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When neuron animations are displayed, there are frequently seen neurons, axons arranged in a lattice with a lot of empty space between. I'm interested if there is indeed empty space in the brain, or if it is filled with some sort of fluid? I've checked an article on cerebrospinal fluid but am not sure that it is present all throughout the brain.

The reason I'm asking is that I'm thinking of neurotransmitters- they are released in synapses, but I'm not sure how they stay there - are they suspended in some liquid as well?

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Not so empty, actually.

The human brain has a mass of ~1.5kg, and volume ~1200cc (a little bigger for men, a little smaller for women). So is heavier than water by a good margin.

While it has Cerebrospinal fluid, that only occupies the subarachnoid space (the space below the skull and above the cortex, contained between two layers: pia matter and arachnoid membrane) and the ventricular system (several spaces inside the brain, remnants of the embryological development of the brain).

Neuron density may vary widely, depending mainly on the particular characteristics of neuron cell types and their interconnections. But besides neurons, there's a lot of infrastructure inside the brain. For example:

  • Astroglia: They are a type of glial cells which participate in the formation of the blood-brain barrier (supporting the endothelial cells), nourishing of neurons, maintenance of ion and neurotransmitter concentrations, among others. They also keep in place most of the tissue.
  • Microglia: Small cells with immune (phagocitic) functions inside the brain.
  • Radial glia: A more specialized precursor cell, that also participates in neuronal migration in the brain.
  • Oligodendrocites: Cells responsible for the insulation (myelination) of axons.
  • Neuroepithelial cells: The stem cells in the brain.

Neuroglia, which includes the first four cell types above, accounts for ~90% of tissue in human brain (http://classes.biology.ucsd.edu/bipn140.WI13/documents/Gliamorethanjustbrainglue.pdf).

Also, when you collect some billions of axons, they can be quite representative in terms of mass and volume. The ratio between white matter and gray matter is close to one for humans, but lower for smaller mammals (http://www.pnas.org/content/97/10/5621.full.pdf).

About the last part of your question, the synaptic cleft (the space between pre-synaptic and post-synaptic neurons) is a salty solution: water with high concentration of sodium and chloride ions (and also calcium ions, neurotransmitters and a lot more). These ion concentrations are fundamental to the generation of action potentials, neural signaling, and the general dynamics of the brain.

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  • $\begingroup$ To get an idea of how crowded the brain is, play around with the eyewire project (eyewire.org). I always thought the brain was mostly space, like it is in diagrams. But it's more like densely packed spaghetti than a fishing net. $\endgroup$
    – Resonating
    Jan 29, 2015 at 23:25
  • $\begingroup$ I really wonder which of these special cells (or anything else) is responsible for getting nutritions to the neurons for creating electrical potencial. Or are the ions just freely released into the brain? $\endgroup$
    – Probably
    Aug 28, 2015 at 12:24
  • $\begingroup$ The answer here again is water. Please see my answer to the main question. Electrical gradients IN the water is what drives the work done in cells. $\endgroup$ Mar 7, 2022 at 23:22
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Apologies for just referencing Wikipedia, but this is a very old bit of science.

The neuron gap junction - the point where the axon touches the next cell over - is 4 nanometers.

This is actually a controlled channel where the neuro transmitter molecules are transferred from one cell interior to another - its a controlled cellular - cellular channel so they really are as close to touching as they could be.

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The reality is a variety of very dense arrangements of axons and neurons: enter image description here This image is from the cremi challenge website, where you can find some varieties of neuron maps from different regions.

Here's a video explanation of the process for building the neural images: https://youtu.be/OoU_GF4fc6w?t=659

There are some efforts to map the neuron circuits of mice and drosophile fruit flies, to be able to add them into computerized physical versions of the animals. It's still many years away for even a fruit fly. There's a good video of nanometer slices here https://www.simonsfoundation.org/2021/03/22/the-connected-connectome-part-2-beyond-the-fly/#

This pic answers the title of the question, here we see 10 micron wide cell bodies of neurons, the cell bodies are distanced by 15-35 in that region. enter image description here

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The question is what's BETWEEN neurons. Most of these answers don't actually even answer. They just say "it's not very empty". Yes, the gaps between neurites are quite small, but not compared to a water molecule. In the images that I took below, it may appear that there is no spacing, but this is misleading as the tissue and areas shrink quite a bit as a result of the processing required for imaging. If you use cryo-fixation however, you can see the gaps more clearly. These gap sizes are not trivial. While they may only span anywhere from a few nanometers to a few hundred, SOMETHING occupies this space. The answer is water. Water molecules are about 1/4 of a nanometer. If you do an Xray analysis in these spaces (as I have done), you will find that its overwhelmingly filled with oxygen and hydrogen, aka water. I could go on, but that is all the question asks of and I hope I have demonstrated sufficiently that water is indeed the most prevalent substance occupying the space in between neurons.

enter image description here

enter image description here

enter image description here

It's mostly filled with water. But not ordinary water, it's like a fluid liquid crystal, 4th phase water. It's quite dense and the carrier of cellular components. 99% of the molecules are water and the distance between subcellular surfaces is usually only a few nanometers and water occupies those gaps in its sticky viscous state

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