Review of Medical Physiology - William F. Ganong 2002

Physiology of Nerve and Muscle Cells
Excitable Tissue: Nerve
Glia

In addition to Neurons, The Nervous system contains glial Cells (neuroglia). These cells are numerous; as noted above, they outnumber neurons by 10 to 50 times. Schwann cells, which wrap around axons in peripheral nerves, also belong to the glia. The CNS contains three MAIN TYPES OF glial cells. Microglial cells (Fig. 2-14) are scavenger cells that resemble tissue macrophages. They presumably originate in the Bone Marrow and enter the nervous system via Blood Vessels. The function of oligodendrocytes is The formation of myelin (see Fig. 2-3). Astrocytes are distributed throughout the Brain and are divided into two subtypes. Fibrous astrocytes, which contain abundant Intermediate filaments, are found predominantly in the White matter. Protoplasmic astrocytes are located in the Gray matter and are characterized by granular Cytoplasm. The processes of both astrocyte subtypes extend toward blood capillaries, where, together with the capillaries, they participate in forming the blood-brain barrier (see Chapter 32). Their processes also form synaptic contacts On the surface of Nerve Cells.

Astrocytes possess a Membrane Potential that depends on the extracellular concentration of K⁺, yet they do not generate propagating action potentials. Astrocytes release substances that exert a trophic effect on neurons and ensure the maintenance of optimal ion and neurotransmitter concentrations by taking up K⁺ and the Neurotransmitters glutamate and γ-aminobutyric acid (GABA) (see Chapter 4).

The Nature of the interaction between astrocytes and glutamatergic neurons is illustrated in Fig. 2-15. Glutamate released by neurons is taken up by astrocytes and converted into glutamine. Glutamine is then returned to the neurons and converted back into glutamate, which is released as a synaptic transmitter.