Brain Injury and the Nervous System
A brain injury is a disruption of brain function caused by an outside force, running a spectrum from brief concussion to lasting coma. Across the whole spectrum the same thing is damaged: the brain's regulation of its own chemistry, blood flow, pressure, and arousal. The Unified Model of Tone reads every severity as one injury at different doses: a collapse in the range the nervous system can move through. The collapse unfolds on a time course that starts after the blow ends.
A traumatic brain injury is a disturbance of brain function produced by an external force, graded mild, moderate, or severe by the depth of unconsciousness it causes.
After the blow, the brain must pump displaced ions back across billions of membranes while defending its blood flow inside a rigid skull. It must keep its accelerator and brake coordinated at the same time, for weeks. Tone is the organization that carries that work. What the force disrupts, at every severity, is the organization rather than only the tissue, which is why function can be devastated while structure looks intact.
Brain injury expresses all of tone. Load, coupling and time course carry its signature.
The remaining foundations each read a specific part of the injury. Set point: the injured brain defends a shifted arousal baseline, braced high in one survivor and blunted low in another. Gain: the sensory volume jams high, which is why ordinary light and noise arrive as glare and din. Oscillation: the injury fragments the sleep rhythms that time the brain's own cleanup. Prediction: a brain that has learned to expect threat keeps allocating for threat long after the danger is gone. Constraint: the skull cannot expand, so swelling that would be trivial in any other tissue becomes the emergency of the severe end. Input quality: the insula misreads the body after injury, and a system fed distorted reports regulates toward the distortion. The autonomic nervous system: the anatomy where the disturbance is measured, in heart rhythms disturbed from concussion to coma.
- A 2013 review by Victoria Johnson, William Stewart, and Douglas Smith established that diffuse axonal injury runs across the whole severity spectrum. Axons strained by the impact degenerate over hours to years rather than snapping at the moment of the blow. The injury to the wiring is a process, not an event.
- Christopher Giza and David Hovda mapped the neurometabolic cascade that follows the impact: potassium floods out of the cells, calcium floods in, and the energy cost of repumping them spikes while blood flow falls. The mismatch lasts 7 to 10 days in animal models and longer in humans. The acute symptoms track a crisis of cellular regulation, not a structural defect.
- Marcus Raichle and Debra Gusnard's 2002 appraisal of the brain's energy budget found the organ takes about 20 percent of the body's energy at roughly 2 percent of its mass. An organ living that close to its limits has no reserve to absorb an injury-induced energy gap, which is why the gap produces symptoms.
- A 2008 review led by Leonardo Rangel-Castilla and Claudia Robertson found cerebral pressure autoregulation frequently impaired after traumatic brain injury. Flow that a healthy brain holds steady across a perfusion pressure range of roughly 50 to 150 millimeters of mercury instead follows pressure passively. The injured brain loses the coupling that protected its own blood supply.
- A 2018 systematic review led by Jon Pertab concluded that autonomic dysfunction is a consistent, measurable consequence of brain injury even at the mild end. Max Hilz's 2011 frequency analysis confirmed disturbed cardiac autonomic balance after mild injury. The blow reaches the body's regulatory machinery even when it spares the scan.
- Lulu Xie and Maiken Nedergaard showed in 2013 that sleep drives metabolite clearance, widening the brain's interstitial space by more than 60 percent and clearing waste roughly twice as fast as waking. Jeffrey Iliff then showed a brain injury cuts this glymphatic clearance by about 60 percent for at least a month. The injury raises the debris and disables the cleanup at the same time.
- John Leddy's 2019 randomized trial of 103 adolescents found subthreshold aerobic exercise shortened concussion recovery to a median of 13 days against 17 for stretching. Graded demand reached the autonomic machinery that rest alone left idle, which is what a regulation injury predicts and a pure structure injury does not.
Brain injury runs one spectrum from concussion to coma
Medicine grades a traumatic brain injury by the Glasgow Coma Scale: 13 to 15 is mild, 9 to 12 is moderate, 3 to 8 is severe. The grades describe one injury at three doses, not three different injuries.
The scale scores how awake a person is: whether the eyes open, whether speech makes sense, whether the body obeys a command. A concussion with a moment of dazed confusion sits at the top. A person who cannot be roused sits at the bottom. Most brain injuries, by a wide margin, sit in the mild range, and most of those are concussions.
