Glial

Glial cells, commonly referred to as glia, are non-neuronal cells that support, protect, and modulate the activity of neurons throughout the nervous system. They are found in the central nervous system, which comprises the brain and spinal cord, and the peripheral nervous system. The main CNS glia are astrocytes, oligodendrocytes, and microglia; in the PNS, the primary glia are Schwann cells and satellite cells, with additional roles played by ependymal cells lining brain ventricles.

Glial functions are diverse and essential for nervous system operation. Astrocytes help regulate the extracellular ion and neurotransmitter environment, maintain the blood-brain barrier, supply metabolic support to neurons, and participate in synaptic signaling. Oligodendrocytes form the myelin sheath that insulates axons in the CNS, while Schwann cells perform a similar myelinating role in the PNS. Microglia act as resident immune cells, surveilling for damage and clearing debris through phagocytosis. Ependymal cells line ventricles and contribute to cerebrospinal fluid production and circulation. Glia also guide neural development, influence synaptic formation and pruning, and contribute to plasticity and repair after injury.

Glial cells originate from distinct embryonic lineages; CNS astrocytes and oligodendrocytes arise from neural progenitors, whereas Schwann cells come from the neural crest. Satellite cells encase sensory and autonomic ganglia. Although once thought mainly structural, glia are now recognized as active participants in neural signaling, metabolism, and immune responses, shaping neuronal networks.

Clinical relevance includes gliomas, tumors derived from glial cells, and demyelinating diseases such as multiple sclerosis. Glial function is also implicated in neurodegenerative conditions and neuroinflammation.