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Chapter 4. Excitable Tissue: Nerve

In an experiment, researchers engrafted human glia progenitor cells into neonatal mice and allowed them to mature. The mature mice demonstrated high numbers of human glial progenitors, mature human astrocytes and all types of mouse glial cells. The human and mice glial cells intercommunicated, with the human cells retaining larger structure. Interestingly, these human-grafted animals displayed increased neural network signaling and scored better in four independent learning tests when compared to vehicle- or mouse glial progenitor-engrafted mice. These data support the idea that glia contribute greatly to learning and cognition. When considering glia in the normal brain, which of the following statements is true?

A. Microglia arise from macrophages outside of the nervous system and are physiologically and embryologically similar to other neural cell types.

B. Glia do not undergo proliferation.

C. Protoplasmic astrocytes produce substances that are tropic to neurons to help maintain the appropriate concentration of ions and neurotransmitters by taking up K+ and the neurotransmitters glutamate and GABA.

D. Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the peripheral and central nervous systems, respectively.

E. Macroglia are scavenger cells that resemble tissue macrophages and remove debris resulting from injury, infection, and disease.

The correct answer is C. Astrocytes, one of the many glia phenotypes, play important roles in modulating extracellular neurotransmitters and ions. Microglia do arise from macrophages, but are physiologically and embryologically unrelated to other neural cell types, eliminating A. Glia do undergo proliferation, eliminating B. Oligodendrocytes are involved in myelin formation in the CNS and Schwann cells provide myelin formation around axons in the peripheral nervous system, eliminating D. The statement in E describes microglia, not macroglia.

Primary erythromelalgia, which may be due to a peripheral nerve sodium channelopathy, was diagnosed in a 13-year-old girl who was experiencing frequent episodes of red, painful, warm extremities. Which part of a neuron has the highest concentration of Na+ channels per square micrometer of cell membrane?

A. Dendrites

B. Cell body near dendrites

C. Initial segment

D. Axonal membrane under myelin

E. Node of Ranvier

The correct answer is E. The nodes of Ranvier along the axon are regions between the myelin sheaths that include high concentrations of Na+ channels to enhance propagation of the action potential. The dendrites A and cell body near the dendrites B function more as receptor areas, and do not have high concentrations ...

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