How can that be? We’re supposed to believe that thousands, even millions, of miles are covered each day just within our brain? It’s impossible for all that distance to be traveled in such a small space. Isn’t it?
Again, it seems counter-intuitive, but consider that linear space isn’t the same as 3D space. Synapses can share cleft space with additional glial cells (the “glue” between the neurons which serve a number of functions and hold them in place). Myriad neurotransmitters will not only traverse that shared space to get from their originating vesicles to their appropriate receptors, but also get taken up by astrocytes or diffused into the space around the neurons. Even though the actual gap between dendrites and axons may be ~20 nm, the distance traveled by each individual neurotransmitter can be longer.
So, a 20 nm space, crossed just once by 2,000 neurotransmitters, and then again by 500 of them on the return trip to uptake will involve cumulative linear travel of 50,000 nm, or .005 centimeters, or a little less than 2/100th of an inch. And that’s the math for just one synaptic vesicle (out of hundreds), on just one neuronal synapse (of billions), each of them firing around 200 times/second to release neurotransmitters.
Here’s a (rough and simple) visual approximation of that “travel”:
20 nm synaptic cleft x 2000 neurotransmitter "trips"
- 20 nm synaptic cleft x 500 neurotransmitter “return trips”
x 200 releases per second
= ~10 million nm crossed => .3937 inches per synapse
That’s a lot of travel. Especially for minuscule entities of .5 nm in size. And that’s not even counting all the activity in the spinal column, which has its own many millions of chemical neuronal connections. When you consider the constant movement and flow of neurotransmitters involved in even the simplest activities, which multiplies the totals even more, my math completely fails me. It all adds up to a number that’s better quantified by a computer. Or a savant.