Anatomy of the cranial meninges and dural venous sinuses

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Introduction0:00–1:17

The brain and spinal cord are covered by the meninges, which are 3 layers or membranes of connective tissue that not only protect the brain and spinal cord, but also form a framework for vessels and venous sinuses.
Just think of this as the brain needing 3 layers of blankets when going to bed at night to make sure it is extra cozy and secure.
These three layers, from superficial to deep, are the dura mater, arachnoid mater, and pia mater. The dura mater is a tough, thick, fibrous external meningeal layer.
Deep to the dura mater is the arachnoid mater. The dura and arachnoid mater are separated from each other by a potential space called the subdural space.
Deep to the arachnoid mater is the pia mater. The pia mater is a delicate vascular layer that is intimately adhered to the brain, covering the gyri and extending along different sulci and fissures.
Together, the arachnoid and pia mater are collectively known as the lepto meninges. Between the arachnoid mater and the pia mater is the subarachnoid space, also known as the lepto meningeal space, which is a true space between the arachnoid and pia mater, which contains cerebrospinal fluid or CSF for short, as well as major vessels in cranial nerves.

Dura mater1:17–1:28

OK, let's dive in and take a closer look at the dura mater, which is the thickest, outermost meningeal layer. The dura mater itself is further divided into two layers.
The first, more superficial layer is called the endosteal layer or periosteal layer of the dura mater. It is located on the interior surface of the skull bones and ends at the foramen magnum.

Endosteal Layer1:28–2:01

The endosteal layer does not continue with the dura mater of the spinal cord, but instead becomes continuous with the periosteum on the external aspect of the skull bones.
Between the endosteal layer of the dura mater and the skull bones, there lies a potential space called the extradural or epidural space, which is not a natural space, but may become a pathological space during bleeding.

Inner Meningeal Layer2:01–2:37

The second deeper layer of the dura mater is the inner meningeal layer, which is continuous with the dura mater of the spinal cord and ends at the level of the S2 vertebra.
For the most part, these two dural layers are fused together and cannot be separated. But there are two main exceptions to this.
One, there are locations where spaces are created between the layers to house the dural venous sinuses. Secondly, the inner meningeal layer of the dura mater reflects away from the endostrial layer at certain regions to create dural enfoldings, or reflections, which divide the cranial cavity into compartments.
These dural enfoldings or reflections consist of the Falc cerebrae, Falc cerebelli, tentorium cerebelli, and diaphragma cella.

Dural infoldings/reflections2:37–3:14

The largest dural enfolding is the falch cerebrae, which lies in the longitudinal fissure and separates the two cerebral hemispheres from each other.
It attaches anteriorly to the frontal crest of the frontal bone and the crista galley of the ethmoid bone. It contains the superior sagittal sinus in its fixed superior margin and attaches posteriorly to the internal occipital protuberance, where it also blends with the upper surface of the tentorium cerebelli.
Speaking of the tentorium cerebelli, this crescent-shaped reflection is the 2nd largest dural enfolding, with a fixed margin posterolaterally and a free margin more anteriorly.

Tentorium Cerebelli3:14–3:40

The fixed margin is attached at 3 sites, bilaterally at the posterior linoid processes, to the superior parts of the petrus portion of the temporal bone, and to the grooves of the transverse sinus on the inner surface of the occipital bone.

Tentorial Notch3:40–4:29

In contrast, the anterior part of the tentorium cerebelli is connected to the linoid process of the sphenoid bone at its most rostral end.
Otherwise, its margins are free, forming a U-shaped space. This space between the free margins of the tentorium cerebelli is called the tentorial notch, and it allows for passage of the brain stem.
The tentorium cerebelli spans a transverse plane over the cerebellum, which forms a roof over the posterior cranial fossa.
This effectively separates the cerebellum from the cerebrum, and divides the cranial vault into an infratentorial compartment below the tentorium cerebelli and a supratentorial compartment above.
As you can see, the falk cerebra blends with the tentorium cerebelli posteriorly, helping to maintain its position. Now, inferior to the tentorium cerebella, we will find the fel cerebelli.

