Long term potentiation and synaptic plasticity
How do you learn? This is a tougher question than it seems. You might know that you learn by studying, for example, but what changes occur in your brain when you read a chapter for the first time or when you test yourself on your knowledge? You probably realize that your brain doesn't just grow new cells to store your memories, otherwise our heads would just get bigger and bigger. What does happen is that connections between neurons strengthen. This is called long-term potentiation, or LTP and it's one example of synaptic plasticity, which is the ability of synapses to change their strength. Let's look at this process in more detail. Neurons communicate using electrochemical signals, that means a combination of electricity and chemicals. When neurons communicate, they actually don't connect to each other directly, there's a junction in between called the synapse. So we have a pre-synaptic neuron, which is a neuron leading up to the synapse, and we have a post-synaptic neuron, which is the neuron immediately following the synapse. When the pre-synaptic neuron is stimulated, it releases neurotransmitters, or special chemicals which then bind to receptors on the post-synaptic neuron, opening channels to allow ions such as sodium and calcium to flow in. All neurons have a surrounding membrane and the interior has a different electrical charge than the exterior of the membrane. This difference in charges is called the neuron's potential. The number of ions that flow into the post-synaptic neuron affect that difference. This is how we measure the strength of the synapse, by how much the post-synaptic neuron's potential changes as a result of pre-synaptic stimulation. With repeated stimulation, the same level of pre-synaptic stimulation converts into <i>greater</i> post-synaptic potential. In other words, after the pre-synaptic neuron gets a lot of practice firing and sending signals to a specific post-synaptic neuron, it gets <i>better</i> at sending those signals, and will get better at opening the channels to allow more ions in. When this happens, we say that the synapse strength is increasing. When this increased strength lasts for a long time, meaning anywhere from a few minutes to many months, it's called long term potentiation. This is thought to be the physiological mechanism by which learning occurs. As synapses are strengthened and they retain that strength, we're able to more easily recall previous experiences.