Overloaded: How Every Aspect of Your Life is Influenced by Your Brain Chemicals

A NOTE ON THE AUTHOR

Ginny Smith is a science writer and presenter with expertise in psychology and neuroscience. She has a talent for making the complex comprehensible, and a passion for bringing her love of brain science to audiences around the world.

Ginny founded Braintastic! Science, which produces spectacular science shows and resources to help young people understand and get the best out of their brains. She is regularly found on stage at schools, festivals and events, and relishes answering childrens’ questions about the brain. Ginny also produces content about the brain for adults. She teaches at the University of Cambridge’s Institute of Continuing Education, and is a regular blogger and video presenter for the Cosmic Shambles Network, alongside some of the biggest names in science. She hosts the Psychological Society’s Psych Crunch Podcast, and shares her skills as a science communication trainer and consultant.

ginnysmithscience.com / @GinnySmithSci

Some other titles in the

Bloomsbury Sigma series:

Sex on Earth by Jules Howard

Spirals in Time by Helen Scales

A is for Arsenic by Kathryn Harkup

Suspicious Minds by Rob Brotherton

Herding Hemingway’s Cats by Kat Arney

The Tyrannosaur Chronicles by David Hone

Soccermatics by David Sumpter

The Planet Factory by Elizabeth Tasker

Catching Stardust by Natalie Starkey

Seeds of Science by Mark Lynas

Nodding Off by Alice Gregory

Turned On by Kate Devlin

Borrowed Time by Sue Armstrong

The Vinyl Frontier by Jonathan Scott

Clearing the Air by Tim Smedley

The Contact Paradox by Keith Cooper

Life Changing by Helen Pilcher

Sway by Pragya Agarwal

Kindred by Rebecca Wragg Sykes

Our Only Home by His Holiness The Dalai Lama

First Light by Emma Chapman 

Models of the Mind by Grace Lindsay

Handmade by Anna Ploszajski

Beasts Before Us by Elsa Panciroli

Our Biggest Experiment by Alice Bell

Sticky by Laurie Winkless

Racing Green by Kit Chapman

Wonderdog by Jules Howard

Growing Up Human by Brenna Hassett

Contents

Chapter 1: The Chemical Brain

Chapter 2: Thanks for the Memories

Chapter 3: Getting Motivated

Chapter 4: Mood Swings and Scary Things

Chapter 5: Sleep, the Brain’s Greatest Mystery?

Chapter 6: Food for Thought

Chapter 7: Logic, Emotion or Chemicals?

Chapter 8: You’ve Got the Love

Chapter 9: A Pain in the Brain

Conclusion

Glossary

Diagram of the brain

Acknowledgements

Index

CHAPTER ONE

The Chemical Brain

This is a book about some of the most fundamental questions in neuroscience; perhaps in science as a whole. How does our brain produce our everyday experiences, and drive our behaviour? How can a kilo and a half of jelly allow us to learn a foreign language, decide what to have for lunch, or even fall in love? While the cells that make up our brains are important for this, it is the chemicals that bathe them, and allow them to communicate, that paint the complex details which colour every aspect of our daily lives. But how exactly can these tiny molecules cause the full spectrum of human experience, with all its richness, its highs and lows, its joys and sorrows? And how does the brain ensure it isn’t overloaded by this melee of molecules?

In this book, we will uncover cutting-edge research and meet leading scientists aiming to better understand the complex and intricate workings of your brain, and the molecules that control it. We will explore drugs, both medicinal and recreational, which alter the levels of these molecules, and investigate how a better understanding of our brain’s workings might help us improve treatments for common conditions, without overloading the delicate balance of our chemical brain. We will even touch on ideas about free will, consciousness, and how our brain enables us to control our instincts.

Along the way we will dig into the history of neuroscience, uncovering stories of scientists’ curiosity and persistence in the face of huge challenges. There are also tales of accidental discoveries which have revolutionised our understanding of the brain. I find these stories of how science is done as fascinating as the science itself, as they help us get a deeper understanding of the way knowledge develops over the years as well as giving us a glimpse into the characters behind the chemicals.

My hope is that this book arms you against the over-simplification rife in the media. As we will see, the idea that ‘serotonin is the happiness chemical’ or ‘dopamine is addictive’ misses so much of the important nuance as to render these statements pointless. Instead, here, we will explore that complexity and celebrate it, keeping things comprehensible and cutting through the scientific jargon to examine the underlying concepts. While this book may not be able to provide all the answers to these fundamental questions (in many cases we just don’t know them yet), I hope that it sparks your curiosity, and encourages you to want to find out more about your incredible brain.

