The Limbic System
The limbic system plays a large role in the biological functions of emotions. It shows the cortical control of emotion, as well as how other areas of the brain and outside factors influence emotions. These processes control our physical and emotional responses to environmental stimuli. The limbic system includes the olfactory bulbs, hippocampus, amygdala, anterior nuclei, fornix, column of fornix, mamillary body, septim pellucidum, habenular commisure, cingulate gyrus, parahippocampal gyrus, limbic cortex, and limbic midbrain areas (Figure 1). This system organizes the neurobiological explanation of a human emotion as a pleasant or unpleasant mental state. Neurochemicals such as dopamine, noradrenaline, and serotonin increase or decrease, causing the brain's activity level to fluctuate, which then makes visible displays in body movement, gestures, and poses.
Papez Circuit Theory
James W. Papez was the first researcher who tried to explain emotions in a neurofunctional way. He discovered what became known as the Papez Circuit in 1937 after injecting the rabies virus into a cat's hippocampus and observing its effects on the brain. Through his experiment he discovered that the hippocampus is chiefly involved in the cortical control of emotion (i.e., the control of emotions by the cerebral cortex).
The Papez Circuit involves several regions in the brain (Figure 3). The hippocampus projects to the fornix, and through this to the corpus mamillare. From here, neurons project via the fasciculus mamillothalamicus to the nucleus anterior of the thalamus, and then to the gyrus cinguli. Due to the connection of gyrus cinguli and the hippocampus, the circuit is closed.
Now, however, the amygdala - a part of the Papez Circuit added in 1952 - is also thought to play a key role in emotion.
Paul MacLean and the Amygdala
The Papez Circuit theory was extended by Paul MacLean in 1949 when he hypothesized that regions like the amygdala and the orbitofrontal cortex work together with the circuit, and form an emotional brain. However, this causes the theory of the circuit to fail because some regions of the circuit can no longer be related to functions to which they were ascribed primarily. Additionally, current research indicates that emotion has its own circuit, and constitutes more than just the limbic system (Figure 1). Other cortical and non-cortical structures of the brain have an enormous bearing on the limbic system, and thus emotion is an interaction between many parts of the brain.
The amygdala is located in the left and right temporal lobe. It belongs to the limbic system, and is essentially involved in the emergence of fear. In addition, the amygdala plays a decisive role in the emotional evaluation and recognition of situations, as well as in the analysis of a potential threat. It handles external stimuli and induces vegetative reactions.
The Autonomic Nervous System
The autonomic nervous system (ANS; part of the peripheral nervous system in humans) has two components: the sympathetic nervous system and the parasympathetic nervous system. When activated, the sympathetic nervous system prepares the body for emergency actions, controlling the glands of the endocrine system. Activation of the sympathetic nervous system causes the production of epinephrine (adrenaline) from the adrenal glands, increased blood flow to the muscles, increased heart rate, and other responses known as "fight-or-flight." Conversely, the parasympathetic nervous system functions when the body is relaxed or at rest, and helps the body store energy for future use. Effects of the parasympathetic nervous system include increased stomach activity and decreased bloodflow to the muscles. The ANS is regulated by the hypothalamus, and thus together regulate pulse, blood pressure, breathing, and arousal in response to emotional circumstances.
The Reticular Activating System
The reticular activating system (RAS) is a network of neurons that runs through the core of the hind-brain and into the midbrain and forebrain. The RAS is believed to first arouse the cortex and then to stimulate its wakefulness so that it may more effectively interpret sensory information. The RAS is made up of the midbrain reticular formation, mesencephalic nucleus (mesencephalon), thalamic intralaminar nucleus (centromedian nucleus), dorsal hypothalamus, and tegmentum (Figure 2). The RAS is involved with arousal and attention, sleep and wakefulness, and the control of reflexes. It helps us fulfill goals by directing our concentration towards them.