Astroglia, also known as astrocytes, are a prominent type of glial cell found within the central nervous system. These cells reside in both the brain and spinal cord, making them the most numerous glial cells in these regions. For many years, scientists largely viewed astroglia as merely supportive “glue” that held neurons in place. However, current understanding reveals them as active participants in brain function, performing many tasks.
Structural and Metabolic Support for Neurons
Astroglia provide a physical scaffold that helps organize and maintain the brain’s intricate architecture. Their processes extend throughout the neural tissue, enveloping neurons and synapses. This branching network contributes to the stability and positioning of neuronal circuits.
Beyond structural support, astroglia play a role in providing energy to neurons. They take up glucose, the brain’s primary fuel, from blood vessels. This glucose is processed into lactate through a metabolic pathway called aerobic glycolysis.
Lactate is then released by astroglia and absorbed by nearby neurons. Neurons can convert this lactate into pyruvate, which enters their mitochondria to produce ATP, the main energy currency of the cell. This metabolic shuttling ensures a consistent energy supply, allowing neurons to maintain their high energy demands for signaling.
The Blood-Brain Barrier Gatekeepers
A specialized structure called the blood-brain barrier (BBB) functions as a selective interface, protecting the brain from harmful substances in the bloodstream. This barrier prevents toxins, pathogens, and large molecules from entering the neural tissue. It achieves this through tightly connected endothelial cells that line the brain’s capillaries.
Astroglia are necessary for the formation and maintenance of this protective barrier. Their extensions, known as astrocytic endfeet, wrap around the brain’s capillaries, forming a close association with endothelial cells. While astroglia do not directly form tight junctions, their presence and signaling molecules induce and reinforce them.
This interaction strengthens the barrier, ensuring its integrity and controlling substance passage into the brain. Astroglia release factors that promote the expression of proteins involved in tight junction formation, directing endothelial cells to maintain a secure and impermeable seal.
Regulating Synaptic Activity
Astroglia are active participants in neuronal communication, forming the “tripartite synapse,” which includes pre-synaptic and post-synaptic neurons, and surrounding astrocyte processes that ensheath the synaptic cleft. This proximity allows them to influence signal transmission.
One of their roles includes clearing neurotransmitters from the synaptic cleft after a neuronal signal. For instance, astroglia possess transporters that absorb glutamate, the brain’s main excitatory neurotransmitter. This removal is important for terminating the synaptic signal and preventing overstimulation, which can lead to cell damage.
Astroglia modulate synaptic strength and plasticity through gliotransmission. They release signaling molecules, known as gliotransmitters, into the synaptic space. These molecules, such as D-serine or ATP, bind to receptors on neurons, influencing their excitability and synaptic transmission efficiency.
Role in Brain Injury and Disorders
When the central nervous system experiences injury, such as a stroke, trauma, or infection, astroglia undergo a transformation known as reactive gliosis. During this response, astroglia change shape, increase in size, and proliferate, extending processes into the damaged area. They form a dense network that encapsulates the injury, creating a “glial scar.”
The glial scar protects by walling off damaged tissue, limiting inflammation and preventing neuronal death. However, this scar can also be detrimental long-term, as its dense structure can inhibit regrowth and repair of damaged axons. This dual nature makes the astrocytic response complex.
Astroglial dysfunction is also recognized in neurodegenerative diseases. In conditions like Alzheimer’s disease, astroglia can lose their ability to provide metabolic support or clear toxic protein aggregates, contributing to neuronal decline. In Amyotrophic Lateral Sclerosis (ALS), reactive astroglia may release harmful factors that contribute to motor neuron degeneration, accelerating disease progression.