Over the last century, knowledge of the anatomy and functions of the various regions of the brain has increased considerably. Scientific research has allowed us to have at least some clues about how our brain works and, consequently, our cognitive and physiological processes.
In this article we will talk about the functions and nerve connections of the septal nuclei , a part of the brain that is essential for memory, emotional expression, pleasure and other processes characteristic of human beings and many different animals.
What are septal cores?
Septal nuclei are a collection of subcortical structures that lie between the hypothalamus, corpus callosum, and septum pellucidum , a membrane that separates the left and right side ventricles of the brain. It is also possible to find references to this brain region with the terms “septal area” and “medial olfactory area.
This concept is used not only to refer to the nuclei themselves, but also to a series of regions that are closely interconnected at the morphological and functional level: the nucleus accumbens (which plays a key role in the activity of the neurotransmitter dopamine), the nucleus of the terminal striae and the diagonal band of Broca.
The septal nuclei connect the limbic system with subcortical structures in the region of the diencephalon, thus allowing an exchange of nerve impulses between these . Specifically, the subcortical areas referred to are the hippocampus, the amygdala and the hypothalamus.
When lesions occur in the septal nuclei, symptoms related to excessive reactivity to food and sexual stimuli appear. This is associated with the connections of this structure with the hypothalamus, which we will discuss in the following sections.
Connections to other brain regions
The septal nuclei receive input from many different areas of the brain . One of the most important connections is with the prefrontal cortex; on this region depend the higher cognitive functions, such as working memory, inhibition of inappropriate behaviour, moral thinking, planning and expectation creation.
The arch-shaped structure that is known as the fornix connects the septal nuclei with the hippocampus, a nucleus of grey substance fundamental for the consolidation and recovery of memories, as well as for the perception of space.
The medial olfactory stria, a bundle of nerve fibers, acts as a link between the septal nuclei and the olfactory bulb, which receives information from the olfactory sense receptors located in the olfactory mucosa.
The septal nuclei are also connected to the amygdala , a structure of the limbic system on which emotional learning and memory depend. In this case the grouping of axons that links both regions is called the “terminal striae”.
On the other hand, this structure is also linked to the hypothalamus, which controls the release of hormones , and the epithalamus or pineal gland, which produces melatonin. Unlike other pathways involving septal nuclei, this connection is bidirectional, as there are afferences and efferences in both directions.
The efferences of the septal nuclei
The pathways that start from the septal nuclei and project towards the hypothalamus and epithalamus have different characteristics, despite the anatomical proximity of both structures.
The efferences to the hypothalamus take place through the medial telencephalic fascicle , a set of fibres with a low degree of myelinization that reaches the tegment of the brain stem. Myelin is a substance that coats the axons of many neurons, protecting them from the extracellular environment and promoting the transmission of electrochemical impulses.
In contrast, projections to the epithelium are made by the medullary striation of the thalamus. When they reach the nucleus of the habenula, a relay is produced in the neuronal transmission; from here the efferences travel along the habeno-interpeduncular tract until they reach the interpeduncular nucleus and the brain stem.
Functions of this structure
Research suggests that the septal nuclei serve a variety of functions. Two of the main ones seem to be the expression of pleasure responses, including those related to sexuality , and the inhibition of sensations of fear, which depend on the amygdala.
The septal nuclei are believed to be involved in the regulation of the activity of the limbic system , as well as in that of cerebral alertness. In this sense, their function would allow mental preparation for the expectation of a specific significant event, favouring the execution of responses to its appearance.
On the other hand, these cores are very important for the codification of new information, and therefore for learning and long-term memory. This function is associated with the connections that the septal nuclei maintain with the hippocampus.
It has been generally stated that the septal nuclei act by fulfilling a role of integration of different physiological and cognitive processes , such as emotion, memory, pleasure, alertness, attention and reactivity to external stimuli. This is due to the multiple afferences that this structure receives from other regions of the brain.