ELECTROPHYSIOLOGY OF VESTIBULOSPINAL AND VESTIBULORETICULOSPINAL SYSTEMS IN LAMPREYS

Both vestibulospinal and reticulospinal neurons are excited by stimulation of the labyrinths in the lamprey, a primitive vertebrate. The roles of these neurons in vestibular reflexes were investigated with intracellular and extracellular recordings in isolated preparations of the lamprey brain and spinal cord. The vestibulospinal tracts of the lamprey consist of medium-sized axons that extend only 1-3 cm behind the brain. Stimulation of the anterior vestibular nerve excited the neurons of the ipsilateral uncrossed tract, and stimulation of the posterior nerve excited the neurons of the crossed tract. Antidromic stimulation of each vestibulospinal tract produced centrifugal discharges in the corresponding vestibular nerve. Transmission through the vestibular nuclei was mediated electrically since it was bidirectional and resistant to low external calcium. Myotomal motoneurons and lateral interneurons in the rostral spinal cord were excited by vestibular stimulation. The early EPSPs were produced by monosynaptic electrical transmission from the vestibulospinal tracts. The late EPSPs were correlated with discharges of reticulospinal neurons. Excitation from the reticulospinal system was usually stronger than that from vestibulospinal neurons. Reticulospinal neurons of the V group in adult lampreys were strongly excited by electrical and rotatory stimulation of the labyrinths. One source of excitation of V-cells was electrical transmission from vestibulospinal axons. The Mauthner cell in larval and adult lampreys was excited by stimulation of the anterior vestibular nerves on both sides. Ipsilateral excitation was monosynaptic and was transmitted electrically. The adequate stimulus for firing of the Mauthner cell was vibration of the otic capsules. The Mueller cells M3 and I1 were excited by electrical stimulation and downward roll of the contralateral labyrinth; ipsilateral vestibular stimulation inhibited these cells. Mueller cells of the bulbar group were excited by more generalized vibration and rotation. The various vestibulospinal pathways of the lamprey illustrate redundancy in motor systems. The most direct pathway is disynaptic excitation of neck motoneurons. Similar excitation of lateral interneurons recruits a descending propriospinal pathway, which may promote complementary undulatory movements. Vestibular stimulation also excites the prominent neurons of the reticulospinal system. For some cells, excitation appears to be nonspecific, as during arousal from other sensory modalities. However, the cells M3 and I1 and those of the V group are excited by specific directions of rotation and, when stimulated, can produce righting movements in the body and fins. Thus, in addition to what other motor functions they may have, the reticulosponal neurons of the lamprey can also participate in polysynaptic vestibular reflexes.