Craniofacial Muscles

4 Motor Control of Extraocular Muscle
59
Fig. 4.5 Schematic showing
anatomical connections
between abducens and
oculomotor nuclei that result
in the generation of
coordinated movements of
the right and left eyes. Note
that the abducens nucleus
contains abducens
motoneurons ( green ) and
abducens internuclear
neurons ( black ). CN VI
cranial nerve VI, abducens
nerve; CN III cranial nerve
III, oculomotor nerve; MLF
medial longitudinal
fasciculus; LR lateral rectus;
MR medial rectus
the medial longitudinal fasciculus to synapse onto medial rectus motoneurons in the
contralateral oculomotor nucleus which in turn innervates the medial rectus muscle
ipsilateral to the oculomotor nucleus via the oculomotor nerve (CNIII) (Buttner-
Ennever and Akert 1981 ) . This pattern of interconnection between the abducens and
oculomotor nucleus is illustrated in Fig. 4.5 . Therefore, excitation of one abducens
nucleus and inhibition of the contralateral abducens nucleus due to pre-motor signals
results in the almost simultaneous contraction of the ipsilateral eye lateral rectus
muscle and contralateral eye medial rectus muscle and relaxation of the
contralateral eye lateral rectus muscle and ipsilateral eye medial rectus muscle,
thereby producing a coordinated and conjugate eye movement. This, in its essence,
is the anatomical basis for generating conjugate eye movements wherein the two
eyes are controlled as one by a single pre-motor conjugate command. From a neuroanatomical
standpoint, there are no major differences between the abducens
motoneurons and the abducens internuclear neurons (McCrea et al. 1986 ) . It may be
that the internuclear neurons are slightly smaller than the motoneurons and that they
have axon collaterals while the motoneurons do not. Comparison of the neural
response characteristics of motoneurons and internuclear neurons is provided later.
4.3.1.2 Implication of Anatomical Connection Between Abducens
and Oculomotor Nuclei
The anatomical connection between the abducens and oculomotor nucleus is clearly
critical for binocular coordination of eye movements in the horizontal plane.
Although perhaps not as easily perceived, these neural interconnections also play