Lecture Notes on Glial Cells

Introduction

• Focus on glial cells: Structure and function.
• Nervous tissue consists of two main components: neurons and glial cells.
• Glial cells are found in:
  • Central Nervous System (CNS): brain and spinal cord.
  • Peripheral Nervous System (PNS): all nerves including somatic, sensory, and autonomic.

Types of Glial Cells

1. Astrocytes

• Location: Only found in the CNS (brain and spinal cord).
• Functions:
  • Blood-Brain Barrier (BBB):
    • Composed of three layers:
      • Endothelial cells (with tight junctions).
      • Basal lamina (connective tissue).
      • Foot processes of astrocytes.
  • Role in BBB:
    • Controls permeability of molecules between blood and nervous tissue.
    • Prevents unwanted substances like proteins from entering.
    • Astrocytes secrete growth factors to increase tight junctions.

2. Role of Astrocytes in Potassium Buffering

• During action potentials, potassium exits neurons.
• Astrocytes absorb excess potassium, preventing excitability issues:
  • Can store and redistribute potassium to maintain balance.

3. Neurotransmitter Regulation

• Astrocytes help regulate neurotransmitters (e.g., glutamate and GABA):
  • Reuptake excess neurotransmitters from synapses.
  • Convert glutamate into glutamine for recycling.

4. Metabolism Support

• Control glycogen and glucose metabolism:
  • Provide lactate to neurons when ATP levels decline.

5. Synaptic Interaction

• Astrocytes may enhance synapse formation and function.

6. Satellite Cells (PNS Counterpart)

• Functions similar to astrocytes but located in:
  • Dorsal root ganglia.
  • Autonomic ganglia (sympathetic and parasympathetic).

Myelination

1. Oligodendrocytes vs. Schwann Cells

• Oligodendrocytes (CNS):
  • Myelinate multiple axons (30-60).
  • Damage leads to no regeneration (linked to multiple sclerosis).
• Schwann Cells (PNS):
  • Myelinate single axon segments.
  • Damage allows for regeneration (linked to Guillain-Barré syndrome).

2. Myelin Function

• Acts as an insulator, increasing conduction velocity:
  • Saltatory conduction: Action potentials jump between nodes of Ranvier.
  • Continuous conduction: Occurs in unmyelinated axons.

Ependymal Cells

• Specialized cuboidal cells in CNS ventricles contributing to:
  • Blood-CSF barrier.
  • Production and circulation of cerebrospinal fluid (CSF).

Microglia

• Immune cells of the CNS derived from monocytes in bone marrow:
  • Act as first responders to injury or pathogens.
  • Release destructive molecules and may cause inflammation.
  • Can phagocytose pathogens and present antigens to T cells.
  • Overactivity (e.g., in HIV) can lead to damage and demyelination.

Conclusion

• Review of the structure and function of glial cells is vital to understand their roles in the nervous system.
• Importance of maintaining glial cell health for overall neural function.