Neuroglia, or glial cells, are the non-neuronal cells of the nervous system. Their name means “nerve glue,” reflecting an old belief that they were merely passive support structures for neurons. This view has been overturned, as neuroglia are now known to be active participants in nervous system function. They are responsible for maintaining the brain’s environment, influencing how neurons communicate, and make up about half the volume of the brain and spinal cord.
The Different Kinds of Glial Cells
The nervous system is divided into the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which encompasses nerves outside the CNS. Different types of glial cells populate each area. The four main types of neuroglia in the CNS are:
- Astrocytes: Star-shaped cells that are the most numerous in the brain.
- Oligodendrocytes: Cells that create insulation for nerve fibers.
- Microglia: The primary immune defenders within the brain.
- Ependymal cells: Cells that line the fluid-filled ventricles of the brain.
In the PNS, two other types of glial cells perform supportive roles. Schwann cells are the PNS equivalent of oligodendrocytes, forming the myelin sheath around nerve axons. Satellite cells surround the cell bodies of neurons in PNS ganglia and have a function similar to that of astrocytes in the CNS.
Essential Roles of Neuroglia
Astrocytes play a significant role in maintaining homeostasis, or a stable environment, for neurons. They regulate the concentration of ions like potassium in the extracellular fluid and control the uptake of chemical messengers called neurotransmitters at the synapse. This ensures that neurons can signal effectively without interference.
A primary function of certain glial cells is myelination. Oligodendrocytes in the CNS and Schwann cells in the PNS wrap their membranes around neuronal axons, forming a fatty, insulating layer called myelin. This sheath is not continuous and has small gaps called nodes of Ranvier. This structure allows electrical signals to jump from node to node in a process called saltatory conduction, which increases the speed of nerve impulse transmission.
Microglia act as the brain’s resident immune defenders, constantly surveying the neural environment for pathogens or cellular debris. When they detect a problem, microglia can engulf and destroy threats and clean up waste. Another specialized function is carried out by ependymal cells. They are involved in producing and circulating cerebrospinal fluid (CSF), the clear liquid that bathes and cushions the brain and spinal cord.
Neuroglia Compared to Neurons
The primary distinction between neuroglia and neurons is their function. Neurons are specialized for processing and transmitting information through electrical signals, known as action potentials. In contrast, neuroglia support, protect, and maintain the optimal environment for neurons. Glial cells do not generate action potentials and communicate more locally through chemical signals.
Another major difference is their capacity for cell division. Most neurons are post-mitotic, meaning they lose the ability to divide and replicate after reaching maturity. This is why recovery from brain and spinal cord injuries can be so challenging. Glial cells, however, retain the ability to divide throughout a person’s life. This allows them to respond to injury and disease by proliferating.
It is estimated that the human brain contains roughly the same number of glial cells as neurons, with some estimates suggesting glia outnumber them. The ratio of glia to neurons can vary significantly between different brain regions. For example, the cerebral cortex has a much higher ratio of glia to neurons compared to the cerebellum.
The Role of Glial Cells in Health and Disease
When neuroglia malfunction, it can lead to serious diseases. Because glial cells can replicate throughout life, they are the origin of most primary brain tumors. These tumors, known as gliomas, are classified based on the specific type of glial cell from which they originated.
The process of myelination is also susceptible to disease. Multiple Sclerosis (MS) is a well-known demyelinating disorder of the central nervous system. In MS, the body’s own immune system mistakenly attacks and destroys oligodendrocytes. This loss of the myelin sheath disrupts the ability of neurons to transmit signals efficiently, leading to a wide range of neurological symptoms.
Neuroglia also play a role in neurodegenerative diseases like Alzheimer’s. In a healthy brain, microglia and astrocytes help maintain a non-inflamed state. However, in response to injury or disease, these cells can become overactivated. This chronic activation can lead to neuroinflammation, where glial cells release substances that can be toxic to neurons, contributing to progressive nerve cell death.