Approximately 90% of fibers in the GL are fast, twitch-generating
SIFs, whereas 10% are slow, nontwitch MIFs resembling those of
amphibians.
3 The SIFs are often divided into three types,
red, intermediate, and white, distinguished by their density of
mitochondria and fatigue resistance.
3 The largest and most
granular red SIFs, constituting approximately 33% of all global
fibers, are very similar to orbital SIFs and are highly fatigue
resistant, whereas the intermediate and white SIFs have progressively
lower fatigue resistance.
3 The predominant static loading
of the GL by the moderate contractile force of antagonist EOM accounts
for the GL’s higher overall recruitment threshold than the
OL
18 and the lesser oxidative, vascular, and
fatigue-resistant features of orbital SIFs.
3 However,
during saccades the high viscous loading of the GL by the relaxing
antagonist EOM requires the high transient force that intermediate and
white SIFs are well suited to provide. In the GL of monkey rectus EOMs,
80% of MIFs terminate in innervated myotendinous cylinders at the
musculotendinous junction called palisade endings (PEs).
21 Similar PEs have been demonstrated in all human EOMs.
22 Although their potential for proprioception has been
controversial,
21 23 studies in cat confirm PEs to be
sensory.
24 Human EOMs also contain neuromuscular spindles
that are thought to be sources of proprioceptive input to the
brain,
25 although the EOMs of monkeys are poorly endowed
with spindles.
26 A careful study based on serial sections
of isolated human EOMs from orbits more than 65 years of age
demonstrated a mean of 16 to 34 spindles per rectus and four per
oblique EOM.
25 In humans, spindles occur mainly in the
proximal and distal regions at the border between the OL and the
GL.
2 23 25