More questions than answers

I have always been curious, wanting to understand why and how the things around me work. As a child I was lucky to have parents who encouraged this curiosity, and did their best to answer my questions. In one infamous story, my mum remembers us walking into a public toilet and me, aged around three, turning to her and asking: ‘Mummy, why are sounds louder in here?’

As she began explaining to me how sound is absorbed by soft surfaces, but bounces off hard ones, of which there are far more in a public toilet, another mum came out of the cubicle and gave her an admiring (or perhaps astonished) look, saying: ‘I’m glad my daughter doesn’t ask me questions like that!’

I always loved science, soaking up knowledge and understanding, but it wasn’t until university that I discovered my fascination with brain science. As so often seems to happen, the decisions that change the course of our lives aren’t those huge, momentous ones we agonise over, but the seemingly small, inconsequential ones. And this was definitely the case for me. It was just a couple of lectures, given as part of a course called ‘Evolution and Behaviour’, which I picked on a bit of a whim,¹ that opened my eyes to the wonders of the human brain. Here was a real challenge. An incredibly complex system, full of mysteries and unknowns, that needed breaking down, and understanding at the most fundamental level. And not only that, but it was something that affected us all, every day. I was hooked, and decided to change the focus of my degree from chemistry to psychology.

Over the summer before my second year, worried that my lack of biology A-level would cause problems for me, I refreshed my limited knowledge of the nervous system. I had learnt a little about the structure of neurons, or nerve cells, at GCSE. I knew they were specialised cells that send messages around the brain and body, but I needed to learn more. So I began reading.

Neurons, I discovered, come in a range of types. In the body, there are sensory neurons, which carry information from your senses to the central nervous system, which is made up of your brain and spinal cord. Motor neurons carry information in the opposite direction, allowing your brain to control your movements. Then there are tiny interneurons, which connect the two and allow complex circuits to form. In the brain, things are more complicated, and we can’t categorise neurons quite so neatly, as they come in so many different shapes and have so many different functions. But there are some similarities between neurons in the brain and the body.

Just like most other cells, neurons have cell bodies. These contain a nucleus (which has a range of roles, including storing the DNA) and mitochondria (which produce energy). It is in this cell body that new proteins are made, allowing the cell to function, and repair itself when needed. But it was the differences between neurons and ‘typical’ animal cells that I found most fascinating: the dendrites, tree-like branches, reaching out from the neuron to allow it to connect with many other neurons, and the long ‘axons’ that allow it to send messages quickly and easily.

Jennifer Aniston and Frankenstein’s frogs

Our understanding of how messages pass along neurons began in 1780, with a scientist called Luigi Galvani. Trained in medicine and surgery, after graduating Galvani became a lecturer at the University of Bologna. Alongside his teaching, he carried out research, and developed an interest in the ways in which electricity could affect the body. He discovered that when electricity was passed through the leg of a dead frog, it began to twitch, as if it was coming back to life.² Galvani was amazed, and named his discovery ‘animal electricity’. He believed he had found the animal’s ‘life force’, and, to some extent at least, he was right. But he was wrong to think that ‘animal electricity’ was a specialised form of electricity. Alessandro Volta, a professor of experimental physics in the University of Pavia, Italy, realised this, and publicly criticised Galvani’s theory. He proved that animal electricity was the same form of electricity that flowed through the fluid in his ‘voltaic pile’, which went on to become the batteries we still use today. In both the battery and the body, electricity is carried by a flow of charged particles, or ions.

We now know that when activated, electricity travels from one end of a neuron to the other,³ a bit like a crowd doing a Mexican wave at a football match. This is known as a ‘spike’ or ‘action potential’, and is often referred to as the neuron ‘firing’. Action potentials are all or nothing, more like a digital watch where it can be either 3.46 or 3.47 than an analogue one where the hands can point to anywhere in between. You can’t have a small or large action potential; either the neuron reaches its threshold and fires, or it doesn’t. So rather than the strength of firing, it is the rate of the spikes that provides information about, for example, the intensity of a sensation.

