It seems likely that an important contribution to the phase-lagging component of the response of contralateral neck muscle is made by reticulospinal neutrons which, no doubt, also play an important role in reflexes of the limbs. Phase-lagging Deiters' neurons may influence neck motoneurons, but probably provide a more important input to limb motoneurons. For example, the shape of the phase plots of Figs. 3 and 7 resembles that of the response of ipsilateral forelimb muscles (15); neurons and forelimb muscles are usually not well modulated at frequencies higher than 1.5 Hz. Phase-lagging neurons of this type can make little contribution to the high frequency, often phase advanced, modulation of neck muscle. This high-frequency behavior must be caused by other premotor neurons. Lesion experiments and the lability of the high-frequency component suggest that it is not entirely due to the direct action of second-order neurons.
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