A Broken Chain with No Missing Links
Perhaps one of the most amazing thing about this process is that all the communication doesn’t take place in a single “stream” of signals between the sensed world and our systems. Our nerve cells are in contact with each other, but they’re not all continuous. Some neurons are long enough to transmit a signal from the point of sensation to the spinal cord (neurons that go from the toe to the spinal cord can be up to three feet long). But billions of neurons are much shorter; some are only a fraction of an inch. To do their part, they pass their information over “synapses” (gaps between the axons and dendrites of separate but connected neurons).
The term “synapse” comes from the Greek synapsis which means “conjunction” or something that “fastens together”. Somewhat contrary to the idea of fastening, synapses don’t always firmly attach axons and dendrites. In the case of electrical synapses, tiny protein “gap junctions” do connect axons and dendrites to pass information at dizzying speeds. But with chemical synapses (the vast majority of synapses in our bodies), there is always a minuscule gap, a distinct and unavoidable separation between neuronal transmitters and receivers.
So what does connect those separated nerve cells? It’s not a physical bridge (like the gap junctions mentioned above). Rather, it’s a biochemical process that’s part of each and every chemical synapse. Biochemical neurotransmitters are released from “synaptic vesicles” at the “button” ends of axons and travel across the synaptic cleft to dendrites which have receptors specific to those particular chemicals. Once they reach the post-synaptic dendrite, those neurotransmitters stimulate more impulses which continue the lightning-fast relay of sense information to the spinal column and brain.
The brain/body then decodes it and decides how to respond to it. And the resulting reaction – a quick stop before striking a pedestrian, a step back from a wet field, or a dash to the shelter of the car – completes the loop.
Each neuron may have thousands of synapses, and with billions of neurons in the human body (an estimated 86 billion of them in the brain alone), it’s commonly estimated that we have an average of 150 trillion synapses – each with its own tiny gap. Not every single synapse is constantly activated, of course. Our systems might short out if that happened. But cumulatively speaking, that’s a lot of gaps built into our systems. Even if you conservatively count the active clefts in terms of thousands (or millions), that’s a considerable cumulative distance for all the information to travel.