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Anatomy of the limbic system

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Anatomy of the limbic system

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A 47-year-old woman sustains a brain injury after a motor vehicle accident. The patient cannot form memories of events taking place after the accident, including her hospital stay and subsequent time spent recovering at home. However, she can recall events that occurred before the accident.  

Which of the following brain structures was most likely injured in this patient?


Transcript

Content Reviewers:

Viviana Popa, MD

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.

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.

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. 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.

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. 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!

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.