Neuroglia and the Neurovascular Unit: The Cells That Set a Neuron's Chemistry

01Counting the glia

Glia and neurons run close to matched in the brain and far from matched in the cord

The adult male human brain holds 86.1 billion neurons and 84.6 billion non-neuronal cells Azevedo 2009. The count came from an isotropic fractionator that dissolves a whole brain into a suspension of nuclei, and it ended a century of estimates. Only 19 percent of those neurons sit in the cerebral cortex.

A review of 150 years of cell counting puts the whole-brain glia to neuron ratio below one to one von Bartheld 2016. The same review puts the glial total below 100 billion, and notes that individual histological studies have spanned 40 to 130 billion. The figure of one trillion glia at ten per neuron spread as a quotation and was never a measurement. Glia were dismissed as glue for most of that century. The correction runs both ways. The count is smaller than the myth, and the work each cell does is larger.

The spinal cord is the most glial tissue in the neuraxis

The human spinal cord is 74.8 percent glial, at 5.6 to 7.1 glia per neuron Bahney 2018. The isotropic fractionator and stereology counted 1.5 to 1.7 billion cells and 197 to 222 million neurons in that tissue, taken from one human cord and one cynomolgus monkey cord. Cervical, thoracic, and lumbar levels did not differ significantly. The whole-brain ratio sits close to one to one, so the glia to neuron ratio is a regional quantity.

The Unified Model of Tone reads that gradient as a statement about where regulation is dense. Mechanical and sensory information entering the spine arrives in the most glial tissue of the neuraxis, where six or seven non-neuronal cells surround every neuron carrying the signal onward.

The human astrocyte is not a scaled rodent cell

Protoplasmic astrocytes in human neocortex measure 2.6-fold larger in diameter than rodent astrocytes and extend 10-fold more GFAP-positive primary processes Oberheim 2009. In slices of surgically resected human cortex they propagate calcium waves at 36 micrometers per second, roughly fourfold faster than rodent. Human cortex also carries astrocyte subclasses absent from rodents, including interlaminar and varicose projection astrocytes.

One human astrocyte governs a territory no rodent experiment measured. Mouse work establishes the mechanism and understates the scale on which a human astrocyte runs it.

02Findings

What the research shows

What the research shows about glial cells and the neurovascular unit

03Astrocyte clearance and coverage

Kir4.1 channels carry 80 percent of the current a synapse evokes in the astrocyte wrapped around it

Astrocytes clear potassium from the extracellular space, pull glutamate and GABA out of the cleft, and return those transmitters through the glutamate glutamine shuttle. That housekeeping writes much of the central integrative state, the running sum that decides how a neuron answers the next signal to arrive.

Potassium clearance runs through inwardly rectifying Kir4.1 channels. Simultaneous recordings from astrocytes and CA1 pyramidal cells in hippocampal slices isolated that current. A single stimulus to the Schaffer collaterals evokes a prolonged inward current in the surrounding astrocyte, and Kir4.1 carries 80 percent of it Sibille 2014.

How much astrocyte a synapse actually has around it

Serial-section reconstruction of rat hippocampus found astrocytic processes apposed to 57 plus or minus 11 percent of CA1 synapses, and where they were present they surrounded 0.43 of the synaptic interface Ventura 1999. The transporters inside that sleeve are not evenly spread either. In rat hippocampal stratum radiatum the glutamate transporter GLT sits at about 12,000 molecules per cubic micrometer against 3,200 for GLAST. The proportion inverts in the cerebellar molecular layer, where GLAST reaches 18,000 against 2,800 for GLT Lehre 1998. Counting the astrocyte as a third element alongside the two neurons is what the tripartite synapse names Araque 1999.

Coverage and transporter density decide how long glutamate stays in the cleft and how far it spills to the neighboring synapse. Two synapses of identical anatomy sitting under different sleeves do not behave alike.

Potassium buffering is synaptic tuning

Glial conditional Kir4.1 knockout mice show what the uptake was doing. Astroglial potassium uptake reduces synaptic responses to repetitive stimulation and post-tetanic potentiation Sibille 2014, so a synapse left without the channel answers a train of stimuli more strongly than one that has it. Inhibiting astrocytic Kir4.1 raises extracellular potassium and glutamate at the synapse together and raises neural excitability, because the channel couples to glutamate transport Ohno 2021.

