Meninges, ventricles and brain blood supplyWe know that we couldn’t live without our central nervous system (CNS). But did you know that the structures surrounding the brain and spinal cord are equally important. By this, we mean the meninges, ventricles, cerebrospinal fluid (CSF) and blood supply. Together these comprise the maintenance system of the brain.
Meninges, ventricles and vasculature of the brain can often bring confusion to students. That’s why we’ve decided to cover them all in one place for you, introducing you to their anatomy and function. Enjoy the ride.
|Meninges||The three membranes that envelop and protect the surfaces of the brain and spinal cord.
Meninges are: Dura mater, pia mater and arachnoid mater
|Ventricles of the brain||
A system of interconnected spaces within the brain through which the cerebrospinal fluid criculates.
The ventricles are: the two lateral, third and fourth ventricle.
|Brain blood supply||Internal carotid artery (anterior circulation), vertebral artery (posterior circulation), and their hexagonal anastomotic network called circle of Willis|
- Ventricular system
- Cerebrospinal fluid (CSF)
- Blood supply to the brain
Meninges are the three membranous layers that surround the brain and spinal cord. They function to protect the central nervous system, support blood vessels and enclose a cavity filled with CSF. From superficial to deep, they are:
- Dura mater
- Arachnoid mater
- Pia mater
Dura mater is the outermost layer lying directly under the skull. It consists of two layers; an outer periosteal layer which adheres tightly to the skull, and an inner meningeal layer. Arachnoid mater is the middle layer. It lies closely against, but not attached to, the above lying dura matter. In some places, the arachnoid mater shows small protrusions into the dura mater, called arachnoid granulations. These function to allow the outflow of CSF.
Pia mater, the deepest of the meningeal layers, adheres to the tissue of the brain entering all its grooves and fissures. Together the arachnoid and pia mater may be termed the ‘leptomeninges’. The inner meningeal dura mater, arachnoid and pia extend through the foramen magnum to surround the spinal cord, forming the spinal meninges.
Three spaces exist between the meningeal layers. The epidural (extradural) space, a potential space between the two dural layers. The subdural space, a potential space between the dura and arachnoid mater. And the subarachnoid space, a real space between the arachnoid and pia which contains cerebral vasculature and cerebrospinal fluid (CSF).
The two dural layers separate in places to form fibrous septa. By separating, dura mater forms the folds that house dural venous sinuses. The four fibrous septa are the:
- Falx cerebri: separates the left and right cerebral hemispheres and contains the superior and inferior sagittal sinuses
- Tentorium cerebelli: separates the cerebrum from the cerebellum and houses the transverse, straight and superior petrosal sinuses
- Falx cerebelli: separates the two cerebellar hemispheres and contains the occipital sinus
- Diaphragma sellae: forms a roof over the hypophyseal fossa within which the pituitary gland sits, contains the anterior and posterior intercavernous sinuses
Master the brain meninges by going through our study materials:
The ventricular system is a network of CSF filled cavities (ventricles) within the brain.
There are four ventricles:
- Two lateral ventricles held within the lobes of the cerebrum
- Third ventricle found between the thalami
- Fourth ventricle located over the pons and medulla, and underneath the cerebellum
These ventricles are connected by foramina through which the CSF passes.
- Interventricular foramen (of Monro) between the lateral ventricles and third ventricle
- Cerebral aqueduct (of Sylvius) between third and fourth ventricles
- Two lateral apertures (of Luschka) between the fourth ventricle and cisterna magna
- Median aperture (of Magendie) between the fourth ventricle and central canal of the spinal cord
Solidify your knowledge about the ventricular system of the brain with this great study material we have for you.
Cerebrospinal fluid (CSF)
Cerebrospinal fluid (CSF) circulates through the ventricles, subarachnoid space of the brain and spinal cord and the central canal of the spinal cord. The CSF function is to absorb mechanical force, cushion and protect the CNS, and supply nutrients to neural tissue. In addition, it removes metabolic waste from neural tissue, enables transportation of neurotransmitters and neuromodulators and can be a useful diagnostic tool for some CNS disorders.
The CSF is produced by specialized tissue called the choroid plexus, which is found within the walls of the ventricles. Production of CSF is equal to its removal, so at any time there is around 150ml of CSF circulating through the ventricular system.
How does this circulation happens? The CSF flows from the lateral ventricles to the third ventricle through the interventricular foramen (of Monro). From the third ventricle, the CSF passes through the cerebral aqueduct (of Sylvius) to the fourth ventricle.
From the fourth ventricle, some CSF flows through a narrow passage called the obex and enters the central canal of the spinal cord. Whilst most passes through the fourth ventricles’ median aperture (of Magendie) and two lateral apertures (of Luschka) and enters the interpeduncular and quadrigeminal subarachnoid cisterns. From there, the CSF flows through the subarachnoid space around of the brain and spinal cord. It is finally reabsorbed into the dural venous sinuses through the subarachnoid granulations.
Brain cisterns by definition are localised expansions of the subarachnoid space, through which CSF circulates.
The major cisterns are:
- Cisterna magna (cerebellomedullary cistern)
- Pontine cistern
- Chiasmatic cistern
- Quadrigeminal cistern
- Interpeduncular cistern
- Ambient cistern
- Crural and carotid cisterns
- Cistern of lateral cerebral fossa (Sylvian cistern)
- Cerebellopontine cistern
- Cistern of lamina terminalis
- Lumbar cistern (L1-S2); clinically significant as it is the site of lumbar puncture (extraction of CSF for analysis)
Do you want to learn more about the subarachnoid cisterns? Go through this study unit:
Blood supply to the brain
Considering its vital functions, it is understandable why the central nervous system shows high demand for oxygen rich blood and proper drainage of deoxygenated blood. Knowledge of the vascular network of the CNS is necessary, as even minimal disturbances of CNS blood supply can cause significant damage to the CNS tissue, and life-threatening conditions.
