The nervous system allows animals to communicate with the external environment and internal environments by sensing a broad variety of stimuli, processing the information, and sending orders, usually a muscle contraction that moves some parts of the organism or the whole body. Neurons, together with glial cells, are the cells that form the central and peripheral nervous systems. Axons that are covered by sections of myelin are called myelinated, whereas axons without myelin sheaths are called unmyelinated.Cell types. Myelin is a very good electrical insulator, like the plastic or rubber that encases an electrical cord. The myelin sheath is composed of lipid layers that surround the axon. Many axons (especially the long axons of nerves in the peripheral nervous system) are covered by sections of myelin (also called the myelin sheath).An axon may branch hundreds of times, but there is never more than one axon per neuron. A small gap called a synapse (also called a synaptic gap or synaptic cleft) is located between the end of the axon terminal and the surface of the receiving cell. Branches of the axon terminal end in axon terminal buttons (also called axon endings, synaptic end bulbs, synaptic buttons/boutons, bouton terminaux, etc.) These are the points where the message is transmitted to other cells (via the release of chemicals called neurotransmitters), often to the dendrites of other neurons. ![]()
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