412 Neuroanatomy: Draw It to Know It
Final Common Pathway
Here, we will draw the anatomy of the fi nal common
pathway for conjugate horizontal eye movements. Note
that our diagram is a simplifi ed schematic: the topo-
graphic anatomy of the oculomotor and abducens nuclei
is more accurately depicted in Chapters 11 & 12. First,
draw a coronal view of the brainstem and label the mid-
brain, pons, and medulla. Th en, draw axial sections
through the eyes. Now, label the left side of the page as
left and the right side as right, and also denote the mid-
line of the diagram. Next, attach a lateral rectus muscle to
the left eye and a medial rectus muscle to the right eye.
Now, let’s draw the cranial nerve nuclei involved in
the fi nal common pathway for horizontal eye move-
ments. In the pons, on the left side, draw the abducens
nucleus of cranial nerve 6. Show that it comprises pools
of motoneurons and interneurons. Indicate that the
motoneurons innervate the left eye’s lateral rectus muscle,
which drives the left eye to the left (laterally). Next, draw
the right oculomotor nucleus of cranial nerve 3 in the
midbrain. Now, let’s show the internuclear connection
between these two nuclei: the right medial longitudinal
fasciculus. Th en, show that the left abducens interneu-
rons project fi bers across midline that ascend the right
medial longitudinal fasciculus and synapse in the right
oculomotor nucleus. Finally, show that the right oculo-
motor nucleus innervates the right eye’s medial rectus
muscle and drives the right eye to the left (medially).
Next, to enhance our understanding of this pathway,
let’s address the common lesions that disrupt it. First,
redraw our diagram. Th en, show that injury to the
abducens motoneurons causes loss of ipsilateral eye
abduction. Next, show that injury to the abducens
interneurons causes loss of contralateral eye adduction.
Finally, encircle the entire abducens nucleus to indicate
that in a complete abducens nuclear injury, there is loss
of gaze to the side of the lesion: for example, in a left
abducens nuclear injury, the eyes are unable to deviate to
the left.
Now, again, redraw our diagram. Show that when the
medial longitudinal fasciculus is injured, the ipsilateral
eye is unable to adduct. Th is is called an internuclear
ophthalmoplegia (or MLF syndrome) — the ipsilateral
eye is unable to adduct and the opposite eye has horizon-
tal nystagmus when it abducts.
Next, redraw our diagram, again, but here include the
bilateral eye movement circuitry: draw the bilateral
medial and lateral recti muscles and draw eye movements
for both horizontal directions of gaze. Th en, show that
when both medial longitudinal fasciculus tracts are
injured, neither eye can adduct: the right eye can’t turn
horizontally to the left and the left eye can’t turn hori-
zontally to the right. Th is is called bilateral internuclear
ophthalmoplegia. Pathologic processes that cross mid-
line, such as demyelinating plaques, hemorrhages, or
tumors, can cause this form of injury because the medial
longitudinal fasciculus tracts run close together in the
midline of the brainstem.
For the last diagram, redraw the bilateral fi nal
common pathway arrangement. Show that when the left
abducens nucleus is injured, there is loss of gaze toward
the side of the lesion (left gaze palsy), and then show that
when the adjacent medial longitudinal fasciculus is
injured, the ipsilateral eye (the left eye) can’t adduct.
Th us, when both the abducens nucleus and the adjacent
medial longitudinal fasciculus are injured, the only intact
movement is right eye abduction (and it has nystagmus
from the left medial longitudinal fasciculus injury); thus,
one-and-a-half of the two complete eye movements are
impaired, so the injury pattern is called one-and-a-half
syndrome.^1 , 2