Abstract
Considerable evidence shows that the vestibular system contributes to adjusting sympathetic nervous system activity to maintain adequate blood pressure during movement and changes in posture. However, only a few prior experiments entailed recordings in conscious animals from brainstem neurons presumed to convey baroreceptor and vestibular inputs to neurons in the rostral ventrolateral medulla (RVLM) that provide inputs to sympathetic preganglionic neurons in the spinal cord. In this study, recordings were made in conscious felines from neurons in the medullary lateral tegmental field (LTF) and nucleus tractus solitarius (NTS) identified as regulating sympathetic nervous system activity by exhibiting changes in firing rate related to the cardiac cycle, or cardiac-related activity (CRA). Approximately 38% of LTF and NTS neurons responded to static 40° head up tilts with a change in firing rate (increase for 60% of the neurons, decrease for 40%) of ~50%. However, few of these neurons responded to 10° sinusoidal rotations in the pitch plane, in contrast to prior findings in decerebrate animals that the firing rates of both NTS and LTF neurons are modulated by small-amplitude body rotations. Thus, as previously demonstrated for RVLM neurons, in conscious animals NTS and LTF neurons only respond to large rotations that lead to changes in sympathetic nervous system activity. The similar responses to head-up rotations of LTF and NTS neurons with those documented for RVLM neurons suggest that LTF and NTS neurons are components of the vestibulo-sympathetic reflex pathway. However, a difference between NTS/LTF and RVLM neurons was variability in CRA over time. This variability was significantly greater for RVLM neurons, raising the hypothesis that the responsiveness of these neurons to baroreceptor input is adjusted based on the animal's vigilance and alertness.
Highlights
The neural pathways through which baroreceptor and other inputs affect sympathetic nervous system activity have been determined in a variety of mammalian species [1,2,3]
A linear regression analysis failed to show a strong correlation between the significance of responses to the light cue and headup tilts for lateral tegmental field (LTF) (P = 0.11, R2 = 0.23) or nucleus tractus solitarius (NTS) (P = 0.03, R2 = 0.21) neurons. These experiments included the first recordings in conscious animals from NTS and LTF neurons with changes in firing rate correlated with the cardiac cycle, which presumably participate in regulating cardiovascular responses mediated by the sympathetic nervous system [4]
They showed that the activity of an appreciable fraction (38%) of both NTS and LTF neurons with cardiac-related activity (CRA) was significantly modulated by 40◦ static head-up tilts, with most (60%) having higher firing rates in the head-up position
Summary
The neural pathways through which baroreceptor and other inputs affect sympathetic nervous system activity have been determined in a variety of mammalian species [1,2,3]. 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 compared peak and trough firing rates of RVLM neurons between cardiac cycles and determined that they can 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]
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