Core Brain Structures
Emotions, moods, pleasure, sexual arousal, pain, smell, hunger, thirst, curiosity, body temperature, consciousness, sleep and wakefulness, memory formation, fight- or-flight response The limbic system consists of structures within the forebrain, interconnected with the endocrine and autonomic nervous systems. Regulating aggression and fear, the amygdala scans for dangerous, threatening situations and can invoke the fight-or-flight response. Incoming sensory signals (apart from those of smell) go through the thalamus en route to sensory cortex areas, but parallel signals also reach the amygdala. The amygdala filters sensory signals, seeking to identify danger, using inherited and learned patterns. We often receive false amygdalic alarms. You might find yourself jumping in response to a twig on the ground because visual processing in the amygdala triggered a flight response to a snake-like pattern. The amygdala quiets down as we recognize there is nothing to fear, but it takes time to calm down, because the amygdala has triggered a stress hormone release through
multiple pathways. These hormones will remain blood borne for some time. Throughout this process, there is a dialogue between the amygdala and the cerebral cortex to negotiate our environment. Part of this dialogue may be conscious, such as when we experience stress and decide how to handle it. Part of it may be subconscious, such as when the amygdala decides to trigger a response and we don’t yet know what it is about. The amygdala also participates in recognizing emotions in facial expressions, so we directly see people as being happy, sad, and so on, without interpretation.
The limbic system evolved as a survival system geared for actively responding to the environment, triggering emotional experiences through hormones, serving to steer the animal away from dangerous situations and pull it toward those conducive to survival. With cortical development came an ability to modulate these experiences through thought. However, there are more upward connections from the limbic system to the cortex than downward ones, suggesting a balance of power in favor of the limbic system.
The limbic system is also involved in smell. We can perceive and recall smells (and tastes, which depend on smell) with immediacy and vividness. This may be because olfactory nerves from the olfactory bulb go straight to the limbic system, bypassing the thalamus. Olfaction evolved early without thalamic involvement, along with basic reflexive limbic responses.
The limbic system supports our memory, with the hippocampus being crucial for memory formation. Someone suffering hippocampal damage may be unable to form memories—although memories prior to the damage might be accessible. The person can rely on working and long-term memory for holding a conversation but might remember nothing of what someone said five minutes ago.