Limbic System

In 1878, the French neurologist Paul Broca called attention to a group of distinct subcortical structures surrounding the thalamus in the mammalian brain. He called these interconnected neuronal components the limbic lobe (limbic, from the Latin, limbus, meaning surrounding, as in encircling). This was later called the limbic system by Paul MacLean. The limbic system is a loosely defined anatomical grouping of subcortical and cortical nuclei that, acting together, increase our chances of survival from predators. The limbic system generates both a heightened state of awareness and attention and, when required, activates other brain systems to enable our bodies to run or fight. In addition, it amplifies information processing and memory encoding that relates to a threat so that stimuli that recall the original learning moment are easily retrieved. The system lowers the threshold for salience and increases vigilance when danger is sensed, allowing us to identify a predator so that we can avoid becoming prey. In a moment of danger, the limbic system prunes the number of options under consideration. In short, the limbic system is a coordinator of changing perceptions so that we make the correct response to a perceived threat.

The nuclei that compose the limbic system have been preserved throughout mammalian evolution. In the human, however, the enlarged neocortical [Page 350]structure called the prefrontal cortex specifically expands our emotional range and our cognitive storage space. The anatomical components (not all shown) assigned to the limbic system by Sitoh and Tien (1997) include the following (see Figure 1):

• Amygdala
• Fornix
• Hippocampus
• Thalamus
• Cingulate gyrus
• Hypothalamus
• Parahippocampal gyrus
• Prefrontal cortex
• Olfactory bulb

Figure 1 Anatomy of the Limbic System

The cingulate gyrus connects to the amygdala and regulates our emotional reaction to pain and aggressive behavior. It is also important for attentiveness. The parahippocampus receives input from the cortex and is important for scene recognition and recall (the context). The olfactory bulb (not shown), unlike other senses, has a direct pathway to the amygdala. This provides for longdistance sensing because early detection increases the number of safe options. The prefrontal cortex, although not belonging to the traditional limbic circuit, has intense bidirectional connections with the other limbic nuclei, most importantly the amygdala and the thalamus. The prefrontal cortex is, in essence, an inhibitor of our limbic system and is involved with the evaluative aspects of a threat.

We now know that fear is generated as a consequence of limbic activation. Aggressive behavior and rage are modulated by the limbic system as well. These two primitive emotional states, fear and rage, are associated with physiological changes that involve the viscera, the soma, and the endocrine system. How does this occur?

Threatening stimuli (unconditional fear stimuli, UFS) enter the thalamus through our senses and activate the amygdala (an exception to this pathway occurs if a predator is sensed by smell, whereby the olfactory neurons directly activate the amygdala; see Figure 2). These UFS are innately hardwired; that is, no learning is required.

The hypothalamus, which connects to the pituitary gland, is simultaneously activated and causes the release of the stress neurochemical cortisol, critical for encoding emotional information.

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