The term limbic system designates the entire neuronal circuitry and forebrain structures that control emotional behavior, motivational drives and the processing of present and past sensory experiences. The brain structures of the limbic system are located around the middle edge of the brain. Several limbic structures are involved in determining the affective nature of sensory inputs, i.e., whether the sensations are pleasant or unpleasant. The emotional qualities we attach to the input provided by our five senses are also called reward (when they are pleasing to us and therefore we crave more of them) or punishment (when they are unpleasant and therefore we seek to avoid them), or satisfaction or aversion. Neurobiological research on the functions of the limbic system dating back to its XIX century pioneer Pierre-Paul Broca (1861), later expanded by James Papez (1937), Giuseppe Moruzzi and Horace Magoun (1949), and Ross MacLean (1949, 1952) identified the “reticular” and “limbic” systems as regulating the energizing and expressive roles in the central nervous system.
The limbic system is comprised of numerous structures, the most important of which are the hypothalamus, the amygdala, the hippocampus, the cortex, the cingulate gyrus, the striatum, the pallidum, the thalamus, and Meynert’s nucleus basalis. Each of these structures performs a specific function, and often also serves to receive, transmit and amplify communication within the limbic system, with other areas of the brain, and with other parts of the central nervous system.
The Hypothalamus: The Central Autonomic Controller
A major component of the limbic system is the hypothalamus and its related substructures. The hypothalamus complex controls the internal state of the body, such as temperature, osmolality of the body fluids, appetite and thirst and the regulation of body weight. Despite its very small size of only a few cubic centimeters (which represents less than 1% of the brain mass), the hypothalamic complex has two-way communicating pathways with all levels of the limbic system and is the key structure for higher level coordination of autonomic and endocrine functions. There would not be a stress reaction, with its almost instantaneous activation of physical and psychological defense mechanisms, without the hypothalamus providing the critical signal activation.
The Amygdala: The CPU of Emotional Response
The amygdala is a group of nuclei embedded in the anteromedial temporal lobe, which receives input from all five senses. It performs the analysis of form and color and facilitates the recognition of complex stimuli such as human faces. The amygdala can influence heart rate and blood pressure, gut and bowel function, respiratory function, bladder function, and many more instinctive physical reactions. It is in the amygdala and its connection to other limbic structures that the determination of the affective value of sensory stimuli (rewarding or aversive) is made and our mood (or feelings about something) is determined. Stimulation of the amygdala produces the defense reaction that prepares us for fight, flight or freeze, along with complex sensory and experiential phenomena, which may include fear, sensory hallucinations, feelings of deja vu, and memory-related flashbacks and nightmares. The amygdala receives neuronal signals from all portions of the limbic cortex and is the “central processing unit” in which the limbic system produces an emotional response to events, people and situations. The amygdala also interacts with higher brain regions that govern such processes as directed attention, declarative memory, and response inhibition (Davidson, Putnam, & Larson, 2000; LeDoux, 1995).
The Hippocampus: Memory Chips and Orientation
The hippocampus is a highly specialized region of the cerebral cortex, which along with surrounding areas of the parahippocampal gyrus is directly involved in memory processing and spatial orientation. The hippocampus provides the neural mechanism for association of different parameters that is necessary for the moment-to-moment incorporation of experience into our short- and long-term memory banks. Almost any type of input from the five senses causes activation of at least part of the hippocampus, which in turn distributes many outgoing signals to the anterior thalamus, to the hypothalamus, and to other parts of the limbic system, especially through the fornix, a major communicating pathway.
The Orbital and Medial Prefrontal Cortex: Food and Personality
The cortical areas of the limbic system are divided into two interconnected networks with related but distinct functions. Many of these functions are related to food or eating (e.g., olfaction, taste, visceral afferents, somatic sensation from the hand and mouth, and vision), and neurons in the orbital cortex respond to multisensory stimuli involving the appearance, texture, or flavor of food. Therefore, the orbital and medial prefrontal cortex have the function of evaluating feeding-related sensory information and to stimulate appropriate visceral reactions. More importantly, damage to the ventromedial frontal lobe can produce dramatic behavioral changes, which suggests that the visceral reactions evoked through this cortical area are critical in evaluating alternatives and making choices. As the well-publicized 19th-century case of Mr. Phineas Gage’s accidental head impaling by a steel rod demonstrates, individuals with damage to the ventromedial prefrontal cortex have no problem with their motor or sensory function, their intelligence or cognitive function, but show devastating changes in personality and choice behavior.
The Cingulate Gyrus: The Cement of Society
Intriguing data and ideas have been proposed by several researchers seeking to identify specific functions of the cingulate gyrus. In what has been termed the affiliation/attachment drive theory, Everly (1988) has shown experimentally that the removal of the cingulate gyrus eliminates both affiliative and grooming behaviors. MacLean (1985) has argued that the affiliative drive may be hard-coded in the limbic system and may be the anatomical underpinning of the “concept of family” in humans and primates. The drive toward other-oriented behaviors, such as attachment, nurturing, affection, reliability, and collaborative play, which has been referred to as the “cement of society” (Henry and Stephens, 1977), appears to originate in this relatively small limbic system structure.
The Ventromedial Striatum, Ventral Pallidum, and Medial Thalamus
The nuclei of the ventromedial striatum are also related to reward and reward-related behavior, whereby they inhibit or suppress unwanted behaviors while allowing other behaviors to be freely expressed. The dorsolateral striatum and related areas of the globus pallidus appear to be involved in switching between different patterns of motor behavior, whereas the ventromedial striatum and pallidum may allow changing of stimulus–reward associations when the reward value of a stimulus has changed. These areas are examples of the complexity and redundancies built into limbic system structures that permit multiple iterations of signal transmission and reception, and a much more complex and refined analysis of sensory inputs from the five senses.
Nucleus Basalis (of Meynert)
The nucleus basalis of Meynert is a prominent group of large cells located in the basal forebrain, most of which are involved in the activation of acetylcholine or GABA neurotransmitters, indispensable in activation of the stress reaction and our defense mechanism when a physical or psychological threat is perceived. The magnocellular basal forebrain nuclei are well situated to modulate brain activity in relation to limbic activity.
Disorders of the Limbic System
Although lesions to limbic structures do not necessarily result in sensory or motor deficits, any loss of function in these structures is usually associated with a variety of psychological problems, including depression, bipolar disorder, obsessive–compulsive disorder, and schizophrenia.
Structural changes have been noted in the hippocampal formation, medial thalamus, and prefrontal cortex in schizophrenic subjects. Images obtained through positron emission tomography scans show that the amygdala, prefrontal cortex and medial thalamus are abnormally active in patients suffering from severe unipolar and bipolar depression.
The complete removal of the amygdala and other nearby structures in laboratory settings causes specific changes in animal behavior called the Klüver-Bucy syndrome, whose characteristic symptoms are a complete lack of fear of anything, extreme curiosity about everything, rapid loss of short-term memory, tendency to place everything in the mouth and sometimes even trying to eat solid objects, and a sex drive so strong that it leads to attempts to copulate with immature animals, animals of the wrong sex, or even animals of a different species. Although similar lesions in human beings are rare, afflicted people respond in a manner not too different from that of the affected animal.