Spinal segmentai levels innervated by different types of vestibulo-spinal tract neurones in rabbit

Two separate fiber bundles, medial and lateral, are distinguished in the vestibulospinal t ract (MVST and LVST, cf. Nyberg-Hansen, 1964). Recent studies in rabbit have shown that cells of origin of bo~h MVST and LVST are contained in Deiters' nucleus (Akaike, Fanardjian, Ito, Kumada and Nakajima, 1973a; Akaike, Fanardjian, I to and Ohno, 1973 b). Further, these cells have been classified into six subgroups according to their axonal courses, their conduction velocities and also to the labyrinthine influences which they receive. The aim of the present investigation was to compare the spinal segmental levels which these subgroups of cells innervate. Adult rabbits (2--3 kg) were anesthetized by pentobarbitone sodium (40-50 mg/kg i.v.). Dissection and experimental procedures were similar to those already described (Akaike et al., 1973a). Arrangement of stimulating electrodes is shown in Fig. 1A. At C1 level, two monopolar stimulating electrodes were implanted, one in the medial portion of the left ventral funieulus and the other in the lateral portion of the right ventral funiculus, and used to stimulate MVST and LVST fibers differentially (Akaike et al., 1973a). At each of the C4, T1, L1 and L s or L 6 levels, one monopolar stimulating electrode was inserted into the medial portion of the ventral funiculus where MVST and LVST fibers run together (cf. Nyberg-Hansen, 1964). Another monopolar electrode was placed in I I I r d nucleus to excite ascending axons of vestibular neurones. The right labyrinth was stimulated with a bipolar electrode. Mieroelectrodes were inserted into Deiters' nucleus on the right side. Figure 1 B F shows recording of uni tary spikes from a cell in Deiters' nucleus in response to spinal cord stimulation. On C1 stimulation (B), the spikes occurred with a brief constant latency (0.3--1.2 msec) and with faithful following of high stimulation frequencies (tested up to 400/sec). Hence they were presumed to be induced antidromically. When threshold for this antidromic activation was compared between the medial and lateral electrodes at C1 segment, there was usually a significant difference. When it was smaller by a factor of 2.8 or more with the medial electrode, the tested cells were judged to belong to MVST (Akaike et al., 1973a). Inversely, when the threshold was smaller by a factor of 2,8 or more with the lateral electrode, they were assumed to belong to LVST. Thus, 249 cells were classified as LVST and 98 as MVST. As examplified in Fig. 1 C--F , stimulation