Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the meninges of the brain. In this tutorial, we'll be discussing the meninges of the brain which are then connective tissue membranes. They enclose the central nervous system or CNS and are made up of three layers. We will also be looking at the arterial and venous systems closely related to the meninges of the brain. And to do so, we're going to mainly use this image here of the superior view of the brain where we just have all these membranes here that we' re going to be looking at and a few of the structures that are related or nearby.
In this tutorial, we'll be discussing the meninges of the brain which are then connective tissue membranes. Now, the outermost layer is known as – this one that you see here highlighted in green – the dura mater. As I mentioned, this is the outermost layer which is the first structure we will look at. This name means "strong or tough mother" due to the fact that it is actually the strongest of the membranes that cover your brain and spinal cord. It is derived from a mesoderm and it forms the dural venous sinuses which transfer blood from the superficial veins of the brain to the jugular vein and then to your heart. The cranial and vertebral dura mater has two layers. Now, the first or superficial layer forms the skull's periosteum – it is known as then the periosteal layer – while the second layer forms the supporting capsule of the brain – this is known as the meningeal layer.
The inner meningeal layer of the dura mater forms reflections and infoldings known as – the first one that you see here highlighted in green – the falx cerebri, which is a sickle-shaped part of the dura mater projecting downward into the longitudinal cerebral fissure and separating the two cerebral hemispheres. We also have then this one that you see here highlighted, the tentorium cerebelli – which you also see here from this image of the base of the skull. This is the part of the dural sheath that spreads out between the ridges of the petrous part of the temporal bone and the transverse sinus separating the cerebellum and the brainstem from the occipital lobes. It can be said that the tentorium cerebelli canopies over the posterior cranial fossa covering the cerebellum like a tent and, for this reason, we call it then tentorium.
The next infolding that we're going to be seeing here highlighted is known as the falx cerebelli. It's also an infolding of dura mater. This is a small sickle-shaped dural sheath between the right and left hemispheres of the cerebellum inferior to the tentorium cerebelli in the posterior part of the posterior cranial fossa. And, finally, this one that you see here highlighted in green, the diaphragma sellae, which is a small circular horizontal sheath of dura spreading out between the clinoid processes above the hypophysis and forming a partial groove above the hypophyseal fossa. It covers the pituitary gland and has an aperture for the infundibulum and the hypophyseal veins to pass. The dura mater is innervated by branches of the trigeminal nerve, also the vagus nerve, and the first 3 cervical nerves. It is very sensitive to stretching – this dura mater – which then produces sensation of headache.
The second layer of meninges that we're going to be highlighting here is known as the arachnoid mater – so, the second layer of meninges. This is one is a delicate impermeable and avascular membrane covering the brain and the spinal cord. It lies between the dura mater externally and the pia mater internally. It is separated from the dura mater by the subdural space and from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid. Now, the cerebrospinal fluid is produced by the choroid plexuses inside the ventricles of the brain which are in direct communication with the subarachnoid space. The spaces that I just mentioned – the subdural and the subarachnoid – are very important from a clinical point of view because here blood can be accumulated due to several reasons either traumatic or non-traumatic and then we have then clinical conditions which are known as subdural haematoma or subarachnoid hemorrhage respectively.
Finally, we're going to be seeing this one here which is known as the pia mater. This is the innermost layer of the meninges. It is a delicate matrix bearing blood vessels which covers the surface of your brain as well as extending into its deepest sulci. It also covers the spinal cord but we will not discuss the spinal pia mater in this tutorial. The name pia mater comes from the Latin which means "tender mother". The pia mater, just like the arachnoid mater, is impermeable to fluid.
Now, having discussed the basic features of the meninges, let's look at some other structures that are directly related to the meninges starting with the sinuses. The venous sinuses are formed by separating of two layers of dura mater along certain lines creating then the sinuses. We now see then the superior sagittal sinus here highlighted in green. Now, these sinuses in general are venous channels that form the major drainage pathways from the brain predominantly to the internal jugular veins.
We're now going to cover all the sinuses in this tutorial but for more information, please check out our tutorial on the dural venous sinuses here at Kenhub.
The first of the venous sinus that we're now looking at highlighted in green is then the superior sagittal sinus. It is an unpaired structure along the attached margin of the falx cerebri as you can see from the image to the right of your screen. It extends from the crista galli to the confluence of sinuses and allows blood to drain from the lateral aspects of the anterior cerebral hemispheres to the confluence of sinuses which is found here. Now, cerebrospinal fluid drains through the arachnoid granulations into it and is returned to the venous circulation.
The next structures we're going to be highlighting here back to this image are known as the lateral lacunae of the superior sagittal sinus. They are located on either side of the superior sagittal sinus which we can see here on our illustrations and they are small pits. They are irregularly shaped venous spaces which communicate with the superior sagittal sinus as you can see here that's not highlighted in green. You can see how they communicate with the sinus.
There are usually 3 on either side of the sinus and arachnoid granulations project into them as you can also see here – these granulations – which we will talk about on the next slides transporting cerebrospinal fluid from the subarachnoid space to then the blood.
We have just mentioned the arachnoid granulations in reference to the lateral lacunae and the superior sagittal sinus but what are they really? The arachnoid granulations are basically small protrusions of the arachnoid through the dura mater into the venous sinuses of the brain. They act as one-way valves allowing then cerebrospinal fluid to exit the subarachnoid space and enter the bloodstream.
Now, let's look at another structure that belongs to the sinuses called then the confluence of sinuses. So far, in this tutorial we have only looked at the superior sagittal sinus of the brain; however, it should be noted that there are 6 paired dural venous sinuses and 4 unpaired dural venous sinuses of the brain. The confluence of sinuses is the site of union of the superior sagittal, straight, occipital and transverse sinuses at the internal occipital protuberance.
The superior sagittal sinus which we discussed earlier on receives blood from an extended network of veins known as the superior cerebral veins which are seen highlighted in green on this image. Numbering them from between 8 to 12 veins, they are arise from the lateral, medial and anterior aspects of the inferior surface of the cerebral hemispheres and will be draining blood into the superior sagittal sinus. You can also see these see these veins here on this image of the lateral view of the brain and how they go all the way to drain blood into the superior sagittal sinus.
Finally, let's see two important arteries that can be seen in this view of the brain. First, let's look at the middle meningeal artery which is the third branch of the first part of the maxillary artery. This artery passes through the foramen spinosum medial to lateral pterygoid muscle into the middle cranial fossa where it ramifies. The middle meningeal artery – through its branches – supplies then the supratentorial part of the dura mater.
The final structures that we will be covering on this tutorial are the branches of the middle cerebral artery. The middle cerebral artery itself is a branch of the internal carotid artery and its branches which supply blood to the majority of the lateral surface of the hemispheres, the insula, the internal capsule and the basal ganglia. And please note that although the left and right middle cerebral arteries are connected to the anterior communicating arteries and the posterior communicating arteries, they are not part of the circle of Willis.
Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.
Now, good luck everyone, and I will see you next time.