Synaptic actions of individual vestibular neurones on cat neck motoneurones

Abstract

Unitary synaptic potentials evoked by the activity of single vestibulocollic neurons were recorded by spike-triggered signal averaging in neck extensor motoneurons of decerebrate cats. Properties of the vestibulocollic neurons which produced the potentials were examined. Vestibulocollic neurons were first identified as projecting to the C3 grey matter by antidromic microstimulation within the C3 extensor motoneuron pool. The spontaneous or glutamate-driven activity of the vestibulocollic neurons was then used to trigger the averaging computer. In this way 10 inhibitory and 2 excitatory neurons were identified (20% of neurons tested). Action potentials in local branches of vestibulocollic neurons were usually recorded in the vicinity of motoneurons. Mean orthodromic conduction time from the foot of the extracellular spike, recorded in the vestibular nuclei, that triggered the average was 0.72 ms. Mean synaptic delay was 0.4 ms. IPSP [inhibitory post-synaptic potential] had a mean time to peak of 0.81 ms and were readily reversed by injection of hyperpolarizing current. These data, together with the shape indices of IPSP indicate that they are generated proximally on motoneurons. All vestibulocollic neurons making synapses with motoneurons were monosynaptically driven by stimulation of the ipsilateral vestibular nerve. Of 7 tested 4 were inhibited by stimulation of the contralateral vestibular nerve (commissural inhibition). Two excitatory neurons were located in Deiters'' nucleus or on the Deiters''-descending border. Inhibitory neurons were found relatively medially in the vestibular complex in the medial, descending and Deiters'' nuclei. Vestibulocollic neurons acting on motoneurons were tested for axon branching to more caudal levels of the spinal cord with electrodes placed at C5-7. Both of the excitatory and 2 out of 9 inhibitory neurons branched.