Grey Matter

A map of the brain, from the ion to the thought, and of why it sometimes lights up on its own. A second brain about the brain, by Yago Mendoza.

Grey Matter. A map of the brain, from the ion to the thought, and of why it sometimes lights up on its own.

Grey Matter is a second brain about the brain: concepts joined by the questions between them, written one at a time and checked before they go in. It exists because I think reading the brain cheaply will be the next revolution after the language models, and a revolution is easier to follow when you know what the tissue underneath is doing.

The four pillars:

- Readable psychology: How a state of mind (frustration, sustained effort, stress, sleep) becomes a response of the limbic system, the prefrontal cortex and the hormones the hypothalamus sets off. - The machine: The neuron as an electrochemical device: ion gradients held by a pump that never stops, the spike, the synapse and the balance of excitation against inhibition. - Getting the data: Why reading a brain through the skull is hard (scattering, signal to noise, the inverse problem, heat) and what electrodes on and in the cortex buy instead. - Ethics: Who owns a recording of a brain, what may be inferred from it and who may refuse, written beside every device that makes the question real.

Biology:

- absence seizure (phenomenon): An absence seizure is a generalised seizure of a few seconds in which awareness switches off and back on, with a blank stare and an interrupted action, and it is the clearest case of a seizure produced by the loop between the cortex and the thalamus. - acetylcholine (molecule): Acetylcholine is the transmitter that makes muscles contract at the neuromuscular junction and, inside the brain, a neuromodulator of attention, arousal and learning released from the basal forebrain and brainstem. - action potential (process): An action potential is the brief, all-or-none reversal of a neuron's membrane voltage that carries a signal along its axon to the next cells, and it is the only output a neuron has. - adrenaline (molecule): Adrenaline (epinephrine) is the hormone the adrenal glands pour into the blood within seconds of a threat or effort, preparing the body to act (faster heart, more glucose, redirected blood flow), and it reaches the brain only indirectly, through nerves. - adult neurogenesis (process): Adult neurogenesis is the birth of new neurons in the mature brain, and its interest is that it would add whole new cells to circuits, beyond changing synapses; in rodents it clearly happens in the hippocampus, while in adult humans how much of it happens is still disputed. - AMPA receptor (molecule): The AMPA receptor is the glutamate-gated ion channel that carries nearly all fast excitation in the brain: when glutamate binds it opens within a millisecond, lets sodium in, and depolarises the receiving neuron for a few milliseconds. - amygdala (region): The amygdala is an almond-sized group of nuclei deep in each temporal lobe, in front of the hippocampus, that decides quickly whether something matters emotionally, above all whether it is a threat, and starts the body's response before deliberate thought has caught up. - anterograde amnesia (disorder): Anterograde amnesia is the inability to form new lasting memories of events and facts after a brain injury, while what was learned before the injury, the capacity to hold something in mind for a few seconds and the ability to learn new skills largely remain. - astrocyte (cell): An astrocyte is a star-shaped glial cell that keeps the space around neurons fit for signalling: it clears the glutamate released at synapses, soaks up the potassium that firing neurons leave behind, and links active tissue to its blood and energy supply. - aura (phenomenon): An aura is a symptom the person notices at the start of an attack, before or instead of anything an observer can see, and it is the name used for two different events: the opening of a migraine attack and the first, still-aware part of a focal seizure. - autoreceptor (molecule): An autoreceptor is a receptor on a neuron that responds to the neuron's own transmitter, usually on its terminals, and it works as a thermostat: when release runs high, the transmitter in the cleft turns release down. - axonal conduction (process): Axonal conduction is how an action potential travels from the cell body along the axon to the terminals, and its speed (from under a metre per second to over a hundred) decides how long a signal takes to cross the brain or reach a muscle. - basal ganglia (region): The basal ganglia are a set of deep nuclei that decide which of the actions the cortex proposes is allowed to start, and that learn from reward which actions to repeat until they become habits. - bitemporal hemianopia (disorder): Bitemporal hemianopia is the loss of the outer half of the visual field in both eyes, the typical sign of something pressing on the middle of the optic chiasm, most often a tumour of the pituitary gland growing upward. - blood-brain barrier (phenomenon): The blood-brain barrier is the sealed wall of the brain's capillaries, which lets in only what the brain needs (oxygen, glucose, selected amino acids) and keeps out most other molecules in the blood, so it protects neurons from fluctuations and toxins and also blocks most drugs. - brain energy budget (phenomenon): The brain energy budget is the account of how much energy the brain consumes and what it spends it on: about 20 W in an adult at rest, a fifth of the body's resting use from 2 % of its mass, spent mostly on restoring the ion gradients that signalling runs down. - brain (region): The brain (the encephalon) is everything of the central nervous system that sits inside the skull: the cerebrum with its two hemispheres, the cerebellum behind and below it, and the brainstem that continues into the spinal cord. - brainstem (region): The brainstem is the stalk that joins the cerebrum to the spinal cord and keeps the body alive without thought: it drives breathing, sets heart rate and blood pressure, runs swallowing, coughing and vomiting, and keeps the cortex awake. - Broca's aphasia (disorder): Broca's aphasia is a disorder