Myelin Governs Hippocampal Network Function through the Extracellular Space it Occupies, Not Only the Axons it Insulates

The brain is not a solid block of tissue; it is a sponge-like structure filled with cells and the fluid that surrounds them. This fluid, known as the extracellular space, acts as a vital highway. It carries nutrients to cells, removes waste, and allows chemical messengers called neurotransmitters to travel from one neuron to the next. For decades, scientists have understood that the insulation around nerve fibers, called myelin, is essential for speeding up electrical signals. However, a new line of inquiry suggests that myelin does something else just as important: it physically shapes the size and shape of the fluid-filled spaces between cells. If the insulation changes, the highway changes, and that change could alter how the brain communicates, clears waste, and functions as a whole.

A team of researchers has now built a detailed computer model to explore exactly how this works in the hippocampus, a brain region critical for memory. They created a virtual environment that mimics the microscopic architecture of this area, filling it with reconstructed nerve cells, support cells, and the fluid that flows between them. By simulating the physics of fluid flow and the electrical activity of neurons, they tested what happens when the myelin insulation is stripped away, a condition known as demyelination. Their work reveals a surprising dual role for myelin: while it insulates wires to make signals fast, it also occupies space to keep the fluid channels narrow. When that insulation is lost, the channels widen, and the consequences are immediate and profound for both the flow of fluid and the strength of neural signals.

The researchers began by simulating the movement of fluid through the tiny gaps between cells. In a healthy, well-myelinated state, the fluid moves slowly through a tight, winding maze. When they removed the myelin layers in their model, the volume of the fluid space expanded dramatically, growing from about 28 percent to as much as 70 percent across different layers. This expansion acted like opening a valve. The fluid began to move much faster, speeding up by nearly six times. However, this speed came with a twist. The flow became more chaotic and swirling, creating small eddies rather than a smooth, directed stream. While the fluid moved faster locally, the overall efficiency of moving waste in a straight line actually dropped. The model showed that the loss of myelin fundamentally reorganizes the fluid dynamics, turning a slow, steady current into a faster but more disordered flow.

This change in fluid dynamics had a direct impact on how chemical signals travel. In the brain, neurotransmitters are released into the fluid space to jump the gap between neurons. The researchers found that when the fluid space expands due to a loss of myelin, the neurotransmitters become diluted, much like a drop of dye spreading through a larger bucket of water. Because the chemical concentration is lower, the signal reaching the next neuron is weaker. In their simulations, this dilution was severe enough to reduce the strength of the electrical response in the receiving neuron significantly. In the most extreme cases, where the myelin was reduced to fewer than four layers out of sixteen, the signal became so weak that transmission failed entirely, and the connection between neurons effectively broke.

The team then scaled up their model to a network of 200 interacting cells to see how these microscopic changes affected the brain's overall activity. They compared healthy networks with those suffering from demyelination, while also testing whether changes in the number of support cells, a common feature of inflammation, played a role. The results were striking. The demyelinated networks fired at roughly half the rate of healthy ones, dropping from 7.9 to 4.4 electrical impulses per second in pyramidal cells. The pattern of firing also changed; instead of a steady rhythm, the activity became dominated by chaotic bursts. The electrical power of the network, which reflects the intensity of its activity, plummeted by up to eight times in certain frequency bands. Crucially, the researchers found that simply changing the number of support cells did not cause these effects. The disruption was driven entirely by the loss of myelin and the resulting expansion of the fluid space.

To ensure these findings were not just a snapshot of two fixed states, the researchers ran a continuous simulation where myelin was gradually removed and then restored. They observed that as the insulation disappeared, the strength of the signals declined steadily and predictably, tracking the loss of myelin layer by layer. When the myelin was restored, the network function recovered completely, returning to its original firing rates and patterns. This reversibility confirmed that the mechanism was a direct physical consequence of the space myelin occupies, rather than permanent damage to the cells. The study concludes that myelin governs brain circuit function not only by insulating the wires but also by defining the space in which the fluid flows. This dual role creates a delicate balance: the same structure that speeds up electrical signals also concentrates the chemical messengers that drive them, and losing it disrupts both the flow of waste and the clarity of communication.