The human brain is approximately 1130-1260 cubic cm in volume. Since 1.13 septillion (1,130,000,000,000,000,000,000,000) nanometers will fit into a 1130 cubic cm space, we actually have plenty of space for those paltry 4500 trillion nanometers – especially because they are scattered throughout the space, in a dithering of chemical potential.
Now, when we add a third dimension, things look even more feasible. According to Alonso-Nanclares, et al, every cubic millimeter (mm) of cerebral cortex contains roughly a billion synapses. So, 150 trillion synapses takes up 150 cubic cm. In terms of volume, that’s about 3.5 cm x 3.5 cm x 12 cm – about the size of a stick of butter.
So, that’s not so massive, considering it’s spread throughout the 1130-1260 cubic cm of the brain. And yet, that’s enough room for 150 trillion synapses, shared by 86 billion neurons. In the brain alone.
When you think about it, it’s pretty amazing just how much is folded into that relatively small space. Our bodies are highly adept at embedding length into compact spaces. The small intestine, which shares space in our abdomens with other internal organs, measures some 23 feet (7 meters) when you stretch it out. And the DNA in a single human cell totals about 6 feet (2 meters) in length. This all works because of 3D space, as well as the incredibly tiny dimensions of the elements in question. So, our nerves and synapses are simply following suit, fitting a whole lot of functionality and information in a compact container.
But let’s get back to linear distance. Remember the illustrations of the synapse earlier? At the end of each axon, we find a “bouton”, or a terminal, which is the pre-synaptic “start” of the synapse. Nerve terminals in certain parts of the brain typically contain approximately 200-500 synaptic vesicles per terminal. Those vesicles are the “pockets” at the ends of the axons holding the biochemicals which travel across the synaptic cleft.