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Hippocampus, fornix and neighbouring structures.
Hey everyone! It's Nicole from Kenhub, and today we're going to talk about the hippocampus and the fornix. Before we go on to talk about the hippocampus and the fornix, let's first introduce you to the image we'll be studying today. This image is a transverse section of the left cerebral hemisphere at the level of the temporal lobe which you can see just here as well as the midbrain which you can see just here. The midbrain is cut roughly at the level of the superior colliculus which you can see around about here and in this cross-section, you can see the red nucleus and the substantia nigra. Also visible in this image is an isolated corpus callosum which you can see just here. However, the main structures that we will be concerned within this video are the hippocampal formation which is highlighted in green and the fornix, now highlighted in green. Both of these structures are part of the limbic system which is an extended neural network that is related with several psychological functions but we'll have a closer look at this system over the next few slides.
First, let's now have a brief overview of what we're going to talk about in this tutorial. During the course of this video, we'll discuss some of the structures that make up and are associated with the limbic lobe starting with the neuroanatomy of the hippocampal formation and its parts, the neuroanatomy of the fornix, and finally we'll move on to talk about the Circuit of Papez and its role in memory and what happens when it is disrupted. But before we go on to talk about the neuroanatomy, let's have a brief chat about the limbic system.
The limbic system often referred to as the limbic lobe isn’t actually a separate region of the brain but rather a collection of neural structures from various brain regions that all work together towards a variety of common functions. Some of these functions are emotion, behavior, motivation, memory, and learning.
Now that we've briefly discussed the limbic system, let's have a chat about the neuroanatomy of the hippocampal formation.
The hippocampal formation, a major component of the limbic system, plays an important role in memory. As its name suggests, it is a formation of several structures within the region of the hippocampus. Though there is some argument as to whether other complexes are included, for today's tutorial, we're going to work with the four structures below – the hippocampus proper which is essentially the hippocampus, the dentate gyrus, the subicular complex, and the entorhinal complex. The hippocampal formation is located within the temporal region and is thought to be involved in the formation of long term memories. To find out more about how it's involved in memory functions, let's have a look at the above components a bit more closely starting with the hippocampus proper.
The hippocampus proper is a bundle of grey matter which resides in the floor of the temporal horn of the lateral ventricle. As you can see in our image, the anterior hippocampus is wider than the posterior extension and its areas of indentation makes the structure resemble a paw. This region of the hippocampus is also known as pes hippocampus. The hippocampus proper is also important histologically where it can be divided into four regions – CA1, CA2, CA3 and CA4 – the CA standing for cornu ammonis. These divisions have relevance to the connections of the hippocampus.
As the hippocampus proper winds along its seahorse shape, it eventually forms a structure called the dentate gyrus highlighted in green on our image. The dentate gyrus is an integral region of the hippocampal formation and is responsible for the formation of our episodic memory as well as the exploration of new environments. It can be seen as a chain of small prominences along the medial aspect of the hippocampus and you can see it just here as I'm pointing it out with my arrow.
The subicular complex is the most inferior component of the hippocampal formation and lies between the hippocampus proper and the entorhinal cortex. As it's only visible in coronal section, it's not visible in this image but is around about here where my arrow is pointing. The subicular complex is implicated in working memory, that is, short term memory. And it contains pyramidal neurons which project into the entorhinal complex and other parts of the hippocampal formation. Like the hippocampus proper, the subicular complex can also be divided histology into parts and these parts are the subiculum which is located adjacent to the histological hippocampal region CA1; the presubiculum which contains a layer of granular cells, and the presubiculum which lies between the subiculum and the entorhinal cortex.
The final part of the hippocampal formation is the entorhinal cortex highlighted in green in this image of the inferior aspect of the cerebral hemispheres which you can see here on my right. The entorhinal cortex is formed from part of the anterior pole of the parahippocampal gyrus which you can see here highlighted in green, and the uncus, which is now highlighted in green and is the most anterior part of the parahippocampal gyrus. The uncus is separated from the parahippocampal gyrus by a small fissure and you can see that fissure just here, where my arrow is pointing. The entorhinal cortex also plays a role in the Circuit of Papez which we'll discuss further towards the end of the tutorial.
Now that we've finished talking about the neuroanatomy of the hippocampal cortex, let's now move on to the fornix. So as you can see in this image, the fornix is a bilateral structure consisting of bundles of nerve fibers that travel from the posterior portion of the hippocampus all the way to the mammillary bodies and you can see me tracing this route so from the posterior portion of the hippocampus all the way to the mammillary bodies which are just here. Like the hippocampus, the fornix can be divided into four different parts – the fimbria which you can see on our right here, the crus or the legs which is now highlighted in green, the body, and the column. Let's now have a chat about each of these parts.
