6: Neuroglia functions and types

6: Neuroglia Functions and Types

Neuroglia (glial cells) are essential non-neuronal cells in the nervous system, outnumbering neurons by roughly $10:1$. They provide critical structural and functional support, maintaining homeostasis, enabling efficient neural signaling, and protecting neurons. Unlike neurons, glia do not generate action potentials but are vital for nervous system health.

Major Types and Functions in the CNS

1. Astrocytes (Star Cells):

   • Structure: Highly branched star-shaped morphology with end-feet contacting blood vessels and neurons.
   • Functions:
     • Blood-Brain Barrier (BBB): End-feet induce and maintain tight junctions between endothelial cells, regulating substance passage into the brain.
     • Ion & Neurotransmitter Buffering: Uptake excess potassium ions ($K^+$) and neurotransmitters (e.g., glutamate) from the synaptic cleft, preventing excitotoxicity and maintaining ionic balance.
     • Metabolic Support: Supply nutrients (like lactate) to neurons via the "lactate shuttle," especially during high activity.
     • Structural Support: Form a scaffold for neuronal migration during development and provide physical support in the adult brain.
     • Synaptic Modulation: Participate in "tripartite synapses," influencing synaptic strength and plasticity by releasing gliotransmitters.

2. Oligodendrocytes:

   • Function: Myelinate central nervous system (CNS) axons. Each oligodendrocyte extends multiple flat processes that wrap concentrically around segments of several different axons.
   • Significance: Myelin sheaths act as electrical insulators, enabling saltatory conduction – the rapid, jumping propagation of action potentials along axons. This dramatically increases conduction speed and efficiency.

3. Microglia:

   • Function: Resident immune cells and macrophages of the CNS.
   • Roles: Constantly survey the CNS environment. Upon detecting damage, infection, or abnormal proteins (e.g., in Alzheimer's), they activate, phagocytose debris, pathogens, or dead cells, and release inflammatory or anti-inflammatory signals. They also play roles in synaptic pruning during development.

4. Ependymal Cells:

   • Location & Structure: Ciliated, cuboidal/columnar epithelial cells lining the brain's ventricles and the central canal of the spinal cord.
   • Functions: Help produce cerebrospinal fluid (CSF) in the choroid plexus (formed by modified ependymal cells and capillaries). Their cilia beat to circulate CSF within the ventricular system.

Major Types and Functions in the PNS

1. Schwann Cells:

   • Function: Myelinate peripheral nervous system (PNS) axons. Each Schwann cell myelinates a single segment of one axon. They also enclose non-myelinated small-diameter axons in Remak bundles.
   • Significance: Enable saltatory conduction in the PNS. Crucially, they guide and support axonal regeneration after PNS injury by forming regeneration tubes (Büngner bands).

2. Satellite Glial Cells:

   • Location: Surround neuronal cell bodies within PNS ganglia (e.g., dorsal root ganglia, autonomic ganglia).
   • Function: Analogous to CNS astrocytes. They regulate the microenvironment of ganglionic neurons, controlling nutrient/waste exchange and modulating neurotransmission.