Contributions of the vestibular nucleus and vestibulospinal tract to the startle reflex

The startle reflex is elicited by strong and sudden acoustic, vestibular or trigeminal stimuli. The caudal pontine reticular nucleus, which mediates a coustic startle via the reticulospinal tract, receives further anatomical c onnections from vestibular and trigeminal nuclei, and can be activated by v estibular and tactile stimuli, suggesting that this pontine reticular struc ture could mediate vestibular and trigeminal startle. The vestibular nucleu s, however, also projects to the spinal cord directly via the vestibulospin al tracts, and therefore may mediate vestibular startle via additional fast er routes without a synaptic relay in the hindbrain. In the present study, the timing properties of the vestibular efferent pathways mediating startle -like responses were examined in rats using electrical stimulation techniqu es. Transient single- or win-pulse electrical stimulation of the vestibular nuc leus evoked bilateral, startle-Eke responses with short refractory periods. In chloral hydrate-anesthetized rats, hindlimb electromyogram latencies re corded from the anterior biceps femoris muscle were shorter than those for stimulation of the trigeminal nucleus, and similar to those for stimulation of the caudal pontine reticular nucleus or ventromedial medulla. In awake rats, combining vestibular nucleus stimulation with either acoustic stimula tion or trigeminal nucleus stimulation enhanced the whole-body startle-like responses and led to strong cross-modal summation without collision effect s. In both chloral hydrate-anesthetized and awake rats, combining vestibula r nucleus stimulation with ventromedial medulla stimulation produced a symm etrical collision effect, i.e. a loss of summation at the same positive and negative stimulus intervals, indicating a continuous connection between th e vestibular nucleus and ventromedial medulla in mediating vestibular start le. By contrast, combining trigeminal nucleus stimulation with ventromedial medulla stimulation resulted in an asymmetric collision effect when the tr igeminal nucleus stimulation preceded ventromedial medulla stimulation by 0 .5 ms, suggesting that a monosynaptic connection between the trigeminal nuc leus and ventromedial medulla mediates trigeminal startle. We propose that the vestibulospinal tracts participate strongly in mediatin g startle produced by activation of the vestibular nucleus. The convergence of the vestibulospinal tracts with the reticulospinal tract within the spi nal cord therefore provides the neural basis of cross-modal summation of st artling stimuli. (C) 2001 IBRO. Published by Elsevier Science Ltd. All righ ts reserved.