Falx Cerebelli4:29–4:50

This small dural enfolding attaches to the internal occipital crest and contains the occipital sinus in its fixed posterior margin.
It extends in the sagittal plane and partially separates the cerebellum into two cerebellar hemispheres. The diaphragmacelli is the last and smallest dural enfolding.

Diaphragma Sellae4:50–5:19

It is a flat layer between the linoid processes that forms an incomplete roof over the hypophyseal fossa of the cella urica, which is a part of the sphenoid bone and contains the pituitary gland.
The diaphragmacei has a circular opening in the middle, which allows the passage of the pituitary stalk or infundibuum to connect the hypothalamus above to the pituitary gland below.
Now, as we said before, the two layers of the dura mater, for the most part, are strictly fused together. However, there are areas between the endostrial and meningeal layers that form spaces to accommodate the dural venous sinuses.

Dural venous sinuses5:19–5:56

These dural venous sinuses contain venous blood from the cerebral veins and also cerebral spinal fluid from the subarachnoid space.
The cerebrospinal fluid enters the sinuses through structures called arachnoid granulations, which protrude through the meningeal dura mater into the dural venous sinuses.
The contents of the dural venous sinuses ultimately drain into the internal jugular vein. The dural venous sinuses can be either paired or unpaired.

Paired Sinuses5:56–6:14

Paired sinuses include the transverse sinus, cavernous sinus, superior petrosal sinus, inferior petrosal sinus, sphenoparietal sinus, and sigmoid sinus.
On the other hand, unpaired sinuses include the superior sagittal sinus, inferior sagittal sinus, straight sinus, occipital sinus, and intercavernous sinus.

Unpaired Sinuses6:14–6:27

The superior sagittal sinus is the largest dural venous sinus, and throughout its course, it receives venous blood from the superior cerebral veins.

Superior Sagittal Sinus6:27–7:10

As the name suggests, it runs in the sagittal plane along the border of the fal cerebrae. The superior sagittal sinus extends from the foramen cecum of the frontal bone rostrally to the internal occipital protuberance caudally.
Where it drains into the confluence of sinuses. Keep in mind that the confluence of sinuses is also the point where the straight and occipital sinuses merge with the superior sagittal sinus.
It then deviates to one side, usually the right, to connect with the transverse sinus. The left transverse sinus and right transverse sinus begin at the internal occipital protuberance, from the confluence of sinuses, and pass laterally to run in the lateral border of the tantorium cerebelli.

Left and Right Transverse Sinus7:10–7:40

While doing so, these sinuses create impressions in the occipital and parietal bones. The transverse sinuses also receive blood from the superior petrosal sinuses.
The two transverse sinuses eventually continue as the right and left sigmoid sinuses on each side. The right and left sigmoid sinuses follow an S-shaped course in the posterior cranial fossa.

Right and Left Sigmoid sinuses7:40–7:57

During their course, they turn anteriorly and continue inferiorly as the internal jugular veins, which travel through the jugular foramen.

Inferior Sagittal Sinus7:57–8:21

Deeper to the superior sagittal sinus lies the smaller, unpaired inferior sagittal sinus, which runs along the inferior free border of the falch cerebrae.
Similar to the superior sagittal, it has a rostral to caudal extent. During its course, it receives venous blood from small veins draining the medial surface of the cerebral hemispheres and ultimately drains into the straight sinus.
The straight sinus is formed by the merger of the inferior sagittal sinus in the great cerebral vein. The straight sinus runs posterior inferiorly along the attachment between the fal cerebra and the tentorium cerebelli to eventually meet the superior sagittal and occipital sinuses at the confluence of sinuses.

