This episode is sponsored by Fabulous, an app that helps you form healthy habits that stick.

Click the link in the description to get a free week trial and 25% off a Fabulous subscription! [♪ INTRO]. What if I told you your brain was bigger than you think?

When you think of a brain, you probably imagine that pink, wrinkly organ in your skull. But… we don’t have to stop there. Because neither the brain’s functions, nor its cells, are confined to the organ we normally think of as the brain.

And expanding this definition could have mind-bending consequences for how we study and understand neurobiology, and even how we treat disease. See, understanding where the brain begins and ends is part of understanding, well, the brain. By comparing different kinds of brains and investigating how brains interact with other parts of the body, scientists can discover how that results in behaviors like eating and moving.

This allows them to develop helpful technologies or treatments for neurological disorders. But if we don’t know what a brain even is, these advances become more difficult. It’s hard to assess how the brain interacts with other parts of the body if we don’t even know where the brain stops and other body parts start.

So let’s start from the rough idea that a brain is a dense clump of specialized cells localized to an animal’s skull. Neurons are the cells most commonly associated with being brain cells. So we could define the brain as the place where neurons are located.

But neurons are not exclusively found in your skull. You have neurons that are partially in your brain, and partially in places like your fingertips. That’s because the same cell is used to carry sensory information all the way from your fingertip to your brain.

The signal-carrying arm of these cells, called an axon, can be up to a meter long! But despite having part of a neuron in it, your finger is probably not part of your brain. A more tricky example is your eyes.

The light-sensing rods and cones in your eyes are neurons called photoreceptors. When these cells are stimulated by light, they send electric signals through the optic nerve and into the brain. If your eyes have neurons in them, then they might be part of your brain.

But there’s other ways we can answer the question. Let’s look deeper into what constitutes a brain. Rather than cell type, why don’t we try to define the brain by how it functions?

The brain works by sending chemical signals called neurotransmitters back and forth between neurons. Those messengers are essentially responsible for the brain’s functioning. So you could say the brain is where neurotransmitters are released.

Except that we can find them in plenty of places outside your skull… like, once again, your eyes. Neurotransmitters like dopamine and melatonin are released from photoreceptors. A 1992 study in chickens shed some light on what those neurotransmitters are doing there.

And while chicken brains are obviously not human brains, studying them can help us learn what’s probably also true in humans. The researchers found that melatonin stimulates cellular signals to adapt their eyes to the dark, while dopamine halts those signals to adapt their eyes to the light. And these neurotransmitters are involved in sleep.

Melatonin is a sleep helper, while dopamine keeps you alert. In other words, these neurotransmitters in the eyes don’t just regulate vision. The balance between the two is probably regulating sleep.

And a lot of that is going on in your eyes, even though it seems like a pretty brainy responsibility! But beyond your eyes, other parts of your body also release neurotransmitters, like your heart. The neurotransmitter adrenaline is released from cardiac nerve cells.

It’s possible that your fight or flight response isn’t just your brain telling your heart to beat faster, but it’s also your heart preparing itself for action. So this definition would include organs like your eyes and heart as part of your brain. A third way scientists have thought to define the brain is the organ inside your skull.

In medicine, a traumatic brain injury is specifically one that affects the brain inside the skull. This is a bit simpler and eliminates things like your eyes, which are on the outside of your skull and not protected by it, or your heart, which is someplace else entirely. But if a brain has to be inside of a skull, can invertebrates have brains at all?

This definition would influence how we view other organisms. Consider the small, pond-dwelling hydra. As far back as 1968, researchers found that hydra use neurotransmitters like glutamate to activate neurons and orchestrate eating, which sounds like something an organism with a brain would do.

But hydra have neural nets of diffuse brain cells, rather than a dense organ like we have. And they don’t have skulls. Another skull-less animal is the octopus, which does have a dense brain organ in its head.

But they also have more complex neural regulation in their limbs than we have. Two-thirds of octopus movement control happens in their limbs! If we define the brain as contained in the head, we could be excluding two-thirds of the octopus brain.

In 2011, researchers found that an octopus’s head-brain uses information from what it sees to guide limb movements, while its limb-brains can guide their own movements based on what they feel. These brain systems can accomplish goals either in tandem, or independent of each other. Earlier, I said your finger is not part of your brain.

Maybe for an octopus it is! Ultimately, it doesn’t seem that there’s a hard and fast brain definition. So what is a brain?

If you wanted to call it a dense clump of neurons localized in an animal’s skull, you wouldn’t be wrong. And if you’re using that definition, then eyes and fingertips are not part of the brain because they’re not encased by the skull. It’s also possible that a human brain could have a different definition from an octopus brain.

Other animals still have pretty impressive brain systems, even if you don’t think of them the same way that you think of our brains. And even though there’s no one perfect definition for our brains, asking where our brains start and end can help us understand how they interact with our bodies and help us do everything we do. Maybe you’re not in the habit of thinking of eyes as brains, but you do want to work on other habits, and make them stick.

That’s why there’s Fabulous, a self-care app that can help you add new things to your routine. It was developed by researchers at Duke University and uses behavioral science to gently support your goals. Whether that’s exercising more, cooking at home instead of eating out, or just staying hydrated!

You’ll be able to choose a self-guided approach or use their daily habit coaching. Either way, you'll be able to go at your own pace. If you’re interested, the first 100 people who click on the link in the description will get a free week trial and 25% off a Fabulous subscription. [♪ OUTRO].