Video: Learning and memory

One… olfactory nerve; two… optic nerve; three… trochlear nerve? No, three is oculomotor… ahhh, I'll never remember these cranial nerves.

Wouldn't it be great if we could read something once and remember it forever? Unfortunately, or fortunately, this is not how our brains work. Let's find out why, as we talk about… wait, what were we talking about? Oh, right -- learning and memory.

Our memories are a fundamental part of our self. They help us understand the world, so they shape how we behave. Memories are created as we experience the world around us and interact with it. We can think of learning as the acquisition of new information, skill, or behavior, and memory as the storage and retrieval of this knowledge.

Learning and memory are also interdependent: learning relies on memory to retain what's acquired and memory depends on learning to find content to store.

Memories can be classified into short- and long-term based on how long they're stored for. Our short-term memory can temporarily hold just a handful of pieces of information for seconds or minutes. This may not seem very useful or important, but short-term memory is fundamental for everyday living.

It would be impossible to have a conversation if we couldn't remember what the other person has just said. Finding our wallet would be hard if we couldn't remember where we've already searched. And we would never cross the street if as soon as we look to the left, we forget that there are no incoming cars from the right.

Luckily, our working memory circuits in the prefrontal cortex use the information stored in the short-term memory to help us with our everyday tasks.

Most short-term memories are simply forgotten, but those that are particularly relevant can be stored as long-term memories through a process called consolidation. These memories stay with us potentially for years, or even, for our whole life.

Long-term memories can be classified into explicit or implicit depending on the type of information they contain. Since explicit and implicit memory have different functions and different pathways, it's best if we discuss them separately. Let's start with explicit memory.

Explicit memories require conscious attention to be recalled. They are also called declarative because the information they contain can be described verbally. Explicit memories can be further divided into semantic and episodic.

Semantic memories recall facts, general pieces of knowledge that exist without a context, like knowing what a ball looks like, what it sounds like, and what it feels like to the touch. Episodic memories instead recall specific events, like receiving a new ball as a gift for your birthday.

Different brain structures are required to form, store, and retrieve explicit memories. The first step to understand these pathways was the discovery that removing the hippocampus and nearby structures for medical reasons made patients unable to form new explicit memories. However, they could still remember events from their youth.

Over time, this led to two key discoveries. First, that the hippocampus and nearby structures in the medial temporal lobe are important for the consolidation of explicit memories. Second, that long-term memories are not stored in the hippocampus itself, but instead are located in the sensory cortex that processed the information.

Memories of the shape and color of a ball are stored in the associative visual cortex in the occipital lobe, its sounds in the associative hearing cortex in the temporal lobe, and its texture and weight in the associative somatosensory cortex in the parietal lobe.

Two other brain regions help us manage explicit memories. The prefrontal cortex appears to be involved in both memory consolidation and retrieval. The amygdala facilitates the consolidation of memories associated with strong emotions. This is why we forget the hundreds of shots we perform when we practice, but we remember the one shot that made us win an important match.

Speaking of shooting a ball, the sequence of muscle activation required to perform skilled movements is an example of implicit memory. Implicit memories are automatic and require an action to be repeated over and over to be memorized. Implicit memories are also called non-declarative because they are difficult to describe in words as they don't relate to facts or events.

Driving a car, navigating a familiar place, and playing an instrument are all examples of implicit memories. If explicit memory helps us remember facts and events, our implicit memory mainly helps us remember how to do things. Yes, even our so-called muscle memory actually takes place in the brain.

As mentioned before, the structures needed for consolidation and storage differ between implicit and explicit memory. Patients without their hippocampus could still learn motor sequences and memorize their way around a building, suggesting that implicit memory consolidation does not require the hippocampus. Instead, the main structures involved in the consolidation and storage of motor skills are the cerebellum, the basal ganglia, and the motor cortex. The amygdala is also involved in implicit memories that evoke an emotional response.

We have learned that the consolidation of information from short- to long-term memory is crucial for storing knowledge for a long time. But how does our brain turn a quick thought into a lasting memory?

Much of the current evidence points to changes in how neurons synapse. Memories seem to be consolidated through a process called long-term potentiation, or LTP. You can picture it like strengthening a bridge between neurons. The more the traffic or signals that cross that bridge, the stronger it gets. This is the LTP that strengthens the memories so they stick around.

But what if a memory or a connection needs to fade? That's where long-term depression, or LTD, comes in. It's like letting that bridge become weaker over time when it's not used. This helps your brain clear out stuff you don't need to remember.

These mechanisms play a fundamental role in learning and memory consolidation, so let's explore in more detail how long-term potentiation works in the hippocampus.

Here we have a presynaptic and postsynaptic neuron. The glutamate released by the presynaptic neuron acts on two types of receptors on the postsynaptic neuron -- AMPA receptors and NMDA receptors. When the glutamate binds to the ligand-gated AMPA receptor, sodium ions flow into the cell. However, glutamate, binding to the NMDA receptor, is not enough to let ions through because a magnesium ion lodged inside the channel blocks access to the cytoplasm.

If the postsynaptic neuron becomes sufficiently depolarized by the sodium that enters through the AMPA receptors, the cell membrane depolarization removes the magnesium ion from the NMDA receptors. This increases the flow of sodium ions and also lets calcium ions into the cell.

The higher intracellular concentration of calcium ions activates a second messenger system that strengthens the synaptic connection between the neurons in three ways. First, new AMPA channels are moved to the cell membrane. This facilitates the flow of sodium ions through the cell membrane. Second, the conductance of the AMPA channels increases, which also facilitates the flow of sodium. Third, a chemical compound is sent to the presynaptic neuron, signaling to increase the amount of glutamate released.

Since now there are more neurotransmitters, more receptors, and the receptors are more effective, the connection between the two neurons is strengthened. This long-term potentiation mechanism was predominantly researched in the hippocampus, specifically in the Schaffer collateral neurons. Scientists believe that similar forms of long-term potentiation may be responsible for memory consolidation in different areas of the brain and for implicit memories, even if the specific cellular pathways may be different. Both the ability to remember and the ability to forget are key to our well-being.

To conclude this tutorial, let's see what happens when these functions are compromised.

A loss of memory is called amnesia, and it is generally caused by traumatic brain injuries or neurodegenerative diseases. Impairment of the memory consolidation areas leads to an inability to form new memories, or anterograde amnesia. Retrograde amnesia instead refers to the loss of memories that were already stored before the onset of the amnesia and is often associated with damage to the brain areas involved in memory storage or retrieval.

A slowing of memory functions is common in older age. But in people with neurodegenerative disorders, memory loss becomes debilitating to the point that the person needs continuous assistance. The most common of these disorders is Alzheimer's disease, where memory loss is due to the accumulation of proteins in the brain.

And that concludes our tutorial on learning and memory. Can you imagine how much long-term potentiation is happening in your hippocampal neurons right now to make sure that all this new knowledge is consolidated in time for your next exam? Still, don't forget to check out the quiz and other learning materials in the study unit on this topic.

See you next time!