of language production after damage to the left frontal lobe, in which speech becomes slow, effortful and stripped of grammar while the understanding of everyday conversation is relatively preserved. - Broca's area (structure): Broca's area is the part of the left inferior frontal gyrus that helps turn an intended message into spoken, grammatical language: it is involved in planning the articulation of words and in building and parsing sentence structure. - cerebellar ataxia (disorder): Cerebellar ataxia is the loss of coordination that follows damage to the cerebellum: strength is normal, but movements become poorly aimed, poorly timed and unsteady, because the system that predicts and corrects them no longer works. - cerebellum (region): The cerebellum is the folded structure at the back of the brain, below the occipital lobes and behind the brainstem, that makes movements smooth and accurate by predicting what each command will do and correcting it before the error shows. - cerebral cortex (region): The cerebral cortex is the folded sheet of grey matter, two to four millimetres thick, that covers both hemispheres and does the brain's perceiving, acting, remembering and deliberating. - cingulate cortex (region): The cingulate cortex is the band of cortex that wraps around the corpus callosum on the inner face of each hemisphere, connecting what the brain feels and wants with what it attends to and does. - corpus callosum (structure): The corpus callosum is the thick bridge of white matter that joins the two cerebral hemispheres, letting each side share what it perceives and plans with the other. - cortical blindness (disorder): Cortical blindness is the loss of sight caused by damage to the primary visual cortex of both occipital lobes, with eyes, optic nerves and pupillary reflexes that still work normally. - cortical spreading depression (phenomenon): Cortical spreading depression is a slow wave of almost complete depolarisation that travels across the cortex at a few millimetres per minute, silencing the tissue it passes, and it is the event thought to cause the aura of migraine. - cortisol (molecule): Cortisol is the steroid hormone of the slow stress response: released by the adrenal cortex at the command of the brain, it mobilises energy, damps inflammation and changes how neurons work for hours, and it rises every morning in a daily rhythm. - credit assignment (phenomenon): Credit assignment is the problem every learning system faces of working out which of its many connections, among all those active before an outcome, deserve to change and by how much, and how the brain solves it is one of the open questions that separate brains from today's artificial networks. - deep brain stimulation (device): Deep brain stimulation is an implanted system that delivers continuous or scheduled electrical pulses through electrodes placed deep in the brain, and for epilepsy it targets a hub of the thalamus to make seizures less frequent when drugs and surgery are not options. - dopamine (molecule): Dopamine is a neuromodulator made by a small group of midbrain neurons that broadcasts, to the striatum and the frontal cortex, whether things are going better or worse than expected, and that signal drives learning from reward, the vigour of movement and motivation. - dorsolateral prefrontal cortex (structure): The dorsolateral prefrontal cortex (DLPFC) is the upper outer part of the prefrontal cortex that holds a goal and its rules in mind while a task runs: it keeps a phone number for the seconds it takes to dial it, switches strategy when the rule changes and reappraises a situation when an emotion needs to be reined in. - endocannabinoids (molecule): Endocannabinoids are lipid messengers that a neuron makes on demand when it is strongly activated and sends backwards across its synapses, telling the terminals that feed it to release less; they are the brain's own version of the molecules that cannabis mimics. - epilepsy surgery (method): Epilepsy surgery removes, destroys or disconnects the part of the brain where seizures start, and it is the treatment most likely to stop seizures altogether in a person whose focal epilepsy has resisted medication. - epilepsy (disorder): Epilepsy is a lasting tendency of the brain to produce epileptic seizures, which are brief episodes of abnormal, excessive or synchronous neuronal activity whose signs depend on where in the brain they start and how far they spread. - epileptiform discharge (phenomenon): An epileptiform discharge is a sudden, brief wave on the EEG that stands out sharply from the background and is typical of tissue prone to seizures, and reading these waves is how a neurologist supports a diagnosis of epilepsy in the long stretches between seizures. - episodic memory (process): Episodic memory is the memory of personal events, each tied to its own time and place, which lets a person mentally travel back to a moment and re-experience it: what happened at breakfast, where the keys were left, the last meeting with a friend. - excitability landscape (phenomenon): The excitability landscape is a working map, used in this section as a hypothesis, of where a patch of cortex sits relative to its two ways of failing, so that a disturbance in it can be predicted to fade, to become a slow wave of spreading depression, or to become a fast synchronous discharge. - excitation-inhibition balance (phenomenon): Excitation-inhibition balance is the way cortical circuits keep the excitatory drive a neuron receives matched by inhibition that arrives with it, so that the network can respond strongly to an input without tipping into runaway activity. - excitotoxicity (process): Excitotoxicity is the death of neurons caused by too much excitation, above all by glutamate that keeps its receptors open too long, and it is a common final path of damage in stroke, brain injury and prolonged seizures. - exocytosis (process): Exocytosis is how a neuron releases its transmitter: a small vesicle filled with neurotransmitter fuses with the membrane of the axon terminal and empties its contents into the synaptic cleft, triggered by the calcium that enters when an action potential arrives. - frontal