If there’s one thing that fascinates me most, it’s the brain. How such a drab pinkish-grey organ can achieve so much—the perception and interpretation of everything around us—is amazing. And as an aspiring psychologist soon to complete my Bachelor’s degree in Psychology and (hopefully) go on to graduate school, I happen to think that there is much we can learn about the brain. But of course, because the brain is a complex organ that even scientists do not fully understand yet, many people know very little about how it functions. In addition, the brain is sometimes used as the “magic bullet” of intelligent design. Some say, “The brain is too complex to have evolved by chance!” Others claim, “Science cannot explain consciousness, which suggests that it is a non-physical process that can only be explained by the existence of a soul.” Truly, the brain is shrouded in mystery, which allows for all manner of conjectures about what its “ultimate purpose” is.
My goal in this series is to explain, as simply as possible (in the terms of a layperson, in other words), how the brain functions and how it could have developed through a gradual process of evolution. This is an ambitious goal, but I think that it is important that even non-scientists have at least a basic understanding about how their own brain works. So, this will be a bit of “science for the common people.” I am not an expert in this subject myself, though I have taken many courses on the subject, so hopefully I can bridge the gap and convey in simple terms how the brain functions. Obviously, there will be much more that can be said on the topic, but those who are interested can find any number of resources to learn more. I only offer this series as a first step. So come on with me, and let’s explore the brain!
The Humble Neuron
The first thing one might ask about the brain is, “What is it made of?” It, like everything else in your body, is made up of cells. But these are a specific type of cells, called neurons. Neurons are cells that, through the awesomeness of chemical processes, can create an electrical signal. In this sense, they’re a little bit like electrical wires. When you connect a wire between two devices, the electrical signal flows from one to the other. And that’s it. Literally, for everything that your brain controls, from perception to motor control to memory to planning to learning to regulating digestion and more, it does so using simple electrical signals flowing from one end of neurons to the other end. A signal is received at the dendrite of the cell, is relayed down the axon, and then is passed on to the next neuron(s) through the terminal buttons. The trick comes in how the neurons are connected. With about 50-100 billion neurons in the human brain, there is plenty of room for connections between them. Each connection with another neuron is called a synapse, and it is estimated that the human brain has as many as 1000 trillion of these, with neurons connected to multiple other neurons. This is where the real power of the brain comes from. These myriad connections allow the brain to send strong and weak signals, feedback loops, divergent and convergent pathways, fast and slow responses, and other neat little tricks. Let’s take a look at a couple of those tricks that help the brain to function.
When a neuron “fires”—sends an electrical impulse down its axon—it sends it to each of the neurons to which it is connected. Then, the signal is passed on from the second neuron to the third neuron, etc. However, wouldn’t that just lead to a chain reaction with neurons firing all the time? It would, except that not all neurons will cause the next neuron in line to fire. We have to know a little bit about excitatory and inhibitory action potentials.
As I’ve mentioned, each neuron sends its message to several other neurons. However, each neuron also receives messages from several neurons as well. So when a neuron receives a number of signals from several neurons, what does it do? Well, some neurons send on excitatory signals, and others send inhibitory ones. If a neuron receives enough excitatory signals, it will itself fire. If a neuron receives inhibitory signals that outweigh the excitatory ones, though, it won’t fire. This is a simple decision-making process. Of course, when I say “decision-making”, I don’t mean some sort of conscious process of deliberation. It is just a mechanistic process. Each neuron needs to reach a certain amount of excitation before it will fire. If it reaches this amount, it fires. If it doesn’t, it doesn’t fire. But the addition of excitatory signals and the subtraction of inhibitory signals allows for complex calculations to take place using just the simple mechanisms present in each neuron. As each neuron “decides” whether or not to fire, it influences other neurons to fire or not, and so on. This allows some parts of the brain to tell other parts what to do and when. What emerges is a complex organization that includes different modules of the brain to work together.
The Learning Process
Neurons engage in a very simple and mechanistic form of “learning” known as long-term potentiation. To say that neurons can learn is a bit of a misnomer—when we think of learning, we think of gaining knowledge of some sort. But neurons have the ability to adapt over time based on what is happening frequently. The process is simple, and can be summarized by the phrase, “Cells that fire together, wire together.”
