For many years, the nervous system was understood primarily through its electrically active cells, the neurons. Glial cells, far more numerous, were considered passive support structures or the “glue” holding the brain together. This perspective has been updated, as science now reveals that glial cells are active participants in nervous system function. They are now understood to be important for brain health, development, and activity, fulfilling duties that maintain the environment for neuronal communication.
Astrocytes and Their Supportive Roles
Astrocytes, named for their star shape, are the most abundant glial cell in the central nervous system (CNS). These cells are master regulators of the neuronal environment, providing physical and metabolic support to neurons. Their processes interact with both blood vessels and neurons, allowing them to transport nutrients like glucose from the blood to energy-demanding neurons.
A primary function of astrocytes is maintaining chemical homeostasis in the fluid surrounding neurons. They manage the concentration of ions, particularly potassium, and neurotransmitters like glutamate. By rapidly taking up excess glutamate from the synapse, astrocytes prevent neurotoxicity that can arise from over-stimulation.
Astrocytes are also integral to forming and maintaining the blood-brain barrier (BBB). This barrier is a selective membrane that protects the brain from harmful substances in the bloodstream. Astrocytic processes, called “end-feet,” wrap around brain capillaries, reinforcing the barrier and regulating what enters the CNS.
These cells also participate in synaptic activity, a concept known as the “tripartite synapse.” Astrocytes respond to neurotransmitters from neurons and, in turn, release their own signaling molecules, called gliotransmitters. This communication allows them to modulate synaptic transmission and plasticity, the cellular basis for learning and memory.
Myelination by Oligodendrocytes and Schwann Cells
A specialized function of certain glial cells is forming myelin, a fatty substance that insulates neuronal axons. This myelin sheath is segmented, allowing for a rapid form of nerve impulse transmission known as saltatory conduction. This process increases the speed of electrical signal propagation by up to 100 times compared to unmyelinated axons.
In the central nervous system, oligodendrocytes are responsible for this myelination. A single oligodendrocyte has multiple processes, each wrapping around a segment of an axon or even multiple different axons. This structure allows for efficient insulation and high-speed communication between different brain regions.
The peripheral nervous system (PNS) relies on a different type of glial cell for myelination: the Schwann cell. Unlike oligodendrocytes, a single Schwann cell myelinates only one segment of a single axon by wrapping its entire cell body around it. This one-to-one relationship is a distinction between myelination in the CNS and the PNS.
This insulating layer is comparable to the plastic coating on an electrical wire, preventing the signal from dissipating. The breaks in the myelin sheath, known as nodes of Ranvier, are where the electrical signal is regenerated, allowing it to “jump” from node to node. The myelin sheath also provides metabolic support to the axon, ensuring its long-term health.
Microglia as the Brain’s Immune System
The central nervous system is protected by the blood-brain barrier, meaning it requires its own immune defense system. This role is filled by microglia, the resident immune cells of the brain and spinal cord. These cells act as the primary line of active immune defense in the CNS, constantly surveying the environment.
In their resting state, microglia monitor the health of surrounding neurons and synapses by extending and retracting their processes. When they detect pathogens, cellular debris, or injury, they transform into an activated state. This activation involves changing their shape and migrating to the site of the problem.
Once activated, microglia perform functions similar to macrophages. They are capable of phagocytosis, a process where they engulf and digest harmful invaders, plaques, or the remains of damaged neurons. They also release molecules like cytokines and chemokines to help manage the inflammatory response.
Beyond responding to injury and disease, microglia are also involved in healthy brain development. They contribute to synaptic pruning, where unnecessary or weak synaptic connections are eliminated. This refinement of neural circuits is a part of learning and development.
Specialized Glial Cells and Functions
Beyond the more abundant glial cell types, others have highly specialized jobs. In the central nervous system, ependymal cells line the brain’s fluid-filled ventricles and the spinal cord’s central canal. These cells play a direct role in producing and circulating cerebrospinal fluid (CSF).
Cerebrospinal fluid serves as a cushion for the brain, provides nutrients, and removes waste. Ependymal cells have cilia, small hair-like projections that beat to help circulate the CSF. This circulation is important for maintaining a stable chemical environment for the CNS.
In the peripheral nervous system, neuron cell bodies are often clustered in ganglia. Here, satellite glial cells perform a supportive role. These cells surround individual neuron cell bodies, providing structural support and regulating the local chemical environment. Their function is thought to be similar to that of astrocytes.
Glial Cells in Neurological Conditions
When glial cells malfunction, it can lead to serious neurological conditions. Many brain disorders once thought to be exclusively neuronal are now understood to have a significant glial component.
Multiple sclerosis (MS) is a classic example of a disease linked to glial cell pathology. In this autoimmune disorder, the body’s immune system attacks and destroys oligodendrocytes, the myelin-producing cells of the CNS. This loss of myelin, known as demyelination, disrupts the ability of neurons to transmit signals efficiently, leading to a wide range of symptoms.
In neurodegenerative diseases such as Alzheimer’s, the role of glial cells is complex. Chronic activation of microglia can lead to sustained neuroinflammation, which contributes to neuronal damage. Astrocytes are also implicated; they can become reactive, fail to perform their neuroprotective functions, and potentially release toxic substances. Additionally, aggressive brain tumors known as gliomas arise from the uncontrolled division of glial cells.