Glial Cells in the Nervous System

Glial Cells: Supporting Cast of the Nervous System

These cells are critical for supporting neurons in their specialized impulse-conducting function.

Origins and Abundance

• Etymology: Glial cells are named from the Greek word for "glue," reflecting their initially perceived role in holding neurons together.
• Quantity: Glial cells outnumber neurons in the brain.
• Function: Glial cells do not conduct electrical impulses themselves; instead, they support neurons in this function.

Four Major Functions of Glial Cells:

• Structural Support: They hold neurons in place.
• Nourishment: They supply neurons with oxygen and nutrients.
• Insulation: They electrically insulate neurons for efficient impulse conduction.
• Protection: They protect neurons from pathogens, acting as a specialized immune defense within the central nervous system.

Astrocytes: Guardians of the Central Nervous System

• Location: Found exclusively in the central nervous system.

• Shape: Star-shaped, hence the name "astrocyte."

• Primary Roles:

  • Support and protect neurons.

  • Create the blood-brain barrier.

  • Mediate inflammatory responses in the brain.

  • Influence other glial cell types.

  • Blood-Brain Barrier: Astrocytes act as bodyguards, positioning themselves between capillaries and neurons.

    • They have foot processes that attach to capillaries.
    • They police substances that might damage nerve cells.

• Reactive Astrocytes: In response to damage or inflammation, astrocytes can change and become reactive astrocytes.

Blood-Brain Barrier Formation

• Capillary Structure: Capillaries consist of endothelial cells joined together.

• Brain Capillaries: In the brain, endothelial cells are tightly bound for reduced permeability.

• Astrocyte Reinforcement:

  • Astrocytes reinforce endothelial cell attachments.
  • They create a thicker layer around capillaries.

• Structure:

  • Red blood cells inside.
  • Endothelial cell layer surrounding blood cells.
  • Astrocyte feet project around the outside of the endothelial cells.

• Limitations: Fat-soluble substances like alcohol and anaesthetics can still cross the blood-brain barrier and affect the CNS.

Schwann Cells: Myelin Producers in the Peripheral Nervous System

• Location: Peripheral nervous system only.
• Function: Produce myelin, a white lipid that insulates axons.
• Myelination Process: Schwann cells wrap around axons to form a myelin sheath, increasing the rate of conductance.
• Structure: Many layers of myelin wrapped concentrically around the axon, pushing mitochondria to the side.
• Nodes of Ranvier and Saltatory Conduction:
  • A single Schwann cell wraps around a segment of axon.
  • Gaps between Schwann cells are called nodes of Ranvier.
  • Electrical impulses jump between these nodes, speeding up conduction in a process called saltatory conduction.

Oligodendrocytes: Myelin Producers in the Central Nervous System

• Location: Central nervous system.
• Function: Produce myelin.
• Structure and Interaction: Unlike Schwann cells, one oligodendrocyte interacts with multiple axons, providing myelin insulation to many different sections across several axons.
  • A single oligodendrocyte can provide a segment of myelin for several axons.

Multiple Sclerosis: A Myelin-Related Disorder

• Pathology: Multiple sclerosis involves immune-mediated destruction of the myelin sheath.

• Consequences:

  • Destruction of myelin-rich areas in the white matter, leading to lesions.
  • Compromised speed and accuracy of nerve conduction.
  • Loss of saltatory conduction.
  • Varied symptoms, including pins and needles, loss of motor function, and visual disturbances.

• Comparison of Myelination:

  • CNS: Oligodendrocytes interact with many axons.
  • PNS: A nerve or axon cell interacts with several Schwann cells.

Microglia: Immune Defenders of the Central Nervous System

• Function: Scavenge and survey the CNS environment for debris or bacteria; act as phagocytes.
• Importance: Provide immune defense for the CNS, as many aspects of the immune system cannot cross the blood-brain barrier.
• Coordination: Often coordinated by astrocytes.
• Action: They have branches that sense and ingest any debris or unwanted substances.

Summing up glial cells and their interactions.

• Myelinating cells (oligodendrocytes).
• Astrocytes forming barrier between capillary and the nerve cell itself.
• Microglia keeping an eye on things.

Glial Cell Ready Reckoner

Irreversible Damage to the Central Nervous System

• Central Question: Why is CNS damage often irreversible despite the presence of a developed support cell network?
• Reactive Astrocytes: Following injury, astrocytes become reactive and invade the injury site.
• Glial Scar Formation: Reactive astrocytes cordon off the area, attract microglia, and create a glial scar (or plaque/cyst).
  • The glial scar physically and chemically obstructs axon regrowth.
  • Reactive astrocytes and myelin breakdown products chemically inhibit axon growth.
• Consequence: The glial response, while initially protective, ultimately makes CNS damage often irreparable.
• Peripheral Nervous System: The PNS, lacking such a specialized glial network, has the capacity to repair and regenerate itself.

Experiment to Induce PNS regeneration:

• Administering drugs helps overcome the inhibitory chemicals expressed by the glial scar.
• Experiment demonstrates that it is possible to overcome glial scar in vitro.