lobe (region): The frontal lobe is the front third of each hemisphere, the part of the cortex that turns goals into behaviour: it holds the plan, chooses among options, keeps impulses in check and finally sends the commands that move the body. - fusiform gyrus (structure): The fusiform gyrus is the long ridge on the underside of the temporal and occipital lobes where the visual system recognises complex shapes as particular things, above all faces and, in readers, written words. - GABA-A receptor (molecule): The GABA-A receptor is the chloride channel opened by GABA, and it delivers the fast inhibition of the brain: within a millisecond of binding it clamps the receiving neuron near or below rest for a few to a few tens of milliseconds. - GABA-B receptor (molecule): The GABA-B receptor is the slow, metabotropic receptor for GABA: through a G protein it opens potassium channels on the receiving cell and closes calcium channels on terminals, giving an inhibition that peaks after tens to hundreds of milliseconds and lasts for hundreds. - GABA-enhancing drug (molecule): A GABA-enhancing drug is an antiseizure drug that strengthens the brain's main inhibitory signal, either by making GABA receptors respond more or by leaving more GABA at the synapse, and it is the fastest way to stop a seizure that is already running. - GABA switch (process): The GABA switch is the developmental change that turns GABA from a depolarising signal in immature neurons into the inhibitory transmitter of the mature brain, by lowering the chloride inside neurons, and it explains why the same transmitter can excite a newborn's neurons. - GABA (molecule): GABA (gamma-aminobutyric acid) is the main inhibitory neurotransmitter of the adult brain: released by interneurons, it makes the next neuron less likely to fire, and it is the brake that keeps excitation from running away. - gap junction (molecule): A gap junction is a cluster of channels that joins the inside of two cells directly, letting ions and small molecules pass from one to the other, and between neurons it forms an electrical synapse that transmits with almost no delay. - glutamate (molecule): Glutamate is the main excitatory neurotransmitter of the brain: released by pyramidal cells, it opens channels on the next neuron that let positive ions in and push that neuron toward firing. - glycine (molecule): Glycine is the main fast inhibitory neurotransmitter of the spinal cord and brainstem, where it keeps motor circuits from overreacting, and in the forebrain it has a second job as the co-agonist that the NMDA receptor needs before glutamate can open it. - glycolysis (process): Glycolysis is the first stage of burning glucose, ten reactions in the cytoplasm that split one glucose into two pyruvates and yield a net 2 ATP, and in the brain it is the fast, oxygen-free start of an energy supply that is completed in the mitochondria. - grey matter (structure): Grey matter is the tissue of the brain and spinal cord where the computing happens: it holds the neurons' cell bodies, their dendrites, the local stretches of their axons and the synapses between them, together with glial cells and a dense bed of capillaries. - Hebbian learning (process): Hebbian learning is the rule that a connection between two neurons grows stronger when the first repeatedly helps to fire the second, and it is the simplest account of how experience writes associations into the wiring. - hemispatial neglect (disorder): Hemispatial neglect is a failure to notice, attend to or act on one side of space, almost always the left after damage to the right hemisphere, although the eyes and the sense of touch on that side still work. - hippocampus (region): The hippocampus is a curved strip of cortex on the inner face of each temporal lobe that turns experiences into lasting memories: it binds the sights, sounds, places and feelings of an episode together so that the episode can be recalled later, and it builds the map that lets an animal know where it is. - histamine (molecule): Histamine is the neuromodulator that keeps the brain awake: it is made by a small cluster of neurons in the posterior hypothalamus that fire during waking and fall silent in sleep, and blocking its receptors is why older antihistamines make people drowsy. - homonymous hemianopia (disorder): Homonymous hemianopia is the loss of the same half of the visual field in both eyes (the left half or the right half of everything seen) caused by damage behind the optic chiasm on the opposite side. - HPA axis (process): The HPA axis is the hormonal chain through which the brain answers stress with cortisol: the hypothalamus signals the pituitary, the pituitary signals the adrenal glands, and the adrenal cortex releases cortisol, which mobilises energy and adjusts the body for a sustained challenge. - hypothalamus (region): The hypothalamus is a small region at the base of the brain, about the size of an almond, that keeps the body's internal conditions in balance: it measures temperature, water, salt, energy and time of day, and corrects them through behaviour, the autonomic nervous system and hormones. - insula (region): The insula is a lobe of cortex hidden inside the lateral fissure that tells the brain how the body feels from the inside: it receives taste and the signals of the heart, lungs, gut, temperature and pain, and turns them into felt states such as hunger, nausea, disgust or a racing heart. - ionotropic receptor (molecule): An ionotropic receptor is a receptor that is itself an ion channel: when the transmitter binds, the pore opens and ions flow at once, so it carries the fast signals of the brain, lasting milliseconds. - ischaemic cascade (process): The ischaemic cascade is the chain of events that follows when blood stops reaching part of the brain, from the loss of oxygen and glucose through the failure of the ion pumps to glutamate release and cell death, and it explains why brain tissue tolerates only minutes without circulation. - juvenile myoclonic epilepsy (disorder): Juvenile myoclonic epilepsy is a common generalised epilepsy that starts in adolescence, recognised by sudden muscle jerks shortly after waking, and it is the clearest