Each of these areas of the brain can be further divided, usually by the function it performs, with each having a name. You need a good memory to be able to keep track of each named area of the brain, what each does, and how it connects to sense organs and other parts of the brain. The intricate ways in which signals are processed and combined to create a simulation of the world are complex and fascinating, but the details are beyond this book. What is central to the brain is the network of connections between brain cells called neurons, and although these networks operate in slightly different ways in different areas of the brain, there are broad similarities in how the networks function.
Your brain consists of between 80 and 100 billion neurons, and at least as many, and perhaps many times more, glial cells that look after the neurons. In the nineteenth century, scientists thought that without glial cells the nervous system would fall apart, so they named them after glia, the Greek word for glue, but that is not what the cells do. Instead, glial cells are to neurons what a team of mechanics are to racing cars, ensuring they are in good working order. The ratio of glial cells to neurons differs between different parts of the brain, and there are highly variable estimates of how many of each type of cell there are. Counting the individual cells in a human brain would be a laborious and not hugely exciting endeavour, so we rely on approximate estimates obtained by scaling up counts from small sections of the brain. Regardless of their number, glial cells are fundamental to brain function, but, as far as we know, their role is not critical to understanding consciousness, and for that reason I do not consider them further.
There are lots of different types of neurons, but all contain the same key components. They have evolved to be able to form large networks of interconnected cells. A single neuron consists of a main cell body, called the soma, where the cell nucleus and other organelles are found. Biology can sometimes be frustrating with its terminology. Soma means body and comes from the Greek sōma. The ‘disposable soma’ refers to our bodies, while the soma of the neuron is the main body part of the cell. Extending from one end of a neuron’s soma there is usually a long, thin tube called the axon. It is covered in a particular type of protein called a myelin sheath that allows it to rapidly transmit an electrical signal. At one end of the cell, typically closer to the soma, is a complex branching network of tubes called dendrites. These look a little like the branches of a tree, in that there are many more tips than there are dendrites leaving the soma. The dendrites are covered with synapses, such that there can be many thousands of synapses on a single neuron. A synapse is the way neurons communicate with one another, a link between cells. They usually link to other neurons, but they can also link to other cells such as those that detect light in your retina. The axon also branches, and at the end of each branch is something called an axon terminal. These terminals form links to dendrites of other neurons via synapses. Each neuron can link to thousands of others, with dendrites of one cell forming synapses with axon terminals of another. The large number of synapses on each neuron allows neurons to create a vast network of interlinked cells, with there being about 600 trillion synapses in your brain. The network is fundamental to how you function, and to consciousness as well.
We have all become familiar with the concept of networks in recent years thanks to online platforms like Facebook and X. Each user of a social network platform forms a set of connections to their friends, acquaintances or people that interest them, and each of the people they connect with has their own set of connections to other folk. In scientific parlance each person is a node, and each connection is an edge. In the network of neurons in the brain, each neuron is a node, and each synapse is an edge. Social networks extend across the planet, and it is claimed that if you were to draw a network of the entire human population, you would find that any two random people are linked via six degrees of separation. In other words, I could be linked to you via five people who are acquainted with one another. I will know one of these people, you will know another, and the remaining three will complete the chain. There will be a few exceptions to this rule such as the isolated tribes on some of the Andaman Islands, but the six degrees of separation will likely be true for the readers of this book. The network of neurons in the brain is even more connected. There are estimated to be between three and four degrees of separation between neurons in the brain, which should give you a hint that the network in the brain is highly connected. It is also extremely large, given there are about fifteen times as many neurons in your brain than there are people on the planet.
Structure of Neuron Call
The social network analogy is useful in providing a little insight into how the brain works. Imagine there are six Instagram accounts devoted to describing what I am wearing and what I am up to. One account is entirely devoted to the shoes I wear each day, a second to my trousers, a third to my shirt, a fourth to where I am, a fifth to what I am doing, and a sixth to what I am saying. These accounts are so unbelievably dull that no one could follow all six without passing out from boredom, but let’s assume that each of my three children follows two accounts. Luke is a devotee of my footwear and trousers, Georgia is interested in my shirt and where I am, and Sophie keeps track of what I am doing and saying. Because I am not very good at keeping Sonya informed, she follows each of the children. On the rare occasions I have a garish new shirt, some snazzy new cowboy boots, am doing something interesting, or have said something of note, Sophie, Luke and Georgia share my posts and Sonya sees them. Sonya consequently acquires useful information from across the simple network and can then decide how to act accordingly, perhaps by suggesting the shirt does not match the boots.
Information flows through the vast network of neurons in the brain in a related way. To understand how this happens, information on how neurons work is helpful. Each neuron operates by receiving inputs from synapses that are then sent as electrical signals that travel along the dendrites to the cell body. Depending on the number of synapses that fire and the number of signals received from the dendrites, the soma may then send an electrical signal along the axon. When this electronic pulse, or action potential as it is referred to by neuroscientists, reaches the axon terminals, in most neurons it results in the cell releasing chemicals known as neurotransmitters. These chemicals bind to receptors on the dendrites on the cell that forms the other side of the synapse. If this is another neuron, the neurotransmitter may result in an electrical signal being sent along the dendrite on its way to the soma. If the cell on the other side of the synapse is a muscle cell, it may contract, or relax. In some cases, electrical ions are passed between axons and dendrites rather than neurotransmitters, but the result is the same – one cell sends a signal to the other. Synapses using electrical ions are faster than those that rely solely on neurotransmitters because electrical currents move faster than molecules of neurotransmitters, but the type of signal that can be transmitted is more restrictive, essentially being just a yes or a no, while a neurotransmitter can convey many degrees of maybe.
The light-sensing cells in your eyes each connect to multiple neurons called retinal ganglion cells. Each one of these neurons connects to about a hundred light-sensing cells, but each does so in a different configuration. Some neurons connect to light-sensing cells scattered across the retina, while others are connected to light-sensing cells in only a small part of it, with yet more connecting to columns or rows of light-sensing cells. The retinal ganglion cells form synapses with large numbers of other neurons, with these neurons receiving information about patterns of light intensity and colour, along with edges and shapes of objects. In turn these neurons form synapses with many others, and with each link a more specific picture of what you are looking at is generated. Eventually some neurons deeper in the network will be triggered that are responsible for letting you know you’re looking at a person, or an animal, or a tree, or a lake, or whatever. In one study, researchers identified a neuron in a subject that fired every time he saw a picture of Jennifer Aniston. If you know what the actress looks like, you too will have neurons like this. In fact, you may have several Jennifer Aniston neurons. You will have several neurons that can fire if they are looking at any familiar object.