As I delved deeper, I realised how complex these seemingly simple structures are. The axon, for example, is coated with a fatty substance called myelin. This acts a bit like the insulation you find around electrical cables, but also helps messages to be sent more quickly along axons. This myelin is what is sometimes referred to as ‘white matter’ in the brain and is made by glial cells.⁴ Although there are ten times as many glial cells as neurons in the brain, they are often ignored in favour of their more famous cousins. This is, at least partly, because they used to be thought of as fairly inert, just there to support the neurons. We are now starting to realise that glial cells take a much more active role in keeping the brain healthy and functioning properly.

The more I learnt about the complexities and intricacies of our brains, the more hooked I became. In my final year of university, I began to realise my interest lay in the boundary between psychology and neuroscience, an area known as cognitive psychology or behavioural neuroscience. This was the area, it seemed to me, that could answer the most important question: why do we behave the way we do? But the deeper I got into it, the more I realised how much we don’t know. For many years, after the invention of MRI scanners, which allowed scientists to peer inside a living human brain, there was a focus on finding out which areas of the brain are involved in which processes. Scientists found areas ‘for’ numbers, faces and laughing when being tickled. They even found a single neuron which responded to pictures of Jennifer Aniston. But more recently, we have started to realise that this is an over-simplification, caused by the resolution of brain scanners. Rather than looking at areas ‘for’ each process, we now search for networks: combinations of neurons which can be distributed widely across the brain.

In a human brain, each neuron can make thousands of connections, building up the dense web of cells which makes up your brain’s grey matter. It is these connections between cells that I find particularly fascinating, because this is what gives our brains their amazing flexibility. We are born with the vast majority of our neurons already in place. There is some debate over whether certain regions of the brain can make new neurons, but this process doesn’t seem to be widespread. But the neurons we already have can make new connections, and the strength of connections can change. Sometimes this happens via neurons growing new dendrites, and new physical connections being made. But this is a slow process, and our brains need a faster way to change the signals that are sent around them. This is where brain chemicals come in. But to understand how these work, we first need to look at how they were discovered – and that means stepping back in time again, this time to the mid-1800s.

The argument of the century

In the middle of the nineteenth century, what brains were made of was still mysterious. Human senses are only so good, and the microscopes of the day weren’t hugely powerful. What was clear was that the brain was a network, consisting of densely packed, intertwining fibres. But what these fibres were, and how they were responsible for the myriad of duties our brain carries out every second we are alive was far from clear. Of course, this didn’t stop scientists putting forward theories and carrying out experiments to try to determine the truth. And one of these scientists was Camillo Golgi.

Golgi was born in Italy in 1843 and grew up to follow his father into medicine. His interest in experimental medicine, however, took him back to a research position and he began to study under Giulio Bizzozero. Amongst other things, Bizzozero was an expert in the nervous system, and studied its structure under a microscope.

At this time, the cell theory of physiology was still relatively young. While the idea that the body consisted of discrete elements, or cells, had been widely accepted, it hadn’t been extended to the brain. Instead, German anatomist Joseph von Gerlach put forward reticular theory, suggesting the fibres of the nervous system formed a continuous network, through which fluid, carrying information, could flow. It was this theory Golgi supported, and this shaped his findings throughout his life.

When money issues forced him to take a job in a hospital once more, Golgi refused to give up his research, setting up a crude laboratory in an old hospital kitchen. In order to see the nervous system under a microscope, scientists of the time created stains, which aimed to dye these structures and make them more visible. But Golgi felt they weren’t good enough to make the complex structures of the brain clear. Frustrated, he set about experimenting, and in 1873, he made a breakthrough. By treating brain specimens first with potassium dichromate and then silver nitrate, he found that just a few of the densely packed brain cells turned black, making their structure easily visible under a microscope. He named his technique la reazione nera (the black reaction), but today it is better known as the Golgi stain, and it is still in widespread use. Golgi published many papers using his stain to support the reticular hypothesis, arguing it allowed us to see the complex network through which information flowed. But there was a controversy brewing, one which would continue to rage for the rest of Golgi’s life. And to understand that, we need to travel across the Mediterranean Sea, to Spain.

In the 1860s, while Golgi was still completing his studies, a Spanish anatomy teacher and his wife were desperately worried about their young son. Kicked out of one school after another for poor behaviour, the young Santiago Ramón y Cajal had even been in trouble with the law for building a cannon that destroyed a neighbour’s gate. After apprenticeships to a shoemaker and a barber did little to curb his anti-authoritarian attitude, his father spent a summer taking him to graveyards to find human remains they could sketch, hoping to spark an interest in medicine. A keen artist growing up, Cajal took to this immediately, and in 1873 he graduated in medicine, before becoming an army medic. This was short-lived, however, as bouts of malaria and tuberculosis contracted in Cuba brought him back to Spain where, after recovering from his illnesses, he turned his attention to research.