example of an epilepsy whose seizures follow sleep, light and fatigue. - ketogenic diet (process): The ketogenic diet is a medically supervised diet very high in fat and very low in carbohydrate that shifts the body to burning fat and ketone bodies, and it is used to reduce seizures in epilepsy that drugs have failed to control, mostly in children. - ketone metabolism (process): Ketone metabolism is the brain's use of ketone bodies, small fuels made by the liver from fat, in place of part of its glucose, and it is what lets the brain keep working through days and weeks of fasting. - lactate shuttle (process): The lactate shuttle is the proposal that astrocytes break down glucose to lactate during activity and pass it to neurons as fuel, coupling the synaptic work an astrocyte sees to the energy a neuron receives; it is an influential and still contested model of how brain cells share their energy. - lateralisation (phenomenon): Lateralisation is the tendency of some functions to depend more on one hemisphere than the other, most clearly language on the left and the spatial attention to both sides on the right. - limbic system (region): The limbic system is a traditional name for the set of structures on the inner edge of each hemisphere that join emotion, motivation and memory to the body's responses: what is felt, what is wanted and what is remembered because it mattered. - locus coeruleus (region): The locus coeruleus is a small nucleus in the pons of the brainstem that supplies almost all the noradrenaline of the cortex, hippocampus, thalamus and cerebellum, and through it a few tens of thousands of neurons set the arousal of the whole brain. - long-term depression (process): Long-term depression (LTD) is a lasting decrease in the strength of a synapse, produced by weak or poorly timed activity, and it is the counterpart that lets learning erase and refine as well as add. - long-term potentiation (process): Long-term potentiation (LTP) is a lasting increase in the strength of a synapse produced by strong, coincident activity of the two cells it joins, and it is the best-studied cellular mechanism of learning and memory. - memory consolidation (process): Memory consolidation is the process that turns a fresh, fragile memory into a lasting one: first by stabilising the synaptic changes that recorded it, then, over weeks to years, by reorganising it so that it depends less on the hippocampus and more on the cortex. - mesial temporal lobe epilepsy (disorder): Mesial temporal lobe epilepsy is epilepsy whose seizures start in the inner face of the temporal lobe (the hippocampus, the amygdala and the cortex around them), and it is the most common focal epilepsy in adults and the one most often treated with surgery. - metabotropic receptor (molecule): A metabotropic receptor is a receptor that, when its transmitter binds, sets off a chain of chemical reactions inside the cell instead of opening a pore itself, so its effects are slower (from tens of milliseconds to minutes), longer-lasting and more varied than those of an ion channel. - microglia (cell): Microglia are the brain's resident immune cells, which patrol the tissue, clear debris and dying cells, and remove synapses that are tagged for elimination, so they keep the wiring tidy as well as defending it. - migraine (disorder): Migraine is a common brain disorder of recurring attacks of headache, usually throbbing and one-sided and made worse by light, sound and movement, that last from hours to a few days; in about a third of people who have it, some attacks begin with an aura. - motor cortex (region): The motor cortex is the strip of the frontal lobe just in front of the central sulcus that turns a decided action into commands for muscles, each hemisphere moving the opposite side of the body. - neural criticality (phenomenon): Neural criticality is the hypothesis that cortical networks run near the boundary between activity that dies out and activity that explodes, a critical point, and that this position gives them the widest range of responses and the richest repertoire of patterns. - neuromodulation (process): Neuromodulation is the slow adjustment of how neurons and synapses respond, made by transmitters that change excitability, release probability and plasticity across a whole region, and it is how the brain switches one fixed set of circuits between states such as alert, drowsy, curious or stressed. - neuron (cell): A neuron is the cell that carries signals in the nervous system: it gathers inputs on its dendrites, adds them up across its membrane and, when the sum is large enough, sends an electrical pulse down its axon to the cells it contacts. - neuropeptides (molecule): Neuropeptides are short chains of amino acids that neurons release as slow, widespread signals (endorphins, substance P, oxytocin, orexin, neuropeptide Y and about a hundred others), and they tune whole circuits to a state such as pain relief, hunger, bonding or wakefulness. - neurotransmitter reuptake (process): Neurotransmitter reuptake is how a synapse ends its signal by pumping the released transmitter back out of the cleft into neurons or glia, and with enzymatic breakdown and plain diffusion it sets how long each message lasts and how far it spreads. - nitric oxide (molecule): Nitric oxide is a gas that neurons make the moment calcium rises inside them and that diffuses freely through membranes to neighbouring cells, where it acts on synapses and on the small blood vessels that bring active tissue its energy. - NMDA receptor (molecule): The NMDA receptor is the glutamate-gated ion channel that opens only when glutamate is bound and the membrane is already depolarised, and the calcium it then admits is the signal that starts most lasting changes in synaptic strength; it is the synapse's coincidence detector. - noradrenaline (molecule): Noradrenaline (norepinephrine) is the neuromodulator of arousal and alertness: released across almost the whole brain from the locus coeruleus, it raises the gain of cortical circuits so that salient inputs stand out, and it rises with novelty, effort and stress. - occipital epilepsy (disorder): Occipital epilepsy is epilepsy whose seizures start in