Septal nuclei: nerve functions and connections

In the last century, knowledge about the anatomy and functions of the different regions of the brain has increased considerably. Scientific research has allowed that today we can at least have 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 characteristic processes of the human being and of many different animals.

• Related article: "Parts of the human brain (and functions)"

What are Septal Nuclei?

Septal nuclei are a set of subcortical structures that lie between the hypothalamus, the corpus callosum, and the septum pellucidum, a membrane that separates the left and right lateral 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 closely interconnected regions at a morphological and functional level: the 

nucleus accumbens (which has a key role in the activity of the neurotransmitter dopamine), the nucleus of the terminal stria, and Broca's diagonal band.

Septal nuclei connect the limbic system with subcortical structures in the diencephalon region, thereby allow an exchange of nerve impulses to occur between these. Specifically, the subcortical areas to which we refer 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 talk about in the following sections.

Connections with other brain regions

Septal nuclei receive input from many different areas of the brain. One of the most important connections is the one with the prefrontal cortex; Higher cognitive functions, such as working memory, inhibition of inappropriate behaviors, moral thinking, planning, and the creation of expectations depend on this region.

The arch-shaped structure that is known as fornix connects the septal nuclei with the hippocampus, a nucleus of gray matter essential for the consolidation and retrieval of memories, as well as for the perception of space.

The medial olfactory stria, a set of nerve fibers, acts as a link between the septal nuclei and the olfactory bulb, which receives olfactory information from sensory receptors located in the olfactory mucosa.

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 “terminal stria”.

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 the melatonin. Unlike other pathways of which the septal nuclei are part, in this one the connection is bidirectional, since there are afferents 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 closeness of both structures.

Effferences to the hypothalamus take place through the medial telencephalic bundle, a set of fibers with a low degree of myelination that reaches the tegmentum of the brain stem. The myelin It is a substance that covers the axons of many neurons, protecting them from the extracellular environment and favoring the transmission of electrochemical impulses.

In contrast, the projections to the epithalamus are made through the medullary stria of the thalamus. When they reach the nucleus of the habenula, there is a relay in neuronal transmission; from here the efferences travel through the habenointerpeduncular tract until reaching the interpeduncular nucleus and the brainstem.

Functions of this structure

Research suggests that the septal nuclei serve a wide 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 fear sensations, which depend on the amygdala.

Septal nuclei are believed to be involved in regulating the activity of the limbic system, as well as in the brain alert. In this sense, its function would allow mental preparation in anticipation of the occurrence of a specific significant event, favoring the execution of responses to the appearance of this.

On the other hand, this set of nuclei is very important for the coding 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.

In general, it has been stated that the septal nuclei act by fulfilling a integration role of different physiological and cognitive processes, such as emotion, memory, pleasure, alertness, attention and reactivity to external stimuli. This is due to the multiple input that this structure receives from other regions of the brain.