What the vagus nerve actually does

The vagus nerve is the longest cranial nerve in the body. It exits the brainstem, runs down each side of the neck, and branches into the heart, lungs, gut, and most of the abdominal organs. Roughly 80% of its fibers are afferent, meaning they carry information from the body up to the brain. The remaining 20% are efferent, carrying signals from the brain down to the organs.

Functionally, the vagus is the principal channel of the parasympathetic nervous system—the “rest and digest” counterpart to the sympathetic “fight or flight” branch. When vagal tone is high, heart rate slows between beats, digestion proceeds, and the body recovers. When vagal tone is low, the system tilts toward sympathetic dominance: faster pulse, shallower breathing, heightened alertness, and a harder time settling.

Because the vagus carries signals in both directions, it is implicated in interoception—the brain’s perception of internal bodily state. That two-way traffic is part of why the vagus has become such a focus of interest in psychiatry, chronic pain, and inflammation research over the past two decades.

HRV as a proxy for parasympathetic tone

Heart rate variability is the variation in time between consecutive heartbeats. A healthy heart does not tick like a metronome; the gaps between beats subtly expand and contract with breathing and autonomic state. Higher HRV is generally associated with better vagal tone, better stress resilience, and better cardiovascular health. Lower HRV correlates with chronic stress, depression, anxiety disorders, and several inflammatory conditions.

HRV has become accessible to ordinary people through wearables. That accessibility is a double-edged sword. The metric is real and clinically meaningful in research settings, but day-to-day readings are noisy, sensitive to sleep, alcohol, illness, hydration, and measurement method. A single number on a wrist device should not drive treatment decisions, and we tell patients that directly.

The depression-HRV literature in brief

There is a substantial body of work linking lower HRV to worse depressive symptoms and to poorer response to antidepressants. Meta-analyses across the 2010s and 2020s consistently show that patients with major depression have lower resting vagal tone than non-depressed controls. Whether the low HRV is a cause, a consequence, or a parallel feature of the same underlying biology is still debated.

Sanacora, Treccani, and Popoli, writing in Neuropharmacology (2012), helped establish the broader glutamate and neuroplasticity model of depression that frames most current ketamine research. Their work is important context here because it sets the dominant explanation for why ketamine works—rapid changes in synaptic plasticity through glutamate signaling and downstream BDNF—against which any vagal hypothesis has to compete.

The autonomic nervous system is not absent from that picture, but in the mainstream depression literature it sits as a downstream correlate, not the engine. Ketamine is FDA-approved as an anesthetic; its use for depression and chronic pain is off-label.

Vagus-nerve stimulation as an FDA-approved TRD therapy

Vagus-nerve stimulation (VNS) is the part of this story where the vagus nerve has unambiguous, regulator-backed evidence. VNS uses an implanted pulse generator, similar to a pacemaker, that delivers electrical stimulation to the cervical vagus nerve. The U.S. Food and Drug Administration originally cleared VNS for treatment-resistant depression in 2005. A 2012 review by Cusin and Dougherty in Annals of General Psychiatry covered VNS alongside other therapeutic neuromodulation approaches for treatment-resistant depression.

VNS response in TRD is typically slow—benefits often emerge over months rather than days—but durable for a meaningful subset of patients. It is not a first-line therapy; the FDA indication is reserved for patients who have failed at least four adequate antidepressant trials. Like any implanted device, it carries surgical risk and requires periodic adjustment.

The reason this matters for a ketamine article is that VNS is the cleanest example we have of a therapy that works through the vagus nerve. Anything we say about ketamine and the vagus has to acknowledge that the device that actually stimulates the vagus is a separate, mature, regulator-cleared technology with a different evidence base and a different patient profile. Ketamine and VNS are not interchangeable, and we do not present them that way.

Where ketamine intersects with the vagal story

Ketamine’s primary mechanism is NMDA receptor antagonism. It binds inside the glutamate-gated ion channel, reduces excitatory signaling, and triggers a cascade of synaptic and neurotrophic changes that the field continues to map. The 2023 review by Krystal, Kavalali, and Monteggia in Neuropsychopharmacology is the most comprehensive recent synthesis: it frames ketamine’s antidepressant effects in terms of synaptic plasticity, glutamate, and BDNF. Vagal and autonomic effects are not framed as a primary mechanism.

Carhart-Harris, in his 2018 update to the entropic-brain framework published in Neuropharmacology, similarly placed ketamine alongside classic psychedelics in terms of central nervous system effects—changes in cortical entropy, top-down control, and network dynamics. Peripheral autonomic mechanisms were not the focus.

So where does the vagus enter? A few honest threads exist. First, several small studies have looked at whether baseline HRV predicts who responds to ketamine, with mixed and preliminary results. Second, sub-anesthetic ketamine has well-documented transient sympathomimetic effects—a modest rise in heart rate and blood pressure during the infusion—which means the autonomic nervous system is clearly engaged in the acute response. Third, many patients describe a felt parasympathetic shift in the hours and days after a session: slower breath, easier sleep, less reactive startle. Some of that is consistent with vagal influence; some of it could simply be the downstream effect of reduced rumination and improved mood. We cannot separate the two from current data.

For a deeper look at the central-nervous-system side of ketamine’s action, our pieces on how ketamine works and on ketamine brain research walk through what the imaging and biomarker literature actually shows.

What's emerging vs. what's speculative

It is worth being explicit about which parts of this picture are settled and which are not.

Wellness marketing tends to collapse those three categories into one. We try not to. Patients deserve the distinction.

What we tell patients honestly

If someone walks in asking whether ketamine will improve their HRV or repair their vagal tone, our answer is the same one we would give about any single biomarker: maybe, but that is not how we measure success. Success in ketamine therapy looks like meaningful improvement in depression, anxiety, or pain symptoms over a course of treatment, sustained between sessions, with side effects that are manageable. HRV is not a primary outcome we treat to.

Patients with cardiac history get a careful conversation, and our pieces on ketamine and heart conditions and ketamine and hypertension cover the specifics of how we think about cardiovascular risk. The transient sympathomimetic effect during infusion is exactly why CRNA-led monitoring matters. Marla Peterson, CRNA, oversees every infusion and provides anesthesia-level monitoring, which is the same standard we describe in how it works.

For patients with high anxiety about the infusion experience itself, our piece on managing anxiety during a ketamine session covers practical preparation. And for readers who have encountered breathless online claims about what ketamine does or does not do, our ketamine myths piece is a useful complement to this one.

Ketamine is FDA-approved as an anesthetic; its use for depression, anxiety, and chronic pain is off-label. We say that plainly because the difference matters when you are deciding whether to pursue treatment. Cost is also a real factor: insurance generally does not cover off-label IV ketamine, and at Music City Ketamine our sessions are $475 each. Never start, stop, or change any medication without talking to your prescribing provider.

The vagus nerve is fascinating, the autonomic nervous system clearly matters, and the science here will keep moving. We are willing to follow the evidence wherever it actually goes. We are not willing to oversell a connection that has not yet been established.