Potassium handling and transmitter clearance are one mechanism with two readouts. A synapse of fixed anatomy delivers a larger or smaller effect according to how fast the surrounding astrocyte clears the last one, which puts the volume control outside the cell that fires. Deleting the astrocytic glutamate transporter GLT-1 costs the mouse its life through spontaneous seizures Tanaka 1997, and what happens inside the neuron once clearance fails belongs to excitotoxicity.

Astrocytes decide which synapses survive

Astrocytes engulf synapses through the MEGF10 and MERTK phagocytic pathways, and the rate tracks neuronal activity Chung 2013. Mice deficient in both pathways failed to refine retinogeniculate connections and kept excess functional synapses. Astrocytes in the adult mouse brain go on engulfing both excitatory and inhibitory synapses.

Pruning continues through adult life. The connectivity a brain carries is maintained by cells that read activity and remove what activity does not support.

04Glycogen and lactate supply

Astrocyte glycogen keeps axons conducting for about 30 minutes without glucose

Glycogen in the brain sits in astrocytes, and the size of that reserve can be measured as time. Switching isolated rat optic nerve to glucose-free artificial cerebrospinal fluid left the compound action potential unchanged for about 30 minutes, after which it failed rapidly Wender 2000. Nerve glycogen fell to a low stable level over the same 30 minutes. Sixty minutes of deprivation caused irreversible injury.

The fuel moves from astrocyte to axon as lactate. Isolated mouse optic nerve kept its compound action potential through the first 30 minutes of an hour without glucose, running on a glycogen-derived substrate Tekkök 2005. MCT2 sits predominantly on axons and MCT1 predominantly on astrocytes, which names the delivery route in molecules. Blocking axonal lactate uptake accelerated the failure.

Autonomic state changes the size of the astrocyte glycogen reserve

Pretreatment with high glucose raised astrocyte glycogen in rat optic nerve and extended the time to conduction failure. Pretreatment with norepinephrine lowered glycogen and shortened it Wender 2000. Norepinephrine spends the reserve before the interruption arrives, which is why the same nerve fails at different times depending on the state it was in.

The Unified Model of Tone takes that result as load measured in minutes. A segment held in sympathetic drive is not simply tense. It is carrying a shorter fuel reserve, so its margin against any interruption of supply is already smaller while everything still looks normal.

Myelinating glia feed the axons they wrap

MCT1, the most abundant lactate transporter in the central nervous system, is enriched in oligodendroglia, and disrupting it produces axon damage and neuron loss in animal and cell-culture models Lee 2012. Oligodendroglia hold axons alive by that route independently of myelination.

MCT1 is reduced in patients with amyotrophic lateral sclerosis and in mouse models of the disease, so the fuel line fails in disease on its own, without any account of the myelin.

05Myelin and conduction speed

One oligodendrocyte maintains up to 50 internodes, which concentrates central fragility in white matter

Maintaining up to 50 internodes is an extraordinary metabolic demand, and it makes the oligodendrocyte one of the most vulnerable cell types in the central nervous system Zeis 2008. Schwann cells myelinate peripherally, each devoted to one internode of a single axon.

That difference in arithmetic decides what a lesion costs. Losing one Schwann cell costs one internode. Losing one oligodendrocyte silences segments on many separate fibers at once.

Myelin exists to make conduction fast. The impulse leaps between nodes of Ranvier instead of crawling along the membrane, and saltatory conduction underlies the whole architecture of fast signaling.

Where early demyelinating lesions carry diagnostic weight

Four locations decide the radiological finding of dissemination in space: juxtacortical, periventricular, infratentorial, and spinal cord. Criteria requiring at least one T2 lesion in at least two of those four locations reached 87 percent specificity and 72 percent sensitivity for conversion to clinically definite multiple sclerosis at three years. The assessment covered 208 patients presenting with clinically isolated syndromes Swanton 2007.

Specificity of 87 percent means 87 in 100 of the patients who did not convert were correctly excluded by the criterion. Sensitivity of 72 percent means it caught 72 in 100 of those who did. The 2005 McDonald criteria scored 88 percent specificity and 60 percent sensitivity in the same cohort, so the two-region rule bought 12 points of sensitivity for one point of specificity. The familiar early sites sit inside those four regions: the cervical spinal cord, the upper brainstem, the optic nerve, and periventricular white matter including that of the cerebellum.

