Anatomy of the limbic system

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Introduction0:00–0:23

The limbic system consists of several parts within the telencephalon, also known as the cerebrum; the diencephalon, which includes the thalamus and hypothalamus; and the midbrain.
The term “limbic” means “border” in latin, so this system’s name originated from its location at the border between the telencephalon and diencephalon.

Functions0:23–1:34

One way to remember the limbic system is to think of the word HOME; Homeostasis, Olfaction, Memory, Emotion. This is because limbic system structures are involved in olfaction, or smell, in the regulation of emotions, like anger and fear, and other behaviors, like aggression and sexual behaviour.
Memory formation and the recollection of those memories are supported by the limbic system, and It can even influence responses of the autonomic nervous system, like cardiovascular or gastrointestinal functions.
To easily recall some of the functions, you can remember the famous 5 F’s, which are: Feeding, Fleeing, Fighting, Feeling and...Fornicating, the last one being, really, just a fancy word for Sex.Structures that are included in the limbic system include the: hypothalamus, olfactory cortex, hippocampal formation, amygdala, subcallosal area, cingulate gyrus, parahippocampal gyrus, mammillary bodies and the basal forebrain.

Hippocampal formation1:34–3:36

Let’s take a closer look at each of them, starting with the hippocampal formation, which is crucial for converting short-term memory into long-term memory, shipping those memories into other cortical regions for long-term storage and assisting in retrieval of memories when needed.
It is also involved in spatial orientation, which is the ability to identify the position of our body relative to the objects around us.
The hippocampal formation consists of the hippocampus, the dentate gyrus, subiculum and entorhinal cortex.Now, on a mid-sagittal section of the brain, the hippocampal formation is hidden within the medial temporal lobe.
It lies posterior to the amygdala and spreads caudally all the way to the splenium of the corpus callosum. On a transverse section of the temporal lobe, we can see the hippocampal formation forming the floor of the inferior horn of the lateral ventricle.
But, the best plane for the visualisation of this structure is actually the coronal plane. Here, we can see the hippocampal formation located within the medial temporal lobe and swooping beneath the inferior horn of the lateral ventricle.
The most inferior part of the hippocampal formation is the subiculum, and it is continuous caudally with the parahippocampal gyrus.
The hippocampus, meaning seahorse in latin, is so named as its contours resemble the shape of a seahorse. The dentate gyrus is contained within the hippocampus, just superior to the subiculum.
The hippocampus is involved in long-term memory formation and helps decide what should be stored within the cerebral cortex as a memory.
It has connections with the olfactory cortex, hence why smells can elicit memories very easily, and also connections with the amygdala to remember emotionally significant events!The next three structures that belong to the limbic system are the subcallosal area, the cingulate gyrus and the parahippocampal gyrus.

Subcallosal area, cingulate gyrus, and parahippocampal gyrus3:36–5:01

These structures influence emotional reactions and support learning and memory formation. On a mid-sagittal section of the brain, we can see the subcallosal area on the medial side of the frontal lobe, just anterior to the lamina terminalis and inferior to the rostrum of the corpus callosum.
Anteriorly, it is continuous with the cingulate gyrus, which arches over the genu, body and splenium of the corpus callosum.
Superior to the cingulate gyrus there is the cingulate sulcus that separates it from the rest of the cerebral cortex. On a coronal section of the brain, we can see the corpus callosum connecting the hemispheres and the cingulate gyrus just superior to it, at the bottom of the longitudinal fissure.On the ventral and medial aspect of the temporal lobe, spreading anteriorly from the splenium of the corpus callosum, we see the parahippocampal gyrus, which is separated from the rest of the temporal lobe by the collateral sulcus.
The anterior portion of the parahippocampal gyrus curves medially and posteriorly, like a hook, and is called the uncus.

Mammillary bodies5:01–6:10

And now let’s look at the mammillary bodies, which are a pair of round nuclei, belonging to the hypothalamus, that can be seen on the ventral surface of the brain.
They are located posterior to the tuber cinereum of the hypothalamus, anterior to the midbrain and between the cerebral peduncles.
Similarly, in the mid-sagittal plane of the brain, they can be seen posterior to the tuber cinereum and anterior to the tegmentum of the midbrain.
In the coronal plane, we can see the mammillary bodies inferior to the third ventricle. Now, all of the hypothalamic nuclei are part of the limbic system, as “homeostasis” is one of their major functions, but the mammillary bodies are especially important as they play a role in learning and memory and are connected to the hippocampus.
These structures assist in episodic memory, which is the ability to recall specific situations, or episodes, from our lives, like the place and time of our first kiss and the emotional reaction associated with it.
Great! Now let’s take a closer look at the amygdala, which means “almond” in latin.

Amygdala6:10–7:28

The amygdala contains numerous nuclei and is connected with various parts of the nervous system, but the important connections with other limbic system structures include the hypothalamus, olfactory cortex, hippocampal formation, frontal lobe and the sensory association cortices.
Because of this, the amygdala is involved in emotional responses, especially fear, rage and aggression, modulation of memory and influences attention and decision making.
Interestingly, the connections between the amygdala and the hypothalamus can elicit various physiological responses to emotional experiences!On a mid-sagittal section of the brain, the amygdala is located in the medial temporal lobe, just deep to the uncus.
On a rostral coronal plane, we can also see the amygdala in the medial temporal lobe, deep to the uncus with the tail of the caudate just superior to it.
Remember that the hippocampus and parahippocampal gyrus would be found more caudally in relation to the amygdala.Let’s take a short break and see if you can recall the role of the hippocampal formation!

