Emotions as they are experienced can be broken into three categories: primary emotions secondary emotions, background emotions.
Primary emotions are experienced as a byproduct of a stimulus-response chain of events; these emotional responses have, to some degree, been hardwired in our brains over the course of evolution. Fear, anger, disgust, sadness, and joy are the "chairs of the board" of primary emotions, and whether or not other board members exist remains under debate.
Secondary and background emotions are the product of an internal feedback loop. While the emotions involved in primary emotional reactions can also play a part in secondary and even as background emotions, non-primary emotions are more likely to be some dues-paying subsidiary of a primary emotion. For instance, fear as a secondary emotion might feel more like anxiety, stress, or shyness; secondary emotions related to joy might be experienced as ecstasy, pleasure, or amusement.
Joseph LeDoux, a professor and researcher at the Center for Neural Science at New York University, maintains that the list of basic (primary) emotions is nothing other than a list of the adaptive behaviors crucial to survival. Fear, for instance, is obviously related to survival: fear helps you react instantaneously to a stimulus you perceive as dangerous (for instance, a snake slithering toward you) and survive the event. So how does it work?
Primary Emotion: Introducing Fear
Evolution has graced us with the amygdala, an almond-shaped component of the ancient basal ganglia that is involved with aspects of emotion and memory formation. The amygdala allows an instantaneous, unthinking reaction in the face of a threat; sort of a survivomatic autopilot. As Joseph LeDoux says, "When it comes to detecting and responding to danger, the brain just hasn't changed much. In some ways we are emotional lizards." A snake advances -- or even wiggles around a bit -- and you jump back. Only after this initial reaction, do you think about what's going on and plot a more reasoned path of hasty retreat.
Two main pathways through the brain are involved in a fear reaction. Sensory input--for instance from the visual cortex -- scurries over the thalamic pathways (which are subcortical: ancient, lizard-like, and not involved in cognition) to the amygdala. The thalamic pathways to the amygdala do not differentiate among stimuli (which helps the message move as quickly as it does), so the information that reaches the amygdala is something along the lines of: "Danger! Danger!" The amygdala sends signals to other regions of the brain -- including the anterior cingulate and the basal ganglia. Nerves pulsing their message of fear reach the gut, heart, blood vessels, sweat glands, and salivary glands, causing the stomach to tighten, the heart to race, blood pressure to rise, the feet and hands to turn clammy, and the mouth to go dry. The skeletal muscles react, tensing up, and the smooth muscles increase activity, contracting the blood vessels and causing pallor. The pituitary gland sends its own missives to the adrenal gland (which will eventually send hormones coursing through the blood back to the brain to help deal with the stress). You jump out of your seat.
The second pathway takes the sensory input--such as that from the visual cortex -- on a relatively leisurely jog over the cortical pathways (more modern, more precise, and slower) to the cortex. The information delivered over the cortical pathways is better defined: "Snake! Slithering toward me! Flicking its tongue! At me!!" This message is time-consuming, both to send and to interpret; you wouldn't want to depend on your cortex to save you. Once this information reaches your cortex, you can start to formulate your backup plan. Your body and brain are primed (via the thalamic pathways), and all you need is a little cognitive input, a little thought, to finesse your escape. The second pathway,