Timing is what integration depends on

Loss of insulation slows or blocks conduction and scrambles the arrival times integration runs on. The Unified Model of Tone treats that as organization rather than volume. A partly demyelinated tract can deliver every impulse it was given and still degrade the state it feeds, because what a converging population computes depends on when its inputs land.

An axon conducting slowly still carries its message. It arrives in the wrong company.

06Microglia and cerebrospinal fluid

Microglia patrol the parenchyma and ependymal cells make the fluid that bathes it

Microglia are the resident immune cells of the central nervous system, and their line is separate from the blood. Fate mapping in mice showed that adult microglia derive from myeloid progenitors arising before embryonic day 8, and that postnatal hematopoietic progenitors do not contribute significantly to adult microglial homeostasis Ginhoux 2010. The immune cells inside the brain were seeded before birth and maintain themselves.

Resting microglia continually survey their surroundings with extremely motile processes and protrusions. Imaged in the living mouse cortex, they sense injury, debris, and pathogen. They then shift to an activated state that clears damage and prunes synapses. Disrupting the blood-brain barrier provoked immediate focal activation, switching microglial behavior out of patrol and into shielding of the injured site Nimmerjahn 2005.

Barrier breach is itself the microglial trigger. The immune line and the vascular line close one loop.

Cerebrospinal fluid is replaced three to five times a day

Blood supplies 80 percent of the molecular constituents of cerebrospinal fluid, and 20 percent is made in the brain or intrathecally Tumani 2017. Aquaporin-4 channels at the blood-brain and brain-CSF interfaces regulate extracellular volume and potassium buffering, which puts the same water channels on the fluid line and on the clearance line.

The route is fixed. Fluid runs from the lateral ventricles through the foramina of Monro into the third ventricle, then through the cerebral aqueduct into the fourth. From there it reaches the central canal or the subarachnoid cisterns through the central foramen of Magendie and the two lateral foramina of Luschka. The circuit removes waste and distributes signaling molecules across the neuraxis.

Fluid production answers to autonomic input

The mammalian choroid plexus receives noradrenergic sympathetic, cholinergic, and peptidergic innervation that modulates secretion, so the rate at which the brain makes its own fluid answers to autonomic state. High water permeability at that barrier depends on aquaporin-1 on the apical membrane, present from early development through adult life Boassa 2005.

In the Unified Model of Tone that puts two glial systems on the same line. Sympathetic drive spends astrocyte glycogen and changes the rate of cerebrospinal fluid production, so one shift in autonomic state reaches the fuel and the fluid together.

07The neurovascular unit

The blood-brain barrier is built by a unit of cells, and its permeability is graded

Pericytes build the blood-brain barrier before astrocytes exist. Mice carrying null and hypomorphic alleles of Pdgfrb generate too few pericytes, and absolute pericyte coverage determines relative vascular permeability Daneman 2010. The barrier forms during embryogenesis as pericytes are recruited to nascent vessels, more than a week before the first astrocyte is generated.

Removing pericytes in mice opens the barrier to water and to tracers of low and high molecular mass Armulik 2010. Endothelial transcytosis rises. Barrier-specific gene expression in the endothelium drifts, and astrocyte end-feet lose their polarization. Three cell types depend on each other to make one wall, and that wall admits glucose and oxygen while excluding many toxins, drugs, and immune cells.

The Unified Model of Tone reads pericyte coverage as constraint. What can reach a neuron at all is decided at the capillary wall. Coverage is a continuous quantity, so one neuron can sit in a more permissive chemistry than its neighbor with nothing about the neuron itself different.

Blood flow follows the glial signal

Astrocyte end-feet wrap capillaries and synapses alike, which couples blood flow to demand. In rat cortical slices, arteriolar dilation triggered by neuronal activity depended on glutamate-mediated calcium oscillations in astrocytes Zonta 2003. Activating a single astrocyte in contact with an arteriole relaxed the vessel. Blocking those calcium elevations in the living rat reduced the blood flow increase in somatosensory cortex during contralateral forepaw stimulation.

Neurons do not open their own vessels. The transmitter reaches an astrocyte, and the astrocyte opens the vessel.

