first group has its peak at 0.5 msec, whereas the second has a peak at about 1.0 msec. Cell B is an example of a cell having an action potential with a short latency, which is about 0.5 msec; cell C has a latency of approximately 0.9 msec. All the cells in Fig. 1K could follow repetitive activation ranging from 200 to 400 stimulations per second and demonstrated a refractory period in the range of 3 to 4 msec with relatively small shift in latency for the second invasion near the refractory period (shift less than 0.5 msec). Figure 1, D to F, illustrates the refractory period of cell potential were smaller than expected (7, 12). The orthedromic activation of Purkinje cells after stimulation of the eighth nerve may be ascribed to at least three pathways; two originate directly from the vestibular bipolar cells (Fig. 1A), and the third is the vestibulo- cerebellar projection through the vestibular nuclei. The two direct pathways end as climbing and mossy terminals in the cerebellar cortex (8). However, since activation of auricular Pur- kinje cells by climbing fibers can be recognized by its short latency and characteristic burst activation (&), the aforementioned response must be evoked by mossy terminals. It has not been determined which of the two sys- tems of mossy fibers, the direct vestibulo-cerebellar system or the disynaptic system through the vestibular nuclei, is responsible for the activation in Fig. 1, G and J. Since the latencyis fairly long, (7.5 and 5.0 msec) however, these action potentials may be evoked by disynaptic vestibulo-cerebellar fibers. On the basis of these data, we con- clude that ‘a number of Purkinje cells from the auricular lobe project directly C which was 3 msec for the antidromic invasion. If the interval between the ‘two stimuli was increased to 3.2 msec (Fig. 1F), a smaller action potential was recorded (see Fig. 1D) which had a larger initial segment-somadendritic (IS-SD) separation (6). The extracellular antidromic action potentials evoked by stimulation of the eighth nerve have characteristics very similar to those already recorded by Matthews et al. (12) who demonstrated that, when a pair of stimuli are de- livered at short intervals to the cerebel- lar white matter, there is a delay in the second response of the Purkinje cell. We frequently observed this phenomenon during our experiments (Fig. 1F). Furthermore, there was, in many instances, a large reduction of the amplitude of the second action potential, suggesting that the cell had not completely recovered from the preceding spike (Fig. 1F). Figure 1, G to J, illustrates extracellular and intracellular recordings from other Purkinje ceils after stimulation of the eighth nerve. In Fig. 1G, an activation of the eighth nerve with a stimulus 1.7 times greater than the threshold stimulus evoked an antidromic as well as an orthodromic action potential. In Fig. 1, H to J, a similar cell from the same electrode tract was impaled intracellularly. As the stimulus strength increased from 1.2 to 1.5 times the threshold stimulus, a subthreshold excitatory postsynaptic potential was seen (Fig. 11). In Fig. 1J an orthodromic activation was obtained with a stimulus strength of 1.8 times the threshold strength. In Fig. 1, I and J, the stimulus artifact interfered with the rising phases of the antidromic action potential. As in previous studies, intracellular impalement of frog Purkinje cells proved very difficult, for which reason both the resting potential (approximately 40 mv) and the action 8 DECEMBER 1967 Fig. 2. Electron micrograph of retrograde changes in a Purkinje cell and degenerating synaptic terminal on to vestibular receptorcells. (Top) Retrograde changes in a Purkinje cell of frog auricular lobe 4 days after ipsilateral transection of the stato-acoustic nerve extracranially (RPC). Note the dark ring formed around the cell nucleus by the rearrangement of the endoplastic reticulum; note also the normal appearance of ‘the adjacent Purkinje cell (NPC) (x 5500). (Bottom) Degenerating synaptic terminal 3 days after ipsilateral removal of cerebellar cortex in the frog (DSB). Note the darkness of the terminal and the clustering of the mitochondria, RC, vestibular receptor cell; SC. supporting cell (> 39000). 1329