No abstract

SUMMARY The location of neurons early in development can be critical for their ability to differentiate and receive normal synaptic inputs. Indeed, disruptions in neuronal positioning lead to a variety of neurological disorders. Neurons have, however, shifted their positions across phylogeny, suggesting that changes in location do not always spell functional disaster. To investigate the functional consequences of abnormal positioning, we leveraged previously reported genetic perturbations to disrupt normal neuronal migration – and thus positioning – in a population of cranial motor neurons, the facial branchiomotor neurons (FBMNs). We used a combination of topographical, morphological, physiological, and behavioral analyses to determine if key functional features of FBMNs were still established in migration mutants, in spite of a dramatic rostrocaudal re-positioning of these neurons in hindbrain. We discovered that FBMNs seem remarkably resilient to a disruption in positioning, suggesting that they may not rely heavily on rostrocaudal positioning to guide their functional development. Thus, the role of positioning may vary across the developing nervous system, with some populations – like facial motor neurons – exhibiting greater resilience to abnormal positioning that permits them to shift location as a part of evolutionary change.

Vestibular responses play an important role in maintaining gaze and posture stability during rotational motion. Previous studies suggest that these responses are state dependent, their expression varying with the environmental and locomotor conditions of the animal. In this study, we simulated an ethologically relevant state in the laboratory to study state-dependent vestibular responses in birds. We used frontal airflow to simulate gliding flight and measured pigeons' eye, head, and tail responses to rotational motion in darkness, under both head-fixed and head-free conditions. We show that both eye and head response gains are significantly higher during flight, thus enhancing gaze and head-in-space stability. We also characterize state-specific tail responses to pitch and roll rotation that would help to maintain body-in-space orientation during flight. These results demonstrate that vestibular sensorimotor processing is not fixed but depends instead on the animal's behavioral state.

McArthur KL, Dickman JD. Canal and otolith contributions to compensatory tilt responses in pigeons. J Neurophysiol 100: 1488 -1497, 2008. First published July 16, 2008 doi:10.1152/jn.90257.2008. Gazestabilizing eye and head responses compensate more effectively for low-frequency rotational motion when such motion stimulates the otolith organs, as during earth-horizontal axis rotations. However, the nature of the otolith signal responsible for this improvement in performance has not been previously determined. In this study, we used combinations of earth-horizontal axis rotational and translational motion to manipulate the magnitude of net linear acceleration experienced by pigeons, under both head-fixed and head-free conditions. We show that phase enhancement of eye and head responses to low-frequency rotational motion was causally related to the magnitude of dynamic net linear acceleration and not the gravitational acceleration component. We also show that canal-driven and otolithdriven eye responses were both spatially and temporally appropriate to combine linearly, and that a simple linear model combining canaland otolith-driven components predicted eye responses to complex motion that were consistent with our experimental observations. However, the same model did not predict the observed head responses, which were spatially but not temporally appropriate to combine according to the same linear scheme. These results suggest that distinct vestibular processing substrates exist for eye and head responses in pigeons and that these are likely different from the vestibular processing substrates observed in primates.

No abstract