6.1 Mechanism of Emotions

We often think of emotions as abstracts. Poets write about them in ethereal ways, and the theatre uses them to draw in the audience. However, like any other trait, emotions also have a mechanistic component. After all, the outward expression of emotions is, in fact, behavior.

While a true explanation of the evolution of emotions would take an entire book to discuss, for the sake of brevity, we will focus on two main aspects – brain structures and neurotransmitters. Brain structures and neurotransmitters fall within the “mechanism” category of Tinbergen’s Four Questions.

Brain Structures

The mammalian brain is incredibly complex, and the more an organism needs to balance competing needs and values, the more complex that brain becomes. But this increase in complexity generally occurs within the prefrontal cortex – the part of the brain involved in complex thinking and reasoning.

Though we may process our feelings (e.g., why did I get angry?) within the prefrontal cortex, emotions are produced, and our body responds in spaces we all share. Here are a few of the key brain structures involved in the creation of the brain.

• Amygdala. Some of the key functions of the amygdala are learning and emotional processing, particularly when related to fear. The amygdala is a small, almond shaped mass that exists inside each hemisphere of the brain. Stimulating the amygdala can cause intense emotional reactions. When paired with a learning experience, an organism can experience a one-time learning event. As a result, anxiety, stress, and even phobias can develop related to specific environmental or emotional triggers, and these can be incredibly difficult to overcome.
• Nucleus Accumbens. The nucleus accumbens is involved in reward processing, aversive responses, and overall regulation of emotion. Depending upon environmental stimuli and neurotransmitter activity, the nucleus accumbens helps to inhibit or disinhibit reactive states. Together, the amygdala, prefrontal cortex, and nucleus accumbens are deeply involved in how an organism behaves after an emotional response is triggered.
• Hippocampus. This seahorse shaped structure is a major player in learning and memory. One study showed that the hippocampi (there is one in each hemisphere) of London taxi drivers are actually enlarged when compared to the general population. This is likely the result of years spent training, navigating streets, and learning the most efficient routes between locations. When the hippocampi and amygdala work together, learning becomes deeply connected to emotional experiences, making it difficult to relearn more appropriate responses to stress.
• Hypothalamus. The hypothalamus is involved in how animals process and express emotions. The lateral (outermost) parts of the hypothalamus are involved in the more extreme emotions (e.g., ecstasy or rage), and the median (innermost) parts of the hypothalamus are associated with slightly more moderate emotions – displeasure, aversion, and uncontrollable, loud laughing. All an organism’s senses (sight, sound, touch) are processed through the hypothalamus except one. The sense of smell bypasses the hypothalamus completely, suggesting it is the oldest of all senses.

Of course, other parts of the brain also play their roles in processing, experiencing, and expressing emotions. For example, once the hypothalamus processes core perceptions, it is the prefrontal cortex that is engaged to control one’s reactions (when possible). However, this is not always the case. For example, a particularly strong fear triggered in the amygdala may result in a flight-or-fight response before the prefrontal cortex has a chance to assess the threat. After all, it is safer to run unnecessarily than to become someone’s lunch.

Watch this video for a walkthrough of how some of these structures work to influence emotional reactivity.

Neurotransmitters and Hormones

The biochemical aspects of emotions are also incredibly complex. Luckily, there are a few specific superstars to highlight. Again, there are entire books written on behavioral endocrinology. But this brief list gives us an important start.

• Serotonin. Serotonin plays a role in many aspects of mammalian behavior – sleep, digestion, sexual desire – but it is most well known for its role in mood regulation. Too little serotonin may result in mood conditions like depression or anxiety, while too much serotonin can actually cause a systemic reaction known as serotonin syndrome. Excess serotonin has also been known to cause extreme mood swings or aggressive behavior. This is why any animal, even humans, taking serotonin regulating medications need to work closely with a medical practitioner.
  *and we actually use SSRIs on depressed animals of other species, too.*
• Dopamine. Dopamine is known as the “feel good” hormone because it is a critical component of the reward system. Dopamine pathways run throughout the brain, working with the hippocampi and prefrontal cortex to increase learning via positive association (did you ever notice you remember more when you enjoy it?). Dopamine is also involved in motivation. When dopamine spikes, a “dopamine high” can occur. As a result, individuals might seek to recapture that experience, resulting in variations of addiction and obsessive behavior. In fact, when we fall in love, it is dopamine that causes those intrusive, nonstop thoughts about the individual to whom we are attracted.
• Oxytocin. Oxytocin is often called the “cuddle hormone,” because it is involved in reinforcing social connectedness. Oxytocin gives us those safe, warm feelings we experience with a loving guardian, our romantic partners, and our desire to protect those for whom we care. Oxytocin has even been connected with stress reduction and cardiac functioning. There is a potential dark side to oxytocin, though. In their book, Survival of the Friendliest, Brian Hare and Vanessa Woods unpack how attachment (mediated largely by oxytocin) can cause a strong desire to protect and defend. While this can lead to incredible feats of courage and strength (like these teenagers who lifted a truck to save their dad), it can also lead to jealousy and aggressive, even violent behavior, to potential rivals in love or attention.
• The HPA Axis. Remember the hypothalamus above? This brain region also works with the pituitary and adrenal glands in something called the hypothalamic-pituitary-adrenal axis (HPA) to moderate stress and stress responses. The HPA axis is responsible for the regulation of cortisol (the stress hormone) by balancing epinephrine and norepinephrine to control nervous system stimulation. When a stressful response occurs, the system is rushed with epinephrine (adrenaline), the nervous system prepares for activity, and cortisol is released. Once the trigger or threat subsides, norepinephrine is released, the nervous system goes off high alert, and cortisol production is reduced.

Science is still discovering all the many ways neurotransmitters and hormones are involved in growth and development, digestion, emotion, and even cognition. We do know that homeostasis is a crucial goal for all our neuroendocrine system. For example, the HPA axis is involved in the immune system, which is why chronic stress leaves us vulnerable to pathogens in our environment. Serotonin is involved in sleep quality, which is why we feel cranky after a restless night.

Watch the following video to further explore the HPA axis.