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FIGURE 6 Optimal area (hatched) for MVMT lesions associated with Holmes tremor. Left, Coronal section; right, sagittal section; GC, central gray; Th, thalamus; TTC, central tegmental tract; LM, medial lemniscus.

touching the mediodorsal edge of the substantia nigra (Figure 6).

In order to verify that the previously mentioned elements are really necessary to produce Holmes tremor, we tried to destroy each element separately one by one, in random order (Ohye et al., 1988). This is schematically illustrated in Figure 7. So, in four monkeys, selective lesions were made (1) within the parvocellular red nucleus (either by radiofre-quency coagulation or by injecting a small amount of kainic acid to destroy cellular elements), (2) at the level of decus-sation of the superior cerebellar peduncle to destroy the cerebellothalamic tract, and (3) at the mediodorsal part of the substantia nigra to interrupt the nigrostriatal fiber at its exit without invading the red nucleus, respectively. Operations were conducted always with the aid of radiological and electrophysiological control, as mentioned previously. Each lesion was made with an interval of several weeks or several months, with observation of whether or not the tremor appeared. Both spontaneous and harmaline-induced tremors were tested. The results obtained in four monkeys and, for comparison, one monkey with a standard MVMT lesion and another monkey with a red nucleus lesion are summarized in Figure 8. It was clearly revealed that only when all three elements were sufficiently (more than 50%) lesioned, irrespective of the order of destruction, a Holmes tremor appeared. Moreover, harmaline-induced tremor was easily manifest in those animals with lesions of all three neural elements, except in a single case with an inadequate lesion of the superior cerebellar peduncle. In this regard and as noted previously, the degree of response to harmaline tremor is a good indicator of damage to the cerebellar peduncle.

C. Physiological Study of Tremor in Monkeys

In order to elucidate the neuronal mechanism of tremor thus produced by MVMT lesion, several physiological studies were conducted.

1. Dorsal Root Section

In three monkeys with postural tremor, the ipsilateral cervical dorsal roots were cut (rhizotomy), to clarify the possible role of spinal reflex loops in tremor maintenance. In the study cited here, monkeys with cerebellar lesions were used (Ohye et al., 1970). Under general anesthesia, the spinal cord was exposed by laminectomy and the cervical dorsal roots from C2 to T2 were cut under direct visual control. The effect of the rhizotomy on the postural tremor was examined frequently by clinical observation, by EMG examination, and, finally, at autopsy. To facilitate the tremor, harmaline (3mg/kg) was also utilized. After rhizotomy, monkeys showed sensory deficits and temporary monoparesis. Importantly, both spontaneous and harmaline-induced tremor persisted after deaf-ferentation. Only a slight increase in the irregularity of tremor amplitude and rate was detected by EMG study. From this experiment the authors concluded that tremor rhythm originates centrally and is transferred to the spinal motor neuron pool via some descending tract. Moreover, the peripheral reflex loop serves to stabilize tremor amplitude and rhythm. Similar conclusions with respect to resting tremors were already made by the pioneering works of Foerster (1911) and Pollock and Davis (1930) in Parkinsonian patients.

Cerebellothalamic Tract

Inferior Olive

FIGURE 7 Schematic illustration of stepwise mesencephalic lesions in relation to the standard MVMT lesion (in the center). The main three tracts essential for Holmes tremor production are shown with numerals.

Inferior Olive

FIGURE 7 Schematic illustration of stepwise mesencephalic lesions in relation to the standard MVMT lesion (in the center). The main three tracts essential for Holmes tremor production are shown with numerals.

Autopsy studies in the deafferented monkeys revealed clear-cut degeneration of ascending tracts in the corresponding part of the ipsilateral cuneate fasciculus. Interestingly, in one monkey, rhizotomy induced concomitant degeneration of the dorsal spinocerebellar and lateral corticospinal tracts by the incidental interruption of the blood supply to the corresponding lateral column of the cervical cord. The findings in this case supported an important proposal that postural tremor can persist after complete interruption of the corticospinal tracts at the peduncular level (Poirier et al., 1969).

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