NERVOUS SYSTEM
GLOSSARY
48 terms, searchable and explained in plain language, from the vagus nerve to the window of tolerance.
Allostasis
The active process of maintaining stability by changing your physiology in response to demand, as opposed to homeostasis, which implies a fixed set point. Allostatic load is the cumulative wear on your body from staying in a stressed, adaptive state for too long.
Autonomic Nervous System (ANS)
The branch of your nervous system that runs largely below conscious control, regulating heart rate, digestion, breathing rate, and your stress response. It splits into the sympathetic (activating) and parasympathetic (calming) branches.
Central Nervous System (CNS)
Your brain and spinal cord. The CNS processes information from the rest of the body and sends back the signals that shape movement, thought, and your automatic stress responses.
Co-Regulation
The process of one nervous system helping to steady another, through a calm voice, steady presence, or physical closeness. It’s often the fastest route back to a regulated state, and is considered foundational rather than a workaround.
Dorsal Vagal State
The most primitive of the three polyvagal states: a shutdown or freeze response that kicks in when fight or flight hasn’t resolved a threat. It shows up as numbness, fog, exhaustion, or disconnection.
Dysregulation
A state in which the nervous system has moved outside its window of tolerance and can’t easily return on its own, showing up as either hyperarousal (anxious, wired, reactive) or hypoarousal (numb, foggy, shut down).
Enteric Nervous System
Sometimes called the second brain, this is the extensive network of neurons lining your gut. It communicates directly with your brain via the vagus nerve, which is part of why stress and digestion are so closely linked.
Exteroception
Sensory awareness of the external world, gathered through sight, sound, touch, smell, and taste. It works alongside interoception (internal body sensing) and proprioception (body position) to build your overall sense of safety.
Fawn Response
A fourth survival response, alongside fight, flight, and freeze, in which a person appeases or people-pleases their way out of a perceived threat. It’s common but frequently left out of mainstream explanations of the stress response.
Fight, Flight, Freeze, Fawn
The four broad survival responses your nervous system can mobilize under threat: fighting the danger, fleeing it, freezing in place, or fawning to de-escalate it. Which one shows up isn’t a choice, it’s shaped by biology, history, and context.
Freeze Response
A survival state in which the body becomes immobilized, often when neither fight nor flight seems possible. It can look like being physically frozen, or mentally blanking out under pressure.
Heart Rate Variability (HRV)
The small, healthy variation in time between heartbeats. Higher HRV generally correlates with higher vagal tone and better stress recovery, and it’s one of the few nervous system markers you can approximate at home without equipment.
Homeostasis
Your body’s baseline drive to keep internal conditions (temperature, blood sugar, arousal level) within a stable range. Where allostasis describes active adaptation, homeostasis describes the stable state being aimed for.
HPA Axis
Short for the hypothalamic-pituitary-adrenal axis, this is the hormonal loop that governs your cortisol-driven stress response. It works on a slower timescale than the nervous system’s immediate fight-or-flight reaction, shaping how long stress lingers in the body.
Hyperarousal
A state of heightened activation outside the window of tolerance: racing thoughts, panic, irritability, a pounding heart. It’s the nervous system’s fight-or-flight response in motion.
Hypoarousal
A state of reduced activation outside the window of tolerance: numbness, fog, exhaustion, disconnection. It reflects a dorsal vagal, shutdown response rather than low energy or laziness.
Interoception
Your sense of your body’s internal state, hunger, heartbeat, tension, the urge to breathe. Strong interoceptive awareness is closely linked to emotional regulation, since many emotions are first noticed as body sensations.
Neuroception
A term coined by Dr. Stephen Porges for the subconscious process by which your nervous system scans for cues of safety or danger, well before conscious thought catches up. It explains why some situations feel unsafe even when nothing is objectively wrong.
Nervous System Reset
An umbrella term for practices, extended exhale breathing, cold exposure, co-regulation, that actively shift the nervous system out of a stress response and back toward the window of tolerance.
Parasympathetic Nervous System
The rest and digest branch of the autonomic nervous system, responsible for slowing your heart rate, supporting digestion, and returning your body to a calm baseline after a stressor passes.
Peripheral Nervous System
Every nerve outside the brain and spinal cord, connecting the central nervous system to your limbs and organs. It includes both the somatic nervous system (voluntary movement) and the autonomic nervous system (involuntary regulation).
Polyvagal Ladder
A visual metaphor for the three polyvagal states stacked like rungs: ventral vagal (safe and social) at the top, sympathetic (mobilized) in the middle, and dorsal vagal (shutdown) at the bottom. Moving down the ladder is a threat response; moving up is regulation.
Polyvagal Theory
A framework developed by Dr. Stephen Porges describing three nervous system states, ventral vagal, sympathetic, and dorsal vagal, and how the vagus nerve governs the shifts between them. It’s widely used in trauma-informed therapy.
Proprioception
Your sense of where your body is in space, without looking. It relies on receptors in muscles and joints, and it’s part of why deliberate, weighted movement (like pushing against something heavy) can feel grounding.
