Brain Protection: Meninges, Ventricles, CSF & Blood-Brain Barrier

Protection of the brain

The skull protects the brain from trauma, movement, and temperature changes; additional protection is provided by the cranial meninges, cerebrospinal fluid (CSF), and blood-brain barrier.

Cranial meninges

Three protective membranes surrounding the brain: dura mater, arachnoid mater, and pia mater.

Dura mater

Outermost, thick, tough meningeal layer surrounding the brain.

Arachnoid mater

Middle meningeal layer located between the dura mater and pia mater.

Pia mater

Innermost meningeal layer that directly contacts the brain and follows its sulci and fissures.

Epidural space

Potential space between the cranial dura mater and skull.

Subdural space

Space deep to the dura mater containing extracellular fluid and veins.

Subarachnoid space

Space between the arachnoid mater and pia mater containing cerebrospinal fluid and blood vessels.

Arachnoid trabeculae

Extensions of the arachnoid mater that cross the subarachnoid space and anchor the arachnoid mater to the pia mater.

Arachnoid granulations (villi)

Projections of the arachnoid mater involved in returning cerebrospinal fluid to the bloodstream.

Pia mater and CSF

Pia mater allows exchange between extracellular fluid of the brain and cerebrospinal fluid.

Infectious meningitis

Potentially life-threatening infection that causes inflammation of the meninges.

Classic signs of infectious meningitis

Headache, lethargy, stiff neck, and fever.

Diagnosis of infectious meningitis

CSF is examined for infectious agents and white blood cells.

Viral meningitis

Generally milder form of meningitis that usually resolves in 1–2 weeks.

Bacterial meningitis

Serious meningitis that can rapidly progress to brain involvement and death without aggressive antibiotic treatment; many forms are vaccine-preventable.

Ventricles

CSF-filled cavities within the brain.

Lateral ventricles

First and second ventricles located within the cerebral hemispheres; each has extensions called horns.

Third ventricle

Ventricle located between the right and left portions of the diencephalon.

Interventricular foramina

Openings that allow CSF to flow from the lateral ventricles into the third ventricle.

Cerebral aqueduct

Canal that carries CSF from the third ventricle to the fourth ventricle.

Fourth ventricle

Ventricle located between the pons and cerebellum that receives CSF through the cerebral aqueduct.

Fourth ventricle connections

Connects with the central canal of the spinal cord and has openings allowing CSF to enter the subarachnoid space.

Cerebrospinal fluid (CSF)

Clear, colorless fluid that fills the brain's ventricles and surrounds the CNS in the subarachnoid space.

Functions of CSF

Cushions the brain, helps regulate temperature, removes waste products, and provides buoyancy.

CSF volume and production

About 150 mL of CSF circulates at one time, while approximately 750–800 mL is produced each day.

CSF replacement

About half of the circulating CSF is replaced every 5–6 hours.

Choroid plexus

Specialized structures associated with the ventricles that produce cerebrospinal fluid.

CSF circulation pathway

CSF flows through the lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct → fourth ventricle → central canal and/or subarachnoid space → returned to blood through arachnoid granulations.

CSF and buoyancy

CSF supports the brain so its effective weight against the floor of the cranial cavity is greatly reduced.

Hydrocephalus

Abnormal accumulation of CSF that increases intracranial pressure and may damage brain tissue.

Causes of hydrocephalus

May result from excessive CSF production, blockage of CSF flow, or failure to properly reabsorb CSF.

Blood-brain barrier (BBB)

Selective barrier that prevents many substances in the blood from gaining access to brain cells.

Structures forming the blood-brain barrier

Brain capillary endothelial cells, their basal lamina, and astrocyte feet surrounding the capillaries.

Brain capillary endothelial cells

Endothelial cells joined by tight junctions that greatly restrict movement of substances from blood into brain tissue.

Tight junctions in the BBB

Connections between brain capillary endothelial cells that prevent many substances from passing between the cells.

Astrocyte feet

Processes of astrocytes that surround brain capillaries and contribute to the environment of the blood-brain barrier.

Substances that readily cross the BBB

Water, oxygen, carbon dioxide, and lipid-soluble compounds can cross the blood-brain barrier.

Substances requiring transport across the BBB

Glucose, amino acids, and ions cross using specific channels or carrier proteins.

Substances restricted by the BBB

Large polar substances and many potentially harmful compounds are prevented from freely entering brain tissue.

Protective benefit of the BBB

Helps prevent toxins and infectious organisms in the blood from reaching brain cells.

Disadvantage of the BBB

The same mechanisms that block harmful substances also prevent many medications from reaching brain tissue.

Location of the BBB

The blood-brain barrier is a property of brain capillaries throughout the brain rather than a physical wall surrounding the brain.