What are the functions of the different types of glial cells?

Glial cells play a supportive role in the nervous system. The four types of glial cells in the central nervous system (CNS) and two types in the peripheral nervous system (PNS). 

Glial cells in the CNS 

1. Astrocytes are star-shaped cells found only in the brain and spinal cord. Their primary responsibility is maintaining the environment around neurons by regulating neurotransmitter levels around synapses. In addition they also form a blood-brain barrier to protect the brain from harmful substances, recycle unused neurotransmitters after a nerve signal is transmitted, and clean up debris after the death of a neuron. 
2. Oligodendrocytes create myelin, a fatty material that wraps around axons of the nerve cells, insulating them and enabling electrical signals to travel faster along axons. Oligodendrocytes also facilitate proper nerve conduction and provide nutrients from the blood to axons.
3. Microglia are small, star-shaped cells that act as immune cells protecting the brain and spinal cord against invading organisms or disease-causing agents. They constantly monitor for atypical or damaged cells. On encountering atypical cells, pathogens or toxins, microglia engulf and destroy them in a process known as phagocytosis. Microglia also play a role in brain development and plasticity by pruning superfluous connections between nerve cells, retaining only the most important ones.
4. Ependymal cells develop in the ventricles of the brain and the central canal of the spinal cord. In addition to providing the lining for these fluid-filled spaces of the central nervous system, ependymal cells also produce cerebrospinal fluid, which transports nutrients, chemical messengers, hormones, and waste to and from the brain and the spinal cord. Cerebrospinal fluid (CSF) also provides the brain with crucial protective cushioning and regulates temperature, which helps maintain homeostasis. 
5. Radial glia are capable of generating new cells and serve as precursors to neurons, oligodendrocytes, and astrocytes. They are considered as a type of stem cell in the brain. In the embryo, radial glia provide a framework that guides new brain cells into place during developmental stages. In later life, they contribute to the brain’s plasticity and ability to adapt. 

Glial cells in the PNS 

The peripheral nervous system (PNS) encompasses the nerves outside the brain and spinal cord. Glial cells in the PNS play a vital role in carrying signals to and from the CNS.

There are two main types of glial cells in the peripheral nervous system - Schwann cells and satellite cells.

1. Schwann cells are a type of satellite cell that function in a manner similar to oligodendrocytes in the CNS. They create a myelin sheath around nerves, which helps insulate them enabling faster transmission of electrical signals along the nerves. Schwann cells help repair and/or regenerate damaged axons.
2. Satellite cells help maintain the environment around neurons by providing the peripheral nerves with nutrition and support and regulating their calcium levels, which are crucial for proper nerve conduction. Satellite cells also release neurotrophic factors, which protect nerve cells from damage or death.