Neural circuitry and neutrotransmitters that mediate the acoustic startle reflex in the rat

Abstract

Because the acoustic startle reflex has such a short latency (e.g., 5 ms measured electromyographically in the neck, 8 ms in the hindleg), it must be mediated by a relatively simple neural pathway. We believe that the acoustic startle pathway is mediated by three synapses. These include auditory nerve inputs onto cochlear root neurons, which are cells embedded in the auditory nerve that receive the first afferent information in the auditory system. They send exceedingly thick axons (e.g., 7 μm in diameter) to cells in the nucleus reticularis pontis caudalis which in turn project directly to motoneurons in the facial motor nucleus (pinna reflex) or spinal cord (whole body startle). Chemical lesions of cochlear root neurons or of cells in the ventral lateral nucleus reticularis pontis caudalis eliminate startle. Local infusion of either NMDA or non-NMDA antagonists into either the nucleus reticularis pontis caudalis or the spinal cord, via intrathecal infusion, markedly depress acoustic startle, suggesting that both of these glutamate receptor subtypes may mediate the acoustic startle reflex.