Branching of olivary axons to innervate pairs of sagittal zones in the cerebellar anterior lobe of the cat.

Four of the eight sagittal zones in the cortex of the cerebellar anterior lobe, the x, c1, c3 and d2 zones, receive similar climbing fibre input from the ipsilateral forelimb through pathways ascending in the dorsal funiculus (DF-SOCPs) and have disynaptic relays in the main cuneate nucleus (Ekerot and Larson 1979a). The present investigation demonstrates that the forelimb areas of these four zones are innervated by climbing fibres from three groups of olivary neurones with branching axons (Fig. 6). The termination sites of climbing fibre branches were determined by identifying the low-threshold spots on the cerebellar surface from which short-latency (3.1-7.5 ms), "direct" climbing fibre responses could be evoked in Purkinje cells. In some Purkinje cells these responses were followed by late (8.2-13.5 ms) climbing fibre responses ("olivary reflex" responses). Each group projects to a pair of these zones which is separated by an intervening zone innervated from a private group of olivary neurones. The three groups of olivary neurones have been denoted the x-c1 group, the c1-c3 group, and the c3-d2 group in accordance with the zones they innervate. They project to the following areas: (a) The x-c1 group to the x zone in the vermal cortex and the lateral part of the c1 zone in the intermediate cortex; (b) the c1-c3 group to the medial parts of the c1 and c3 zones in the intermediate cortex; and (c) the c3-d2 group to the lateral part of the c3 zone in the intermediate cortex and the d2 zone in the extreme lateral part of the anterior lobe. Olivary axons belonging to the x-c1 and c1-c3 groups often send several climbing fibres to each projection area, whereas only single termination sites were found in the c3 zone for olivary axons belonging to the c3-d2 group. The two projection areas of each group of olivary neurons have a similar topographical organization: the areas innervated by the x-c1 group lack distinct somatotopical organization, whereas the areas innervated by the c1-c3 and c3-d2 groups have a detailed somatotopical representation of the ipsilateral forelimb. The low threshold spots for the late climbing fibre responses ("olivary reflex" responses) were restricted to the same sagittal strips as the low-threshold spots for the "direct" responses.