MCAT Chemical and Physical Foundations of Biological Systems 16 — Flashcards

Use the following information to answer questions:
 

Myelin is an essential electrically insulating material that forms a layer around the axon of a neuron. The production of this myelin sheath is called myelination, and it begins in humans several months into fetal development. Although myelin is approximately 40% water, its dry mass is approximately 70% to 85% lipid and approximately 15% to 30% protein mass. The main lipid found in myelin is galactocerebroside, which can also be sulfated at carbon 3 on the galactose ring by sulphotransferase (Figure 1). The primary function of the myelin sheath is to increase the speed at which impulses propagate along the myelinated fiber, decreasing capacitance and increasing electrical resistance across the cell membrane. Impulses move continuously as waves along unmyelinated fibers, but saltatory conduction occurs along myelinated fibers, with intracellular and extracellular fluid separated strongly. The nodes of Ranvier, unlike the myelin sheath, contain many voltage-gated sodium channels that allow enough sodium into the axon to create an action potential.

Diseases like multiple sclerosis and Guillain-Barré disease are caused by the destruction of the myelin sheath.