The grades matter because they predict different immediate dangers. At the severe end the emergency is physical: bleeding, swelling, and pressure inside a skull that cannot expand. At the mild end the scan is usually clean and the trouble is functional. But the same processes run at every grade. The wiring strains, the cells fall into an energy crisis, the blood flow control falters, and the autonomic machinery loses its coordination. Severity is how much of each, and for how long.
The concussion page covers mild traumatic brain injury in depth, including the full metabolic cascade, the return-to-activity protocol, and the persistent symptoms that follow some concussions. What a concussion shares with a coma explains both better than either alone, and that shared machinery is the severity spectrum itself.
The severe end announces itself, and it is an emergency
A brain injury with a worsening headache, repeated vomiting, one pupil larger than the other, deepening confusion, weakness, seizures, or a person who cannot be woken is an emergency. These signs point to bleeding or swelling inside the skull, structural injuries that must be found and treated fast. The emergency evaluation clears a far larger territory: the brain injury that is real while the pictures are normal.
The blow is an instant; the brain injury is a time course
Every traumatic brain injury has two halves. The primary injury is the mechanical event, over in milliseconds. The secondary injury is the cascade of dysregulation it launches, running for hours, days, and weeks.
The primary injury is physics. A moving head stops hard, or something strikes it, and the soft brain shifts and strains inside the skull. Nothing can be done about that instant once it has happened, at any severity.
The secondary injury is control. The strained cells fire chaotically, the chemistry tips out of balance, the blood flow falters, pressure can rise inside the closed skull, and the systems that would restore order are themselves among the injured. This half is where most of the lasting harm accumulates, and it is the half medicine can still influence. Neurocritical care for severe brain injury is, almost entirely, the management of secondary injury.
This division is the first place the time course of a brain injury shows itself. The person who walked away from the collision can be worse on day three than on day one, because the injury is still unfolding. The severity grades of section one are snapshots of a process, and the process, not the snapshot, is what recovery has to reverse.
The brain's wiring strains at every severity
The long fibers that carry signals between brain regions are called axons, and the rapid stretch of an impact strains them without snapping them. The strain then plays out over time, and the dose of it separates a concussion from a coma.
Victoria Johnson, William Stewart, and Douglas Smith, a neuropathology and bioengineering team, reviewed the fate of axons in head trauma. Their account of diffuse axonal injury describes wiring that is deformed by the stretch, then undergoes a slow internal breakdown that can disconnect a fiber hours or days after the impact. Degeneration in injured white matter remains detectable years later. The wires fail on a schedule, not at the moment of the blow.
The same axonal strain, at a low dose, hides below the resolution of a scan. At a high dose it is a principal cause of coma.
Dose is the whole difference. In a mild brain injury the strain is scattered thinly across the wiring, each damaged axon invisible to CT and routine MRI, and the person is awake and symptomatic with a clean picture. In a severe injury the same process affects enough of the deep connecting pathways to disconnect the networks that sustain consciousness itself.
Diffuse axonal injury is one of the principal reasons a person with severe head trauma does not wake. One mechanism spans the spectrum, which is why brain injury is one injury at different doses rather than three separate diagnoses.
Demand spikes while supply falls inside the injured brain
The stretch of a brain injury throws open the ion gates of the neurons, and the cost of repumping potassium out and calcium back creates an energy crisis that runs for days to weeks.
Christopher Giza and David Hovda at UCLA mapped this sequence, first in their 2001 description and again in their updated neurometabolic cascade. The impact forces a mass discharge. Potassium floods out of the cells, calcium floods in, and the signaling chemical glutamate surges, driving the firing harder.
Restoring order means pumping every displaced ion back, and pumping costs fuel. Demand spikes at the precise moment cerebral blood flow, the supply line, has dropped. The mismatch lasts 7 to 10 days in animal models and runs longer in humans.
The reason a fuel gap in this particular organ is so consequential was measured by Marcus Raichle and Debra Gusnard. Their appraisal of the brain's energy budget found that the brain consumes about 20 percent of the body's energy while making up roughly 2 percent of its mass. An organ running that close to its ceiling has no reserve. When a brain injury opens a gap between need and supply, function fails even though no tissue has been torn.
This cascade is the engine of the mild end of the spectrum, and the concussion page teaches it in full. The crisis is a failure of cellular regulation, temporary at a low dose. At a high dose it compounds, feeding the swelling and rising pressure inside the closed skull.
The skull is a closed box, and the injured brain stops guarding its own blood supply
The skull cannot expand, so swelling after a severe brain injury raises the pressure inside it and squeezes the blood supply. At the same time the injury disables autoregulation, the reflex that held that blood supply steady.
