Glial cells

Glial Cells Overview

• Glial cells, also known as neuroglia or glia, are essential cell types within nervous tissue.

• Their primary function is to support neurons in their communication functions.

• The term "glia" is derived from the Greek word for "glue," introduced by German pathologist Rudolph Virchow in 1856.

• Modern research has highlighted their diverse and complex roles.

• There are six types of glial cells: four in the Central Nervous System (CNS) and two in the Peripheral Nervous System (PNS).

Types and Functions of Glial Cells

Glial Cells of the CNS

1. Astrocytes

   • Star-shaped appearance under a microscope.

   • Help maintain chemical concentrations in the extracellular space.

   • Remove excess signaling molecules, respond to tissue damage, and contribute to the formation of the blood-brain barrier (BBB).

   The Blood-Brain Barrier (BBB)

   • A physiological barrier that limits the entry of substances from the bloodstream into the CNS.

   • Allows essential molecules (e.g., glucose, amino acids) to pass while blocking white blood cells and most other pathogens.

   • Composed of endothelial cells that restrict movement of substances and relies on active transport mechanisms.

2. Oligodendrocytes

   • Insulate axons within the CNS by wrapping around them with myelin.

   • One oligodendrocyte can myelinate multiple axon segments.

3. Microglia

   • Smaller glial cells believed to originate from macrophages.

   • Function as CNS-resident macrophages, ingesting and digesting damaged cells and pathogens.

4. Ependymal Cells

   • Line brain ventricles and filter blood to produce CSF.

   • Possess cilia that help circulate CSF throughout the CNS and assist in maintaining the BBB.

Glial Cells of the PNS

1. Satellite Cells

   • Located in sensory and autonomic ganglia, they envelop neuron cell bodies and support their functions without creating a BBB.

2. Schwann Cells

   • Insulate single axon segments with myelin in the PNS.

   • Each Schwann cell encases one axon segment, differing from oligodendrocytes that insulate multiple axons.

Myelination

• Myelin, a lipid-rich sheath, facilitates rapid electrical signal transmission along axons.

• The myelin sheath consists of glial cell membranes and is critical for nerve signal efficiency.

• Myelination mechanisms are consistent between CNS (oligodendrocytes) and PNS (Schwann cells).

Myelin Sheath Structure

• Myelin sheath wraps around axons multiple times in oligodendrocytes with minimal cytoplasm between layers.

• In Schwann cells, myelin sheath encompasses the axon entirely, with the nucleus located on one side.

Myelin Length vs. Axon Diameter

• The length varies, extending between one to two millimeters, depending on axon diameter (1-20 micrometers).

Disorders of the Nervous Tissue

• Multiple Sclerosis (MS): An autoimmune disorder leading to demyelination in the CNS, resulting in inflammation and scarring.

  • Symptoms include motor and sensory deficits due to compromised axonal insulation.

• Guillain-Barré Syndrome: An autoimmune demyelinating disorder affecting the PNS, possibly causing motor deficits and autonomic symptoms.

Terms to Know

• Cerebrospinal Fluid (CSF): A clear fluid in the CNS that cushions, protects, and provides nutrients to nervous tissue.

• Demyelination: The loss or damage of the myelin sheath, causing issues with nerve signal transmission, associated with diseases like MS.