There’s a question that has puzzled neuroscientists for generations. How can we store memories for a lifetime when proteins in our brain typically only last for minutes? We may now know the answer and it’s not what anybody expected!
How Does Memory Work In The Brain?
First, let’s take a look at how memories are formed.
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Types Of Memory
We have two main types of memory – short-term and long-term. Short-term memory is where you keep those bits of information that are temporarily important. Like that phone number you’re trying to remember before you write it down or enter it into your phone. Memories tend to last less than 30 seconds, although you can increase the time by rehearsing it. That’s why you keep repeating the number until you can find that pen or unlock your phone.
Short-term memory is fleeting and it can be interrupted. That’s why it’s fun to shout random numbers at someone who is counting!
When we talk about memory we usually mean long-term memory, however. These are the memories of learned responses, new sensations, our skills and habits as well as facts and experiences.
There are two types of long-term memory; Declarative (or Explicit) Memory and Non-Declarative (Implicit) memory.
Declarative memories are the facts and events that we remember. Like that great meal you had on your last holiday. Or why you might know Pythagoras’ theorem without knowing how to use it…
Non-declarative memories are the skills and habits that you learn. The stuff you can do on auto-pilot, like riding a bicycle or driving a car.
Both of these types of long-term memory involve different biological processes. Someone who is not able to make new long-term memories may be able to learn new skills. And vice versa.
Different types of memory reside in different areas of the brain. For example, the amygdala is central to motor memories. Whereas facts and events involve the medial temporal lobe, the thalamus and the hypothalamus.
Plasticity And The Brain
Plasticity is the key to memory. It is the process by which we learn new skills and can unlearn bad habits.
Remember the phrase about how the brain is like a sponge? (Guess where the name of this blog comes from?) It’s plasticity that allows us to soak up knowledge like that. Our brains are very plastic when we’re young, which is why kids are so good at learning. We front-load our education in our early years to take advantage of this fast learning.
We lose some of that plasticity as we age and we literally become set in our ways. Our habits become ingrained the more we do the same things day after day. But we never lose the ability to learn or to set down new memories.
Plasticity is the process by which the neurons in our brains make new connections and strengthens existing connections. It also describes the process by which they weaken and break connections.
Certain life-events can cause an increase in plasticity. When we fall in love or become parents, we generate oxytocin which helps to re-organise our brain. We need to unlearn some behaviours and to learn others. Being in a loving relationship or becoming a parent requires different skills to being single.
Where Are Memories Stored?
Your brain contains approximately 85 million brain cells called neurons. They consist of a nucleus surrounded by many branches known as dendrites. These dendrites are what the neuron uses to receive signals. The cell also has one long extension called the axon. This is what the neuron uses to send signals. At then end of the axon are more extensions, allowing the neuron to communicate with many others. Neurons never touch each other, the space between them is called a synapse.
When we experience or learn something new we process it via these nerve cells. Neurons listen to each other through their dendrites and they talk through their axons. They make new connections in the vast network of our brains.
I’m drinking a cup of mushroom soup while I write this. I love mushroom soup and have experienced it many times. Each time I have some, it strengthens a particular pathway of neurons that signify that this is what mushroom soup tastes like. I’m not likely to forget the taste!
Thinking about mushrooms soup gives me a nice satisfied feeling. That’s my brain attempting to recreate the memory of the soup by sending a signal through the very same pathway. The fact that I’m writing about it tells my brain that there is something special about this soup and it may need to be remembered. More neurons are firing, tying it into my other senses. In a year’s time the only thing that I might remember about this article is that I had some mushroom soup while writing it!
Do Memories Exist?
Believe it or not, there is a mind-blowing fact buried in that banal anecdote. Memories themselves do not physically exist in the brain! Our brains are different to computers and our memories are not like files and folders. They are more like maps in a very large maze. There are many possible paths and each unique path is used to encode information.
Basically, memories are just patterns in our mind. Pathways between cells. When something triggers a memory what actually happens is that signals pass between these different neurons in the same order. That pattern gets stronger the more we remember the memory. And a stronger pattern means a stronger memory, skill or habit.
Is Memory An Ancient Virus?
When we learn something new these pathways are created. When we recall these events or facts these pathways are reinforced. But how?
Each neuron on the pathway signals to the next neuron to make more neural receptors to “listen” to signals from it. Each neuron also increases the amount of neurotransmitters it releases to the next neuron in the line.
The details of how neurons communicate at a molecular level has confounded neuroscientists. Now, research from the University of Utah Health may have some answers, and it’s a big surprise!
The researchers focused on a protein called Arc, made by a gene (confusingly, also called Arc). Previous research showed that the Arc protein is vital for brain plasticity and learning. Mice without the Arc protein cannot form memories of events 24 hours earlier.
The surprise is how Arc transmits the information between brain cells. Arc travels to an end of the axon and forms a capsule-like structure that breaks away from one neuron and enters another. This method of information transfer looks remarkably similar to the way that retroviruses infect host cells! In fact, the virus that Arc most resembles is HIV!
The structure that Arc forms is called a capsid. Capsids are the vehicles by which viruses travel between cells. The viral genetic information is stored inside the capsid which then enters host cells and infects them. However, Arc capsids are triggered by the neuron itself and they exchange RNA between brain cells.
How Did Memory Evolve?
No one has seen this behaviour outside of viruses. But it leads to an intriguing question? Why do our brain cells communicate like viruses?
The researchers believe that it happened by a chance infection. Between 350-400 million years ago one of our ancestors got infected from an early form of retrovirus, called a retrotransposon. The retrotransporon inserted its genetic material into the host animal’s DNA. And that chance genetic alteration gave us the ability to remember…
I promised you that it would be mind-blowing!
This has opened a rich vein of potential research. No one suspected that memory might have come from an outside source and scientists are keen to explore the implications in depth.
For me, this shows how remarkably intertwined life is on this planet. Who knows how much of our DNA came from other life-forms? All life on this pale blue dot shares a rich and shared history.