Inputs from regularly and irregularly discharging vestibular nerve afferents to secondary neurons in the vestibular nuclei of the squirrel monkey. II. Correlation with output pathways of secondary neurons.

1. Intracellular recordings were made from secondary neurons in the vestibular nuclei of barbiturate-anesthetized squirrel monkeys. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the ipsilateral vestibular nerve (Vi) were measured. An electrophysiological paradigm, described in the preceding paper (26), was used to determine the proportion of irregularly (I) and regularly (R) discharging Vi afferents making direct connections with individual secondary neurons. The results were expressed as a % I index, an estimate for each neuron of the percentage of the total Vi monosynaptic input that was derived from I afferents. The secondary neurons were also classified as I, R, or M cells, depending on whether they received their direct Vi inputs predominantly from I or R afferents or else from a mixture (M) of both kinds of Vi fibers. The neurons were located in the superior vestibular nucleus (SVN) or in the rostral parts of the medical or lateral (LVN) vestibular nuclei. 2. Antidromic activation or reconstruction of axonal trajectories after intrasomatic injection of horseradish peroxidase (HRP) was used to identify three classes of secondary neurons in terms of their output pathways: 1) cerebellar-projecting (Fl) cells innervating the flocculus (n = 26); 2) rostrally projecting (Oc) cells whose axons ascended toward the oculomotor (IIIrd) nucleus (n = 27); and 3) caudally projecting (Sp) cells with axons descending toward the spinal cord (n = 13). Two additional neurons, out of 21 tested, could be antidromically activated both from the level of the IIIrd nucleus and from the spinal cord. 3. The Vi inputs to the various classes of relay neurons differed. As a class, Oc neurons received the most regular inputs. Sp neurons had more irregular inputs. Fl neurons were heterogeneous with similar numbers of R, M, and I neurons. The mean values (+/- SD) of the % I index for the Oc, Fl, and Sp neurons were 34.7 +/- 24.7, 51.9 +/- 30.4, and 61.8 +/- 18.0%, respectively. Only the Oc neurons had a % I index that was similar to the proportion of I afferents (34%) in the vestibular nerve (cf. Ref. 26). 4. The commissural inputs from the contralateral vestibular nerve (Vc) also differed for the three projection classes. Commissural inhibition was most common in Fl cells: 22/25 (88%) of the neurons had Vc inhibitory postsynaptic potentials (IPSPs) and 1/25 (4%) had a Vc EPSP. In contrast, Vc inputs were only observed in approximately half the Oc and Sp neurons.(ABSTRACT TRUNCATED AT 400 WORDS)