the occipital lobe, and because that lobe builds vision, its seizures are seen before they are felt: lights, colours and shapes, or a patch of blindness, that the person knows are not really there. - oligodendrocyte (cell): An oligodendrocyte is the glial cell that wraps axons of the brain and spinal cord in myelin, making them conduct fast, and it is what turns grey matter's local wiring into the white matter's long-range cables. - optic chiasm (structure): The optic chiasm is the X-shaped crossing under the front of the brain where the two optic nerves meet and exchange half their fibres, so that each hemisphere receives the opposite half of the visual field from both eyes. - oxidative phosphorylation (process): Oxidative phosphorylation is the process in the mitochondria that uses oxygen to make most of a cell's ATP, the molecule that pays for the pumps, and in the brain it supplies nearly all the energy behind every signal. - oxytocin (molecule): Oxytocin is a small peptide hormone, nine amino acids long, that makes the uterus contract during birth and the breast release milk during suckling, and that also acts inside the brain on social behaviour and bonding. - parietal lobe (region): The parietal lobe is the upper back part of each hemisphere that builds the brain's model of where things are: where the body is, where objects are around it, where attention should go and, by extension, how large a quantity is. - pituitary gland (region): The pituitary gland is a pea-sized gland hanging below the hypothalamus that carries the brain's orders to the rest of the endocrine system: on instruction from the hypothalamus it releases the hormones that set the activity of the thyroid, the adrenal glands and the gonads, and those that regulate growth, milk and wat - prefrontal cortex (region): The prefrontal cortex is the front of the frontal lobe, the region that keeps a goal in mind and bends perception, memory and action toward it: it plans, holds information for the next few seconds, weighs options against their consequences and stops actions that do not serve the goal. - primary auditory cortex (region): The primary auditory cortex (A1) is the patch of the upper temporal lobe where sound first reaches the cortex, organised by pitch so that low and high frequencies land at opposite ends of a map. - primary somatosensory cortex (region): The primary somatosensory cortex (S1) is the strip of the parietal lobe just behind the central sulcus where touch arrives in the cortex: pressure, vibration, temperature, pain and the position of the limbs from the opposite half of the body each reach their own spot on a map of the body. - procedural memory (process): Procedural memory is the memory of how to do things (ride a bicycle, type, tie a knot, play a scale) that shows itself in performance and cannot be fully put into words. - proteostasis (process): Proteostasis is the cell's upkeep of its proteins (folding them correctly, refolding or destroying the damaged ones and clearing aggregates), and in neurons, which must last a lifetime without dividing, its slow failure is the common ground of the major neurodegenerative diseases. - reflex epilepsy (disorder): Reflex epilepsy is epilepsy in which seizures are reliably set off by a specific stimulus or activity, and it is the rare case where a seizure can be provoked on demand, which makes it both a clinical problem and a window on how a network tips over. - release probability (phenomenon): Release probability is the chance that a spike arriving at a synapse makes a vesicle fuse and release its transmitter, and at most synapses in the brain it is well below one: a spike often releases nothing, which makes synaptic transmission noisy and adjustable at once. - resting membrane potential (phenomenon): The resting membrane potential is the steady voltage across a neuron's membrane when it is not signalling, about −70 mV with the inside negative, and it is the stored charge every signal spends: inputs move the cell away from it, and a spike is a brief, controlled collapse of it. - saltatory conduction (process): Saltatory conduction is the way an action potential travels along a myelinated axon, jumping from one gap in the insulation to the next, and it is how the nervous system gets fast signals from thin fibres. - scotoma (phenomenon): A scotoma is a patch of the visual field in which vision is lost or distorted while the rest of the field stays normal, and in migraine it is the visible footprint of a wave crossing the visual cortex. - seizure threshold (phenomenon): The seizure threshold is how much of a push a brain needs before it produces a seizure, and it is the idea that explains why any brain can seize under enough stress while some brains seize with much less. - semantic memory (process): Semantic memory is the store of general knowledge about the world (what words mean, what a lemon tastes like, that Paris is in France) held without any memory of when or where it was learned. - septal area (structure): The septal area is a small group of nuclei on the midline of the forebrain, under the front of the corpus callosum, that adjusts the intensity of emotional and motivated behaviour and sets the rhythm of the hippocampus. - serotonin (molecule): Serotonin (5-HT) is a neuromodulator released across the brain from the raphe nuclei of the brainstem, which adjusts mood, patience, sleep, appetite and how strongly circuits respond to stress and punishment. - short-term plasticity (process): Short-term plasticity is the change in a synapse's strength from one spike to the next during a train, lasting from milliseconds to minutes, and it makes every synapse a small filter that responds differently to a single spike, a burst or a steady stream. - SNARE complex (molecule): The SNARE complex is the bundle of three proteins that pulls a synaptic vesicle onto the membrane of the terminal and fuses the two, and it is the motor of transmitter release: without it, vesicles dock but cannot open. - sodium channel blocker (molecule): A sodium channel blocker is an antiseizure drug that binds the voltage-gated sodium channels which produce the action potential, and it is the oldest and most widely