Brain

The human brain is the most complex organ in the body. It regulates your basic body functions; enables you to interpret and respond to everything you experience; and shapes your thoughts, emotions, and behavior.

The brain is made up of many parts that all work together as a team. Different parts of the brain are responsible for coordinating and performing specific functions.

• The brain stem controls basic functions critical to life, such as heart rate, breathing, and sleeping.
• The limbic system contains the brain's reward circuit - it links together a number of brain structures that control and regulate our ability to feel pleasure.  
• The cerebral cortex is divided into areas that control specific functions. Different areas process information from our senses, enabling us to see, feel, hear, and taste. The front part of the cortex, the frontal cortex or forebrain, powers our ability to think, plan, solve problems, and make decisions.

The brain is a communications center consisting of billions of neurons, or nerve cells. Networks of neurons pass messages back and forth to different structures within the brain, the spinal column, and the peripheral nervous system. These nerve networks coordinate and regulate everything we feel, think, and do.

• Neuron to Neuron
  Each nerve cell in the brain sends and receives messages in the form of electrical impulses. Once a cell receives and processes a message, it sends it on to other neurons.
• Neurotransmitters
  The messages are carried between neurons by chemicals called neurotransmitters. (They transmit messages between neurons.)
• Receptors
  The neurotransmitter attaches to a specialized site on the receiving cell called a receptor. A neurotransmitter and its receptor operate like a "key and lock," an exquisitely specific mechanism that ensures that each receptor will forward the appropriate message only after interacting with the right kind of neurotransmitter.
• Transporters
  Located on the cell that releases the neurotransmitter, transporters recycle these neurotransmitters (i.e., bringing them back into the cell that released them), thereby shutting off the signal between neurons.

Source: Addiction: Drugs, Brains, and Behavior - The Science of Addiction (National Institute on Drug Abuse)