Brain Support Systems: Meninges, Ventricular System, Glia, and Blood Supply

Supporting the Brain

Meninges

  • Tough, protective tissue/sheath that covers the CNS and some of the PNS (as peripheral needs to be more flexible).

  • Consists of three layers (four counting the fluid-filled space):

    1. Dura mater – outer layer; thick, tough, and flexible but not stretchable.

    2. Arachnoid membrane – middle layer, located between dura mater and pia mater, is soft and spongy.

    3. Pia mater – clings to the surface of the brain and spinal cord; thin and delicate, smaller surface blood vessels are found here.

    4. Subarachnoid space – fluid-filled space between the arachnoid membrane and pia mater, cushions the brain. Contains cerebrospinal fluid (CSF).

CNS

  • Covered by 3 layers of meninges:

    • Dura mater

    • Arachnoid membrane

    • Pia mater

PNS

  • Two layers fuse:

    • Dura and pia mater fuse.

    • The sheath protects spinal and cranial nerves and the autonomic ganglia.

    • The arachnoid membrane is not present.

Ventricular System and Cerebrospinal Fluid (CSF)

  • The ventricular system sits in the sub-cortex.

  • The brain is delicate and needs protection from shock; therefore, it floats in a pool of CSF which is produced and found in the ventricular system.

  • Ventricles are hollow, interconnected spaces within the brain, filled with cerebrospinal fluid.

  • Cerebrospinal fluid is a clear fluid.

Four Ventricles

  1. Lateral ventricles – sited in the center of the telencephalon (largest).

  2. 3rd ventricle – sited at the midline in the center of the diencephalon.

  3. Cerebral aqueduct – a long tube in the mesencephalon connects the 3rd and 4th ventricles.

  4. 4th ventricle – found between the cerebellum and pons.

Two Central Functions of Ventricles

  1. Production and flow of CSF

  2. Protection of the CNS and maintenance

  • Sole function: to produce and secrete cerebrospinal fluid to protect and maintain the CNS.

Cerebrospinal Fluid

  • Extracted from blood.

  • Constantly being produced and flowing around the brain and spinal cord.

  • Consists of ions, water, protein, and glucose.

  • Produced constantly from the choroid plexus.

  • Total volume of CSF is ~125ml.

  • Takes 3 hours for half of the CSF to be replaced.

CSF Flow

  • Produced by the choroid plexus of lateral ventricles.

  • Flows to the 3rd ventricle where more is produced.

  • Flows through the cerebral aqueduct to the 4th ventricle.

  • Leaves ventricles to flow into the subarachnoid space around the CNS, where it is reabsorbed into the bloodstream through arachnoid granulations.

Four Vital Functions of CSF

  1. Protection – reduces shock caused by sudden head movement.

  2. Buoyancy – reduces weight on the base of the brain, allowing the brain to ‘float’ in the skull.

  3. Waste reduction – removes waste and harmful chemicals from the brain, particularly during sleep.

  4. Transport – hormones throughout the brain.

Hydrocephalus

  • Accumulation of CSF within the cerebral ventricles.

  • Leads to ventricular dilatation.

  • Classified into two types:

    • Obstructive – caused by a blockage to the natural ventricular drainage system and CSF flow.

    • Communicating – reduced absorbance of CSF by the arachnoid villi.

Glia

  • Supporting cells of the nervous system

  • Several types, each with a special role

Astrocytes

  • Physical support ‘neuron glue’.

  • Nourish neurons (wrap blood vessels to receive, store, and release nutrients to neurons).

  • Help control the chemical composition of extracellular fluid (sodium chloride).

  • Surround and isolate synapses.

Microglia

  • Smallest glial cells.

  • Are phagocytes (clean up the brain).

  • Representative of the immune system in the brain (protect the brain from invading microorganisms).

  • Primarily responsible for the inflammatory reaction in response to brain damage.

  • Also play an important role in keeping the brain clean.

Phagocytosis

  • Where a cell binds to something it wants to engulf (destroy it).

  • Astrocytes bind to dead cells in the CNS and form scar tissue in place of dead tissue.

Amyotrophic Lateral Sclerosis (ALS)

  • Most common form of motor neuron disease.

  • Attacks nerve cells, rapidly progressive, leading to fatality.

  • Lose all voluntary muscle control.

  • Typically impacts respiration (unable to breathe for ourselves).

  • Thought to have a role in neurodegenerative disorders (Alzheimer's and Parkinson's) and viral infections (HIV).

Oligodendrocytes

  • CNS

  • Wraps several axons

  • Contains fatty tissue called myelin that wraps around neuron axons

  • Forms insulating coating (myelin sheath) which helps with the propagation of the AP.

Schwann Cells

  • PNS

  • Wraps individual axons

  • Same role as oligodendrocytes.

Blood Supply to the NS

Brain Blood Supply

  • The brain receives 15-20% of the body’s blood supply.

  • Nutrients and oxygen are carried to the brain by blood vessels.

Basic Blood Function

  1. Blood brings materials necessary for brain function

    • Oxygen

    • Nutrients

    • Hormones

  2. Blood removes materials from the brain

    • Carbon dioxide

    • Lactate

    • Hormones

    • Ammonia

  • These become toxic if left in the brain.

Blood-Brain Barrier (BBB)

  • Semi-permeable barrier between blood and brain.

  • Too much fluid in the brain may disrupt the brain and its balanced chemicals from doing its job.

  • Some substances can cross the barrier, and others can’t.

  • In most parts of the body, the smallest blood vessels (capillaries) are lined with endothelial cells.

  • There are small spaces between each endothelial cell to allow substances to move across capillary walls; however, in the brain, there are no gaps (BBB).

BBB - 3 Main Functions

  1. Protects the brain from “foreign substances” in blood that may injure the brain.

  2. Protects the brain from hormones and neurotransmitters in the rest of the body.

  3. Maintains a constant environment for the brain.

Substances and BBB

  • Lipid-soluble molecules can penetrate through the BBB relatively easily via lipid membranes of cells.

  • Water-soluble molecules can only use specialized carrier-mediated transport mechanisms.

  • Active transport allows some substances to move across capillary walls.

  • The barrier is weaker in some areas.