Memories Are Made of What? Part 2

In my last post I began describing how memory or learning might occur in the brain of a mammal, as described by Dennis Bray in his wonderful book, Wetware. 

         It seems that learning and memory are somehow encoded in synapses between neurons.  The more often a synapse is used, the stronger and more developed it becomes.  But this strengthening depends on neuron A sending a signal to neuron B at the same moment that neuron B is also sending a signal.

         Here’s how the chemistry may work.  When A sends a signal to B, B may not be aware of the signal because its receptors are plugged up.  But if B is also signaling, B’s electrical change may unplug its receptors.  Then it can receive the signal from A.

         If the signal from A to B is repeated, a protein called Cam II kinase starts changing itself in permanent ways, so that the synapse between A and B becomes permanently stronger and more sensitive.  The changed protein is “…like a toggle switch that, once thrown, remains on forever.”  This switch then turns neuron B on at the slightest twitch from neuron A.

         But how is it possible for this situation to last a lifetime?  For this is how long many memories and much learning do last.  More on this next time.



Memories Are Made of What? Part 1

How do we learn?  How do we remember?  How can a mass of neurons in the brain produce bodies of knowledge from our experiences?

         Recently I came across an excellent description of what we know about this in Dennis Bray’s wonderful book, Wetware, which I have referred to a number of times in this blog.

         Neurons communicate with other neurons via synapse.  A synapse is a tiny gap (cleft) between the sending part of one neuron (the axon) and a receiving part of another neuron (a dendrite or cell body).  When an electrical signal travels to the end of an axon, it causes the axon to spill out molecules of neurotransmitter.  The neurotransmitter molecules spill into the tiny synaptic cleft.  Then they diffuse across the synaptic cleft and lodge in receptors on the receiving neuron. 

         So far, all we have is a signal from neuron A to neuron B.  But it turns out that if neuron B is simultaneously sending a signal (say to neuron C or D), the synapse between A and B will be strengthened.

         Bray’s example here is the Pavlov experiment with dogs.  The sound of the bell makes neuron A fire a signal so that the dog hears the bell.  The smell of food makes neuron B fire a signal so that the dog salivates.  The strengthened synapse results in the dog salivating when it merely hears the bell.

         How does this synapse strengthening happen?  More on this in my next post.



Dopamine and Eating Disorders: Obesity

Judging by popular beliefs about obesity, we might think weight and eating, like drug use, are also controlled by dopamine, but this is only partly true.

         We have a built-in “weight thermostat.”  Our bodies store energy in the form of fat.  When our fat cells are full, they secrete the hormone “leptin” and we feel less hungry. 

         Also, when we lack nutrients, our brain secretes “orexin” to make us feel hungry.  When we eat and have plenty of circulating nutrients, the brain secretes “CRH” to make us feel full.

         However, eating results in higher levels of dopamine as well.  So the news isn’t completely clear when it comes to dopamine and obesity.

         Genetic components of weight also include sensitivity to all these signals.  We may have more or fewer receptors for the signals, or more or less sensitive receptors.

         So weight regulation is more complex than drug addiction.  We don’t “need” drugs and alcohol, so leaving them out of our lives is a more straightforward cure.  But we do need to eat, so it’s not surprising to discover that regulation of eating is complicated.   



Dopamine and Eating Disorders: Anorexia

Recently, I’ve been posting about dopamine and addictions.  Since dopamine is a “feel good” neurotransmitter, and has connections to food addiction, I’ve been wondering what it has to do with anorexia.

         Research on eating disorders is really in its infancy.  But there seem to be connections to dopamine.  Apparently, people who suffer from anorexia have an unusual reaction to this substance.  When dopamine flows in their brains, instead of feeling pleasure, they feel anxiety and fear.

         Not only do anorexics feel anxiety when they eat, they feel anxiety and worry about other things as well.  Some of their fears are similar to what happens in the brains of people suffering from obsessive/compulsive disorder.  The lives of anorexics are unpleasant and scary, to say the least.

         There are so many myths about anorexia, it’s a relief to realize that anorexia is very much a product of the anorexic’s brain chemistry.  But there is clearly a great deal more to be learned about eating disorders in general and anorexia in particular. 

         Stay tuned 

Dopamine, Learning, and Addiction

Yet more about brain chemistry and addiction:  A few years ago, the Harvard Mental Health Letter pointed out that the dopamine link is important with respect to learning.

         That is, when we do something pleasurable, the neurotransmitter dopamine is released in our brain and tells us to repeat the experience. 

         But dopamine is also involved in parts of the brain having to do with forming memories and learning.  So not only do we get the message to repeat the pleasurable experience, we also truly “learn” to repeat it.  Hence, we may become addicted.

         Of course, from the standpoint of evolution, we need to feel pleasure in learning, because learning was and is essential to our survival.  And we still enjoy learning new things, whether they are in in academics or entertainment or gossip.  I know I get a kick out of learning about the connection between dopamine and addiction—it explains a lot.

         Dopamine apparently is part of becoming addicted to alcohol, drugs, gambling, food, thrill seeking, etc.

         But ironically, dopamine also rewards use of recovery techniques, especially in twelve-step programs.  Recovering addicts experience the pleasures of spiritual practice and fellowship.  Clearly, we have a lot to learn about dopamine, the human brain, neurotransmitters, and who knows what else?