Straka, H., S. Holler, and F. Goto. Patterns of canal and otolith afferent input convergence in frog second-order vestibular neurons. J Neurophysiol 88: 2287-2301, 2002 10.1152/jn.00370.2002 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 ϳ30% 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 2°VN (3%). In contrast to the considerable number of 2°VN that received an afferent input from the utricle or the lagena as well as from one of the three canal nerves (ϳ30%), smaller numbers of 2°VN (14% of each type of 2°otolith or 2°canal neuron) received an afferent input from only one particular otolith organ or from only one particular semicircular canal organ. Even fewer 2°VN received an afferent input from more than one semicircular canal or from more than one otolith nerve (ϳ7% each). Among 2°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 2°VN were determined by the presence of antidromic spikes. 2°VN mediating afferent utricular and/or semicircular canal nerve signals had ascending and/or descending axons. 2°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. I N T R O D U C T I O NComponents of linear and angular head acceleration are detected separately by the different otolith and semicircular canal organs in the labyrinth. These signals are mediated in parallel pathways by afferent nerve fibers to the vestibular nuclei, the brain stem reticular formation, and the cerebellum. Afferent nerve fibers from individual semicirc...

It is notoriously difficult to study population interactions among highly mobile animals that cannot be meaningfully confined to experimental plots of limited size. For example, migratory water birds are believed to suffer from competition with resident fish populations for shared food resources. While observational evidence in support of this hypothesis is accumulating, replicated experiments addressing this issue at the proper spatial scale are lacking. Here, we report from a replicated whole-system experiment in which we stocked large (0.07 km2), shallow (< or =2.5 m deep), highly eutrophic ponds in the bird protection area "Ismaninger Speichersee mit Fischteichen" with different densities of carp and assessed the responses of water birds and their food resources during summer over several years. In all years, the biomasses of benthic macroinvertebrates, macroalgae, and macrophytes as well as the densities of herbivorous, carnivorous, and omnivorous water birds were reduced in carp ponds compared to fishless ponds. The negative effects of carp on food resources and on the numbers of water birds feeding in carp ponds increased over the season (May-September) and were stronger at high than at low stocking densities of carp. Consequently, differences in resource densities between ponds with and without carp increased, and the ranking of ponds with respect to resource densities became more predictable over the season. These factors may have contributed to a seasonal improvement of the birds' abilities to track resource densities across ponds, as suggested by tight correlations of bird numbers on ponds with resource densities late, but not early, in the season.

. Activation maps of pre-and postsynaptic field potential components evoked by separate electrical stimulation of utricular, lagenar, and saccular nerve branches in the isolated frog hindbrain were recorded within a stereotactic outline of the vestibular nuclei. Utricular and lagenar nerveevoked activation maps overlapped strongly in the lateral and descending vestibular nuclei, whereas lagenar amplitudes were greater in the superior vestibular nucleus. In contrast, the saccular nerveevoked activation map coincided largely with the dorsal nucleus and the adjacent dorsal part of the lateral vestibular nucleus, corroborating a major auditory and lesser vestibular function of the frog saccule. The stereotactic position of individual second-order otolith neurons matched the distribution of the corresponding otolith nerve-evoked activation maps. Furthermore, particular types of second-order utricular and lagenar neurons were clustered with particular types of second-order canal neurons in a topology that anatomically mirrored the preferred convergence pattern of afferent otolith and canal signals in second-order vestibular neurons. Similarities in the spatial organization of functionally equivalent types of second-order otolith and canal neurons between frog and other vertebrates indicated conservation of a common topographical organization principle. However, the absence of a precise afferent sensory topography combined with the presence of spatially segregated groups of particular second-order vestibular neurons suggests that the vestibular circuitry is organized as a premotor map rather than an organotypical sensory map. Moreover, the conserved segmental location of individual vestibular neuronal phenotypes shows linkage of individual components of vestibulomotor pathways with the underlying genetically specified rhombomeric framework.

Commissural inputs of identified second-order semicircular canal neurons were studied by separate stimulation of each of the three canal nerves on either side in the vitro frog brains. The spatial pattern of these inputs was further investigated in those second-order canal neurons that received a monosynaptic input from only one ipsilateral canal nerve (91%). Since similar results were obtained in the presence as in the absence of the cerebellum, commissural inputs must have been relayed via fibers crossing in the brainstem. Following stimulation of individual semicircular canal nerves, commissural inputs were either inhibitory or excitatory. A commissural inhibition was evoked in the majority of the recorded neurons (79%) by stimulation of the coplanar semicircular canal nerve on the contralateral side. In the remaining neurons, a commissural excitatory input was evoked. A commissural excitation, originating from the two noncoplanar semicircular canals, predominated in most (68%) of the recorded neurons and was independent of the type of second-order canal neuron. The onset latency of the canal plane-specific commissural inhibitory potentials was di- or trisynaptic. Stimulation of the contralateral VIIIth nerve evoked excitatory commissural responses. The canal plane-specific commissural inhibition therefore might have been masked by commissural excitatory responses as in earlier studies. The similar organization of the canal plane-specific commissural inhibition in frog and cat corroborates the notion of a phylogenetically conservative, basic vestibular organization. The presence of a canal plane-unspecific commissural excitation, however, appears to be a feature that is specific to frogs. The functional implications of these similarities and differences are discussed.

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