Cajal’s early work focused on inflammation, cholera and the study of the skin, but in 1887, in Barcelona, he heard about Golgi’s new staining method, which sparked an interest in the brain that would continue throughout his life. Cajal was not just a scientist but also an artist, and his drawings of neurons are as beautiful as they are detailed. And it was while sitting at the microscope, making these incredible drawings, that Cajal realised Golgi had got something wrong. The brain wasn’t one continuous web, but was made up of discrete cells. This provided support for what was known as the neuron theory. Despite the pair sharing the Nobel Prize in 1906, Golgi never accepted Cajal’s evidence for neuron theory, and continued to support the reticular theory until his death in 1926.

Of course, modern imaging techniques have proved without a doubt that Cajal and other proponents of the neuron theory were correct. Neurons don’t quite connect, leaving a small gap between the end of one and the start of the next. This gap is called a synapse, and is vitally important in the way our brain works.

Mind the gap!

In some synapses, the electrical current can pass straight from the first (or presynaptic) neuron to the second via something called a gap junction. But much more common in the adult brain are chemical synapses, and it is these we will be focusing on in this book.

When a neuron is activated, as we have seen, an electrical signal flows along it from one end to the other. When this action potential reaches the end, it causes chemicals called neurotransmitters to be released. These travel across the synapse until they reach the next neuron. At the very edge of this neuron are special structures called receptors, which sit half in and half out of the cell. When the chemicals bind to these receptors, they cause changes in the second cell. What changes they cause depends both on the chemical released and on the type of receptor, but they fall into two main categories. The first is fast and direct, and the second slower, operating through a cascade of messengers within the neuron.

In the first category, when the neurotransmitters bind to the receptors, they directly change the flow of ions inside the neuron. This can make the cell more ‘excitable’, and, if enough neurotransmitters bind, can cause the neuron to fire. If a neurotransmitter has this effect, we call it excitatory. Through a similar process, neurotransmitters can have the opposite effect, changing the flow of ions to make it harder to activate a neuron. These are inhibitory neurotransmitters.

These processes are relatively fast and short-lasting, but the second type of receptor allows chemicals to have slower and longer-lasting effects, triggering a cascade of changes within the cell. These can work to make the cells easier or harder to activate, change the way neurotransmitters are released, or alter the receptors found in the neuron. There can be multiple types of these receptors for each neurotransmitter, meaning the same chemical can have different effects depending on which receptors it binds to.

The most common neurotransmitter in the human brain is called glutamate. In most cases, glutamate has an excitatory effect on the second neuron, making it easier for the signal to be sent and activating the second cell. If enough glutamate is released, the signal is transmitted from the first cell to the second, and can continue on its journey. As we’ll see, this chemical is vital for all sorts of processes, from learning to pain.

Found widely throughout the brain, Gamma Aminobutyric Acid, known as GABA, has the opposite effect to glutamate. When it binds to receptors, it makes it harder for the neuron to send an action potential. This makes GABA an inhibitory neurotransmitter. Neurons that use GABA, then, have a calming effect on the brain, reducing the activity of other neurons. This means they are important for sleep, and for counteracting anxiety.

Most of the brain chemicals we come across in this book, however, won’t be this simple. They may have different effects in different parts of the brain, based on which types of receptors are found there. There can even be multiple types of receptor found on the same neuron, so the same chemical may have different effects depending on how much of the chemical is released, or how long for. This might sound confusing, but as we will discover, each process alone is relatively simple, and it is by combining them that we reach the greater levels of complexity that make our brain such an amazing machine.

Then there is the question of reach. In the simple examples of neurotransmission we have discussed here, GABA or glutamate is released into a synapse and affects one or maybe a couple of neurons that are on the other side of it. But sometimes brain chemicals are released more widely and affect a whole group of neurons, making it easier or harder for them to pass messages between them. This extra level of complexity is what allows our brains to change so quickly, and gives us such amazing flexibility in our behaviour.

This brings us back to that fundamental question, the one that hooked me all those years ago. How do our brains make us behave the way we do? It seems to me that the answer lies not in the wiring of our brains, but in the chemicals that bathe them. Because while, as we will see, the connections between neurons can and do change,⁵ this process is slow. This means it can’t be responsible for the millisecond-by-millisecond changes we all experience: the split-second decisions, fluctuations in emotion and the temptations we encounter. Instead, these are all controlled by our brain chemistry.

