Much of psychology includes the observation of behavior, however what triggers the behavior observed? Everything from the decisions we make, our personalities, movement and emotions can be studied and explained by looking at our brains and central nervous system (CNS). Scientist have been able to 'map' our brains to show which areas can be attributed to our humor, speech pronunciation and movement of our hands, just to name a few. 
All of these elements can be traced down to the very neurons (nerve cells that send messages to fulfill brain functions) which account for everything that we do. Right this second, energy is traveling through the tens of millions of axons (extensions of the neurons that receive signals) at speeds of ~220 miles per hour! Disease, injuries, psychoactive drugs can effect these connections which can cause changes in behaviors, mental degeneration/activation even treatment to illnesses such as Parkinson's and Alzheimer's disease.
Many people question how some of their personality and emotional traits came to be which introduces the study of Nature vs. Nurture. It's true that some of our genetic information (genes) can be amplified or turned off (pruned) by the environment that we grow up in. There are also predispositions we may have to traits and diseases which can be attributed to our hereditary. Nevertheless, the study of Nature vs. Nature has been found to be far more intertwined than separate.
FUN FACT: In ancient times, people believed the heart to be the source of mental activity because when we get excited, scared, upset, etc. our heart beats more rapidly!
