The efferent system of cranial nerve nuclei: a comparative neuromorphological study.

A number of inconsistencies and controversies are inherent in the classification of cranial nerve nuclei based on the concepts of the various head-theories. The assumption of head segmentation, which is common to these theories, serves as the basis for designating the dorsomedial nuclei as the somatomotor column, although they innervate striated muscles of a viscus and a specific sense organ. The ventrolateral nuclei are called the specific visceromotor column; they innervate striated muscles in the branchiogenous area, but many of these muscles insert on skeletal elements. A series of comparative neuromorphological studies investigating the dendritic arborization pattern and axonal trajectory in the frog, lizard, and rat suggests a much more delicate classification in which nine morphologically and functionally different neuron groups can be discerned: 1. The hypoglossal nucleus appears coincidentally with the muscular tongue in amphibia. The spindle-shaped perikaryon, the bipolar dendritic arborization, and the straight ventral trajectory of the axon are characteristic morphological features in all three animal species investigated. 2. The oculomotor, trochlear, and abducens nuclei present a remarkably conservative topography and organization in all vertebrates with a moving eye. With their oval-shaped or polygonal perikarya and radiating dendritic arborization, these neurons distinctly differ from hypoglossal neurons. The ipsilateral axons follow a straight ventral course, the contralateral axons form a dorsal loop before crossing the midline, and the crossing is not consequence of neuron migration to the contralateral side. 3. The accessory abducens nucleus is present in tetrapods except apes and human. The elongated perikaryon and the dorsoventral dendritic orientation distinctly distinguish these neurons from other cranial motoneurons, the nucleus is found in the lateral part of the reticular formation. The neurons differentiate in situ, they do not migrate from the main abducens nucleus. 4. In the submammalian trigeminal and facial nuclei, two basic neuron types can be distinguished on the basis of their morphology. The first type is larger and accumulates in the rostral part of the trigeminal nucleus. This type innervates the jaw closer muscles. The second type is found in the caudal part of the trigeminal nucleus and in the facial nucleus. These neurons innervate the muscular floor of the mouth and the facial contingent supplies the jaw opener muscle. A very characteristic feature in the axonal trajectory is an initial medial course and a hairpin turn, or dorsal loop, at the lateral aspect of the medial longitudinal fasciculus. In addition to the two types of neurons, there is a third type in the frog trigeminal nucleus. This innervates an orbital muscle.(ABSTRACT TRUNCATED AT 400 WORDS)