The Bedrock Theory of Memory: How Memories are Formed, where they are Stored

Introduction

Learning and memory are two of the most remarkable faculties of our mind. Learning is the biological process of acquiring new knowledge about the world, and memory is the process of retaining, reconstructing and accessing that knowledge over time. Ask most people where in their body memory resides and they’re most likely to look at you as if to say, “What a dumb question! It’s in the brain, of course.”

So, let’s take a deep dive into the human brain and see where memories are stored.

Meet Your Brain

The average adult brain weighs about 3 pounds. It is made up of around 75 per cent water. The brain consists of roughly 100 billion neurons, as many as the stars in our galaxy, embedded in a scaffolding of a 100 billion glial cells. Each neuron may have 1,000 - 10,000 synapses (connections with other neurons). The most active period of neuron proliferation takes place during the middle of the second trimester, when 250,000 neurons are created every minute.

Early experiences have decisive impact on the architecture of the brain, and on the nature and extent of adult capacities. Brain development is non-linear: there are prime times for acquiring different kinds of knowledge and skills. By the time children reach age three, their brains are twice as active as that of their pediatrician. But be warned: if you have young children, brain activity levels drop during adolescence.

The basic unit of the Central Nervous System is the neuron or nerve cell. Each neuron has several thousand dendrites – up to 10,000 – tiny hair like strands of tissue that receive signals and one axon, a more robust structure through which the neuron sends signals to other cells. Neurons function in networks.

Neurons do not actually touch. Each axon produces about 160 different neurotransmitters that cross a miniscule gap, the synapse to insert themselves in the receptors of dendrites that are structured to receive a particular neurotransmitter and no other. Sort of like a space shuttle docking to a space station.

There is good evidence that specific neurotransmitters such as epinephrine, dopamine, serotonin, glutamate, and acetylcholine are involved in the formation of memory. Although we don’t yet know which role each neurotransmitter plays in memory, we do know that communication among neurons by way of neurotransmitters is critical for developing new memories.

It is also believed that strong emotions trigger the formation of lasting memories, and weaker emotional experiences form weaker memories; this is called arousal theory.

Neurons comprise only 15 per cent of the brain. The other 85 per cent is made up of glial cells. Glial cells continue to grow in number until a few years after birth. They guide early brain development and keep the neurons healthy throughout life. Glial cells provide the scaffolding for neurons and as the origin of their name implies (Greek for glue) they help to keep the neurons together. Glial cells can affect the functioning of neurons even though they cannot discharge electrical impulses of their own.

Human neurons are very similar to those of other animals, right down to using the same neurotransmitters. But as one compares the brains of animals ascending the evolutionary tree, one sees that the higher you climb, the more non-neuronal glial cells these animals’ brains contain in proportion to the number of neurons. For years glial cells were dismissed as mere putty. Actually, Glial cells control communication between neurons and play a central role in learning.

The Bedrock Theory of Learning and Memory

The accepted scientific hypothesis regarding memory is what I call “The Bedrock Theory of Learning and Memory”. According to this theory, incoming signals from our sense organs initiate the production of specific proteins in neurons that make their synapses grow stronger. These proteins not only build up the synapse but also encode memories. Just like physical exercise leads to greater muscle mass through the production of new proteins, so experience builds memories in synapses, potentially whole neural networks in an ever-changing plastic brain. Short-term memory is linked to functional changes in existing synapses, while long-term memory is associated with a change in the number of synaptic connections and strengthening of the brain’s existing circuitry.

Eric Kandel, who shared the Nobel Prize in the year 2000 with Arvid Carlson and Paul Greengard for “their discoveries concerning signal transduction in the nervous system” and is professor of biochemistry and biophysics at Columbia University conducted his studies on the marine snail, Aplysia which has only about 20,000 nerve cells compared with about a 100 billion in the human brain. The snail has a simple reflex by which it protects its gills and Kandel used that reflex to study how the snail learnt and remembered stimuli. He showed that short-term memory involves increased levels of neurotransmitters at the synapses and long-term memory requires changes in the levels of proteins in the synapse.

After learning how these simple animals functioned, he then experimented on mice. This work helped him understand how the same processes that occurred in nerve cells of slugs could be seen in mammals, which includes humans.

Kandel concluded that the basic building block of memory is the synapse, where both pre- and postsynaptic elements together with associated glial processes form an integral unit with an individual identity and distinct “neighborhood.” The increase in connectivity strength within a diffuse group of cells in a more complex feed forward circuit results in the emergence of an engram (engrams are complex stored memories, within a cell assembly.

Eric Kandel, declared in 2006 that “in the study of memory storage, we are now at the foothills of a great mountain range… To cross the threshold from where we are to where we want to be, major conceptual shifts must take place.” I totally agree. As we shall see in the next part of this paper, it is time that Kandel’s reigning neuroscientific view of memory storage is modified.

Key Takeaways

 Kandel concluded that the basic building block of memory is the synapse.

 Kandel proposed that the increase in connectivity strength within a diffuse group of cells in a more complex feed forward circuit results in the emergence of an engram (engrams are complex stored memories, within a cell assembly).

 In the next part of this paper, I shall show that the synapse acts as a conduit to neurons not as a depository for memories.

References

Bruel-Jungerman, E., Davis, S., and Laroche, S. (2007). Brain plasticity mechanisms and memory: a party of four. Neuroscientist 13, 492–505.

Kandel, Eric R. (2002). The Molecular Biology of Memory Storage: A Dialog Between Genes and Synapses. Bioscience Reports. V. 21, No. 5. Plenum Publishing Corporation p. 567.

Kandel, E. R. (2007). In search of memory: The emergence of a new science of mind. WW Norton & Company.