Glia have a close relationship with neurons but this relationship has not been fully understood. Glia are essential partners to neurons and may influence many functions including neuronal and metabolic activities, neurotransmitter kinetics, as well as the blood flow. Glial activities are gradual and their impulses spread slower, compared with the fast signals generated by neurons. Recent studies found that the total number of neurons and the total number of glial cells are about the same.
The astrocytes are the major glial cells. One astrocyte can interact with multiple neurons to regulate the metabolism and neuron excitability, and to supply pre-processed food for the energy needed by neurons. Astrocytes are involved in the regulation of the cellular environment, the blood flow for the energy used by neurons, and the communication and signaling.
Oligodendrocytes are the cells that generate myelin around axons with layers of cell membranes. The gaps in the myelin sheath are called nodes. With the myelin sheaths, action potentials can jump between the nodes and make the signaling faster. Oligodendrocytes can also respond to synaptic events and generate spikes similar to action potentials. Such responses make the functional difference between neurons and glia unclear.
Schwann cells play the same role as the oligodendrocytes outside the central nervous system. They generate myelin around the axons of peripheral nerves. Similar to the conditions of oligodendrocytes in the central nervous system, there are nodes and gaps between the myelin sheath of peripheral nerve axons also. Schwann cells are also involved in the regulation of the extracellular environment and the neurotransmitter activities, especially at the junctions between the neural tissues and muscular tissues.