Glial Cells & Neuronal Activity

Glial Cells & Neuronal Activity

• Glial cells support neuronal survival and function, significantly more abundant in the mammalian brain than neurons.
• Glial cells originate from progenitor cells in the embryonic neural plate.
• Glial cells surround neuronal cell bodies, axons, and dendrites, filling spaces between neurons.
• The CNS has minimal connective tissue, with glial cells playing a connective tissue-like role.
• Glial cells create a specialized microenvironment surrounding neurons for optimal activity.
• The intercellular network surrounding CNS cells resembles collagen but is a network of cellular processes (neuropil).

Types of Glial Cells

• Oligodendrocytes:

  • Small cells that produce myelin sheaths around axons in the CNS.
  • Their sheet-like processes wrap around parts of several axons, forming myelin sheaths.
  • Predominate in the white matter of the CNS due to the lipid concentration in the myelin sheaths.
  • Crucial for electrical insulation of neurons.
  • Damage in multiple sclerosis (MS) affects oligodendrocyte function, exacerbating the disease.

• Astrocytes:

  • Abundant, star-shaped glial cells with radiating processes.
  • Two types: fibrous (white matter) and protoplasmic (gray matter).
  • Extensive processes associate with numerous synaptic sites.
  • Contain glial fibrillary acidic protein (GFAP), a marker for astrocytes (commonly associated with brain tumors).
  • Functions include:
    • Forming the blood-brain barrier (BBB) by associating with blood vessels.
    • Supporting synapses, affecting their formation, function, and plasticity.
    • Forming the glial limiting membrane, a barrier layer surrounding the CNS.
  • Allow for communication between astrocytes through gap junctions.

• Ependymal Cells:

  • Columnar or cuboidal cells lining ventricles and central canal of the CNS.
  • Some have cilia that facilitate cerebrospinal fluid (CSF) movement.
  • Involved in CSF absorption.

• Microglia:

  • Small, macrophage cells with irregular processes.
  • Distributed throughout gray and white matter.
  • Migrate through the neuropil, scanning for damaged cells and pathogens.
  • Secrete immunoregulatory cytokines.
  • Primary immune defense mechanism in the CNS, similar to other antigen-presenting cells.

• Schwann Cells:

  • Found only in the peripheral nervous system (PNS).
  • Differentiate from neural crest precursors,
  • Myelinate axons in the PNS, one Schwann cell per segment of axon, in contrast to oligodendrocytes which can myelinate parts of several axons.

• Satellite Cells:

  • Surround neuronal cell bodies in ganglia of the PNS.
  • Derived from the embryonic neural crest.
  • Form an intimate covering layer over neuronal cell bodies.
  • May exert trophic or supportive effects on neurons.

Central Nervous System (CNS)

• Composed of cerebrum, cerebellum, and spinal cord.
• Protected by meninges (three connective tissue layers).
• Consists of white matter (primarily myelinated axons) and gray matter (neuronal cell bodies, dendrites).
• White matter tracts contain myelinated axons and myelin-producing oligodendrocytes.
• Gray matter contains abundant neuronal cell bodies, dendrites, and unmyelinated axons.
• Gray matter typically occupies the surface (cortex) of the cerebrum and cerebellum, and deeper regions may have nuclei.
• Cerebral cortex contains neurons organized in layers, including Purkinje cells in the cerebellum.
• The spinal cord has a central canal lined by ependymal cells, with gray matter in the shape of an "H" and white matter surrounding it. Gray matter houses motor neurons in anterior horns and sensory neuron cell bodies in the posterior horns.