How does the brain affect behavior and regulate emotional states?
Emotions are regulated by the complex interaction between various brain components and the environment in a feedback loop that allows for both the environment to impact brain structure and function and the brain to impact on the environment through action. More than being a two-way street, however, the brain is more like a superhighway. This highway consists of a variety of environmental inputs (some that are available to our consciousness but many that are not) and our ultimate responses to those inputs. Environmental inputs available to our consciousness are those we typically associate with the five senses: sight, smell, taste, hearing, and touch. The mere words conjure up a myriad of emotional memories for experiences we have had in the past. A certain odor or song can suddenly take a person back to a previous relationship or situation. The connection between a current environmental cue and memories is caused by actual structural changes in the brain. In fact, long-term memories are long term because of those structural changes. The brain is not a computer but is a dynamic organ capable of physical change throughout one's life.
Although sensory inputs are generally obvious, a multitude of environmental inputs occurs without conscious awareness. The brain is also constantly monitoring our body's internal environment, the available nutrients and chemicals, blood pressure, pulse, temperature, and respiration, and it adjusts itself accordingly. It is also monitoring the external environment in ways that are not immediately apparent. These unconscious inputs can affect the emotional state in ways that are not always obvious.
Interpretations of these inputs that prompt actions are also influenced by two important factors influencing the brain long before inputs are received. Built into the brain are sets of biases, some of which are determined by genes and the biologic (uterine) environment in which development occurs and others by past experiences. Although genes do not cause behavior, they are the foundation for a person's entire organic makeup. Genes  code for proteins, which are the building blocks for both the structure and function of the human organism. Genes guide neuroanatomy, and, in turn, neuroanatomy and neurophysiology guide actions. Past experiences, on the other hand, are carved into brains through a process conceptualized as neuronal plasticity. Nerves are literally pruned away like tree branches through learning and experience as the brain attempts to create more efficient and faster communication pathways through those repeated experiences. By the nature of genetics and developmental experiences, people are biased to respond to the environment in certain ways. Although bias can predispose people toward negative actions and may be one of the mechanisms behind the development of some types of depression, it is merely biology's way of simplifying behavioral strategies to create more rapid and efficient actions. Without emotions one cannot prioritize; priorities to action must be linked to a preconceived template of what one considers important in decision making. This is the bias based on one's emotional experiences and constitutional nature (genes and nongenetic biologic effects).
In terms of defining the specific areas of the brain—or the anatomic locations—that control emotions, the division of regions is not clear-cut. One of the oldest and easiest to understand (but not necessarily the most accurate) theories divides the brain into three regions or layers. The most primitive is the brainstem and basal ganglia, followed by the limbic system, and then the rational brain, composed of the cortex. The first layer is that part responsible for self-preservation. It is where the "fight or flight" response is generated in response to perceived danger. The brainstem is also where control of certain visceral or "vegetative" functions (sleep, appetite, libido, heart rate, blood pressure, etc.) are generated. The limbic region (from the Latin word limbus for ring or surrounding, because it forms a kind of border around the brainstem) is better known as the reward center, where emotions or feelings such as anger, fear, love, hate, joy, and sadness originate. The limbic system is also responsible for some aspects of personal identity as related to the emotional power of memory. The third cerebral region is considered the "rational brain," which is capable of producing symbolic language and developing intellectual tasks such as reading, writing, and performing mathematical calculations. These neuroanatomic distinctions are really not that distinct but are integrated into function as a unified whole such that an assumption cannot be made of any one system taking priority over the other. The notions of brain regions as "primitive versus advanced" and "inferior versus superior" have not been supported by modern science. Brain structures are not hierarchical but are egalitarian. Brain function is more akin to an orchestra rather than to the more common notion of a military command center, as each component is required for the entire symphony to work where the conductor is merely a "ghost in the machine."
-  a memory evoked by a sensory experience.
-  DNA sequence that codes for a specific protein or that regulates other genes. Genes are heritable.
-  the structural makeup of the nervous system and nervous tissue.
-  the part of science devoted specifically to the physiology, or function and activities, of the nervous system.
-  the act of nerve growth and change as a result of learning. Mental exercise alters neuronal growth in the same manner physical exercise alters muscle growth.
-  referring to a person's biopsychological makeup, that is, the personality and the traits.
-  a region of the brain consisting of three groups of nerve cells—the caudate nucleus, putamen, and the globus pallidus— that are collectively responsible for control of movement. Abnormalities in the basal ganglia can result in involuntary movement disorders.
-  the part of the brain thought to be related to feeding, mating, and most importantly to emotion and memory of emotional events. Brain regions within this system include the hypothalamus, hippocampus, amygdala, and cingulate gyrus as well as portions of the basal ganglia.
-  the outer portion of the brain, which is composed of gray matter and made up of numerous folds that greatly increase the surface area of the brain. Advanced motor function, social abilities, language, and problem solving are coordinated in this area of the brain.
-  a reaction in the body that occurs in response to an immediate threat. Adrenaline is released, which allows for rapid energy to run (flight) or to face the threat (fight).
-  the anatomic part of the brain that connects the brain cortex to the spinal cord. It contains the major centers that regulate what are known collectively as "vegetative functions," that is, sleep, appetite, blood pressure, temperature, and respiration.
-  a bodily sensation usually referencing the gut; also a feeling or thought attributed to intuition rather than reason, such as "a gut instinct."