Straight Sinus8:21–8:40

Speaking of the occipital sinus, this sinus is located on the anterior aspect of the occipital bone, along the caudal attached border of the phal cerebelli.

Occipital Sinus8:40–8:55

It ends postio superiorly at the confluence of sinuses. Up next we have the cavernous sinuses, which are located within the middle cranial fossa on either side of the selaturica of the sphenoid bone.

Cavernous Sinuses8:55–10:02

The cavernous sinus extends from the superior orbital fissure to the apex of the petrus part of the temporal bone. The left and right cavernous sinuses are connected to each other via intercavernous sinuses, which are quite variable, but usually consist of anterior and posterior parts.
The cavernous sinuses then diverge into the superior petrosal sinus and the inferior petrosal sinus. The superior petrosal sinus drains into the transverse sinus at the site where it continues as the sigmoid sinus, and the inferior petrosal sinus drains into the sigmoid sinus at its transition to becoming the internal jugular vein.
Off note, there are bridging vessels called emissary veins that connect the dural venous within the cranium to veins outside of the cranium.
One example is the parietal emissary vein, which runs through the parietal foramen and connects the superior sagittal sinus with veins in the scalp.

Quiz10:02–10:15

OK, that was a lot. Let's take a short break and see if you can recall these dural venous sinuses.

Cavernous Sinus10:15–10:46

Of all the sinuses that we have talked about, the cavernous sinus is the most clinically relevant because there are several important structures related to it.
Again, the cavernous sinus lies on both sides of the salaturica of the sphenoid bone. The pituitary gland lies in between the sinuses, along the midline, within the hypothesceal fossa.
Looking at a coronal section, the cavernous sinus contains numerous fibrous trabeculi that result in the sinus having a spongy, cave-like appearance.

Medial and Lateral Border of Cavernous Sinus10:46–11:13

The cavernous sinus has a medial and lateral border, a floor, and a roof. The medial border of the cavernous sinus is formed by the sphenoid bone, and the lateral border is formed by the meningeal layer of the dura mater.
The roof of the cavernous sinus is the diaphragmacelli, which is also the meningeal layer of the dura mater. And lastly, the floor is formed by the endostrial layer of the dura mater overlying the sphenoid bone.
There are several key anatomical structures that travel either through the cavernous sinus or very close to it. The structures passing through the cavernous sinus are the internal carotid artery and the postganglionic sympathetic fibers of the carotid plexus that run with it, as well as the abducens nerve, or cranial nerve 6.

Key anatomical structures of the Cavernous Sinus11:13–11:52

In contrast to running through the cavernous sinus, the ocular motor nerve or cranial nerve 3, the trochlear nerve or cranial nerve 4, and the ophthalmic or V1, and maxillary or V2 branches of the trigeminal nerve, or cranial nerve 5, travel along the lateral wall of the sinus.

Dural vascularization11:52–12:41

Now, the dura mater is highly vascularized with its own intricate arterial circulation. The major artery that supplies the dura is the middle meningeal artery, which is a branch of the maxillary artery.
It passes through the skull in the foramen spinosum and enters the middle cranial fossa. From here, the middle meningeal artery runs laterally and then superiorly on the greater wing of the sphenoid bone, where it divides into the frontal or anterior branch and the parietal or posterior branch.
The parietal branch runs posteriorly and superiorly to supply the posterior part of the dura and cranium. On the other hand, the frontal branch heads superiorly to the teron and eventually runs posteriorly to give off several branches that distribute throughout the dura up to the vertex of the skull.

