Efferent Receptors and Synaptic Mechanisms
In reptiles, birds, and mammals, efferent vestibular neurons can provide synaptic input to three targets: type II hair cells, their bouton afferents, and afferent calyces innervating type I hair cells. Stimulation of efferent neurons would be expected to exert their effects at one of these three locations. In our system, we have mainly focused on three classes of vestibular afferents and their different responses to efferent stimulation.
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Modification of Vestibular Output During Efferent Stimulation
Most studies of efferent action have dealt with its influence on background or spontaneous afferent discharge. However, it is important to examine how efferent activation modifies the afferent’s response to vestibular stimuli. In most cases, it is thought that efferent activation increases the conductance of hair cells and afferent terminals thereby shunting synaptic currents underlying the afferent’s response to vestibular stimulation.
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Characterizing the morphophysiological properties of vestibular efferent neurons
In order to define a functional role for vestibular efferents, it is essential to know what their discharge properties are. This certainly includes descriptions of any “spontaneous” or background activity, discharge regularity, and what the upward limits of their discharge might be. It also becomes important to determine if we can modify efferent discharge by modulating afferent activity. Vestibular efferents may exhibit background activity which responds to both vestibular and extralabyrinthine sensory inputs. Such activity, by recruiting efferent receptors on hair cells and afferent fibers, would, in turn, modify afferent physiology.
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Behavioral Assessment of Efferent Function
The first three projects provide us with a wealth of information regarding the activity and synaptic actions of vestibular efferent neurons and how they might impinge upon vestibular processing. However, there is very little information about how efferents are working in animals performing normal vestibular tasks such as walking or turning their head. We have a number of pharmacological agents that can be used to interact with different components of the vestibular efferent system simply by local application into the peripheral vestibular labyrinth.
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