Contraversive turning movements of the head were elicited in alert cats by unilateral electrical stimulation of the entopeduncular nucleus. To determine the relative functional importance of ascending and descending pathways activated by the stimulation, the animals were submitted to thalamocortical or midbrain lesions and to combinations of the two lesions interrupting the pathways. Head turning was hindered considerably only after combinations of the two lesions and not after either lesion alone. Taking into account the results of previous studies, showing that most efferent fibers of the entopeduncular nucleus branch to the thalamus and to the midbrain, it is concluded that not only one but two main efferent pathways, corresponding to each branch, are used by the entopeduncular nucleus to control motoneurons, each pathway carrying enough redundant information to mediate head turning.
Afferent Pathways Animals