Limbic Functions

In 1937, Papez suggested that the limbic lobe formed a neuronal circuit that provided the anatomical substratum for emotions. Based on experimental results suggesting that the hypothalamus plays a critical role in the expression of emotions, Papez argued that since emotions reach consciousness and thought and, conversely, higher cognitive functions affect emotions, the hypothalamus must communicate reciprocally with higher cortical centers.

The representation of the outside world and the internal milieu are superimposed in the limbic system. All the sensory information about the perceiver's environment is inscribed in the neuronal network of the limbic cortex, the hippocampus, and the amygdala. The vegetative, nervous, and humoral functions that contribute to homeostasis are represented simultaneously in the limbic system. Moreover, the hippocampus has been described as a gatekeeper embodying the brain's ability to commit things to lasting memory. Evidence for such a role of the hippocampus is clear. For instance, the neurosurgical removal of the hippocampus on both sides of the human brain, as a treatment of otherwise intractable forms of epilepsy, leads to a central disorder called hippocampal amnesia. The patient retains the memories he or she collected well before the surgery but cannot collect new ones.

Other evidence suggests that the amygdala is not only essential for olfactory discrimination but also commands a number of adaptative responses. Lesions and electrical stimulations of the amygdala produce a variety of effects on autonomic responses, emotional behavior, and feeding. Consequently, the amygdala has been implicated in the process of learning, particularly learning those tasks that require coordination of information from different sensory modalities or the association of a stimulus and an affective response.

Finally, it has been extensively described that the interplay between the neural activity of the hypothalamus and the neural activity of higher centers results in emotional experiences that we describe as fear, anger, pleasure, or satisfaction. For example, the behavior of patients from whom a part of the limbic system (frequently the prefrontal cortex) has been removed supports this idea. Indeed, these patients are no longer bothered by chronic pain or, alternatively, when they do perceive pain and exhibit appropriate autonomic reactions the perception is no longer associated with a powerful emotional experience.

In summary, neurons from the limbic system form complex circuits that collectively play an important role in numerous behavioral responses, such as learning, memory, and emotions. Such a role played by motivational states in homeostasis is discussed in the next section.

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