Final answer:
Astrocytes do not form myelin sheaths; this function is performed by oligodendrocytes in the CNS and Schwann cells in the PNS.
Step-by-step explanation:
The function that is not a role of astrocytes is b) Forming myelin sheaths. Astrocytes perform several crucial functions within the central nervous system (CNS), including providing nutrients to neurons, maintaining the blood-brain barrier, and regulating the concentrations of ions and chemicals in the extracellular fluid. They also provide structural support for neurons, cushioning, and protecting the brain. However, the formation of myelin sheaths around axons in the CNS is actually the responsibility of oligodendrocytes, not astrocytes. In the peripheral nervous system (PNS), this job is done by Schwann cells.