Glial cells, also known as neuroglia, are non-neuronal cells in the central and peripheral nervous systems. Historically, they were thought to primarily serve as a “glue” holding neurons in place. However, current understanding recognizes their importance for brain function, revealing diverse and dynamic roles that extend far beyond structural support. These cells are integral to the nervous system’s health and activity, facilitating communication, maintaining balance, and responding to challenges.
Providing Essential Support for Neurons
Glial cells provide foundational support to neurons and contribute to the nervous system’s structural integrity. Astrocytes, which are star-shaped and the most abundant glial cell type in the central nervous system (CNS), offer physical support and maintain the extracellular environment surrounding neurons.
Beyond structural roles, glial cells supply vital nutrients and clear metabolic waste. Astrocytes provide neurons with nutrients by extending processes that contact blood vessels and neurons. They transport glucose from the blood-brain barrier to neurons and can metabolize glucose into lactate, which is then transferred to neurons as an energy source. This metabolic support is important for neuronal activity and survival.
Oligodendrocytes in the CNS and Schwann cells in the peripheral nervous system (PNS) form myelin sheaths. Myelin is a fatty substance that insulates neuronal axons, similar to insulation on an electrical wire. This insulation increases the speed and efficiency of nerve impulse transmission, a process known as saltatory conduction, allowing electrical signals to jump between gaps in the myelin called nodes of Ranvier. Oligodendrocytes can myelinate multiple axons, while a single Schwann cell typically myelinates one axon segment.
Regulating Brain Environment and Communication
Glial cells actively maintain the brain’s internal balance and directly influence neuronal communication. Astrocytes contribute to the blood-brain barrier (BBB), a protective physiological filter that controls substance passage from blood into the brain. Their end-feet surround brain vasculature, reinforcing the barrier and regulating metabolite flow.
Astrocytes also regulate ion concentrations and neurotransmitter levels at synapses, influencing neuronal communication and synaptic plasticity. They rapidly remove excess neurotransmitters like glutamate from the synaptic cleft, preventing accumulation and limiting “spill over.” This reuptake is important for proper synaptic transmission and preventing excitotoxicity. Astrocytes also regulate potassium ion balance, which is important for synaptic function.
Microglia function as the brain’s resident immune cells, continuously surveying the brain environment for threats. These dynamic cells extend and retract their “branches” to patrol their surroundings, detecting pathogens, damaged cells, or foreign substances. Upon recognizing a threat, microglia activate, changing morphology and function to initiate an immune response. This surveillance maintains brain homeostasis.
Responding to Injury and Disease
Glial cells, particularly microglia and astrocytes, play a role in the nervous system’s response to injury, infection, or neurodegenerative diseases. Following nervous system damage, these cells become reactive, undergoing morphological and molecular changes. For instance, astrocytes undergo astrogliosis, characterized by increased glial fibrillary acidic protein and proliferation.
These reactive glial cells initiate inflammatory responses and clear cellular debris. Microglia migrate rapidly to lesion sites within minutes of injury to phagocytose dead cells and tissue debris, preventing damage and supporting tissue repair. Astrocytes also contribute to debris engulfment, especially when microglial function is impaired.
A notable response to severe CNS injury is the formation of a glial scar, primarily composed of reactive astrocytes and extracellular matrix molecules. This dense barrier seals the injured area, isolating damaged tissue and preventing inflammation and cellular damage. While protective in acute situations by limiting infection, the glial scar can impede axonal regrowth and functional recovery in chronic conditions by forming a barrier and secreting inhibitory molecules. Chronic glial activation, particularly of microglia, can also contribute to neurodegenerative disease progression by releasing neurotoxic factors and promoting sustained inflammation.