In the brain, the limbic system consists of several structures that help to regulate memory and emotion. One component of this system, known as the septal nuclei or septal area, is involved in the inhibition of fear and the expression of pleasurable responses. This structure receives input, as well as sends output, to many parts of the brain, including other limbic system structures, and the thalamus, which routes incoming stimuli from most sensory systems.
Near the center of the brain just beneath the corpus callosum, is the septal nuclei, located near many of the structures it interacts with. This is a bundle of nerves connecting the hemispheres of the brain. From an evolutionary perspective, this area seems to have been an offshoot of the hippocampus, a structure in the limbic system involved in memory. Consequently, there are strong connections between these two regions.
Nerve projections mediate the hippocampal connections with the brainstem and hypothalamus, which are both involved in automatic processes. The medial and lateral portions of the septal area seem to regulate the interaction of these three areas. Functionally, this allows these nuclei to regulate hippocampal memory formation and retrieval in response to arousal, which is mediated by the hypothalamus and brainstem.
A region of the brain involved in the expression of fear and other emotions called the amygdala, also is regulated through the septal nuclei. Like its connections with the hippocampus, these nuclei regulate interaction with the hypothalamus. Arousal and the activation of the amygdala can result in sexual behavior and physical contact. Inhibitory signals sent by the septal area modulates this response, and promotes more discriminating contact. Some researchers believe that this inhibition contributes to forming closer, longer-lasting emotional bonds.
Portions of the septal nuclei serve as a reward center, which means that it generates pleasant responses to certain stimuli. These responses help to create a sense of reward for certain behaviors. Animal studies have shown that when devices are implanted that allow rats to stimulate this area directly, they will do so repeatedly to experience the pleasant sensation.
Certain mental disorders may involve changes to the septal nuclei. Individuals with bipolar disorder and schizophrenia were found to have lower cell density in this area when compared to healthy subjects. There was also a correlation found in individuals with major depressive disorder, where the length of time they had their disorder was linked to having lower cell densities. This interaction is likely due to the importance of the septal area in emotional regulation.