In this note, we are going to discuss the anatomy and function of the limbic system.

Functions

The limbic system is a group of different structures and, most importantly, connections between them that are involved in several various functions. These are our emotional responses, olfaction, memory, learning, sleep, some types of behavioral activities, eating, sexual survival behavior, and of course, autonomic and endocrine regulation.

Parts

It’s important to understand that the limbic system is a functional, not an anatomical unit. So, its functions rely more on connections and mechanisms of interaction between various structures rather than on a specific location.

For better understanding, all components of the limbic system can be divided into three groups:

• Cortical structures
• Nuclear structures
• Nerve fibers or white matter structures that provide connections between all the elements

Cortical structures

Let’s start with the cortical structures. The main component here is the so-called limbic lobe.

If we look at the medial surface of the cerebral hemispheres, we can see that the limbic lobe looks like a C-shaped structure. What can we notice within it? Let’s go from anterior to posterior:

• The subcallosal gyrus located below the corpus callosum

• The cingulate gyrus

• Then a narrow area called the isthmus

• And the last one is the parahippocampal gyrus, at the end of which we can find a little curved extension called the uncus

The region of the parahippocampal gyrus corresponds to several areas such as the entorhinal cortex, perirhinal cortex, prepiriform cortex, and periamygdaloid cortex. All these areas are predominantly, but not exclusively, involved in olfaction and memory.

Other cortical components adjacent to the limbic lobe include the prefrontal, orbital, frontal, insular cortex, and some other regions of the frontal, temporal, and parietal lobes.

Nuclear structures

And now let’s discuss the nuclear structures of the limbic system.

Slightly above the parahippocampal gyrus, we can find a very important structure called the hippocampus.

It has a specific seahorse-shape visible on the frontal section and is also called the cornu ammonis.

The right and left hippocampi are connected by the commissure of fornix, also called the hippocampal commissure.

As a reminder, the fornix is a kind of huge bundle of nerve fibers.

In the cerebrum, the hippocampus, or “hippocampus proper” is closely connected to the dentate gyrus located medially,

and to the subiculum, which is the basal part of the hippocampus.

All these three structures form the so-called hippocampal formation involved in memory processing, particularly in converting our short-term memory into long-term memory.

And in this system, the dentate gyrus acts as an input. It receives afferent impulses, whereas the subiculum and the hippocampus provide the outflow. They send efferent information to the other regions of the brain.

The next significant component is the amygdaloid body, or just amygdala.

It is an almond-shaped structure located right above the hippocampus, and it is responsible for our emotional responses and behavior, particularly related to survival, such as rage, fear, aggression, anxiety, sexual drive, all this stuff. Moreover, the amygdala is part of the olfactory system, so it’s involved in olfaction.

The next three structures: the nucleus accumbens, the septal nuclei, and the habenula, are parts of the so-called reward system, or reward pathway.

It is responsible for the emergence of positive emotional activity in response to certain actions. The same mechanisms play a role in the development of addictive behavior.

The nucleus accumbens is adjacent to the anterior part of the corpus striatum.

This area is sometimes called the ventral striatum. It is also involved in the regulation of slow wave sleep.

The septal nuclei are located near the septum pellucidum, which is stretched between the fornix and the corpus callosum.

The habenula, which is part of the epithalamus, contains a group of habenular nuclei and attaches the pineal gland to the thalamus on each side.

The next nuclear component, the hypothalamus, is discussed in detail in its own topic. But for now, we need to know that it has groups of special limbic and autonomic nuclei involved in limbic and autonomic functions, respectively. And some of these functions are controlled by releasing hormones via the anterior pituitary.

Some nuclei of this thalamus, in particular the anterior nuclear group and the dorsomedial nucleus, are also components of the limbic system involved in memory and emotional responses, as you can see in the diagram (next page).

White matter tracts

And now we are going to talk about the white matter tracts that connect all the elements of the limbic system. There is a very special mechanism of interaction between the limbic structures when we are talking about memory and learning. It is called the Papez circuit, and it starts off at the subiculum. As we said, it is an output portion of the hippocampal formation, and it sends impulses to the mammillary bodies of the hypothalamus via the fornix. Then information gets into the anterior thalamic nucleus via the mammillothalamic tract. From there it goes to the cingulate gyrus via the internal capsule, then to the parahippocampal gyrus, particularly to the entorhinal cortex, and eventually returns back to the hippocampal formation, to be more specific, into the dentate gyrus.

The involvement of the Papez circuit in learning is possible because the cingulate gyrus has extensive connections with the prefrontal cortex, which is responsible for our reasoning, decision-making, judgment, and various personality aspects.

Olfactory structures

All the olfactory structures are discussed in more detail in the topic about the cranial nerve one. But for now, we only need to know that information from the olfactory receptors travels via the olfactory pathway and eventually gets into the primary and secondary olfactory cortex. These areas correspond to the region of the parahippocampal gyrus and provide processing, analysis, and recognition of the olfactory sensory input.

Another cortical region that receives smell information is the orbitofrontal cortex. This is the area on the inferior surface of the frontal lobe where the conscious perception of smell takes place.

Golosary

Limbic system

limbic system

systema limbicum

limbic lobe

lobus limbicus

subcallosal area

area subcallosa

cingulate gyrus

gyrus cinguli

isthmus of cingulate gyrus

isthmus gyri cinguli

parahippocampal gyrus

gyrus parahippocampalis

uncus

uncus

insular cortex

lobus insularis

prefrontal cortex

cortex prefrontalis

entorhinal cortex

cortex entorhinalis

hippocampus

hippocampus

commissure of fornix

commissura fornicis

fornix

fornix

dentate gyrus

gyrus dentatus

subiculum

subiculum

hippocampal formation

formatio hippocampalis

amygdala

corpus amygdaloideum

nucleus accumbens

nucleus accumbens

septal nuclei

nuclei septales