Craniofacial Muscles

34 L.K. McLoon et al.
Fig. 3.4 Speci fi cally with reference to the right eye, the photographs demonstrate ( a ) elevation
and extortion, ( b ) elevation, ( c ) abduction, and ( d ) primary position of gaze. ( a ) To direct the gaze
to look right, up and out, that is elevate and extort, the inferior oblique muscle of the right eye must
contract. As the eyes move conjugately, the left eye must intort and elevate which is performed
by the superior rectus muscle of the left eye. ( b ) To look directly overhead, without bending the
neck, the superior rectus and inferior oblique muscles must co-contract. Both elevate, and their
co-contraction will essentially “cancel out” the extorsion and intorsion components of each of
these muscles seen when they contract alone
of the eyes needed to fi xate on a distant object in primary gaze (Fig. 3.4d ), it is clear
that a certain amount of muscle tension must be maintained on the medial rectus
muscles bilaterally. In addition, due to the complex vectors of each muscle, for the
eyes to look up at the ceiling (without bending your neck), bilaterally the superior
rectus muscle must contract—which elevates and intorts the eye—and the inferior
oblique must contract—which extorts the eye and elevates. The combination of
contraction of these muscles results in elevation directly superiorly (Fig. 3.4b ).
What this means is that the EOM are continuously active, even in the primary direction
of gaze (Fig. 3.4d ).
The EOM are innervated by three pairs of cranial nerves: the oculomotor nerve
(CNIII) innervates the superior rectus, medial rectus, inferior rectus, and the inferior
oblique muscles, the trochlear nerve (CNIV) innervates the superior oblique muscle,
and the abducens nerve (CNVI) innervates the lateral rectus muscle. The EOM are
densely innervated, resulting in very small motor units, with typical fi ring rates an
order of magnitude higher than seen in spinal motor neurons (Fuchs et al. 1988 ) .
3.3 Embryological Origins
The EOM arise from non-segmented cranial mesoderm in contrast to the body and
limb skeletal muscles, which are derived from somites. While all of the craniofacial
muscles except tongue develop from non-segmented cranial mesoderm, the EOM
have a distinct genetic program that controls their initial formation in development.
These genes essentially have little overlap with the embryological development of
other craniofacial muscles (see Chap. 2 , Harel and Tzahor 2012 ) . Limb and body
skeletal muscle somite formation is dependent on the transcription factor Pax3