Glial Cells - Structure and Function

Introduction

• Components of Nervous Tissue:
  • Neurons
  • Glial cells
• Locations of Glial Cells:
  • Central Nervous System (CNS): Brain and spinal cord
  • Peripheral Nervous System (PNS): Somatic motor, sensory, autonomic nerves

Astrocytes

• Location: Exclusively in the CNS
• Functions:
  • Blood-Brain Barrier (BBB):
    • Composed of endothelial cells, basal lamina, and astrocytes' foot processes
    • Controls molecule movement between blood and nervous tissue
    • Lipid-soluble molecules and respiratory gases pass easily; larger molecules need transport proteins
    • Astrocytes secrete growth factors enhancing tight junctions in endothelial cells
    • Damaging astrocytes increases BBB permeability
    • BBB is broken in specific brain areas (e.g., area postrema, hypothalamus-pituitary axis) allowing selected molecule crossings
  • Potassium Buffer:
    • Astrocytes uptake excess potassium during neuronal activity preventing hyperexcitability and neuronal inactivation
  • Neurotransmitter Regulation:
    • Uptake of excess glutamate and conversion to glutamine, preventing excitotoxicity
    • Conversion and recycling of glutamate and GABA
  • Glucose and Glycogen Metabolism:
    • Store, mobilize, and convert glucose to lactate, supplying neurons during high energy demand
  • Synapse Support:
    • Enhance synaptic interactions between neurons

Satellite Cells

• Location: PNS
• Function:
  • Similar to astrocytes, but found in:
    • Dorsal root ganglion
    • Autonomic ganglia (sympathetic and parasympathetic)

Oligodendrocytes vs. Schwann Cells

• Oligodendrocytes:
  • Myelinate axons in the CNS and optic nerve
  • One oligodendrocyte myelinates multiple axons (up to 60)
  • Damaged oligodendrocytes cannot regenerate
  • Demyelination results in Multiple Sclerosis
• Schwann Cells:
  • Myelinate axons in the PNS (spinal and cranial nerves 3-12)
  • One Schwann cell myelinates segments of one axon
  • Capable of regeneration, facilitated by the neurolemma
  • Demyelination results in Guillain-Barre syndrome

Myelination

• Function: Insulation and speed enhancement for action potentials
• Types of Conduction:
  • Myelinated: Saltatory conduction
  • Unmyelinated: Continuous conduction
• Role in Conduction:
  • Myelin reduces membrane capacitance, preserving ion movement and increasing speed
  • Nodes of Ranvier contain high densities of voltage-gated ion channels
• Factors affecting Conduction Velocity:
  • Myelination (more myelin, faster conduction)
  • Axon Diameter (larger diameter, faster conduction)

Ependymal Cells

• Location: Line ventricles in the CNS
• Function:
  • Part of the blood-CSF barrier
  • Composition: Fenestrated endothelial cells, basal lamina, and tight-junctioned ependymal cells
  • Control molecule movement, producing cerebrospinal fluid (CSF)
  • Cilia on ependymal cells help circulate CSF

Microglia

• Origin: Derived from monocytes (bone marrow)
• Function:
  • Act as immune cells in the CNS
  • Respond to neuronal damage by releasing destructive molecules
  • Phagocytose pathogens and present antigens to t-cells
  • Excessive activity can lead to demyelination
• Relevance: Implicated in conditions like HIV encephalitis

Summary

• Discussed roles and importance of various glial cells
• Highlighted specific functions of astrocytes, satellite cells, oligodendrocytes, Schwann cells, ependymal cells, and microglia