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Straka, Hans ORCID: 0000-0003-2874-0441; Holler, Stefan; Goto, Fumiyuki (2002): Patterns of canal and otolith afferent input convergence in frog second-order vestibular neurons. In: Journal of Neurophysiology, Vol. 88, No. 5: pp. 2287-2301
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Second-order vestibular neurons (2degrees VN)were identified in the isolated frog brain by the presence of monosynaptic excitatory postsynaptic potentials (EPSPs) after separate electrical stimulation of individual vestibular nerve branches. Combinations of one macular and the three semicircular canal nerve branches or combinations of two macular nerve branches were stimulated separately in different sets of experiments. Monosynaptic EPSPs evoked from the utricle or from the lagena converged with monosynaptic EPSPs from one of the three semicircular canal organs in similar to30% of 2 VN. Utricular afferent signals converged predominantly with horizontal canal afferent signals (74%), and lagenar afferent signals converged with anterior vertical (63%) or posterior vertical (37%) but not with horizontal canal afferent signals. This convergence pattern correlates with the coactivation of particular combinations of canal and otolith organs during natural head movements. A convergence of afferent saccular and canal signals was restricted to very few 2degrees VN (3%). In contrast to the considerable number of 2degrees VN that received an afferent input from the utricle or the lagena as well as from one of the three canal nerves (similar to30%), smaller numbers of 2degrees VN (14% of each type of 2degrees otolith or 2degrees canal neuron) received an afferent input from only one particular otolith organ or from only one particular semicircular canal organ. Even fewer 2degrees VN received an afferent input from more than one semicircular canal or from more than one otolith nerve (similar to7% each). Among 2degrees VN with afferent inputs from more than one otolith nerve, an afferent saccular nerve input was particularly rare (4-5%). The restricted convergence of afferent saccular inputs with other afferent otolith or canal inputs as well as the termination pattern of saccular afferent fibers are compatible with a substrate vibration sensitivity of this otolith organ in frog. The ascending and/or descending projections of identified 2degrees VN were determined by the presence of antidromic spikes. 2degrees VN mediating afferent utricular and/or semicircular canal nerve signals had ascending and/or descending axons. 2degrees VN mediating afferent lagenar or saccular nerve signals had descending but no ascending axons. The latter result is consistent with the absence of short-latency macular signals on extraocular motoneurons during vertical linear acceleration. Comparison of data from frog and cat demonstrated the presence of a similar organization pattern of maculo- and canal-ocular reflexes in both species.