How neurons influence behaviour

Sometimes I’ll be out, minding my own business, sinking a couple of beers, and talking about, you know, shitty landlords or people who don’t pick up their dog’s poop or something. With annoying regularity, someone will turn to me and say something like “I know why people do this thing we’re talking about. It’s because of [insert random Andrew Huberman or whatever podcaster’s nonsense pseudo-brain-science tidbit], isn’t that right Dorian”.

The answer is almost always no. Whatever podcaster you like is almost certainly lying to you, because knowing things about the brain almost never tells you anything about behaviour and even when it does, people who are trying to make you buy their stuff will tell you other, more interesting, blatently false things instead because they get more clicks. But I don’t want to ruin people’s funny anecdote about illicit dog poopers with a brain science lecture, so I just grimace out a smile, and take another sip.

But, people do like to know about brain things, so I thought I’d write about the brain things that do influence our behaviour here. So this is part 1 of that series: what can neurons tell us about human behaviour?

See also my article on making meaning in the brain which talks about brain and behaviour too.

I’ll start with a quick overview of all you need to know about neurons to understand their role in behaviour (and nothing you don’t need to know). Then I’ll show you how they influence us.

¶What are they?

The first thing worth noting is that neurons don’t just exist in the brain. Neurons and nerve cells are the same thing. Nerve cells, when they aren’t in the brain, and they’re bundled together into nerve fibres, are called nerves. So the nerves in your body and the neurons in your brain fundamentally do the same job: they are the basic signalling unit of the body and they’re supposed to transmit and process information—to map perceptions to the appropriate actions.

Generally, neurons have three main parts: dendrites, a cell body, and an axon. Dendrites are branch-like structures that receive signals from other neurons or sensory receptors. These signals are sent to the cell body, which has a bunch of your typical cell organelles inside—the things that power and run the neuron. The axon is a long tube-shaped extension that carries signals from the cell body to other neurons, or perhaps a target tissue somewhere in the body, like to tell a muscle to contract or something.

Neurons communicate with each other at the connections between their axons and the dendrites of other neurons. This connection between neurons is called a synapse.

This is my attempt at drawing a neuron. You have the cell body, with all these dendrites, like hairs coming off, for other neurons to connect to. Inside the cell body you have a bunch of organelles I'm not going to bother telling you about. From the cell body, a long tube-like axon extends and eventually ends up at another neuron, splitting into a bunch of thin fibres that end in synapses---connections to the dendrites of the other neuron.

There are two sort-of stages to a neuron’s communication: a chemical part and an electrical part. In the cell body, specialised organelles create vesicles: little bubbles full of chemical messengers called neurotransmitters. The vesicles get transported towards the synapse, to be stored at the end of the axon. Eventually, these vesicles will be ‘popped’, releasing the neurotransmitters into the synapse to float over to neighbouring neurons. This sharing of the released neurotransmitters between neurons is the chemical part.

This is my attempt at drawing a synapse. So all these bubbles (vesicles) of neurotransmitters produced back at the cell body get transported here to the end of the axon, right next to the dendrites of the next neuron. These bubble then 'pop' into the space between the two so the receptors on the dendrite of the other neuron can gobble them up.

These shared neurotransmitters then trigger the electrical part. When a neuron recieves enough neurotransmitters from other neurons through its dendrites, it will open some channels in its skin (the cell membrane) to allow in some of the electricity that’s washing through your body. This will lead to a sort-of overcharge, which we call an action potential, and this electrical energy will whiz down the axon to the synapse.¹ This action potential is the trigger for the ‘popping’ of the vesicles into the synapse, kicking off the process once more for the neighbouring neurons.

Lastly, there are types of neuron. Three rough categories are useful to think about, I think:

1. Motor neurons take in neurotransmitters from other neurons, and release neurotransmitters to muscle fibers, triggering the muscle fibre to contract.

2. Interneurons are what we call neurons that link neurons at both ends, both recieving neurotransmitters from other neurons and releasing neurotransmitters to their neighbours.

