Brain Meninges and Mechanical Suspension

• Neurons require support to function effectively, relying on ventricles and meninges for protection and cerebrospinal fluid suspension
• They also need a stable blood supply due to high energy demands

Meninges

• Three layers surround the brain: dura mater (outer, collagenous), arachnoid mater (thinner, cellular), and pia mater (inner, thin)
• The intracranial dura has a vascular outer layer and a less vascular inner layer

Mechanical Suspension

• Subarachnoid space exists normally between the arachnoid and pia mater
• The extent changes depending on the separation of the skull and the brain's surface
• It is narrow over gyri, extends into sulci, and enlarges to form subarachnoid cisterns in depressed areas
• The subarachnoid space includes cerebrospinal fluid (CSF), arachnoid trabeculae, and blood vessels
• CSF and arachnoid trabeculae provide mechanical suspension
• The brain is nearly weightless inside the head due to similar density to CSF

Dural Septa

• Infoldings of the inner dural layer create dural septa that continue the mechanical suspension
• Dural septa include the falx cerebri (between hemispheres) and tentorium cerebelli (over cerebellum)

Dural Venous Sinuses

• Spaces are created as the inner dural layer folds inward, which forms endothelium-lined dural venous sinuses
• These sinuses are veins within the dura mater
• The superior sagittal sinus runs along the attachment of the falx cerebri, and a transverse sinus runs along the tentorium cerebelli
• Blood flows posteriorly through the superior sagittal sinus and into transverse sinuses at the confluence of sinuses
• Each transverse sinus curves into the sigmoid sinus, which empties into the internal jugular vein supplying venous outflow from the CNS
• Other venous sinuses include the inferior sagittal sinus and the straight sinus

Potential Meningeal Spaces

• No epidural or subdural spaces are normally present; they are potential spaces created by bleeding
• Epidural bleeding is commonly caused by tearing a meningeal artery
• Subdural bleeding is often caused by tearing a cerebral vein as it enters a dural venous sinus

Spinal Meninges

• Spinal cord meninges are similar to those of the brain, but the dura mater attachment is different
• The intracranial dura's layers split at the foramen magnum, with the periosteal part remaining attached to the skull
• The inner layer continues as a dural sheath around the spinal cord.
• There is an epidural space between the dura and periosteum in the vertebral canal
• Spinal dura is lined with arachnoid and the cord is suspended by connective tissue strands
• Thickened pia forms denticulate ligaments on the sides of the spinal cord
• The dural sheath extends to the S2 vertebra level, while the spinal cord ends between L1 and L2 vertebrae
• A subarachnoid cistern (lumbar cistern) is formed between these levels

Meningeal Extensions Around Peripheral Nerves

• The meninges extend into peripheral nerves to provide mechanical stability
• The dura mater continues as the epineurium, arachnoid mater as the perineurium
• Wisps of connective tissue make up the endoneurium

Ventricles

• The neural tube's cavity persists as a continuous system of ventricles
• Each hemisphere has a C-shaped lateral ventricle with anterior, inferior, and posterior horns
• The lateral ventricle connects to the third ventricle via the interventricular foramen
• The third ventricle funnels into the cerebral aqueduct, which runs through the midbrain
• The aqueduct opens into the fourth ventricle in the pons and medulla
• The fourth ventricle narrows into the central canal
• Ventricles connect with subarachnoid space through three apertures in the fourth ventricle

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless liquid of low protein and has a controlled ionic composition
• Produced in the ventricles by the choroid plexus
• Ependymal layer specializes as choroid epithelium, and pia mater covers the choroid epithelium
• The lateral ventricles contain most of the choroid plexus, in the form of a long strand
• CSF is made of serum components that leak from choroidal capillaries, diffuse past the pial elements, and are transported across the choroid epithelium

Circulation of Cerebrospinal Fluid

• CSF is constantly produced to replace the volume in the ventricles and subarachnoid space
• CSF moves through the ventricles, out of the fourth ventricle apertures, and into the posterior fossa cisterns
• It moves upward through the tentorial notch to reach arachnoid villi in the dural venous sinuses
• Arachnoid villi act as one-way flap valves and are the main sites for reabsorption

Central Nervous System Blood Supply

• Neurons need a constant blood supply of glucose, oxygen, and other necessities
• The CNS accounts for only about 2% of the body weight but gets about 15% of the blood pumped
• Interruption of the blood supply causes irreversible damage
• Blood supply comes from two pairs of arteries: an internal carotid artery and a vertebral artery

Arteries

• The internal carotid artery branches to supply most of the cerebrum
• The vertebral system supplies the rest of the cerebrum, cerebellum, brainstem, and upper spinal cord
• The internal carotid artery divides into the middle cerebral and anterior cerebral arteries
• The middle cerebral artery supplies most of the cerebral cortex on the hemisphere's lateral surface, while the anterior cerebral artery supplies the cingulate gyrus and the medial frontal and parietal lobes
• The vertebral arteries fuse into the basilar artery, which ends by bifurcating into the two posterior cerebral arteries
• Posterior cerebral arteries supply the thalamus and the medial occipital lobe
• Perforating arteries supply nearby deep structures
• The circle of Willis connects the internal carotid and vertebral-basilar systems

Collateral Circulation

• The circle of Willis interconnects the internal carotid and vertebral-basilar systems of both sides
• Spinal cord blood supply is from the anterior and posterior spinal arteries, which are vertebral artery branches
• Anterior spinal arteries form a midline artery supplying the anterior two-thirds of the cord's central area
• Posterior spinal arteries supply the posterior third of the cord and receive blood from radicular arteries

Veins

• Brain veins include superficial veins draining to the superior sagittal sinus and deep veins draining to the straight sinus
• Deep veins join near the interventricular foramen and turn posteriorly as the internal cerebral vein
• The two internal cerebral veins form the great vein (of Galen), which empties into the straight sinus

Barrier Systems

• Diffusion barriers isolate extracellular fluids around neurons
• Tight junctions between endothelial cells of CNS capillaries form the blood-brain barrier
• Arachnoid cells also form tight junctions and an arachnoid barrier
• Choroid epithelium stops leakage out of choroid plexus capillaries
• Blood-nerve barrier forms by barrier properties continuing into peripheral nerves
• Circumventricular organs monitor bloodstream concentrations and lack a blood-brain barrier