And, as it is our brain that makes us who we are, that means that we are controlled by this turbulent sea of neurotransmitters. But how does this work? To find out, I have spent the last 18 months reading books and papers and talking to experts around the world. And while I have found some answers, I have also discovered a myriad of questions. Again and again I have run up against the boundaries of scientific knowledge, and rediscovered just how much there is still to learn about our incredible, complex brains. Neuroscience moves faster, I would argue, than any other science, so while I have attempted to present the current scientific consensus in each topic, there will be people who disagree. And, by the time you are reading this, new studies will have been published that might turn our understanding on its head again. That is the way science works, so it is best to treat this book as a snapshot of our understanding at this moment in time, not as facts, carved in stone for evermore.

Each chapter in this book covers a different aspect of our lives, and how they are affected by our brain chemistry. These are huge topics, and each could have been (and has been) the focus of a book in its own right, so I have had to pick and choose which stories to cover. This means there will, unavoidably, be gaps – topics and areas that could have been included in each chapter, but which I haven’t been able to go into. I have provided a section with further reading suggestions for each chapter at the end of the book, should you want to delve deeper into any one area.

I have attempted to provide the chapters in an order that makes sense, with each building on those that have come before. The intricate relationship between brain areas, networks and chemicals means that there is more in common than you might expect between topics that may seem, on the surface, completely different. You can, of course, dip into any chapter, and read them in whatever order you choose, and each should make sense alone. But I encourage you, if you can, to start at the beginning, because, as you will see, the brain is highly interconnected.

Now, without further ado, I invite you to join me on a journey of discovery, and find out more about the chemical soup that makes you you…

Notes

1 And, if I’m honest, because I thought it would give me the excuse to watch lots of David Attenborough documentaries as ‘revision’. 

2 This is thought to have inspired Mary Shelley’s novel Frankenstein, written in 1818.

3 Interestingly, in most neurons, information can only flow in one direction: the first neuron releases the chemicals and the second has the receptors to receive them. 

4 Glial cells are any cells in the brain that aren’t neurons.

5 And this process itself is controlled by chemicals, as we will see in Chapter 2.

CHAPTER TWO

Thanks for the Memories

Do you remember what you were doing on your 18th birthday? Mine was a Friday and after college my boyfriend drove me, along with a couple of friends, to Reading town centre. It was a chilly January evening as we walked to the Purple Turtle, a characterful (if a touch grimy) cocktail bar a little way off the high street. There, proudly wearing my ‘18 today’ badge, I enjoyed my new-found ability to buy alcohol legally by working my way through their extensive cocktail menu. My memories of this evening (well, the early parts of it at least!) are fairly clear. I even remember queuing for the toilet, where a group of girls asked me if it really was my birthday.¹

But if you picked another Friday from that year and asked me what I did, I wouldn’t have a clue. On a superficial level, it seems obvious that momentous occasions are stored differently, so they can be recalled much more easily than other, more humdrum days. But how is this difference coded in our brains? What is actually going on in my neurons and synapses when I recall that evening that brings the sticky floors and graffiti-covered walls to mind so readily?

To understand that, we have to look at how we learn and remember, a journey that will introduce us to the ethical quagmire that is cognitive enhancing drugs and make us question the reliability of our own memories. But first, let’s start with the basic neuroscience of learning, and this means beginning not with humans, but with a strange creature known as a sea hare.

Don’t poke the… hare

The sea hare is a type of sea slug. This large mollusc lives in shallow water and is named for two long protrusions on its head, which resemble (a little) the ears of a hare. These structures are actually used to detect chemicals dissolved in seawater, so the animal can ‘smell’ its way to food or a mate. Sea hares also have a set of gills on their back (which allows them to breathe) and a siphon which pumps water over the gills. Their siphon and gills are particularly delicate, so when the creature is disturbed it rapidly withdraws them into its body, like a snail retreating into its shell. It is this reflex which has formed the basis for a myriad of learning experiments by neuroscientists all over the world, and even won the Nobel Prize for Eric Kandel from Columbia University in 2000.

As well as this easily seen reflex, sea hares only have around 10,000 neurons, which are very large. This makes it relatively easy to study the circuits of neurons that control the animal’s behaviour, especially learning and memory. And it turns out these simple creatures can, and do, develop memories. Normally,