A healthy brain defends its perfusion. Cerebral autoregulation adjusts the caliber of the brain's vessels so that flow stays nearly constant while the pressure driving it swings across a wide range, roughly 50 to 150 millimeters of mercury of perfusion pressure. Flow and pressure are deliberately uncoupled, which is what lets you stand up quickly without the lights going out.
A neurosurgical team led by Leonardo Rangel-Castilla and Claudia Robertson reviewed what head trauma does to this reflex and found cerebral pressure autoregulation frequently impaired after traumatic brain injury. The uncoupling fails, and flow becomes pressure-passive: when systemic pressure dips, the injured brain's blood supply dips with it, and when pressure surges, the surge reaches fragile vessels directly. Each swing can extend the injury.
Why intensive care for severe brain injury is a regulation service
This is why the neurocritical care of a severe brain injury revolves around pressures: intracranial pressure monitored through the skull, perfusion pressure held inside a target band, swelling managed hour by hour. The clinicians are performing, from outside, the regulation the brain has stopped performing for itself.
The severe end of the spectrum makes the nature of the whole spectrum explicit. What failed is a control loop, and the treatment is a manual replacement for it. At the mild end the same lesson holds with the machines removed: the brain injury is carried in what the brain can no longer regulate.
After a brain injury, the accelerator and the brake lose their coordination
The sympathetic system mobilizes the body and the parasympathetic system settles it, and the regions that coordinate the two sit in the pathways a head injury strains. The disturbance is measurable at every severity.
The measurement uses the heartbeat. The interval between beats varies constantly as the two branches adjust it, and that variation, heart rate variability, is an established index of cardiac autonomic state. A team led by Jon Pertab gathered the scattered studies of autonomic function after concussion into a 2018 systematic review.
The evidence pointed one way: autonomic dysfunction is a common, measurable consequence of even mild brain injury, consistent across studies rather than a stray finding. The neurologist Max Hilz sharpened the point with a frequency analysis of the heart rhythm that demonstrated cardiac autonomic dysfunction after mild traumatic brain injury. The balance between sympathetic drive and the vagal brake was disturbed in people whose injury was labeled mild.
At the severe end the disturbance stops being subtle. Some survivors of severe brain injury develop paroxysmal sympathetic hyperactivity, episodes in which the accelerator fires unopposed: racing heart, soaring blood pressure, drenching sweat, rigid posturing. The coordination between the branches has failed outright. Mild and severe injury sit on the same axis here, a coordination disturbed a little or disturbed completely.
One blow, two opposite outcomes
The disturbance does not run the same direction in everyone. One person is left overdriven: wired, tachycardic, sleepless, anxious. Another is left blunted: flat, exhausted, slow to rouse. The same category of blow produces opposite states because the force lands on a nervous system already tuned a particular way, and the outcome belongs to that meeting.
An account of brain injury as a fixed lesion predicts one deficit per location. The scatter is what a disturbance of regulation predicts, and the scatter is what the studies find.
Why light, noise, and the body itself feel wrong after a brain injury
Two settings distort together after a brain injury: the volume the brain sets on the world, and the accuracy of its reading of the body. The first makes a lit room unbearable. The second makes a normal body feel wrong.
The brain amplifies the signals that matter and suppresses the rest, a setting called neural gain. A group led by Elena Orekhova measured this gain through fast cortical rhythms and found gain control tracked individual sensory sensitivity. The less a brain suppressed incoming signals, the more intensely its owner felt the world.
After a brain injury the suppression fails and the gain jams high. The eyes report ordinary light and the brain receives glare. The ears report an ordinary room and the brain receives din. Nothing in the sense organs has changed. The setting has.
The insula misreads the body it is supposed to report
The brain also keeps a running read of its own body: heartbeat, breath, gut, muscle tension, skin temperature. The neuroanatomist Arthur Craig traced where these internal reports assemble into feeling and mapped the insula as the cortex that reads the body's physiological condition. A brain injury distorts this reading.
A normal heartbeat registers as alarming; an ordinary state feels persistently off in a way no test confirms. This matters beyond comfort. A nervous system regulates the body it perceives, and a system fed a distorted report of its own state regulates toward the distortion. The misread belongs to the machinery of the brain injury itself, part of what keeps a person unwell.
A brain injury raises the debris and breaks the cleanup at once
Sleep is when the brain physically washes out its metabolic waste, and a brain injury impairs that washing for at least a month while producing more waste to wash.
