Mechanism 3: BDNF and Plasticity Reconstruction — Why Light Can Promote Neural Repair

First understand the core: What is “Light → BDNF → Plasticity”?

Simply put, BDNF is the brain’s “repair and growth factor” (similar to a “nutrient solution + growth hormone” for neurons). Near-infrared light acts like a “starting key” that triggers a three-stage chain reaction:

First, it makes the brain produce large amounts of BDNF;
then it delivers it precisely to where it is needed;
and finally it drives neuron repair and reconnection.

The whole process is like installing an “intelligent repair engine” in the brain.

The whole process in one sentence:
Photons → calcium signal burst + energy surge → activate the BDNF gene (production ↑300%) → precise transport and release of BDNF → activate TrkB receptors → synapse growth + neural circuit remodeling

First stage: Minute-level “activation of the BDNF gene engine” (producing the “repair fluid”)

This step makes the brain’s “gene factory” start mass-producing BDNF, and it can begin within just a few minutes:

1. Calcium signal as the “switch”
Light activates the TRPV4 channels on neurons, causing the intracellular calcium ions (Ca²⁺) to increase fivefold, creating a “calcium signaling storm” (similar to pressing the “production start button”).

The calcium ions activate calcineurin, which causes the Ser133 site of the CREB protein to become phosphorylated (like powering on the switch).

The phosphorylated CREB (pCREB) binds with the CBP protein and moves to the “activation region” of the BDNF gene (the promoter IV region), where it recruits RNA polymerase II—equivalent to starting the production line.

As a result, BDNF mRNA expression increases directly by 300% (production triples).

2. “Loosening the constraints” on the gene (epigenetic reprogramming)
Light causes the HDAC2 protein to become phosphorylated and inactivated, which increases acetylation of histone H3K9—similar to removing the restraints around the gene, making it easier to activate.

The chromatin structure where the gene is located becomes 40% more relaxed (like opening the warehouse doors of a factory, making production easier).

Light can also increase TET oxidase, which removes the methylation marks on the BDNF gene—like clearing production obstacles, allowing BDNF production to proceed more smoothly.

Second stage: The “processing and transport revolution” of BDNF (delivering the “repair fluid” precisely)

After BDNF is produced, it still needs to be processed and transported precisely to the places where neurons need it:

1. Subtype conversion (making BDNF more effective)
Light regulates the subtype conversion of BDNF, turning it into mature BDNF (mBDNF)—which is like processing a “semi-finished repair fluid” into a “finished product,” making the repair effect stronger.

2. Intelligent transport system (no mistakes, no delays)
Light activates the KIF5C microtubule motor protein, allowing the vesicles carrying BDNF (similar to “delivery boxes”) to move faster—from 0.5 micrometers per second to 0.8 micrometers per second (a 60% increase in delivery speed).

40 Hz light pulses synchronize with the brain’s gamma oscillations, allowing BDNF to be released locally at “active synapses” (the connection points between neurons). This is like delivering the package precisely to the workstation that needs repair.

Third stage: TrkB signaling “cascade amplification” (starting the “repair construction”)

After BDNF reaches its destination, it activates TrkB receptors, triggering a series of reactions that promote neuron repair and remodeling:

1. Receptor “gate-opening” mechanism
Mature BDNF (mBDNF) binds to the TrkB receptors on neurons, causing two TrkB receptors to pair together (dimerize) and undergo autophosphorylation—this is like opening the gate for repair signals.

Next, SH2 domain proteins are recruited, activating three key signaling pathways:
PLCγ/DAG/PKC, PI3K/Akt, and Ras/MAPK—similar to starting different teams in a construction crew, each handling a specific task.

2. Core repair effects
mTOR pathway activation: Through the TrkB → PI3K → Akt → mTORC1 pathway, the synthesis rate of synaptic proteins increases by 250%—like rapidly producing building materials.

Structural remodeling: Two-photon live imaging shows that the density of dendritic spines (key structures for synaptic connections) in neurons increases by 35%.
At the same time, PSD-95 proteins accumulate at newly formed synapses—similar to new connection points constantly growing, making neuronal connections stronger.

Fourth Stage: Neurovascular–plasticity coupling (“blood supply + repair” coordination)

The repair process requires energy and nutrients, and blood vessels cooperate at the same time, forming a “positive repair cycle.”

1. Blood vessels help deliver BDNF to the right place
Light activates the MMP-9 enzyme in vascular endothelial cells, which widens the channels of the blood–brain barrier, allowing BDNF in the bloodstream to enter the brain more easily. As a result, the BDNF concentration in cerebrospinal fluid increases threefold.

Light can also increase VEGF, promoting the growth of new blood vessels—similar to widening transportation highways, allowing more “repair fluid” and nutrients to reach the repair area.

2. Positive cycle of energy and repair
Light causes mitochondria to produce more ATP (energy), which speeds up the transport of BDNF vesicles (with more energy, the delivery system moves faster).

After BDNF activates TrkB receptors, it promotes synaptic protein synthesis through the mTOR pathway (the repair construction process).

New synapses require more energy, which in turn stimulates mitochondrial proliferation (expanding the cell’s energy factories).

This forms a beneficial cycle:
the more repair occurs, the more energy is available; the more energy there is, the faster the repair progresses.

Summary

The core idea of Light → BDNF → Plasticity is using photons as a key to activate the brain’s own repair system:

First, calcium signaling and epigenetic “unlocking” activate the BDNF gene, producing large amounts of “repair fluid.”
Then it is transported precisely to the damaged areas.
Finally, signaling pathways are activated, allowing neurons to form new connections and repair old damage.

The entire process is like:
Photon signal → factory produces repair fluid → precise delivery → construction teams rebuild, turning light energy into the driving force for brain repair.

Simple Process Diagram (easy to understand)

Photon → calcium signal ↑5× + ATP surge → CREB phosphorylation + epigenetic unlocking → BDNF gene expression ↑300% → faster BDNF vesicle transport → local synaptic release → TrkB dimerization → activation of pathways such as mTOR → synaptic protein synthesis ↑250% → dendritic spine density ↑35% → neural circuit remodeling.