Modification of Vestibular Output During Efferent Stimulation
Addition of efferent stimulation modifies an afferent's response to vestibular
stimulation. Here a BM afferent is first mechanically stimulated with sinusoidal
indentation which mimics head rotation (Vestibular Only). Notice that the
afferent’s discharge rate increases and decreases in response to the mechanical
stimulus. However, when the efferents are simultaneously activated, the same
vestibular stimulus is much less effective (Efferent + Vestibular).
The afferent essentially becomes less sensitive to the mechanical
stimulus 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. However, these studies used relatively high-frequency efferent stimuli and did not investigate the use of sustained efferent stimuli of lower frequencies which may be more physiologically relevant. Furthermore, no studies have used pharmacology to study each efferent synaptic mechanism in isolation and to determine what each component contributes to the observed gain changes.
In our vestibular model, rapid efferent actions in hair cells and afferents are associated with a reduction in afferent sensitivity (see Figure) whereas slow efferent actions enhance afferent responsiveness. We postulate that each efferent receptor mechanism presented in the Efferent Receptors and Synaptic Mechanisms project differentially modulates the transmission of vestibular information. Testing our hypotheses requires first characterizing the interaction between efferent activation and vestibular stimulation by systematically varying the frequency and intensity of both efferent and vestibular stimuli. Then, each receptor mechanism will be targeted pharmacologically to identify its physiological contribution.