Buttner-Ennever J A, Horn A K E, Graf W, Ugolini G
Institute of Anatomy, Ludwig-Maximilian University of Munich, 80336 Munich, Germany.
Ann N Y Acad Sci. 2002 Apr;956:75-84. doi: 10.1111/j.1749-6632.2002.tb02810.x.
The extraocular muscles, unlike the skeletal muscles, contain non-twitch muscle fibers. Recent experiments have located the non-twitch motoneurons. They lie around the periphery of the oculomotor, trochlear and abducens nuclei, separate from the more usual twitch motoneurons that cluster within the boundaries of the classical motor nuclei. The premotor inputs to non-twitch neurons were traced by the injection of rabies virus into the distal tip of the lateral rectus muscle. Retrogradely labeled cells were found in areas associated with the neural integrator, vergence and smooth pursuit premotor areas, but not the saccadic premotor burst neurons or the direct vestibulo-ocular pathways. The rabies tracing emphasizes for the first time that the central mesencephalic reticular formation (cMRF) and the supraoculomotor area exert direct premotor control over the non-twitch motoneurons. Because the two sets of motoneurons do not receive the same afferents, they must have different functions; these are not yet clarified. These results are not compatible with the concept of a single final common pathway from motoneurons to eye muscles. Putative sensory receptors, palisade endings, are located at the tips of non-twitch muscle fibers reminiscent of an inverted muscle spindle, which would make the non-twitch motoneurons, gamma-motoneurons. We propose that twitch motoneurons are the major source of tension used for eye movements, whereas non-twitch motoneurons are more important for fine alignment of the eyes. Furthermore, the non-twitch motoneurons could be controlled through sensory feedback networks (including perhaps proprioceptive signals from the palisade endings) that are relayed through the superior colliculus and via cMRF to the non-twitch motoneurons. The clinical repercussions of these hypotheses are discussed.
与骨骼肌不同，眼外肌含有非抽搐性肌纤维。最近的实验已确定了非抽搐性运动神经元的位置。它们位于动眼神经核、滑车神经核和展神经核的周边，与聚集在经典运动核边界内的更常见的抽搐性运动神经元分开。通过将狂犬病毒注射到外直肌的远端来追踪非抽搐性神经元的运动前输入。在与神经整合器、辐辏和平滑跟踪运动前区相关的区域发现了逆行标记的细胞，但在扫视运动前爆发神经元或直接前庭眼反射通路中未发现。狂犬病毒示踪首次强调中脑中央网状结构（cMRF）和动眼神经上区对非抽搐性运动神经元施加直接的运动前控制。由于这两组运动神经元接受不同的传入神经，它们必然具有不同的功能；这些功能尚未阐明。这些结果与从运动神经元到眼肌的单一最终共同通路的概念不相符。假定的感觉感受器，栅栏状末梢，位于非抽搐性肌纤维的末端，类似于倒置的肌梭，这将使非抽搐性运动神经元成为γ运动神经元。我们提出，抽搐性运动神经元是眼球运动所用张力的主要来源，而非抽搐性运动神经元对眼球的精细对准更为重要。此外，非抽搐性运动神经元可以通过感觉反馈网络（可能包括来自栅栏状末梢的本体感觉信号）进行控制，这些信号通过上丘并经由cMRF传递到非抽搐性运动神经元。讨论了这些假设的临床影响。