3. Sensory neurons work slightly differently. These neurons take in sensory impulses from our sensory organs. These act more-or-less like neurotransmitters, but here the neuron has to convert whatever physical or chemical stimulation the body is sending it into the electrical form that triggers the action potential.

Whenever you have sensory, inter- and motor neurons connected together in a functional loop, we call it a neural circuit.

There is a final category of cell worth mentioning, although they aren’t technically neurons: ‘white matter’ or glial cells. White matter fits all around the neurons (the ‘grey matter’) and takes care of them—protecting them, repairing them, and improving their performance. I’m not going to draw this for you. Just imagine the neuron above wrapped in a bunch of shit, like gooey armour.²

¶What’s a good way to think about them?

So, that’s all well and good, but really what we want to know is how we should think about neurons. Like, what do they do, right?

Basically, nerve cells are responsible for taking in signals from our senses and transmitting them through the nervous system to produce the appropriate responses. They fundamentally map perceptions to actions. This can be simple, like a knee jerk response to someone tapping the knee cap. But it can be complex too, like determining the difference in response between seeing someone running toward you with a knife on TV, vs that same knife-wielder in the dark alley you’re walking in.

Of course, these signals don’t pass through the brain magically. It has to travel a pathway limited by which neurons are connected to which other neurons (and how). The way neurons are wired together determines an enormous proportion of our behaviour. This is why the idea of neural ‘pathways’ is so popular, because it really is the best way to think about how neurons (indeed, all nerve cells) work.

Those pathways are given to you by evolution, and then developed by you as you go through life. The more you train a pathway, the more likely it is that certain sensory inputs will produce the response it’s connected to.

The strongest pathways are the ones you have trained the most and, since you cannot control the electrical firing of your neurons directly, means that there’s only so much you can do to change the pathways that you’re using in the moment.

The white matter, or glial cells, act as infrastructure around the pathways. More glial cells are attracted to pathways that are better trained, to help the pathway do its job of mapping perceptions to actions. The more you practice something, the more white matter will grow around those pathways—all the better to support them in doing this thing that we do all the time. This is wonderful for speed and performance. But it means that these neurons, now surrounded by all this construction, can’t change their connections very easily anymore. It means these neurons are quicker at doing something, but much less good at doing other stuff. They are less flexible. This is, in fact, the main difference between adults and teenagers when people say teenagers’ brains aren’t developed— they have less white matter.

So, a good way to think about neurons are pathways. All grouped together, they’re the information highways of the brain and body, running messages about what perceptions should go to what actions. If we zoomed out from these pathways, both the heavily constructed highways and the lighter animal trails that comprise our perception-action mappings, we’d have exactly that—a map. Our nervous system, and particularly our brain, is a big map of which perceptions should be mapped to which actions. A map of the statistical structure of the world, and our actions within it.

¶Limits of the analogy

For the most part, this road/map analogy will do just fine. Stronger and weaker pathways is a good way to think about a lot of neural activity.

But of course, roads are fixed. You can’t move a road. They go from point a to point b, and back again, and they only do that.

Neural pathways don’t have these kinds of restrictions. Not only can you change your neural pathways, for the most part, it’s also true that each pathway is typically involved in several things at once. Any given neural circuit is likely to be involved in many different mappings of perception to action. And some people think that some neural circuits could be involved in almost anything we do, all at the same time.

What this means is that it might not be possible to simply change one neural pathway at a time. One habit isn’t just a set number of dedicated neural circuits. The neural circuits involved also do a bunch of other things, and by changing them, you might be changing those other things too. Or, because you’re still doing those other things, then you might find you’re having a harder time changing this one.

So there you have it. What neurons can tell us about behaviour: changing a road is easy, changing a neural road isn’t quite so neat and tidy. It’s not sexy, but at least it’s true.

And, as a bonus, now you’re fully equipped to go and learn about how repressed memories work: lots of it seems to be re-used neural circuits at work.