Self-Regulation
The ability to shift your own nervous system state without outside help, using tools like breathwork, movement, or grounding. It typically develops on top of, not instead of, a history of good co-regulation.
Sympathetic Nervous System
The fight or flight branch of the autonomic nervous system. It speeds your heart rate, redirects blood to your muscles, and prepares your body to act quickly in response to a perceived threat.
Upregulation / Downregulation
Upregulation refers to intentionally increasing nervous system activation, useful when you’re in hypoarousal and need more energy. Downregulation refers to intentionally decreasing it, useful when you’re in hyperarousal and need to calm down.
Vagal Brake
The vagus nerve’s ability to rapidly slow or speed up heart rate by adjusting its influence, functioning like a brake pedal for your physiological arousal. A well-functioning vagal brake allows fast, flexible shifts between calm and alert states.
Vagal Tone
A measure of how strongly your vagus nerve influences your heart rate, often estimated through heart rate variability. Higher vagal tone is associated with faster stress recovery and greater emotional resilience.
Vagus Nerve
The tenth cranial nerve, and the main pathway of the parasympathetic nervous system. It runs from the brainstem through the throat, chest, and abdomen, and roughly 80% of its fibers carry information from the body up to the brain, not the other way around.
Ventral Vagal State
The polyvagal state associated with feeling safe and socially connected: calm, curious, and able to think clearly. It’s the state most nervous system practices are ultimately trying to help you access and return to.
Vestibular Sense
Your sense of balance and movement, governed by structures in the inner ear. It works alongside proprioception and is often engaged in somatic and movement-based practices.
Window of Tolerance
A term coined by Dr. Dan Siegel for the zone of arousal in which a person can function at their best, calm enough to think, activated enough to stay engaged. Outside it lies hyperarousal above and hypoarousal below.
Amygdala
A small, almond-shaped structure in the brain that acts as an early threat detector, flagging potential danger before conscious thought catches up. It plays a central role in triggering the fight, flight, freeze, or fawn response.
Amygdala Hijack
A term for the moment the amygdala’s threat response overrides the more measured, reasoning parts of the brain, leading to a reaction that feels bigger or faster than the situation seems to call for.
Cortisol
The primary stress hormone, released via the HPA axis. Short bursts help mobilize energy during a real threat, but chronically elevated cortisol from ongoing stress is linked to a narrower window of tolerance and slower recovery.
Adrenaline (Epinephrine)
A hormone released rapidly during a sympathetic nervous system response, responsible for the fast heart rate, alertness, and energy surge that come with fight or flight.
Prefrontal Cortex
The brain region responsible for reasoning, planning, and impulse control. It tends to go partially offline during high sympathetic activation, which is part of why it’s hard to think clearly when you’re panicked.
Limbic System
A group of brain structures, including the amygdala and hippocampus, involved in emotional processing and the stress response. It works closely with the autonomic nervous system to shape how threat and safety are experienced.
Somatic Nervous System
The branch of the peripheral nervous system that controls voluntary movement and carries sensory information, distinct from the autonomic nervous system, which handles involuntary regulation like heart rate and digestion.
Reticular Activating System (RAS)
A network of neurons in the brainstem that regulates overall alertness and arousal, filtering which sensory information reaches conscious awareness. It plays a role in how quickly you notice potential threats or cues of safety.
Respiratory Sinus Arrhythmia (RSA)
The natural, healthy speeding up of heart rate on the inhale and slowing on the exhale. It’s a direct, felt marker of vagal engagement, and a stronger RSA pattern generally reflects better vagal tone.
Baroreflex
A fast-acting feedback loop that helps regulate blood pressure by adjusting heart rate, working closely alongside the vagus nerve to keep your cardiovascular system responsive to changing demands.
Tend-and-Befriend
An alternative stress response, alongside fight, flight, freeze, and fawn, characterized by seeking social connection and caretaking behavior under threat. It’s been studied particularly in the context of stress responses in women.
Autonomic Flexibility
The capacity to move smoothly between sympathetic activation and parasympathetic calm as circumstances change, rather than getting stuck in one state. Higher flexibility is generally associated with better stress resilience.
Chronic Stress
Prolonged activation of the stress response without adequate recovery time, which narrows the window of tolerance over time and increases allostatic load.
Neuroplasticity
The nervous system’s ability to form new neural connections and reorganize existing ones in response to experience and practice. It’s the mechanism behind why consistent nervous system practice reliably widens your window of tolerance over time.
Insula
A brain region closely tied to interoception, helping integrate signals from the body into a coherent sense of internal state. It’s a key part of the circuitry linking body sensation to emotional experience.
Test Your Knowledge
Quiz yourself on nervous system terms, flashcard style or multiple choice.
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Go Deeper
For the practice-based, body-first side of this work, see our companion Somatics Glossary, covering terms like titration, pendulation, and felt sense.
