12/16/2025
🧠🦠CONCUSSION, THE VAGUS NERVE & THE BRAIN–GUT AXIS:
WHY SEROTONIN, INFLAMMATION & AUTONOMIC BALANCE MATTER MORE THAN YOU’VE BEEN TOLD
When someone suffers a concussion or head injury, the focus is almost always on the brain itself — headaches, dizziness, memory problems, visual strain, brain fog.
But neuroscience is becoming increasingly clear:
👉 The brain does not heal in isolation.
👉 The vagus nerve and the brain–gut axis play a critical role in concussion recovery.
A recent comprehensive review published in the International Journal of Molecular Sciences (MDPI) highlights how gut signaling, vagal pathways, serotonin, and neuroimmune responses directly influence brain health, inflammation, mood, cognition, and recovery after neurological injury.
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🔌 The Brain–Gut Axis: A Two-Way Neurological Highway
The brain–gut axis is a bidirectional communication network connecting:
• The brain and brainstem
• The autonomic nervous system
• The immune system
• The gut microbiome
• The endocrine (hormonal) system
At the center of this network sits the vagus nerve (cranial nerve X) — the primary sensory highway sending information from the gut to the brain.
💡 Up to 80–90% of vagal fibers are afferent, meaning they carry information from the body to the brain, not the other way around.
This makes the gut one of the most powerful sensory organs influencing brain function.
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🧠What Happens to the Vagus Nerve After Concussion?
After concussion or head trauma, several things commonly occur:
đź”» Reduced vagal tone
🔻 Autonomic imbalance (sympathetic dominance / “fight-or-flight”)
đź”» Impaired heart rate variability
đź”» Increased neuroinflammation
đź”» Altered gut motility and permeability
This dysregulation can drive persistent post-concussion symptoms, including:
• Nausea and GI upset
• Anxiety and mood changes
• Poor sleep
• Fatigue
• Brain fog
• Head pressure
• Light and sound sensitivity
• Exercise intolerance
These symptoms are not psychological — they are neurophysiological.
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🦠The Gut, Inflammation & Brain Injury
The MDPI review highlights that after brain injury:
⚠️ The gut microbiome can become disrupted
⚠️ Intestinal permeability (“leaky gut”) may increase
⚠️ Immune signaling from the gut can amplify brain inflammation
This is critical because neuroinflammation delays neural recovery and interferes with synaptic plasticity — the brain’s ability to rewire and heal.
The vagus nerve normally helps suppress excessive inflammation via what’s known as the cholinergic anti-inflammatory pathway.
When vagal signaling is impaired, inflammation can remain unchecked.
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🌟 SEROTONIN: THE MISSING LINK MOST PEOPLE DON’T KNOW ABOUT
One of the most important — and misunderstood — pieces of the brain-gut axis is serotonin.
🧬 Over 90% of serotonin is produced in the gut, not the brain.
Serotonin plays a critical role in:
• Mood regulation
• Sleep–wake cycles
• Pain modulation
• Cognitive flexibility
• Autonomic balance
• Neuroplasticity
Gut-derived serotonin communicates with the brain primarily through the vagus nerve.
After concussion:
đź”» Serotonin signaling can become dysregulated
đź”» Vagal feedback to brainstem nuclei is altered
đź”» Mood changes, anxiety, irritability, and depression may emerge
đź”» Sleep and circadian rhythms are disrupted
This is one reason many post-concussion patients experience emotional and psychological symptoms — even without a prior history.
Again: this is biology, not weakness.
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🧠Brainstem, Vagus & Higher Brain Centers
The vagus nerve projects directly into the nucleus tractus solitarius (NTS) in the brainstem — a key hub that connects to:
• The locus coeruleus
• The raphe nuclei (serotonin centers)
• The hypothalamus
• Limbic and emotional regulation circuits
This means vagal input from the gut can directly influence:
✔️ Arousal and alertness
✔️ Stress responses
✔️ Emotional regulation
✔️ Cognitive clarity
✔️ Recovery capacity
If this system is offline, the brain struggles to regulate itself.
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đź©ş Why This Matters at The Functional Neurology Center (FNC)
At FNC, we recognize that persistent concussion symptoms are often driven by network dysfunction, not structural damage alone.
That’s why our approach looks at:
🔹 Autonomic nervous system balance
🔹 Vagal tone and brainstem integration
🔹 Gut–brain signaling
🔹 Inflammatory load
🔹 Neuroplastic recovery pathways
We don’t just ask “Where does it hurt?”
We ask “Which systems are failing to communicate?”
Because restoring communication is how healing happens.
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🧠✨ The Big Takeaway
Concussion is not just a brain injury.
It is a whole-system neurological event.
The vagus nerve and brain-gut axis — especially serotonin signaling — play a central role in:
• Persistent symptoms
• Mood and emotional changes
• Cognitive recovery
• Autonomic regulation
• Long-term brain health
Understanding and addressing these pathways can be the difference between stalled recovery and meaningful healing.
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đź“© If you or someone you love is struggling with lingering concussion symptoms, know this:
👉 There is more to the story
👉 There is a physiological explanation
👉 And there is hope
TheFNC.com
612 223 8590
https://www.mdpi.com/1422-0067/26/3/1160
Interaction of the Vagus Nerve and Serotonin in the Gut–Brain Axis
by Young Keun Hwang 1ORCID and Jae Sang Oh 1,2,