used way of keeping neurons from firing in the long, fast runs that a seizure is made of. - sodium-potassium pump (molecule): The sodium-potassium pump is the membrane enzyme that spends ATP to push sodium out of a cell and potassium in, keeping the two gradients that every neural signal runs on. - spinal cord (region): The spinal cord is the cable of nervous tissue in the vertebral canal that carries sensation up and commands down between the brain and the body, and that runs simple reflexes by itself. - status epilepticus (disorder): Status epilepticus is a seizure that does not stop by itself, or seizures that follow one another without recovery in between, and it is the emergency of epilepsy: the longer it lasts, the harder it is to stop and the more likely it is to leave damage. - structural plasticity (process): Structural plasticity is the physical rewiring of the brain by growing, enlarging, shrinking and removing synapses, mostly visible as the turnover of dendritic spines, and it is how learning changes which neurons are connected as well as how strongly. - SV2A (molecule): SV2A (synaptic vesicle glycoprotein 2A) is a protein in the membrane of the vesicles that store neurotransmitter, present in nearly every synapse of the brain, and it is the binding site of the antiseizure drugs levetiracetam and brivaracetam. - synaptic transmission (process): Synaptic transmission is how one neuron passes a signal to the next across a synapse, by turning the electrical spike into a chemical released into a narrow gap and back into an electrical change in the receiving cell; electricity inside the neuron, chemistry between neurons. - synaptotagmin (molecule): Synaptotagmin is the calcium sensor on the synaptic vesicle: when calcium floods in after a spike it binds this protein, and synaptotagmin triggers the final fusion that the SNARE complex has prepared, so release happens within a fraction of a millisecond of the spike. - temporal lobe (region): The temporal lobe is the lower side of each hemisphere, below the lateral (Sylvian) fissure, that turns sounds and shapes into things recognised: it hears, it knows what a word or an object means, it recognises faces, and on its inner face it forms new memories. - thalamus (region): The thalamus is a pair of egg-shaped masses of grey matter at the centre of the brain through which almost everything that reaches the cortex passes: it relays the senses, the motor loops of the cerebellum and basal ganglia, and the signals that set the level of wakefulness. - tonic-clonic seizure (phenomenon): A tonic-clonic seizure is the convulsive seizure most people picture, in which both hemispheres are recruited, the body first stiffens and then jerks rhythmically, and awareness is lost throughout; it can begin generalised or start in one area and spread. - trigeminovascular system (phenomenon): The trigeminovascular system is the set of pain fibres of the trigeminal nerve that supply the meninges and their blood vessels, together with the brainstem neurons they feed, and it is the pathway that turns a migraine attack into head pain. - Urbach-Wiethe disease (disorder): Urbach-Wiethe disease (lipoid proteinosis) is a rare inherited disorder of the skin and mucous membranes that, in some patients, calcifies both amygdalae and so provides the clearest human evidence of what the amygdala does for fear. - vagus nerve stimulation (device): Vagus nerve stimulation is an implanted device that sends regular electrical pulses into the left vagus nerve in the neck to reduce seizures, and it is used when epilepsy resists medication and surgery is not possible. - vasopressin (molecule): Vasopressin (antidiuretic hormone, ADH) is the peptide hormone that tells the kidneys to keep water: when the blood becomes too concentrated or its volume falls, it makes the urine more concentrated and, at high levels, narrows blood vessels. - ventromedial prefrontal cortex (structure): The ventromedial prefrontal cortex (vmPFC) is the lower, midline part of the prefrontal cortex that gives options a value and keeps emotion in proportion: it tells the rest of the brain how good or bad a choice is likely to feel, and it damps the amygdala when a threat turns out to be harmless. - visual cortex (region): The visual cortex is the part of the occipital lobe, at the back of the brain, that receives the signals from the eyes (relayed through the thalamus) and builds them into what is seen. - voltage-gated calcium channel (molecule): The voltage-gated calcium channel is the membrane protein that opens when a spike reaches the axon terminal and lets calcium in, and that calcium is the trigger for exocytosis: no calcium entry, no transmitter release. - voltage-gated potassium channel (molecule): The voltage-gated potassium channel is the membrane protein that opens a little after a spike begins and lets potassium flow out, and it is what ends each action potential and sets how fast a neuron can fire again. - voltage-gated sodium channel (molecule): The voltage-gated sodium channel is the membrane protein that opens when the voltage rises past threshold and lets sodium rush in, and it is the part that turns a graded input into an all-or-none action potential. - Wernicke's aphasia (disorder): Wernicke's aphasia is a disorder of language comprehension after damage to the back of the left superior temporal gyrus, in which speech stays fluent and well articulated but loses its sense, and understanding of what others say breaks down. - Wernicke's area (structure): Wernicke's area is the back part of the left superior temporal gyrus that links the sounds of words to their meaning: it is needed to understand spoken language and to choose the right words when speaking. - white matter (structure): White matter is the brain's long-range cabling: bundles of axons, most of them wrapped in myelin, that carry signals between areas of grey matter that are centimetres apart. - working memory (process): Working memory is the brain's small mental workspace that holds a few items for seconds while they are used: a phone number between reading and dialling it, the intermediate result of a sum done in the head, the rule of the task at hand.

