While the axon mainly conducts nerve impulses from the soma to the terminal, the terminal itself secretes chemical substances called neurotransmitters. In the central nervous system the myelin sheath is formed from glial cells called oligodendrocytes, and in peripheral nerves it is formed from Schwann cells ( see below The neuroglia). Concentric layers of these lipids separated by thin layers of protein give rise to a high-resistance, low-capacitance electrical insulator interrupted at intervals by gaps called nodes of Ranvier, where the nerve membrane is exposed to the external environment. Myelin is composed of 80 percent lipid and 20 percent protein cholesterol is one of the major lipids, along with variable amounts of cerebrosides and phospholipids. Large axons acquire an insulating myelin sheath and are known as myelinated, or medullated, fibres. This is the region where the plasma membrane generates nerve impulses the axon conducts these impulses away from the soma or dendrites toward other neurons. The axon arises from the soma at a region called the axon hillock, or initial segment.
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