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The Biology of Learning |
I guess I should apologize before I start. This article has a lot of terms and technical stuff that might leave you saying "Well why is that important?" You must admit however, that it is quite fascinating that we have come to understand just how our brains process and learn. With this perspective we are certainly better able to teach. But I do understand if you're now saying, "Thanks, but no thanks", and "My interest is in learning but not on a molecular level".
The biology of learning, of course, deals with the most essential unit of our nervous system, "the Neuron", and moving to the end of each neuron "the Synapse" or the place where information will electrochemically transfer from one neuron to the next. At birth, each of our neurons have about 2500 synapses or connections, which grow dramatically in number to around 15,000 connections per neuron by age two or three. As we age many of these connections are lost in a process called synaptic pruning. It's always a plus when they come up with a term that tells us exactly what the thing itself does. In this case, prune down, as in the branches of a tree, the synaptic connections to around 10,000 per neuron for the rest of our adult lives. Not bad when you consider our nervous system contains around 100 billion neurons (give or take one or two).
Now what is not true, yet widely believed, is that we never grow new neurons or new synapses. We do! In fact this is how many neurobiologists believed we obtained new learning. But there is a "Big But" in this last statement, because although we do have new neuron and synaptic grow, it really doesn't account for new learning. What does is actually the strengthening of synaptic connections within our nervous system.
Ah ha, now that I have you captivated by this little mysterious tidbit, you're asking "how indeed, are synaptic connections made stronger?" Well, yes indeed, you guessed it, I am about to tell you. But first a bit about how a message travels through the neuron in the first place. For this I plan on using multiple metaphors, as a learning technique that, as a matter of fact, works because of strong synaptic connections that you have made already.
Now first of all, neurons work hard to stay at rest. It's like your mom used to tell you, "It's hard to get a moment's rest with all you kids running around". Mom has to put the kids to bed before she can get any rest. A neuron works to align electrically charged ions (potassium, sodium, chloride ions) inside and outside of the cell's membrane to get to what is termed a "resting potential".
Now I mentioned that each neuron can have up to 10,000 synapses, so it makes sense that at one end of each neuron are these branch-like structures called dendrites (each little branch is referenced as a dendritic spine). The dendrites are like the ears of the neuron, ready to accept little wavelets of excitation from the receptors in your eyes, nose, ears, and skin. But one little excitation does not a message make. In fact, for a neuron to pass on the message, a dendritic spine would have to be excited multiple times (let's say 50 times per second) which is called temporal summation or many of the dendritic spines on the neuron would need to be stimulated at once which is called spatial summation. All of this summing happens in the cell body of the neuron and then is passed to a sort of gateway of the neuron called the axon hillock. Now picture this old curmudgeon running a roller coaster in an amusement park. His rule, "The roller coaster ride does not start until ALL the cars are filled." Now we may have a line of five excited kids wanting to ride, but this old axon hillock is not going to pull the switch until ALL the cars are filled. But once enough excited kids pile on, it's "Katie bar the door." The switch is pulled (this event is called an action potential) and the message is sent screaming down the track, which in this marvelous metaphor is the part of the neuron known as its axon. Now some of these axons can be quite long. Picture a roller coaster track running from your toe all the way up to your brain. Typically the axons in our central nervous system (the brain itself) are tiny, but some of the axons in the peripheral nervous system (other neurons or nerves throughout our bodies) can be quite long. A blue whale has neurons that travel for almost 60 feet. Because of this we have this great insulator called the myelin sheath that covers the axon and works to sort of grease the tracks allowing our messages to travel at lightening speed all the way to the end of the line, which is conveniently called the axon terminal. Now you'll have to admit I've gotten a lot of mileage out of this roller coast metaphor, some might say too much. Anyway, here at the axon terminal is where we go from Magic Mountain to a water park ride. Remember I said that the neuron's messages are electrochemical. Small vesicles, in the axon terminal (also called pre-synaptic terminal), reacting to this powerful wave of electrical energy, move to a cell membrane at the axon terminal and release chemicals which are called neurotransmitters.
