Identification of motoneurons innervating individual extraocular muscles within the oculomotor nucleus in human

摘要

The oculomotor nucleus (nIII) and trochlear nucleus (nIV) of the midbrain contain motoneurons that innervate extraocular eye muscles. The aim of this study is to identify the motoneuronal subgroups of the human nIII and nIV, which innervate individual extraocular muscles using several histochemical stains, and comparing the results with those obtained in monkey. The nIV innervates only the superior oblique (SO) muscle, whereas nIII contains extraocular motoneurons, which innervate the medial rectus (MR), inferior rectus (IR), superior rectus (SR) and inferior oblique (IO) muscles. The organization of the motoneuron subgroups in human is not clear, and is the main subject of this investigation. In monkey the localization of the motoneuron subgroups for individual muscles is well known as a result of tract tracing studies, and analyses of their differential transmitter inputs. The results in monkey provided a useful basis for this study on human nIII.udHuman brainstems were fixed in 4% paraformaldehyde, series of frozen and paraffin sections of 40μm, 10μm and 5μm respectively were stained with choline acetyltransferase (ChAT) antibody to identify cholinergic motoneurons of extraocular muscles. The ChAT sections were then immunostained for the inhibitory transmitter GABA with antibodies against glutamate decarboxylase (GAD) or for the calcium-binding protein calretinin (CR). The cytoarchitecture of nIII was visualized with cresyl violet, gallyas stain or antibodies against non-phosphorylated neurofilaments (NP-NF).udSix subgroups were delineated in nIII: a dorsolateral (DL), dorsomedial (DM), central (CEN), ventral (VEN), lateral group (LAT) and the nucleus of Perlia (NP), in addition to the central caudal nucleus (CCN) and the urocortin-positive centrally projecting neurons of the Edinger- Westphal nucleus (EWcp). The DL, VEN, LAT, EWcp and NP groups receive a strong supply of GAD-positive terminals as the SO motoneurons in nIV. Strong CR- immunoreactivity was found in the CEN group, NP and CCN, but not in nIV.udBased on the staining properties of the subgroups and a comparison to the existing studies on monkey nIII, the CCN was considered to innervate the levator palpebrae; the CEN group was identified as the upgaze motoneuron subgroup for SR and IO, and DL, VEN and LAT showed characteristics of MR motoneurons. For the first time two separate subgroups of motoneurons (DL and VEN) subserving motor pathways for MR were identified in the human nIII, whereby the DL subgroup corresponds to the B-group and the VEN subgroup to the A-group in the monkey nIII. The DM group was considered to innervate IR muscle. The strong CR input to NP revealed characteristics of upgaze motoneurons in nIII. A good correlation was found between monkey and human in CR stains. But surprisingly, there were striking differences between monkey and human nIII with GAD stains. The results indicate that human MR motoneurons may contribute to a specialized function, e.g. during vergence, by its strong GABAergic input.