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Notes on Glial Cells of the Nervous System

Types of Glial Cells in the Nervous System

  • Overview of Glial Cells
    • There are primarily two categories of glial cells: those in the central nervous system (CNS) and those in the peripheral nervous system (PNS).
    • The five types of glial cells: Astrocytes, Microglia, Ependymal cells, Oligodendrocytes (CNS), and Schwann cells (PNS).

Astrocytes

  • Definition: The most numerous type of glial cell in the CNS.
  • Shape: Star-shaped and the largest among glial cells.
  • Functions:
    • Connect neurons to blood capillaries, facilitating nutrient transfer.
    • Create the Blood-Brain Barrier (BBB):
    • Composed of tight junctions between astrocytic end-feet and endothelial cells of capillaries.
    • Protects the brain by regulating which substances can enter from the bloodstream.
    • Some drugs and chemicals can cross the BBB, while others cannot.

Microglia

  • Definition: Small, usually stationary cells in the CNS.
  • Functions:
    • Act as the brain's immune defense mechanism.
    • Undergo changes (enlargement and mobility) during inflammation to carry out phagocytosis, engulfing harmful agents and debris in the brain tissue.

Ependymal Cells

  • Definition: Glial cells resembling epithelial cells, lining fluid-filled cavities in the CNS.
  • Location: Found in the central canal and the ventricles of the brain.
  • Functions:
    • Some ependymal cells produce cerebrospinal fluid (CSF).
    • Assist in the circulation of CSF within the brain and spinal cord, similar to ciliated epithelial cells in the respiratory tract.

Oligodendrocytes

  • Definition: Smaller glial cells in the CNS that wrap around neuron fibers.
  • Functions:
    • Produce the myelin sheath:
    • Composed of fat and protein, insulating neurons and enhancing the conduction speed of nerve impulses.
    • Can wrap around multiple axons simultaneously, providing myelin for several neurons at once.

Schwann Cells

  • Definition: The only type of glial cell found in the peripheral nervous system.
  • Functions:
    • Similar function to oligodendrocytes: wrap around individual neuron fibers and produce myelin.
    • Important for the regeneration and repair of damaged neurons in PNS.
    • The membrane of Schwann cells is known as the neurolemma.

Key Distinctions

  • Oligodendrocytes vs. Schwann Cells:
    • Oligodendrocytes (CNS) can myelinate multiple axons; Schwann cells (PNS) myelinate one axon.
  • Myelin Sheath:
    • Provides insulation to neurons, crucial for increasing conduction speed.
    • Gaps in the myelin sheath called Nodes of Ranvier facilitate rapid signal transmission along the neuron.

Additional Information

  • Importance of the Neurolemma:
    • Essential for the regeneration of neurons in the PNS; if damaged, repair is difficult, often leading to scar tissue formation.

Summary of Glial Cells

  • Central Nervous System: Astrocytes, Microglia, Ependymal cells, Oligodendrocytes.
  • Peripheral Nervous System: Schwann cells.
  • Functions: Support, protect, and maintain neurons, playing critical roles in nervous system health and functionality.