Responses of neurons in the rostral ventrolateral medulla of conscious cats to anticipated and passive movements

Abstract

The vestibular system contributes to regulating sympathetic nerve activity and blood pressure. Initial studies in decerebrate animals showed that neurons in the rostral ventrolateral medulla (RVLM) respond to small-amplitude (<10°) rotations of the body, as in other brain areas that process vestibular signals, although such movements do not affect blood distribution in the body. However, a subsequent experiment in conscious animals showed that few RVLM neurons respond to small-amplitude movements. This study tested the hypothesis that RVLM neurons in conscious animals respond to signals from the vestibular otolith organs elicited by large-amplitude static tilts. The activity of approximately one-third of RVLM neurons whose firing rate was related to the cardiac cycle, and thus likely received baroreceptor inputs, was modulated by vestibular inputs elicited by 40° head-up tilts in conscious cats, but not during 10° sinusoidal rotations in the pitch plane that affected the activity of neurons in brain regions providing inputs to the RVLM. These data suggest the existence of brain circuitry that suppresses vestibular influences on the activity of RVLM neurons and the sympathetic nervous system unless these inputs are physiologically warranted. We also determined that RVLM neurons failed to respond to a light cue signaling the movement, suggesting that feedforward cardiovascular responses do not occur before passive movements that require cardiovascular adjustments.

Responses of Neurons in the Medullary Lateral Tegmental Field and Nucleus Tractus Solitarius to Vestibular Stimuli in Conscious Felines

“…Experimental procedures similar to those used in our prior studies were used to determine if a unit exhibited CRA (7)(8)(9). Trigger pulses coinciding with the R-wave of the ECG were used for the creation of post R-wave interval histograms (10 ms bin width) of unit activity (Datapac software, Run Technologies; Mission Viejo CA USA).…”

“…An exception is the rostral ventrolateral medulla (RVLM), an area of the reticular formation that plays a key role in controlling the firing rate of sympathetic preganglionic neurons that regulate heart rate and constriction of vascular smooth muscle (5,6). Three studies reported the activity of RVLM neurons in conscious felines and how it differs from that in decerebrate animals (7)(8)(9). These studies showed that RVLM neurons whose activity is correlated with the cardiac cycle, and presumably participate in controlling sympathetic nervous system activity, have spontaneous firing rates that vary over time.…”

“…They also compared peak and trough firing rates of RVLM neurons between cardiac cycles and determined that they can also fluctuate during a recording session. It was postulated that these changes in spontaneous firing rate and cardiac-related activity (CRA) of RVLM neurons were dependent on supratentorial inputs and related to the animal's alertness, vigilance, and attentiveness to its environment (4,7,9).…”

“…In decerebrate felines, vestibular inputs elicited by small (<10 • ) sinusoidal head rotations were sufficient to alter the activity of sympathetic efferent fibers in peripheral nerves as well as the firing rate of RVLM neurons (8,11). However, in conscious animals the firing rates of RVLM neurons were unaffected by 10 • sinusoidal body rotations (8,9), although the activity of a third of the neurons was modulated by static 40 • head-up tilts, with an increase in activity in ∼50% of units, and a decrease in the other 50% (9). These changes in unit activity were not correlated with changes in heart rate, suggesting that they were not due to engagement of the baroreceptor reflex.…”

“…The present study examined CRA and responses to 10 • sinusoidal rotations and 40 • static tilts of NTS and LTF neurons in conscious felines. The major goal was to ascertain whether NTS and LTF neurons are likely components of the vestibulosympathetic pathway in conscious animals, with responses to vestibular stimuli similar to those of RVLM neurons presumed to transmit these signals to sympathetic preganglionic neurons in the spinal cord (9). In addition, comparing the responses to whole-body tilts of NTS and LTF neurons with those of RLVM neurons provides insights into the role of neurons at each stage of the vestibulo-sympathetic reflex pathway in transforming signals from the inner ear.…”