Lulu Xie and Maiken Nedergaard measured what sleep does for the brain's housekeeping by tracking how fast the sleeping and waking brain cleared injected tracers. Their finding: sleep drives metabolite clearance. During sleep the brain's interstitial space widens by more than 60 percent and waste clears roughly twice as fast as during waking, through a fluid pathway now called the glymphatic system.
Jeffrey Iliff, working in the same laboratory, asked what a head injury does to this pathway and found it cuts glymphatic function by about 60 percent. The impairment persists at least a month after injury and promotes the accumulation of tau, a protein that aggregates after trauma.
Now add the third element: a brain injury commonly wrecks sleep itself. The injury generates extra debris, disables the pathway that clears debris, and disturbs the sleep state in which that pathway runs. Three failures compound into one loop that turns against recovery.
This loop is where the chronic phase of a brain injury takes root, and where repeated injury becomes more than the sum of its blows. Debris that outpaces clearance accumulates, and the tau finding states that arithmetic in molecular terms. The time course of a brain injury does not end when the acute symptoms fade. What the acute phase leaves unresolved, the chronic phase entrenches.
A brain injury holds its braced state, and the holding has a measurable cost
A crisis that has passed should resolve, and for many people a brain injury does not. Two bodies of work explain the sticking: the cost of constant compensation, and a brain defending predictions the injury wrote.
Bruce McEwen and Eliot Stellar named the cost. Studying why chronic stress produces disease, they defined allostatic load: the cumulative wear a body accrues when stability is purchased through constant, expensive compensation instead of flexible regulation. A nervous system left braced by a brain injury, accelerator engaged around the clock, is paying exactly this price. The load is measurable wear on the systems doing the compensating, and it accumulates for as long as the bracing holds.
Karl Friston's work explains why the bracing holds. His free-energy principle describes the brain as a prediction machine that spends its resources closing the gap between what it expects and what it senses.
A brain whose model of the world was rewritten by injury now expects threat, and a brain expecting threat allocates for threat. It defends the braced state because the braced state is what its predictions call for. The collapsed range does not spring back on its own, because the system is actively maintaining the collapse.
Recovery from a brain injury retunes regulation, at every severity
In most brain injuries no tissue was lost, so recovery cannot be regrowth. What recovers is regulation, retuned prediction by prediction, and the evidence for that runs from the concussion clinic to the rehabilitation ward.
The retuning has anatomy. A review led by Arseny Sokolov characterized the cerebellum as an adaptive-prediction machine that builds internal models, compares expectation against what arrives, and corrects the model when they disagree. Recovery from a brain injury is this correction run across the whole system: relearning that light is safe, that a raised heartbeat is not danger, that the world can be met without bracing. It proceeds through graded, tolerable demand, because demand is the data the retuning needs.
At the mild end the demonstration is experimental. John Leddy's team randomized 103 adolescents with sport concussion to gentle aerobic exercise, held below the level that provoked symptoms, or to stretching. The exercise group recovered in a median of 13 days against 17, with no harm. Rest alone, past the first days, left recoverable range on the table; graded demand reclaimed it. The concussion page carries the full graded return-to-activity framework this trial anchors.
At the moderate and severe end, rehabilitation applies the same principle at a higher dose over a longer time course. Mobilize early, challenge the system to the edge of tolerance, expand the tolerance, and repeat, across months instead of weeks.
Restoring regulation and masking output are different acts
A medication that quiets the headache, dampens the anxiety, or forces sleep manages output, and in the acute phase it is often necessary and right. Restoring regulation is the other aim: widening the range the nervous system can move through so the symptoms ease because the system can settle itself again. The distinction predicts something measurable.
A restoring input should move the overdriven survivor down and the blunted survivor up, both toward the middle, because what returns is the capacity to reach the middle. A sedative moves both down. The direction of change, checked against where each person started, is the test that separates the two. Brain injury, with its two opposite autonomic outcomes on the same measurable index, is a clean place to run it.
What a brain injury expresses in tone
Every condition expresses all of tone, the integrated organization the nervous system holds across the body. In brain injury, three aspects carry the signature.
- Load is the wear of the compensation: an injured nervous system buys its stability with round-the-clock bracing, and McEwen's allostatic arithmetic runs against it for as long as the bracing holds.
- Coupling is what the force disconnects: pressure from flow in the injured brain's vessels, clearance from sleep in its fluid system, accelerator from brake in its autonomic control.
