Oculomotor system: A dual innervation of the eye muscles from the abducens, trochlear, and oculomotor nuclei

The twitch fibers, orSIFs, are the type of muscle fibers that constitute allskeletal muscles, i.e., they respond to electrical excita-tion with an all-or-nothing response that propagatesalong the whole length of the fiber. They are innervatedby large en plaque motor endplates in an endplate zoneoccupying the central third of the muscle. The MIFs arehighly unusual in mammals, occurring only in eyemuscles, the larynx, and muscles of the middle ear. Thefibers are fatigue-resistant and respond to electricalstimulation with a slow tonic contraction, which is notpropagated along the muscle fiber (they are non-twitch).The motor endplates are typically small and are distrib-uted all along the length of the fiber. At the distal tip ofthe eye muscle fiber, as it inserts into the tendon, theglobal layer MIFs are capped by a tangle of nerve ter-minals called palisade endings, or myotendinous cylin-ders.