ANTIDROMIC AND TRANS-SYNAPTIC ACTIVATION OF DEITERS' NEURONES INDUCED FROM THE SPINAL CORD

Abstract

1. Glass microelectrodes were inserted into the lateral vestibular nucleus of Deiters, and potential changes were recorded both extracellularly and intracellularly under stimulation of the spinal cord. 2. When the ipsilateral C₃ segment was stimulated at its ventrolateral surface, negative field potentials of several millivolts were recorded from the area which was histologically identified as Deiters' nucleus. These field potentials were presumed to be caused by the antidromic activation of Deiters' neurones through the vestibulospinal tract. 3. The antidromic field potential of Deiters' nucleus showed different distributions in the ventrodorsal direction for activation at the C₃ and L₁ levels, which is in keeping with the histologically determined somatotopical arrange-ment. 4. Within the nucleus of Deiters specified by the field potentials, a total of 134 units were impaled with microelectrode. They were identified as Deiters' neurones by the characteristics of the antidromic spikes, their afterhyperpolarization and by occurrence of synaptic noise and PSPs. 5. In sixteen selected neurones the resting potential was -55 to -75 mV and the spike height 60 to 100 mV (mean, 80.8 mV). The duration of the spike was 0.4 to 0.6 msec (mean, 0.47 msec). 6. The conduction velocity along the vestibulospinal fibres was determined in fifty Deiters' neurones which were activated antidromically both from the C₃ and L₁ segmental levels. It ranged from 24 to 140 m/sec, the peak frequency being at 90 to 100 m/sec. 7. The falling phase of the spike, after it crossed the base line, reversed to an afterhyperpolarization which reached its summit at 0.7 to 8.8 msec (mean, 2.4 msec), and diminished within 33 to 100 msec (mean, 49 msec). These values are smaller than those for cat spinal motoneurones. As a unique feature in Deiters' neurones the afterhyperpolarization did not show a temporal summation when it was evoked successively at short intervals. 8. The stimulation of the ipsilateral C₃ segment just subthreshold for the axon of the impaled cells induced EPSPs monosynaptically in 15.4% and IPSPs polysynaptically in 7.7% of the examined neurones. The recurrent axon collaterals might be responsible for these IPSPs, but the possibility was not excluded for the other pathways. 9. Stimulation at the C₃ segment supramaximal for the antidromic field potential of Deiters' nucleus produced monosynaptic EPSP in 50% and polysynaptic IPSP in 28.6%. It was suggested that the monosynaptic EPSP was produced through the ascending tract fibres in the spinal cord. 10. In view of the fast time course of the action potential and of rarity of the possible recurrent inhibition, it was postulated that Deiters' neurones would be grouped with the fast type of lumbosacral motoneurones.