As an example, let’s imagine two neurons, one connected to the other via a synapse. Let’s say the first cell receives input from the eyes and detects a certain shape.1 The animal that these neurons are inside of is walking around and currently in the presence of this particular shape. So, the first neuron starts firing. The second neuron receives the message and may be responsible for some other decision about this shape. As the first neuron fires, if the second one does as well, this strengthens the association between the two of them. If the animal continues to be in the presence of this shape for a long time (let’s imagine that it’s an animal that lives in a forest, with lots of “tree shapes”), the strength of the connection between the two neurons will grow. The second neuron will give more weight to the signals it receives from this first neuron, as opposed to perhaps several other neurons that are also connected to it. If the organism then leaves the presence of the shape, over time the connection will weaken.
This process right here is one of the most important parts of brain adaptation. Neuronal synapses are constantly gaining or losing strength based on what’s going on inside and around the brain. For instance, when you learn that neurons are like electrical wires, the connections in your brain between what you know about cells (neurons) and what you know about wires grow in strength just a little bit. When you remember seeing the Eiffel Tower in Paris, the connections between your memories and your concept about large metallic buildings grow in strength. When you learn a new instrument, the connections between the neurons responsible for moving your fingers and those for reading notes on a page grow in strength. Similarly, when you stop playing that instrument for years, the connections lose some of their strength, and you might be a bit rusty when you pick it up again to play. In short, long-term potentiation is one of the most critical elements in how organisms (and the brains that control them) adapt to changing circumstances.
The Bigger Picture
But let’s move on up to a little bit broader picture of what the brain is like. I want to describe, just briefly, the different parts of the brain. Each part is made up of millions or billions of neurons, tightly packed together and connected to each other.
The brain is divided up into two hemispheres, known as the left and right hemisphere. It also has four major “lobes”: the frontal lobe, the temporal lobe, the parietal lobe, and the occipital lobe. Each of these parts, broadly speaking, has different functions. The frontal lobe is responsible for planning, reasoning, problem-solving, and goal-directed behaviour. The temporal lobe is associated with perception of sounds, memory, and speech. The parietal lobe is associated with movement, recognition of objects, and orientation. Finally, the occipital lobe is associated with visual processing. Please note that this divisions are only true in a general sense, but there are plenty of connections between each of these divisions.2 Beyond these lobes, collectively known as the cerebral cortex, the brain also includes many other areas involved in memory, motor control, sensory input and output, hormone balance, and a whole host of other functions that I simply don’t have the time or space to explain. Needless to say, the brain is a complex organ.
However, I’d like to take a special look into an area of the frontal lobe called the prefrontal cortex. This area, located right behind your forehead, handles many of the functions that humans hold dear, like the planning of complex behaviours, goal-setting, personality, and decision-making. While none of these are truly unique to humans, we have a very large capacity for them, due to the large size of our prefrontal cortex. I’ll talk about our comparison to other animals a little more later. But for now, I’d just like to show that this is a very important part of the brain, because it coordinates many other brain functions according to our goals and plans. Individuals who incur damage to this part of the brain become impulsive and indulgent, often experiencing great shifts in personality and a difficulty planning ahead. (If you want to read a remarkable story of someone getting a giant metal rod shot through this part of his brain, read the story of Phineas Gage.) This part of the brain is responsible for many of the things that we consider to be “human nature.”
There is clearly much more that could be said about the brain. But this is only intended to be a very brief summary of some of the important areas that I will be bringing up later. If we want to understand how (or if) the brain evolved, we have to understand at least to some extent how it works. At its heart, despite its extraordinary complexity, the brain is composed of organized groups of neurons that fire based on certain criteria. Like a massive network of telephone wires, calls are sent and received constantly in order to coordinate complex behaviours. It’s important to realize, though, that the abilities of the brain rely on nothing more than billions and billions of copies of essentially the same type of cell. Each neuron functions in virtually an identical fashion.3 What really makes the brain incredible is not the size of the network, but the adaptability of it, shown through processes like long-term potentiation. And although this process seems fairly simple and straight-forward, the compounded effect of billions of neurons each using this process allows the brain to perform the amazing feats that it does.
In Part 2, I will discuss the evolutionary development of the brain, and how it could have arisen through the processes of natural selection. After that, in Part 3, I will discuss the phenomena present in consciousness and free will, and whether these can really be produced by brain functions alone.
- This example is quite simplified; in reality, clusters of neurons work together to detect colours, shapes, sizes, etc. [↩]
- For instance, images from the eye are first sent to the occipital lobe, where they are processed for shapes and colours, but then they are sent to the parietal lobe for processing about locations of objects in space, and to the temporal lobe for processing of object recognition. [↩]
- Neurons do, of course, use different neurotransmitters, but the way these chemicals are used is also virtually identical: They are sent from one end of the synaptic gap to the other, and open or close receptors on the receiving neuron. [↩]