The skeletal system is part of the brain, too

It seems like a fact uniformly forgotten is that the brain is a biological organ just the same as your liver or your spleen or your bones. Its goal – like every other organ – is to keep your stupid collection of cells in on piece. It is one, coherent organism. Just like any other collection of individuals, it needs to communicate in order to work together.

Many different organs are sending signals to the brain. One is your gut, which is innervated by the enteric nervous system. This “other” nervous system contains more neurons (~500 million) than the spinal cord, and about ten times as many neurons as a mouse has in its whole brain. Imagine that: living inside of you is an autonomous nervous system with sensory inputs and motor outputs.

We like to forget this. We like to point to animals like the octopus and ask, what could life be like as an animal whose nervous system is distributed across its body? Well, look in the mirror. What is it like? We have multiple autonomous nervous systems; we have computational processing spread across our body. Have you ever wondered what the ‘mind’ of your gastrointestinal system must think of the mind in the other parts of your body?

The body’s computations about what to do about the world aren’t limited to your nervous system: they are everywhere. This totality is so complete that even your very bones are participating, submitting votes about how you should be interacting with the world. Bones (apparently) secrete neurohormones that directly interact with the brain. These hormones then travel through the blood to make a small set of neurons more excitable, more ready to respond to the world. These neurons then become ready and willing to tell the rest of the brain to eat less food.

This bone-based signaling is a new finding and totally and completely surprising. I don’t recall anyone postulating a bone-brain axis before. Yet it turns out that substantial computations are performed all throughout the body that affect how we think. Animals that are hungry make decisions in a fundamentally different way, willing to become riskier and riskier.

 

A lot of this extra-brain processing is happening on much slower timescales than the fast neuronal processing in the brain: it is integrating information along much longer amounts of time. This mix of fast-and-slow processing is ubiquitous for animals; classification is fast. The body is both fast and slow.

People seem to forget that we are not one silicon instantiation of neural architecture away from replicating humans: we are meat machines.

References

 

MC4R-dependent suppression of appetite by bone-derived lipocalin 2. Nature 2017.