Craniofacial Muscles

58 V.E. Das
4.3 Motor Control of Conjugate Horizontal Eye Movements
Coordinated and conjugate eye movements in the horizontal plane require simultaneous
contraction of the ipsilateral eye lateral rectus and the contralateral eye medial rectus
and simultaneous relaxation of the contralateral eye lateral rectus and the ipsilateral
eye medial rectus muscles. The coordinated contraction and relaxation of the
four horizontal recti is mediated by innervation from the abducens and oculomotor
nerves and is facilitated by the hard-wired interconnections between the abducens
and oculomotor nuclei. Most of our understanding of motoneuron control of eye
movements comes from studies of the horizontal system. Although principles of
operation are the same, the cyclo-vertical system is necessarily more complicated
because the brain must control four pairs of EOM.
4.3.1 Neurons in the Abducens Nucleus
4.3.1.1 Neuroanatomy
Excellent reviews of the neuroanatomy of the cranial motor nerves and the extraocular
nuclei can be found elsewhere (Sharpe and Wong 2005 ; Buttner-Ennever 2006 ) .
Here we give a brief summary to provide a suitable context for the discussion of the
response properties of extraocular motoneurons during different kinds of eye movements.
Due to its critical role in binocular coordination, the abducens nucleus is
sometimes called the center of horizontal gaze (Leigh and Zee 2006 ) . The abducens
nucleus is a spherical-shaped structure that is located just below the fl oor of the
fourth ventricle and at the junction of the pons and medulla. Based on neuroanatomical
tracing, the abducens nucleus is now recognized to have four principal
populations of motoneurons—twitch and non-twitch abducens motoneurons,
abducens internuclear neurons, and fl occulus-targeting neurons. The fl occulustargeting
neurons belong to the cell groups of the paramedian tracts and probably
supply the cerebellum with a copy of the command signal (Langer et al. 1985 ) . They
are not directly responsible for generating an eye movement. The abducens motoneurons
(lateral rectus motoneurons) may be of the “twitch” or “non-twitch” sub-type
depending on the type of muscle fi ber that they innervate. Thus, the twitch abducens
motoneurons mostly innervate the singly-innervated fi bers or twitch fi bers in the lateral
rectus muscle and the non-twitch motoneurons mostly innervate the multiplyinnervated
fi bers in the lateral rectus (Buttner-Ennever et al. 2001 ) . Although such a
categorization is potentially interesting, there is yet no neurophysiological evidence
that differentiates twitch and non-twitch motoneurons. The abducens motoneurons
innervate the ipsilateral lateral rectus via the abducens nerve (CNVI). Thus, these
neurons are directly responsible for abduction of the ipsilateral eye. The other population
of neurons in the abducens nucleus that are critical for control of horizontal
gaze are the abducens internuclear neurons. The axons of the abducens internuclear
neurons cross the midline at the level of the abducens nucleus and then ascend via