The molecule that reports barrier compromise also joins the reaction

At high concentration S100B acts through RAGE as a damage-associated molecular pattern and drives the tissue reaction it also reports Michetti 2019. The protein is a calcium-binding astrocyte product, present also in oligodendrocytes, choroid plexus epithelium, and neurons. It is measurable in cerebrospinal fluid, blood, urine, saliva and amniotic fluid. A raised level marks active neural distress across a long list of disorders, and specificity across them is low.

The relationship to barrier function has been measured directly. At 12 hours after traumatic brain injury, serum S100B tracked the cerebrospinal fluid to serum albumin quotient with an area under the curve of 0.794 Blyth 2011. The cohort was 16 patients with moderate to severe injury against 6 patients with non-traumatic headache. The association held at 12 hours and at no other time point. The same study's headline analyte, the neuron-specific hydrolase UCH-L1, tracked the quotient at 0.76 in that window, which is how the authors argue that peripheral release of S100B after multi-trauma is negligible.

Inflammation opens the barrier further, letting peripheral immune cells and neutrophils enter and raise permeability again. The same glial cells that build the barrier report its compromise and take part in what follows.

08Glia hold the state

Glia hold an altered segmental state in place after the input that caused it has passed

Peripheral nerve injury changes glia in the dorsal root ganglion and the spinal cord, and the glial change carries the pain state. Spinal nerve ligation in GFAP knockout mice produced neuropathic pain with the same onset as wild-type littermates and a shortened duration Kim 2009. Intrathecal GFAP antisense oligonucleotide in injured rats reversed behavioral hypersensitivity along with the GFAP upregulation in ganglion and cord.

Onset did not move. Duration did. Neurons started the state and astrocytes held it in place.

That gives the glial contribution its own clock. A synapse changes in milliseconds and scar tissue in weeks. Astrocyte activation after nerve injury sits between the two, which is the range over which a segment goes on holding a state the original input no longer supplies.

Why the cord is where this matters

Proprioceptive and mechanical information entering the spine arrives in the most glial tissue of the neuraxis, six or seven non-neuronal cells to every neuron carrying the signal onward. We are the practice organized around the nervous system, and the input we deliver is graded mechanical and sensory information into a segment whose regulation is largely glial.

What that input meets is a clearance rate, a glycogen reserve, and a barrier setting, each of them already in some condition before the contact is made. Two segments given the same thrust do not answer alike, because the astrocytes around them are not clearing at the same speed. Correspondence between the input and that standing chemistry decides the result, and the force behind the input is a separate question from it.

What the whole unit means for health

Every action potential draws on astrocyte glycogen, depends on potassium the astrocyte removes, travels along myelin an oligodendrocyte maintains, and fires inside a chemistry the barrier defends. Disturb any partner and the neuron falters while the neuron itself remains intact. Excitability is a shared achievement, and the share the neuron owns is the smaller one.

Tone inside its healthy range is health because the system keeps the freedom to adapt. Potassium returns to its window after every discharge, glycogen refills, the barrier reseals, and microglia go back to patrolling. Tone that drifts outside that range is what manifests as illness. Potassium clears slowly, glutamate lingers, the barrier leaks, and the same glia that restored the state now hold the drift in place.

The claim carries a specific risk in this tissue. It predicts that glial clearance capacity, glycogen reserve, and barrier permeability in a segment move together, and that they move with the autonomic state the segment is held in. Show those three running independently of each other and of that state, and there is no single organization left for them to be readings of. The commitment is stated in full at tone.

09Tone

How this system expresses tone

Every part of the nervous system expresses all of tone. In the neuroglia three foundations carry the signature.

The other foundations read out in the same tissue, each on its own clock. Set point: extracellular potassium is held inside a narrow window, and astrocytes return it there after every discharge. Oscillation: astrocyte calcium travels as waves at 36 micrometers per second in human cortex, a slow rhythm running beneath the fast one. Prediction: astrocytes raise blood flow on the glutamate signal itself, so supply moves with demand instead of behind it. Input quality: peripheral nerve injury drives GFAP up in the dorsal root ganglion and the cord, and the glial change outlasts the injury that started it. Coupling: one astrocyte contacts a capillary and a synapse at the same time, which ties blood flow to transmitter release. Time course: potassium clears in milliseconds, glycogen empties in 30 minutes, and myelin is rebuilt over months.

10Across the library

How this page relates to the rest of the library

Where the glial contribution is read across the rest of the library.

11Frequently asked

Questions about this topic

12The sources

References

Sources: primary literature, linked inline.