Dural innervation12:41–13:51

The dura mater also has an extensive nerve supply, which directly or indirectly comes from the trigeminal nerve or cranial nerve 5.
The dura overlying the anterior cranial fossa is innervated by branches of all three divisions of the trigeminal nerve. The dura overlying the middle cranial fossa is innervated mainly by branches of the mandibular nerve, or V3, with some innervation from the maxillary nerve or V2.
Furthermore, the stentorial dura covering the posterior cranial fossa is innervated by branches of the ophthalmic nerve, or V1.
The infratentorial dura is innervated by branches of the cervical plexus arising from C2 and C3, as well as the vagus nerve, or cranial nerve 10.
Pain signals from the dura travel along the sensory fibers of these nerves and can be referred to the cutaneous areas that are also supplied by the same nerve fibers.
For example, irritation of the dura from an infection in the anterior cranial fossa can be referred to any of the cutaneous areas that are also supplied by those same branches of the trigeminal nerve, such as the skin around the orbit or face.
OK, let's move on to the two layers that form the lepto meninges, the arachnoid mater and the pia mater. The arachnoid mater is situated between the dura and pia mater, creating two spaces.

Arachnoid mater13:51–15:13

The first is the subdural space, which is a potential space between the dura and arachnoid mater. The second is the subarachnoid space between the arachnoid and pia mater, which is filled with CSF as well as cerebral arteries, veins, and cranial nerves.
The arachnoid mater forms protrusions called arachnoid granulations, which pierce the dura mater to enter the dural venous sinuses.
These protrusions allow the CSF to flow from the subarachnoid space into the dural venous sinuses, and from there into the venous circulation.
The largest granulations are present along the length of the superior sagittal sinus, located just below the calvaria. These granulations may erode into the inner surface of the skull, creating small pits known as the granular faveola.
The arachnoid mater is also loosely connected to the pia mater through thin strands of fibrous tissue known as arachnoid trabecula.
These strands create a meshwork that looks similar to a spider web, hence the name arachnoid for these structures. Where the various cranial nerves exit the skull, the arachnoid mater fuses with their outside layer called the epineurium.
Finally, we have the pia mater, which is the deepest meningeal layer, and we don't mean the most profound. This is the most delicate and the thinnest meningeal layer, and it tightly adheres to the entire surface of the brain.

Pia mater15:13–15:47

The cranial pia mater is the only meningeal layer that closely follows the contours of the brain. The pia mater dives into all of the sulci and fissures, blending for a short time with the sheaths of arteries as they enter the brain, and fuses with the epinurium of the cranial nerves within the subarachnoid space.
In this video, we have focused on the cranial meninges. However, please note that there are slight differences between the cranial meninges and the meninges surrounding the spinal cord.

Cranial meninges versus spinal meninges15:47–16:22

Basically, the dura mater of the cranial meninges is double layered with both endosteal and meningeal layers, while the spinal meninges only have a single meningeal layer.
Also, the epidural space, in relation to the cranial meninges, is a potential space, while the epidural space superior to the dure of the spinal cord is a real space.

Review16:22–17:55

All right. As a quick recap, there are 3 meningeal layers that consist of membranous connective tissue that covers the brain and the spinal cord.
The outermost and toughest layer is the dura mater, which has 2 layers, the outer endosteal and the inner meningeal layer.
The endostrial layer is an extension of the periosteal layer of the anterior skull bones, whereas the deeper meningeal layer creates projections that form dural enfoldings or reflections.
The arachnoid mater is found deep to the dura mater, and between them is the subdural space. The arachnoid mater contains arachnoid granulations, which pierce the dura mater to enter the dural venous sinuses.
The deepest meningeal layer is the pia mater, which closely adheres to the contours of the brain. The arachnoid mater is loosely connected to the pia mater through the arachnoid trabecula.
Between these two layers is the subarachnoid space, which contains CSF, cerebral vessels, and cranial nerves. The main dural enfoldings are the Felch cerebrae, the tentorium cerebelli, the fel cerebelli, and the diaphragma cella.
These two duramato layers are normally attached to each other. However, there are areas where pockets occur between them, forming the dural venous sinuses that drain cerebral venous blood and cerebrospinal fluid.
The dura has extensive sensory innervation from different branches of the trigeminal nerve, as well as the vagus nerve and branches from cervical nerve C2 and C3.