Responses of reticulospinal neurons in intact lamprey to vestibular and visual inputs

A lamprey maintains the dorsal-side-up orientation due to the activity of p ostural control system driven by vestibular input. Visual input can affect the body orientation: illumination of one eye evokes ipsilateral roll tilt. An important element of the postural network is the reticulospinal (RS) ne urons transmitting commands from the brain stem to the spinal cord. Here we describe responses to vestibular and visual stimuli in RS neurons of the i ntact lamprey. We recorded activity from the axons of larger RS neurons wit h six extracellular electrodes chronically implanted on the surface of the spinal cord. From these multielectrode recordings of mass activity, dischar ges in individual axons were extracted by means of a spike-sorting program, and the axon position in the spinal cord and its conduction velocity were determined. Vestibular stimulation was performed by rotating the animal aro und its longitudinal axis in steps of 45 degrees through 360 degrees. Nonpa tterned visual stimulation was performed by unilateral eye illumination. Al l RS neurons were classified into two groups depending on their pattern of response to vestibular and visual stimuli; the groups also differed in the axon position in the spinal cord and its conduction velocity. Each group co nsisted of two symmetrical, left and right, subgroups. In group, I neurons, rotation of the animal evoked both dynamic and static responses; these res ponses were much larger when rotation was directed toward the contralateral labyrinth, and the dynamic responses to stepwise rotation occurred at any initial orientation of the animal, but they were more pronounced within the angular zone of 0-135 degrees. The zone of static responses approximately coincided with the zone of pronounced dynamic responses. The group I neuron s received excitatory input from the ipsilateral eye and inhibitory input f rom the contralateral eye. When vestibular stimulation was combined with il lumination of the ipsilateral eye, both dynamic and static vestibular respo nses were augmented. Contralateral eye illumination caused a decrease of bo th types of responses. Group 2 neurons responded dynamically to rotation in both directions throughout 360 degrees. They received excitatory inputs fr om both eyes. Axons of the group 2 neurons had higher conduction velocity a nd were located more medially in the spinal cord as compared with the group I neurons. We suggest that the reticulospinal neurons of group, I constitu te an essential part of the postural network in the lamprey. They transmit orientation-dependent command signals to the spinal cord causing postural c orrections. The role of these neurons is discussed in relation to the model of the roll control system formulated in our previous studies.