Frequently Asked Questions
The sympathetic nervous system drives fight-or-flight activation: faster heart rate, redirected blood flow, heightened alertness. The parasympathetic nervous system drives rest-and-digest recovery: slower heart rate, supported digestion, a return to calm. Both are branches of the autonomic nervous system, and a healthy nervous system moves between them fluidly.
It’s a state where your nervous system has moved outside its window of tolerance, into either hyperarousal (anxious, wired, reactive) or hypoarousal (numb, foggy, shut down), and isn’t easily returning to baseline on its own.
A commonly used proxy is heart rate variability (HRV): higher HRV is generally associated with stronger vagal tone and faster stress recovery. Our vagus nerve guide walks through a simple at-home way to get a rough read on this.
It’s an influential framework, widely used in trauma-informed therapy, and some specific claims within it are still debated among researchers. Its practical framing, naming states and looking for safety cues, is used clinically and reported as helpful by many therapists and clients regardless of the underlying debate.
Extended exhale breathing and cold exposure are two of the fastest tools, both working through the vagus nerve. Co-regulation, connecting with a safe person or presence, often works even faster, though it requires someone else to be available.
Common signs include racing thoughts, irritability, and a pounding heart on the hyperarousal side, or numbness, fog, and disconnection on the hypoarousal side. Both reflect a nervous system that has moved outside its window of tolerance.
Neuroception is the subconscious process by which your nervous system scans for cues of safety or danger, well before conscious thought catches up. It matters because it explains why some situations feel unsafe even when nothing is objectively wrong, the response is happening below awareness.
The autonomic nervous system is the broader network governing involuntary functions like heart rate and digestion, split into sympathetic and parasympathetic branches. The vagus nerve is the main pathway of the parasympathetic branch specifically, not the whole system.
Most people can meaningfully widen their window of tolerance and build a more flexible, resilient nervous system with consistent practice over time. It’s rarely instant, but it is trainable, the same way a muscle strengthens with training.
Freeze is an immobilized survival state that tends to kick in when fight or flight isn’t possible or hasn’t worked. Fight or flight is energized and built for action; freeze is a stillness response, often described as the nervous system’s last-resort option.
It’s the cumulative wear on your body from staying in a stressed, adaptive state for too long. Where allostasis is the active, short-term process of adjusting to demand, allostatic load is what builds up when that adjustment never gets a chance to fully reset.
The vagus nerve is the physical nerve itself, the tenth cranial nerve running from your brainstem through your throat, chest, and abdomen. Vagal tone is a measure of how strongly that nerve influences your heart rate, often estimated through heart rate variability.
The HPA axis (hypothalamic-pituitary-adrenal axis) is the hormonal loop that governs your cortisol-driven stress response. It works on a slower timescale than your nervous system’s immediate fight-or-flight reaction, which is part of why stress can linger in the body long after a stressful event ends.
Yes. Practices like extended exhale breathing, cold exposure, and consistent aerobic exercise are commonly associated with improvements in vagal tone and heart rate variability over time.
It’s a visual metaphor for the three polyvagal states stacked like rungs: ventral vagal (safe and social) at the top, sympathetic (mobilized) in the middle, and dorsal vagal (shutdown) at the bottom. It’s used to help people track which state they’re in and notice movement between them.
Heart rate variability (HRV) is the small, healthy variation in time between heartbeats. Higher HRV is generally associated with stronger vagal tone and faster recovery from stress, making it one of the few nervous system markers you can approximate at home.
The central nervous system is your brain and spinal cord. The peripheral nervous system is every nerve outside of that, connecting the central nervous system to your limbs and organs, including both voluntary movement pathways and the autonomic nervous system.
It’s the extensive network of neurons lining your gut, sometimes called the second brain because it can operate somewhat independently and communicates constantly with your actual brain via the vagus nerve. This is part of why stress and digestion are so closely linked.
The fawn response is understood as an automatic, nervous-system-level survival strategy, appeasing a perceived threat to stay safe, while people-pleasing is often used more loosely to describe a learned personality pattern. In practice the two overlap heavily, and fawning is considered one of the four core threat responses alongside fight, flight, and freeze.
They’re closely related but not identical. Chronic stress is prolonged activation of the stress response without adequate recovery. Left unaddressed, it commonly leads to nervous system dysregulation, a narrower window of tolerance and less flexible movement between states.
The amygdala acts as an early threat detector, flagging potential danger before conscious thought catches up, and helping trigger the fight, flight, freeze, or fawn response. An amygdala hijack refers to this threat response overriding more measured, reasoning parts of the brain.
Roughly 80% of the vagus nerve’s fibers are afferent, carrying signals from your gut, heart, and lungs up to your brain, not the other way around. It means nervous system work is often more effective when it starts with changing a body signal, rather than trying to think your way calm first.
Upregulation means intentionally increasing nervous system activation, useful when you’re in hypoarousal and need more energy. Downregulation means intentionally decreasing it, useful when you’re in hyperarousal and need to calm down. Both are tools for moving back toward your window of tolerance.