The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are anatomical structures that help to protect and isolate the CNS. The arterial blood to the brain comes from the internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding space defined by the meninges, the protective covering of the brain and spinal cord.
The blood that nourishes the brain and spinal cord is behind the glial-cell–enforced blood-brain barrier, which limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is reabsorbed into the blood, removing the waste from the privileged central nervous tissue.
The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains from the head and neck.
anterior spinal artery
blood vessel from the merged branches of the vertebral arteries that runs along the anterior surface of the spinal cord
arachnoid granulation
outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood
arachnoid mater
middle layer of the meninges named for the spider-web–like trabeculae that extend between it and the pia mater
arachnoid trabeculae
filaments between the arachnoid and pia mater within the subarachnoid space
basilar artery
blood vessel from the merged vertebral arteries that runs along the dorsal surface of the brain stem
carotid canal
opening in the temporal bone through which the internal carotid artery enters the cranium
central canal
hollow space within the spinal cord that is the remnant of the center of the neural tube
cerebral aqueduct
connection of the ventricular system between the third and fourth ventricles located in the midbrain
choroid plexus
specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain
circle of Willis
unique anatomical arrangement of blood vessels around the base of the brain that maintains perfusion of blood into the brain even if one component of the structure is blocked or narrowed
common carotid artery
blood vessel that branches off the aorta (or the brachiocephalic artery on the right) and supplies blood to the head and neck
dura mater
tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS
dural sinus
any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins
foramen magnum
large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium
fourth ventricle
the portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures
internal carotid artery
branch from the common carotid artery that enters the cranium and supplies blood to the brain
interventricular foramina
openings between the lateral ventricles and third ventricle allowing for the passage of CSF
jugular veins
blood vessels that return “used” blood from the head and neck
lateral apertures
pair of openings from the fourth ventricle to the subarachnoid space on either side and between the medulla and cerebellum
lateral ventricles
portions of the ventricular system that are in the region of the cerebrum
lumbar puncture
procedure used to withdraw CSF from the lower lumbar region of the vertebral column that avoids the risk of damaging CNS tissue because the spinal cord ends at the upper lumbar vertebrae
median aperture
singular opening from the fourth ventricle into the subarachnoid space at the midline between the medulla and cerebellum
meninges
protective outer coverings of the CNS composed of connective tissue
occipital sinuses
dural sinuses along the edge of the occipital lobes of the cerebrum
orthostatic reflex
sympathetic function that maintains blood pressure when standing to offset the increased effect of gravity
pia mater
thin, innermost membrane of the meninges that directly covers the surface of the CNS
sigmoid sinuses
dural sinuses that drain directly into the jugular veins
straight sinus
dural sinus that drains blood from the deep center of the brain to collect with the other sinuses
subarachnoid space
space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae
superior sagittal sinus
dural sinus that runs along the top of the longitudinal fissure and drains blood from the majority of the outer cerebrum
third ventricle
portion of the ventricular system that is in the region of the diencephalon
transverse sinuses
dural sinuses that drain along either side of the occipital–cerebellar space
ventricles
remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain
vertebral arteries
arteries that ascend along either side of the vertebral column through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum
Watch this animation to see how blood flows to the brain and passes through the circle of Willis before being distributed through the cerebrum. The circle of Willis is a specialized arrangement of arteries that ensure constant perfusion of the cerebrum even in the event of a blockage of one of the arteries in the circle. The animation shows the normal direction of flow through the circle of Willis to the middle cerebral artery. Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left?
If blood could not get to the middle cerebral artery through the posterior circulation, the blood would flow around the circle of Willis to reach that artery from an anterior vessel. Blood flow would just reverse within the circle.
Watch this video that describes the procedure known as the lumbar puncture, a medical procedure used to sample the CSF. Because of the anatomy of the CNS, it is a relative safe location to insert a needle. Why is the lumbar puncture performed in the lower lumbar area of the vertebral column?
The spinal cord ends in the upper lumbar area of the vertebral column, so a needle inserted lower than that will not damage the nervous tissue of the CNS.
Watch this animation that shows the flow of CSF through the brain and spinal cord, and how it originates from the ventricles and then spreads into the space within the meninges, where the fluids then move into the venous sinuses to return to the cardiovascular circulation. What are the structures that produce CSF and where are they found? How are the structures indicated in this animation?
The choroid plexuses of the ventricles make CSF. As shown, there is a little of the blue color appearing in each ventricle that is joined by the color flowing from the other ventricles.
1. What blood vessel enters the cranium to supply the brain with fresh, oxygenated blood?
A) common carotid artery
B) jugular vein
C) internal carotid artery
D) aorta
C
2. Which layer of the meninges surrounds and supports the sinuses that form the route through which blood drains from the CNS?
A) dura mater
B) arachnoid mater
C) subarachnoid
D) pia mater
A
3. What type of glial cell is responsible for filtering blood to produce CSF at the choroid plexus?
A) ependymal cell
B) astrocyte
C) oligodendrocyte
D) Schwann cell
A
4. Which portion of the ventricular system is found within the diencephalon?
A) lateral ventricles
B) third ventricle
C) cerebral aqueduct
D) fourth ventricle
B
5. What condition causes a stroke?
A) inflammation of meninges
B) lumbar puncture
C) infection of cerebral spinal fluid
D) disruption of blood to the brain
D
1. Why can the circle of Willis maintain perfusion of the brain even if there is a blockage in one part of the structure?
The structure is a circular connection of blood vessels, so that blood coming up from one of the arteries can flow in either direction around the circle and avoid any blockage or narrowing of the blood vessels.
2. Meningitis is an inflammation of the meninges that can have severe effects on neurological function. Why is infection of this structure potentially so dangerous?
The nerves that connect the periphery to the CNS pass through these layers of tissue and can be damaged by that inflammation, causing a loss of important neurological functions.