Quiz7:28–7:38

Orbitofrontal cortex7:38–8:18

Ok, now that we saw the primary structures that make up the limbic system, let’s go through just a few more structures that are connected to and work together with it.
First, there’s the orbitofrontal cortex of the frontal lobe, which is involved in planning and decision making in relation to emotion and reward.
It’s connected with the dorsomedial nucleus of the thalamus, which gathers information from other cortical and limbic system structures like the hypothalamus and amygdala.
It relays this information to the orbitofrontal cortex to aid in decision making; reward learning; affective behaviour; emotional reactions; subjective feeling states and personality.
Another part of the thalamus that is connected to the limbic system includes the anterior nuclei of the thalamus. They receive input from the mammillary bodies and project to the cingulate gyrus, supporting the pathway for long-term memory storage.Two more groups of structures that are connected to the limbic system and the basal ganglia include: the basal forebrain nuclei and limbic nuclei in the brainstem.

Anterior nuclei of the thalamus8:18–8:37

Basal forebrain nuclei8:37–9:54

The basal forebrain nuclei include the basal nucleus of Meynert, the nucleus accumbens and the septal nuclei. On a coronal section of the brain, the basal nucleus of Meynert can be seen just below the basal ganglia.
It contains neurons that release acetylcholine as their neurotransmitter. The nucleus accumbens can be seen within the basal ganglia, next to the septal area.
It contains neurons that release GABA as their neurotransmitter. The septal nuclei are believed to be a pleasure area of the brain involved in reward and reinforcement.
These nuclei are part of the basal forebrain and are connected to other limbic system structures such as the amygdala, the olfactory cortex, the hippocampus and the hypothalamus.
In the mid-sagittal plane, we see them located inferior to the rostrum of the corpus callosum, superior to the lamina terminalis and around the anterior commissure.Moving on to the limbic nuclei in the brainstem, these nuclei include the ventral tegmental area, the raphe nuclei and the locus ceruleus.

Limbic nuclei in the brainstem9:54–10:39

The ventral tegmental area is located in the ventral midbrain, medial to the substantia nigra, and its neurons are dopaminergic.
The raphe nuclei can be found throughout the brainstem as part of the reticular formation and their neurons produce serotonin.
The locus ceruleus is located in the pons and contains norepinephrine-producing neurons. All of these act as diffuse modulators within the nervous system.Let’s take another short break and see if you can recall the parts of the limbic system!Lastly, let’s cover a few of the axon tracts that connect the parts of the limbic system and allow for overall communication and function!

Quiz10:39–10:49

Tracts of the limbic system10:49–13:08

Although connections are rich and varied, a few of the major tracts include: the fornix, the mammillothalamic tract, and the stria terminalis.
The fornix is made up of fibers that run from the hippocampus to the mammillary bodies, and it has several parts. First, there are the fimbria, which are fibers that arise from the hippocampus.
They gather and leave the hippocampus posteriorly as the crus of the fornix. The left and right crura of the fornix curve posteriorly and then superiorly, passing posterior to the thalamus and inferior to the splenium of the corpus callosum.
They come together to form the body of the fornix, which lies beneath the corpus callosum and superior in relation to the third ventricle.
Before they merge into the body, the crura are connected by a membrane called the commissure of the fornix, or the hippocampal commissure.
It contains fibers that connect the left and right hippocampi. The body of the fornix then curves anteriorly and inferiorly with the septum pellucidum located superior to it.
The septum pellucidum is a membrane between the lateral ventricles and on a medial view we see it extending between the fornix and the corpus callosum.
Near the anterior commissure, the body of the fornix splits into two columns of the fornix. These pass over the interventricular foramina, and pass through the hypothalamus, before reaching the mammillary bodies where it will terminate.The second major bundle of axons is the mammillothalamic tract, that connects the mammillary bodies with the anterior nuclei of the thalamus.
The last pathway is the stria terminalis, which connects the amygdala with the basal forebrain and the hypothalamus. The fibers of the stria terminalis originate in the amygdala and pass along the medial side of the tail of the caudate nucleus, following it through the floor of the body of the lateral ventricle, before reaching the hypothalamus.Alright, as a quick recap… The limbic system consists of several parts of the telencephalon, diencephalon and midbrain that together influence emotions, behaviour and learning and memory.

Review13:08–14:53

The hippocampal formation consists of the hippocampus, dentate gyrus, subiculum and entorhinal cortex, and it’s involved in long term memory formation and spatial orientation.
These structures, along with the mammillary bodies help to support the consolidation and storage of memories. Then, there is the subcallosal area, the cingulate gyrus and the parahippocampal gyrus, which support emotional responses and learning and memory.
The amygdala regulates emotions like fear and aggression and can elicit physiological responses to emotion via its connections with the hypothalamus.
Then, there are areas that are functionally connected to the limbic system, and these include: the orbitofrontal cortex, and the dorsomedial nuclei and anterior nuclei of the thalamus.
The basal forebrain include the basal nucleus of Meynert, the nucleus accumbens and the septal nuclei. And finally, the limbic midbrain area that includes the ventral tegmental area, which produces dopamine, the raphe nuclei, which produce serotonin, and the locus ceruleus, which produces norepinephrine.
The major pathways that connect the structures of the limbic system include the fornix, the mammillothalamic tract and the stria terminalis.