Meninges, ventricles and brain blood supply

Subarachnoid cisterns

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:

Brain arteries

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 anterior cerebral arteries are joined anteriorly by the anterior communicating artery, whilst the posterior are united by the posterior communicating artery, thus completing the arterial circle. 

The major arteries supplying the brain, cerebellum, brainstem and spinal cord all arise from this circle, such as the cerebral, cerebellar and 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.

Brain veins

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:

1. It prevents water-soluble and large molecules to pass from blood circulation to the CNS
2. 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.