 2. Left Gaze Palsy.  Left PPRF and left abducens fasciculus lesion (giving rise to the left esotropia)


(vv)WrayLeft Gaze Palsy.mp4(tt)

From: Wray S. Unilateral Horizontal Gaze Palsy. Retrieved from  https://collections.lib.utah.edu/ark:/87278/s6w123g6




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