Organization of the Commissural Connections : Physiology

Abstract

Synaptic nature of the commissural inhibition and the neuronal organization of the related pathway mediating the inhibitory action on kinetic and tonic type-I neurons of the horizontal canal have been studied with the unanesthetized and decerebrate cat. These studies were carried out by correlating the intracellular records of the postsynaptic potentials in the vestibular nucleus neurons with the extracellular records of functionally identified type-I neurons. The commissural inhibition is caused by a production of the inhibitory postsynaptic potential (IPSP) in the recipient vestibular neuron. The commissural pathway acting on the kinetic type-I neurons is composed of two neuron chains from the vestibular nerve; impulses along the primary afferent fibers excite monosynaptically the inhibitory commissural neurons, impulses from which in turn cross the midline, eventually producing the IPSP monosynaptically in the contralateral kinetic neurons. The commissural pathway for tonic type-I neurons has one or two more synapses along its path from the primary vestibular nerve, and the inhibitory neuron (probably type-II neuron) is located on the same side as the type-I neuron that receives the commissural inhibition. Extra commissural crossed effects on Deiters neurons responding to lateral tilt of the head are briefly described here; they are consistently excitatory.