Technology:

- Blackrock Neurotech (company): Blackrock Neurotech is a company in Salt Lake City, founded in 2008 on technology from the University of Utah, that makes the Utah array and the recording systems around it, the hardware on which most human intracortical brain-computer interface research of the last two decades has run. - brain-computer interface (method): A brain-computer interface (BCI) records activity from the brain and turns it, in real time, into an action outside the body, such as moving a cursor, typing, speaking through a synthesiser or moving a robotic arm, and today its main use is giving back communication and control to people who are paralysed. - brain waves (phenomenon): Brain waves are the rhythmic fluctuations of voltage that large groups of neurons produce when they are active together, and they are the first thing any recording of the brain is sorted by: the frequency band of a signal says much about the state of the tissue that made it. - calcium imaging (method): Calcium imaging watches neurons fire by filming a fluorescent protein that lights up when calcium enters the cell, and it is the standard way to record thousands of identified neurons at once in a living animal. - Chile's neurorights amendment (phenomenon): Chile's neurorights amendment is the 2021 change to the Chilean constitution, Law 21.383, that made Chile the first country to protect brain activity and the information derived from it in its constitution, and it has already been applied by its Supreme Court against a consumer neurotechnology company. - Connexus (device): Connexus is Paradromics's fully implanted brain-computer interface, designed for the highest data rate of the current implants: a coin-sized module on the cortex pushes hundreds of fine microwires into it, and the recordings travel under the skin to a transceiver in the chest that sends them out wirelessly. - EEG (method): EEG (electroencephalography) records the brain's electrical activity through electrodes on the scalp, and it is the cheapest, most portable way to follow the brain in real time, used to diagnose epilepsy, stage sleep and drive simple brain-computer interfaces. - electrocorticography (method): Electrocorticography (ECoG) records the brain's electrical activity from electrodes laid directly on the surface of the cortex, under the skull, and it is used mainly to find the exact origin of seizures before epilepsy surgery and, increasingly, as the signal for brain-computer interfaces that decode speech or movemen - fMRI (method): fMRI (functional magnetic resonance imaging) maps brain activity across the whole head by detecting the changes in blood oxygen that follow it, and it is the method that made it possible to see, without surgery, which areas of a healthy brain work during a task. - fNIRS (method): fNIRS (functional near-infrared spectroscopy) measures brain activity by shining near-infrared light into the head and reading how much returns, and it is the most portable way to follow blood oxygen in the cortex, used in infants, in moving people and outside the lab. - functional ultrasound (method): Functional ultrasound images brain activity by measuring how much blood flows through the smallest vessels of each spot of tissue, and it offers fMRI-like maps at much finer resolution from a device the size of a hand, provided the skull is out of the way. - heat budget (phenomenon): The heat budget is the amount of power a device can dissipate in or near the brain before it warms the tissue enough to harm it, and it caps how much amplification, processing and transmission an implant can do. - informed consent (process): Informed consent is the process by which a person agrees to a procedure or to take part in research after understanding what it involves, its risks, its likely benefits and their alternatives, and for brain implants it is harder than for almost any other medical research. - inverse problem (phenomenon): The inverse problem is the task of working out which currents inside the head produced the voltages or magnetic fields measured outside it, and it is the step that turns an EEG or MEG recording into a map of brain activity. - Layer 7 cortical interface (device): The Layer 7 cortical interface is Precision Neuroscience's electrode film for the surface of the brain: a sheet thinner than a hair, carrying 1,024 small electrodes, slid onto the cortex through a narrow slit in the skull, and the first part of a next-generation brain-computer interface to receive regulatory clearance. - measurement back-action (phenomenon): Measurement back-action is the change that the act of measuring makes in the brain being measured, and it limits every method that gets close enough to read single cells: the closer and longer the reading, the more the tissue reacts to the reader. - MEG (method): MEG (magnetoencephalography) records the tiny magnetic fields that the brain's electrical currents produce outside the head, and it is used to map where and when activity happens with millisecond timing, most often to locate the source of epileptic activity before surgery. - mental privacy (phenomenon): Mental privacy is the claim that what happens in a person's mind, and the brain activity that carries it, should be protected from being read, recorded or inferred without their consent, and it is the first ethical question that any technology for reading the brain raises. - microwave tomography (method): Microwave tomography sends low-power microwaves through the head from a ring of antennas and reconstructs a map of how the tissue inside slows and absorbs them, and its main use is to tell a bleed from a clot in a suspected stroke with equipment that fits in an ambulance. - molecular recording (method): Molecular recording makes each cell write a record of its own activity into molecules inside it, to be