Abstract
The vagus nerve is the largest autonomic nerve, innervating nearly every organ in the body. “Vagal tone” is a clinical measure believed to indicate overall levels of vagal activity, but is measured indirectly through the heart rate variability (HRV). Abnormal HRV has been associated with many severe conditions such as diabetes, heart failure, and hypertension. However, vagal tone has never been directly measured, leading to disagreements in its interpretation and influencing the effectiveness of vagal therapies. Using custom carbon nanotube yarn electrodes, we were able to chronically record neural activity from the left cervical vagus in both anesthetized and non-anesthetized rats. Here we show that tonic vagal activity does not correlate with common HRV metrics with or without anesthesia. Although we found that average vagal activity is increased during inspiration compared to expiration, this respiratory-linked signal was not correlated with HRV either. These results represent a clear advance in neural recording technology but also point to the need for a re-interpretation of the link between HRV and “vagal tone”.
Highlights
The vagus nerve is the largest autonomic nerve, innervating nearly every organ in the body
While vagus nerve activity has not been directly measured in these diseases, heart rate variability (HRV) is a clinical measure presumed to assess vagal activity, often termed “vagal tone”[8]
The physiological mechanism underlying HRV has been debated in literature: some believe HRV to be representative of overall vagal a ctivity[18,19,20,21], while others claim HRV is only driven by cardiac vagal activity, or a combination of cardiac vagal efferents and baroreceptor a fferents[22,23,24]
Summary
The vagus nerve is the largest autonomic nerve, innervating nearly every organ in the body. “Vagal tone” is a clinical measure believed to indicate overall levels of vagal activity, but is measured indirectly through the heart rate variability (HRV). We found that average vagal activity is increased during inspiration compared to expiration, this respiratory-linked signal was not correlated with HRV either These results represent a clear advance in neural recording technology and point to the need for a re-interpretation of the link between HRV and “vagal tone”. While vagus nerve activity has not been directly measured in these diseases, heart rate variability (HRV) is a clinical measure presumed to assess vagal activity, often termed “vagal tone”[8]. Extraneural cuff electrodes, while useful for stimulation, suffer from small signal-to-noise ratio, and can be difficult to implement in small autonomic nerves such as the rat vagus nerve (diameter ~ 250 to 500 μm). CNTY electrodes are small (10 μm diameter), and highly flexible with low-impedance, creating an axon-like interface that allows high-SNR recordings in small autonomic nerves
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