There are numerous ways our brain “learns” and one of the main ways, and most common, is called long term potentiation (LTP).
What happens is in certain areas of the brain glutamate (the CNS’s main excitatory neurotransmitter) gets released in response to some stimulus. Glutamate binds to an AMPA receptor in the post synaptic neuron which allows sodium to flood the neuron and fire an action potential which proceeds to send the signal for the initial stimulus. This could be anything from hearing a noise or smelling a flower to thinking about an event or trying to memorize a phone number.
When the learning occurs is when that initial stimulus fires over and over again causing a lot of Glutamate to be released and lots of post synaptic binding to AMPA receptors occurs resulting in a large amount of sodium to enter the post synaptic neuron.
Within this post synaptic neuron there are also NMDA receptors. NMDA receptors are both ligand gated and voltage dependent receptors. The NMDA receptors have a magnesium ion lodged in the middle of it. When there is enough sodium in the post synaptic neuron the magnesium ion gets repelled outside of the neuron allowing calcium to enter the post synaptic neuron.
Once in the cell, calcium induces activity in enzymes in the cell. Calcium activates calmodulin which activates CAM-KII, a kinase which will phosphorylate a dendritic vesicle. This dendritic vesicle has an AMPA receptor lodged in the membrane waiting to be activated. The phosphorylation activates it and the vesicle places the AMPA receptor into the post synaptic neuron’s membrane.
Now the post synaptic neuron has more receptors making it easier for the neuron to fire. This is also known as learning.
When it comes to drugs of abuse, this pathway has a very important role. An example could be that this pathway is in the reward center of the brain. The initial stimulus could be a cigarette. This ultimately releases dopamine into the nucleus accumbens. The person really liked that feeling so they start smoking a lot more causing that over stimulation of the neuron making the post synaptic neuron “better” at firing.
Now that learning has occurred, it only takes the smell of a cigarette to trigger this action potential to get the same feeling as one cigarette took the first time. This is an example of craving.
LTP is a very important CNS pathway whether it is for memorizing a fact for an exam or causing craving troubles in an addicted person.
I think it is one of the most interesting topics because scientists have found a way to literally see dendritic growth from LTP meaning if the growth was removed, the memory would be lost. This could be incredible for future treatments for certain disorders such as PTSD.
Artstract:
Image citations:
http://thebluediamondgallery.com/l/learning.html
http://www.nature.com/articles/srep01957
LTP and how learning effects abuse potential