- Time course is the injury's very structure: a millisecond of mechanics launching a secondary cascade over days to weeks, and a chronic phase that entrenches whatever the acute phase left unresolved.
The health side of this reading completes it. Tone within its healthy range is health, because the organization keeps the flexibility to meet a demand and return. A brain injury drives tone outside that range and holds it there. The held distortion is what manifests as the illness: the intolerance of light, the racing or flattened heart, the sleep that will not consolidate, the body that feels wrong. Recovery, at every severity, is tone regaining its range.
How brain injury relates to the rest of the library
Brain injury sits at a junction in the library, because a single blow disturbs nearly every system other pages treat on its own.
- Concussion owns the mild end of this spectrum in depth: the full neurometabolic cascade, the persistent post-concussive syndrome, and the graded return-to-activity framework behind the 13-day recovery of Leddy's 2019 trial all live there.
- Time course is the foundation this page leans on hardest, because primary versus secondary injury is the library's clearest case of a condition that is a trajectory rather than an event.
- Coupling generalizes what autoregulation failure demonstrates here: systems a healthy body holds in step, uncoupled by illness.
- Load carries the full account of allostatic wear that section nine applies to the braced survivor.
- Balance and coordination follows the cerebellum, the adaptive-prediction machine of section ten, into the dizziness and unsteadiness that trail so many brain injuries.
- And neck pain travels with brain injury because the force that accelerates a skull accelerates the neck below it, and the two injuries are routinely acquired together and confused for one another.
Frequently asked
Why is my brain scan normal if I have concussion symptoms?
Because most concussions are a disturbance of how the brain regulates itself, not a hole in the tissue. The wiring is stretched rather than cut, and the cells fall into a temporary energy crisis, and neither shows on a routine CT or MRI. Those scans are built to find bleeding and gross damage, which they do well. A failure of regulation leaves no lesion for them to detect, which is why the picture is clean while you are genuinely unwell.
Can a concussion affect my heart rate, sleep, and anxiety?
Yes. A brain injury disturbs the two controllers that balance the body, the sympathetic accelerator and the vagal brake. Research finds measurable autonomic dysfunction even after mild injury. This is why a concussion leaves one person wired, sleepless, and anxious, and another flat, exhausted, and slow, from the same kind of blow.
Why are light and noise unbearable after a head injury?
The brain sets the volume on incoming signals, amplifying some and suppressing others. After a brain injury that setting, called neural gain, is turned up and the suppression fails. Ordinary light and everyday noise then arrive at full strength and overwhelm the system. Your eyes and ears are reporting normally. The brain is treating their signals as too loud.
When is a brain injury an emergency?
Seek emergency care immediately for a worsening or severe headache, repeated vomiting, one pupil larger than the other, deepening confusion or drowsiness, a person who cannot be woken, slurred speech, weakness, or a seizure. These can signal bleeding or dangerous swelling inside the skull, structural injuries that must be found and treated fast. This page discusses the regulation problems that remain after such emergencies are ruled out, and it is not a substitute for urgent evaluation.
What is the difference between masking symptoms and restoring recovery?
A medication that quiets a headache or forces sleep manages the output, which is useful and sometimes necessary, and it pushes in one direction. Restoring regulation widens the range the nervous system can move through, so symptoms ease because the brain regains its capacity to settle itself. Graded, tolerable activity aims at the second, and the subthreshold aerobic exercise tested in concussion cut median recovery to 13 days from 17. Restoring regulation moves overdriven and blunted people toward a healthy middle from opposite sides, which a one-directional drug does not.
What does the Unified Model of Tone say about brain injury?
The Unified Model of Tone reads a brain injury, at every severity, as a collapse in tone. Tone is the integrated organization the nervous system holds across chemistry, blood flow, wiring, and autonomic control, together with the range it can move through. The blow uncouples systems the healthy brain holds in step, the compensation accumulates wear, and the disturbance unfolds on a time course from cascade to chronic phase. Recovery is the organization regaining its range, which is why graded demand helps and why the scan can stay clean throughout.
References
Every source below links to its publication on PubMed, PubMed Central, or the original journal.
Dr. Jason Dulberg, DC, DACNB, FACFN
Diplomate, American Chiropractic Neurology Board (DACNB), a chiropractic specialty board and not a medical neurology board · Fellow, American College of Functional Neurology · Luxury Chiropractic, Miami. Author of the Unified Model of Tone.