Morphological study of innervation of orbicularis oculi in the cat with respect to motoneurons, nerve fibers and muscle fibers
Sakamoto, N.; Mukuno, K.; Uga, S.
Nippon Ganka Gakkai Zasshi 93(10): 969-977
1989
ISSN/ISBN: 0029-0203 PMID: 2603848 Document Number: 335575
It has been suggested that the orbicularis oculi muscle functions in different modes of action in the orbital portion and in the palpebral portion. To study the anatomical correlation between the localization of muscular division and the size of motoneurons in facial nucleus, the retrograde tracing method was used. A histochemical study on the type of muscle fibers were also performed. The relationship between the motoneurons, nerve fiber and muscle fiber was discussed in terms of the functional organization of the palpebral or orbital orbicularis oculi. We injected a tracer of 50% horseradish peroxidase (HRP) into the orbital and/or palpebral portion of the orbicularis oculi muscle and HRP-labelled motoneurons at the brainstem were studied. In the facial nucleus, four subnuclei have been described: the ventrolateral, ventromedial, dorsolateral and dorsomedial. HRP-labelled motoneurons which innervated both the orbital and palpebral portions were spindle shaped and were found ipsilaterally in the dorsolateral subnucleus, and were distributed rostrocaudally. The histogram of the average soma diameter traced by the injection in the orbital portion presented two peaks at 30.mu.m and 40.mu.m in contrast to one peak of palpebral portion. Secondly, a histochemical study using alkali (pH 10.4) preincubated ATPase and acid (pH 4.3) preincubated ATPase and NADH stain was performed. It was found that the muscle fibers of the orbital part were composed predominantly of large and middle diameter group II B, while the palpebral part consisted mainly of small diameter group II A. By diameter analyses of the facial nerve fibers, it was found that the myelinated nerve fibers consisted of 3 peaks in terms of the magnitude of histogram. It can be inferred that large motoneurons (approximately 40 .mu.m) in the dorsolateral subnucleus innervate large II B muscle fibers of the orbital part via large myelinated nerve fibers, and are possibly involved in the action of strong eye closure and/or winking. On the other hand, small motoneurons (approximately 30.mu.m) in the dorsolateral subnucleus innervate small II A muscle fibers of the palpebral part through small myelinated nerve fibers, and are possibly involved in the motion of weak eye closure, spontaneous blinking and reflex blinking.