Neurotransmitters are actually simple amino acids. They include household names like dopamine, serotonin, norepinephrine, and glycine. They do different things for different neurons located in different nuclei (functional entities) of the brain. For example, dopamine in one part of the brain effects fluid voluntary movement (this location in the brain called the substantia nigra, where the lack of dopamine production results in the second most devastating neurological disease, Parkinson's Disease). In another part of the brain logical, linear thinking, and in yet another part of the brain it is involved with addiction and our pleasure/reward systems. I mentioned that these neurotransmitters are biosynthetically simple. They need to be because they are always in demand, quickly used and just as quickly recycled, or excreted from our bodies. I also mentioned a water park ride, and this has to do with where these neurotransmitters are released. They need to actually float across something called the synaptic cleft. For years neuroscientists believed that messages travel from one neuron, connected to the next (like an actual neural NET). Not so, and the chasm between the axon terminal of one neuron, and the dendritic spine of the next neuron is the synaptic cleft, and where neurotransmitters must "swim" (thus my reference to the water park ride) in order to link up with receptors found in the dendritic spine. They actually seek out these receptors and in a "lock and key" sort of way (a famous neuroscience metaphor) combine to begin the process anew. In this way the message continues on. Eventually the neurotransmitters are either recycled (reuptake) back into the vesicles in the pre-synaptic terminal or dissolved and removed as waste from our bodies. Cocaine is a drug that works by blocking the recycle or reuptake process for the neurotransmitter dopamine. This allows dopamine to double-back, if you will, and continue to bind with receptors, sending messages to our brains of euphoria and of things that are not really there, something called a hallucination. You've got to be getting a little interested now!
Okay, but you're saying, "Gary, when you started this whole thing, you said we were about to find out about the biology of learning. Instead you described how my body knows when I get stuck with a pin, or hears a loud noise or sees a flash of light. What about better information retention, and how these synapses are strengthened? What about all that you've said above tells me anything about how we, as individuals, LEARN?" (I placed the word "learn" in caps to emphasize how annoyed you were becoming at me). You're correct (I defend), above we just described how information travels from neuron to neuron but it will also serve to help explain how we learn. A fellow by the name of Donald Hebb (July 1904 – August 1985), a Canadian psychologist (don't hold this against him), developed theory that explains how synapses are strengthened, how neurons can network, and how we learn. For this, he is venerated as the Father of Neuropsychology.
We understand that learning and memory take place in the cortex (thin covering , Latin for "Bark") and hippocampus of our brain (named for its shape and Latin for "seahorse"). We know this from studying diseases like Alzheimer's disease, and some pretty amazing medical cases. One such case is that of HM, who suffered from unrelenting epileptic seizures which resulted in his hippocampus (both R & L hemisphere) being surgically removed. Remarkably, HM lived, but was left with a 30 second memory. Although HM had and continues to have an average IQ (118) he cannot learn anything new. Basically you can meet HM, talk with him for a time, leave, and re-enter the room, and HM will not have a clue as to who you are! Many folks are like that after they have met me as well.
Remember I said that different neurotransmitters work differently depending upon where they are operating in our brains. Well when it comes to learning, glutamate is the neurotransmitter of choice (recommended by 3 out of 4 neurophysiologists – just kidding). It is the most excitable neurotransmitter known to man, and it has what could be called a dual receptor system. It combines with two types of receptors at the post synaptic dendrite.
To begin, neurons in the brain's hippocampus and the cortex release glutamate which goes on the water park ride, across the synaptic cleft, to bind with receptors in the post synaptic dendrite. Glutamate binds to the first type of receptor but nothing happens with receptor type 2. Add more glutamate, another small wavelet of excitation occurs in first receptor type, but zero in receptor type 2… However introduce enough glutamate, and you get a tidal wave of excitation on the second receptor and now we are cooking! The synapse is transformed, becoming highly receptive, and has been, as we say in the biz, long term potentiated. In essence, the instructor's lecture drones on, you're bored, murmuring "please kill me now". Then … "Hey, what's that he said, that's interesting", and you listen up. "Wow, really!", you listen some more, "HMM, can't be, doesn't make sense, OH MY GOODNESS, HE'S RIGHT, THAT'S AMAZING!!!!". Right there is where the explosion occurs, the AH HA effect, and to your astonishment you are a victim of the bane of humankind; learning. When a synapse is said to undergo long term potentiation, it not only has more receptors, but they remain active for a longer period of time. For neurons in the brain where this occurs, synapses are strengthened, and fire off more easily. This, in a much over simplified explanation is, as promised, the biology of learning. |
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