read out later, and it is a proposed way around the wiring, bandwidth and depth limits of every method that reads the brain from outside the cell. - N1 implant (device): The N1 implant is Neuralink's fully implanted brain-computer interface: a sealed, coin-sized unit set into the skull that records from motor cortex through flexible threads and sends decoded activity wirelessly to a computer or phone, so that a person with paralysis can move a cursor by intending to move. - neural data (phenomenon): Neural data is information generated by measuring the activity of a person's nervous system, and it is the legal object that the first laws on neurotechnology protect, because the mind itself cannot be regulated while the recordings can. - neural dust (device): Neural dust is the idea of reading the brain with many tiny, wireless, battery-free implants scattered through the tissue instead of one large wired array, and it is a research route toward recording from far more sites than today's implants reach. - Neuralink (company): Neuralink is a neurotechnology company, founded in 2016 with Elon Musk among its founders, that builds a fully implanted, wireless brain-computer interface and the robot that implants it, with the stated aim of making implantation routine and eventually scaling it far beyond medical use. - Neuropixels (device): Neuropixels is a thin silicon probe that is pushed into the brain to record the spikes of hundreds of individual neurons at once along its length, and it is the standard tool for recording large populations of single cells across several brain structures in animals. - neurorights (phenomenon): Neurorights are proposed human rights aimed specifically at protecting the mind and the brain from misuse of neurotechnology, and they are the framework behind the first laws, from Chile's constitution to UNESCO's recommendation. - optogenetics (method): Optogenetics makes chosen neurons respond to light by giving them a light-sensitive ion channel from algae or microbes, and it is the main tool for testing what a given set of neurons does: switch them on or off with millisecond precision and watch what changes. - Paradromics (company): Paradromics is a neurotechnology company in Austin, Texas, founded in 2015, that builds Connexus, a fully implanted brain-computer interface designed around the largest number of single-neuron channels it can read and send out, with restoring speech as its first goal. - penetration depth (phenomenon): Penetration depth is how far a wave can travel into the head and still carry back a sharp picture, and it is the first physical limit on reading a brain from outside: every kind of wave trades how deep it reaches against how fine a detail it can resolve. - Precision Neuroscience (company): Precision Neuroscience is a neurotechnology company founded in 2021, among others by the neurosurgeon Benjamin Rapoport, a co-founder of Neuralink, that builds dense electrode films laid on the surface of the cortex without piercing it, and it holds the first regulatory clearance for a component of a next-generation br - recording bandwidth (phenomenon): Recording bandwidth is the amount of data per second that a recording of the brain produces and has to move out of the head, and it grows with every electrode added, until moving the data costs more power and heat than measuring it. - sampling rate (phenomenon): The sampling rate is how many times per second a recording measures its signal, and it sets the fastest event a method can capture: a brain read slower than its spikes can only see what those spikes leave behind. - signal-to-noise ratio (phenomenon): The signal-to-noise ratio is how large the brain's own signal is at a sensor compared with everything else the sensor picks up, and it decides what a recording can resolve long before the electronics or the decoder do. - Stentrode (device): The Stentrode is Synchron's brain-computer interface delivered through the blood vessels: a stent carrying electrodes is threaded up from the jugular vein into a large vein lying against the motor cortex, where it records the cortex's activity without any opening in the skull. - Synchron (company): Synchron is a neurotechnology company, started in Australia and now based in New York, that develops the Stentrode, a brain-computer interface placed through the blood vessels instead of through the skull, and it was the first company to receive US permission to test a permanently implanted BCI. - transcranial magnetic stimulation (device): Transcranial magnetic stimulation (TMS) makes neurons fire from outside the head by inducing an electric current in the cortex with a brief, strong magnetic pulse, and it is used both to probe the brain (what happens if this area is disturbed?) and to treat depression and other disorders. - UNESCO Recommendation on the Ethics of Neurotechnology (phenomenon): The UNESCO Recommendation on the Ethics of Neurotechnology is the first global standard on the ethics of technologies that record or act on the nervous system, adopted by UNESCO's member states at the 43rd session of its General Conference in November 2025, and it gives governments a shared set of values, principles an - US state neural data laws (phenomenon): The US state neural data laws are amendments to state consumer privacy laws, first in Colorado and California in 2024, that classify data measured from a person's nervous system as sensitive, and they are the first laws in the United States written specifically for consumer neurotechnology. - Utah array (device): The Utah array is a small bed of rigid silicon needles, each tipped with an electrode, pressed into the cortex to record single neurons, and it is the device behind most of what is known about brain-computer interfaces in people, from the first cursor control in 2004 to the speech decoders of 2023.