Excitable tissue : Nerve

Motor neuron with myelinated axon

Myelin, a protein-lipid complex that is wrapped around the axon, outside the CNS myelin is produced by Schwann cells, glia like cells found along the axon. Myelin forms when Schwann cell wraps its membrane around an axon up to 100 times. The myelin sheath envelops the axon except at its ending and at the nodes of Ranvier. In CNS of mammals most neurons are myelinated but the cells that form the myelin are oligodendrocytes rather than Schwann cells. Unlike schwann cell which forms the myelin between two nodes of Ranvier on single neuron, oligodendrocytes send off multiple processes that form myelin on many neighboring axons

Top : Relation of Schwann cells to axons in peripheral nerves. On left is unmyelinated axon adn on the right is myelinated axon. Note that the cell membrane of the Schwann cell has wrapped itself around and aroun axon.
Bottom : Myelination of axon in central nervous system by oligodendrogliocytes sends processes to up to 40 axons.

Functional organization of neurons. Non conducted local potentials are integrated in the receptor zone, and action potentials are initiated at a site close to the receptor zone (arrow). The action potentials are conducted along the axon to the nerve endings, where they cause release of synaptic transmitters.