Arterial supply to the brain comes from two sources:
- Anterior circulation: two internal carotid arteries supply the anterior and middle portions of the cerebrum
- Posterior circulation: two vertebral arteries supply the posterior portion of the cerebrum, brainstem and cerebellum
The internal carotid arteries ascend through the anterior neck and enter the skull through the carotid canal of the temporal bone. Inside the skull they run anteriorly, branching into the anterior and middle cerebral arteries. The vertebral arteries also begin at the base of the neck. They rise through the transverse foramina, openings in the transverse processes, of C1-C6 vertebrae. Entering the cranium through the foramen magnum, they join to form the basilar artery, which subsequently divides into the posterior cerebral arteries.
The branches of the internal carotid and vertebral arteries anastomose with each other at the base of the brain, forming a loop of vessels called the circle of Willis. The spinal cord is supplied by branches of the vertebral arteries, as well as branches of the aorta.Arteries of the brain will be a piece of cake for you once you cover these study resources.
Circle of Willis
The circle of Willis, officially termed the ‘cerebral arterial circle’, is a hexagonal anastomotic vascular network at the base of the brain. It has two main sources. The first are the two internal carotid arteries and their branches – anterior and middle cerebral arteries. The second is the basilar artery, formed by the joining of the vertebral arteries, and its branches – the posterior cerebral arteries.
The major arteries supplying the brain, cerebellum, brainstem and spinal cord all arise from this circle, such as the cerebral, cerebellar and spinal arteries.
|Cerebrum||Anterior cerebral artery
Middle cerebral artery
Posterior cerebral artery
|Brainstem||Posterior cerebral artery
Superior cerebellar artery
Anterior inferior cerebellar artery
Posterior inferior cerebellar artery
Anterior spinal artery
|Cerebellum||Superior cerebellar artery
Anterior inferior cerebellar artery
Posterior inferior cerebellar artery
|Spinal cord||Anterior spinal artery
Posterior spinal arteries
Who would’ve known that we’d be talking about hexagons in anatomy classes? Take the circle of Willis quiz to seal your knowledge of the brain’s blood supply.
Venous drainage of the brain occurs through a system of cerebral and cerebellar veins, which in turn drain into the dural venous sinuses. The dural venous sinuses ultimately empty into the internal jugular veins which, together with the external jugular vein (draining the scalp and face), returns venous blood from the head and neck region back to the heart.
Superficial structures of the head, such as parts of the scalp and the skull, also drain into the dural venous sinuses via the emissary and epiploic veins. The spinal cord is drained via a network of epidural veins called the vertebral venous plexus (Batson’s plexus).
Brain veins are divided into the superficial and deep cerebral veins. Superficial veins, such as the superficial middle cerebral vein, drain the brain surfaces into the dural venous sinuses through a network of smaller bridging veins.
Deep veins drain deep brain masses either directly into the dural venous sinuses or via the superficial cerebral veins. Notable deep veins are the basal vein and internal cerebral veins which unite to form the great cerebral vein (of Galen). The great cerebral vein merges with the inferior sagittal sinus, forming the straight sinus.
You can learn the brain veins anatomy here.
Dural venous sinuses
Dural venous sinuses are spaces found between the two layers of dura mater. Unlike the previously mentioned subarachnoid space which contains CSF, the dural venous sinuses contain venous blood. Superficial and deep veins of the brain drain into these sinuses, either directly or indirectly, as does a small amount of CSF via the arachnoid granulations. The dural venous sinuses drain blood from the brain into the internal jugular veins.
There are two groups of sinuses: upper and lower groups. The upper group contains the:
- Superior sagittal sinus - found within the superior portion of the falx cerebri
- Inferior sagittal sinus - located within the inferior portion of the falx cerebri
- Straight sinus - found within the tentorium cerebelli
- Transverse sinuses - placed in the tentorium cerebelli
- Sigmoid sinuses - found in the anterior portion of the tentorium cerebelli from which they drain into the jugular veins
- Petrosquamous sinus - found along the junction of the petrous and squamous parts of the temporal bone
- Occipital sinus - lays in the posterior part of the falx cerebelli
- Confluence of the sinuses - found laterally to internal occipital protuberance of the occipital bone. The superior sagittal, straight and occipital sinuses drain into the confluence of the sinuses.
The lower group of dural sinuses comprises:
- Cavernous sinus - formed by the major brain veins at the base of the skull
- Anterior and posterior intercavernous sinuses - found around the pituitary gland which they specially drain
- Superior petrosal sinus - found in a groove of the petrous part of the temporal bone
- Inferior petrosal sinus - located at the junction of the petrous part of the temporal bone and occipital bone
- Basilar plexus - lies on the basilar part of the occipital bone. It connects the left and right inferior petrosal sinuses
Learn everything about the dural venous sinuses anatomy here.
Blood brain barrier
Blood brain barrier refers to the wall between the brain tissue and blood vessels. Microscopically, it is formed by the endothelium of the blood vessel, capillary basal membrane, glial basal membrane and foot processes of glial cells. The integrity of the blood brain barrier is essential for optimal functioning of the CNS, as it has two very important functions:
- It prevents water-soluble and large molecules to pass from blod circulation to the CNS
- It restricts most immune system cells from entering the brain and causing inflammatory damage of the brain tissue
This barrier is highly permeable for lipid-soluble substances, It has mechanisms of transport for nutrients, such as glucose, essential for the brain.