Neuroplasticity refers to the brain's ability to remould and restructure itself in response to internal and external environments. In this respect it has similarities with epigenetics and this is one of the ways that neuroplasticity works. The idea that the adult brain can change after childhood development is a relatively new concept in science. Until very recently most scientists believed that it didn't occur to a large extent, if any at all. However, it has been proven to occur through brain scans of patients undergoing psychotherapy and also after undertaking meditation exercises regularly for a couple of months (which is how I came to find out about it). While I'll be focusing on the positive aspects of neuroplasticity it is important to note that it can have negative consequences as well. For example, if you continually think that you are incapable of performing a certain action you will start to believe it and it will cause limitation in your life.
How neuroplasticity works is by changes in connections between nerve cells in the brain. Nerve cells (or neurons) form connections with each other depending on the timing of their impulse firing. If two close neurons fire simultaneously they will be naturally inclined to link together and form a connection. The three ways of explaining this process are the following: "Neurons that fire together, wire together", and "Neurons that are out of sync, fail to link" (so if two neurons are not simultaneously excited, they will not become more assosciated with each other). Neurons will connect with each other due to the expression of their genes. This is where epigenetics come in, when certain genes are activated by internal or external environments they will change their expression and guide the axons (nerve fibres that extend from the cell body of nerve cells) towards other neurons. The good thing about neuroplasticity is that we aren't required to experience some profound event in order to change our brains, just thinking differently causes our brains to form different connections between its nerve cells.
One of the foundational principles of neuroplasticity is that you either "use or lose" various connections. As the brain is constantly adapting to its environment it is also constantly breaking old connections and forming new connections. It is estimated that we have 100 billion nerve cells in our brains and each is capable of making 10,000 connections with other nerve cells. With so many potential connections that can be formed, the possibilities of brain plasticity are vast.
Nerve cells form connections at structures called synapses. At synapses, nerve cells release specific molecules called neurotransmitters. Due to the effect of experiences on reforming these synaptic connections even identical twins will have different brain structures.
Pathways of nerve cells are defined by the strength of the synaptic connections, these connections become stronger each time they are used, similar to how muscles are affected by working out at the gym. It is believed that we are forming and recreating 1 million synaptic connections each second.
It is worth noting that although neuroplasticity is slowed by aging, it isn't stopped. Neuroplasticity occurs in your brain throughout your entire life. The above information refers to synaptogenesis, i.e. the creation of new synaptic connections between neurons.
However, neuroplasticity occurs in another recently discovered way; neurogenesis.