The brain relies on the coordinated activity of diverse cell types. While neurons, which transmit electrical signals, often receive the most attention, other cellular components are equally significant. Among these are astrocytes, star-shaped cells abundant in the cerebral cortex, the brain’s outermost layer involved in higher cognitive functions.
Understanding Cortical Astrocytes
Cortical astrocytes are specialized glial cells, a category of non-neuronal cells that support and protect neurons throughout the central nervous system. Their name, derived from the Greek word “astron” meaning “star,” reflects their distinctive morphology with numerous radiating processes. These processes allow astrocytes to interact extensively with neurons, blood vessels, and other glial cells within the cerebral cortex.
Astrocytes are the most numerous glial cells in the brain and are closely associated with neuronal synapses. Unlike neurons, they do not generate electrical impulses directly. Instead, astrocytes play a supportive role, maintaining the environment for effective neuronal operation. Within the cerebral cortex, astrocytes exhibit morphological diversity, including types like protoplasmic astrocytes enclosing synapses.
The Multifaceted Roles of Astrocytes
Astrocytes perform a wide array of functions fundamental to a healthy brain. They provide metabolic and physical support to neurons, forming a close relationship with neuronal synapses. This association allows astrocytes to influence the formation, function, and plasticity of synapses, the connections between neurons that enable communication. Astrocytes can modulate synaptic transmission by releasing signaling molecules.
Astrocytes also contribute to the integrity and function of the blood-brain barrier (BBB), a protective barrier regulating substance passage from the bloodstream into the brain. This barrier, composed of endothelial cells, pericytes, and astrocytes, prevents harmful molecules from entering the brain. Astrocytes provide structural support and influence the transport properties of endothelial cells, contributing to BBB maintenance.
A further role of astrocytes involves regulating the chemical environment around neurons, known as ion and neurotransmitter homeostasis. They regulate the concentration of ions such as potassium and calcium, and neurotransmitters like glutamate. For instance, astrocytes remove excess potassium from the extracellular space during neurotransmission. They also clear excess neurotransmitters, like glutamate, from synaptic spaces to optimize synaptic signaling.
Astrocytes play a role in energy supply to active neurons. They provide neurons with metabolic substrates such as glucose and lactate, supporting their high energy demands.
Astrocytes in Brain Disorders
Dysfunctional or reactive astrocytes are implicated in the progression of various neurological diseases. In response to injury or disease, astrocytes can undergo “reactive astrogliosis,” a change in their state that can be detrimental. This reactive state contributes to neuroinflammation, a common feature across many brain disorders.
In Alzheimer’s disease, astrocytes are involved in amyloid plaque pathology and neuroinflammation. Their reactivity can exacerbate disease progression by contributing to the inflammatory environment and potentially impairing neuronal function. Similarly, in Parkinson’s disease, astrocytes play a role in neurodegeneration and inflammation, influencing the survival of dopamine-producing neurons.
During acute injuries such as stroke and traumatic brain injury (TBI), astrocytes exhibit a dual role, contributing to both initial damage and subsequent repair processes. They can form a glial scar that isolates the injured area, though this scar can also hinder axonal regeneration. In epilepsy, astrocytes contribute to neuronal hyperexcitability, a hallmark of seizures. They are involved in glutamate uptake, and disruptions in this process can lead to increased extracellular glutamate levels, promoting excessive neuronal firing.