As you can see in this image, the fimbria of the hippocampus is a band of white matter that is formed along the medial aspect of the floor of the lateral ventricle's inferior horn. This band of white matter contains axons projecting from cell bodies of the hippocampus and the subicular complex and although it's not quite obvious in this image, there are two fimbriae as there's another one on the other side. And as the fimbriae project posteriorly they remain close to the hippocampus until they reach a point beneath the splenium of the corpus callosum and the splenium of the corpus callosum is just here.
The corpus callosum extends the length of this curvature and is the largest white matter commissure. The splenium on the other hand which I'm now pointing out with my arrow once more is the most posterior portion of the corpus callosum. And as you can see, when the fimbriae reach the splenium they separate from the hippocampus to form the crus, otherwise known as the legs of the fornix. In this image, you can only see the left crus splitting off from the hippocampus, however, there is an identical crus running along the unseen right aspect. As the crura run along the inner curvature of the corpus callosum, the two legs are joined together by a thin band of white matter otherwise known as the hippocampal commissure and although these two cannot be seen in this image, the hippocampal commissure forms a kind of triangular shape as the two crura come together to form the body of the fornix. Once the crura join together, the fornix then continues anteriorly below the level of the septum pellucidum and within the roof of the third ventricle.
In this superior view of the cerebral hemisphere's cut in transverse and coronal section, you can see the body highlighted in green traveling superior to the mass of the thalamus which is just here. And as the mass of the body of the fornix reaches the level of the interventricular foramen of Munroe which is a small communicating foramen that allows cerebrospinal fluid to pass between the third ventricle and the lateral ventricles, a segment of the fornix separates from its contralateral counterpart and descends as the column of the fornix. And you can see a clearer image of the interventricular foramen of Munroe in this sagittal section of the ventricles of the brain as seen from the lateral perspective with the lateral ventricles over here and the third ventricle just here. And over here is our interventricular foramen of Munroe.
And finally in this image you can clearly see the body separating into two bilateral columns. As the columns descend, they move away from the midline and approach an ipsilateral mammillary body which you can see now highlighted in green and the ipsilateral mammillary body is the final destination of the fornix.
Now that we've covered the relevant neuroanatomy regarding the hippocampal formation and the fornix, let's move on to the Circuit of Papez. The Circuit of Papez is a neural circuit involving the hippocampus and the limbic system and is once thought to be connected to the control of emotion. Further research has since modified this concept finding that these structures are in fact to do with memory. The Circuit of Papez is a closed neural circuit beginning and ending at the hippocampus. More specifically, the circuit looks like the flow chart below. So in this chart you can see our hippocampal formation specifically the subiculum sending axons from this region posteriorly to enter into the fornix. And at the fornix, the axons continue along the crus and then into the body and then into the column of the fornix ending in the ipsilateral mammillary body and there the axons synapse then make their way down to the anterior nucleus of the thalamus via the mammillothalamic tract and once at the thalamus they synapse again. The next set of axons traveling superiorly to the cingulate gyrus – the structure just above the corpus callosum – and from here the axons synapse once again to loop back around to the entorhinal cortex following a path similar to the fornix. And at the entorhinal cortex, you can see that they have synapsed for a final time to project back to the subiculum.
And now that we've seen the Circuit of Papez and how it works, let's have a quick look at some clinical notes.
So the main topic we want to discuss today is the Circuit of Papez dysfunction as tumors or damage to the mammillary bodies, the fornix or the mammillothalamic tract can cause dysfunction of the Circuit of Papez. In addition, patients with Alzheimer's disease also sometimes suffer from the degeneration of the components of the Papez circuit. These dysfunctions generally result in amnesia or memory impairment, however, recent studies have shown that dysfunction of the Circuit of Papez may be involved in the pathogenesis of major depressive disorder otherwise known as MDD. Nevertheless, further studies on this phenomenon are still to be conducted.
Now let's take a couple of minutes to summarize everything that we've seen in our tutorial today. So if you may remember, we started with the hippocampal formation which you can see highlighted on our right here and the hippocampal formation is made up of four components – the hippocampus proper which is an important component of the limbic system and long term memory, the dentate gyrus which is responsible for our episodic memory, the subicular complex which is involved in working memory, and the entorhinal complex which plays a role in the Circuit of Papez.
We also talked about the fornix which is a bilateral structure consisting of nerve fibers and also has four components including the fimbriae which is a band of white matter, the crura of the fornix otherwise known as the legs of the fornix, the body of the fornix – a continuation of the crura – and the column of the fornix which approach ipsilateral mammillary bodies. If you remember, the Circuit of Papez is a neural circuit involving the hippocampus and the limbic system and in the clinical notes, we talked about the dysfunctions of the Circuit of Papez which often result in amnesia or memory impairment.
That's all we have for you today. Thanks again for watching.
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