around a third of the peripheral neurones in vertebrates are myelinated.
Most sensory and motor neurons are associated with many Schwann cells, these cells make up a fatty sheet called the myelin sheath. These cells are wrapped tightly around the neuron so that the sheath action consists of several layers of membrane and thin cytoplasm from the Schwann cell.
At intervals of 1 - 3 milimeters along the neuron, there are gaps in the myelin sheath, called the nodes of ranvier; each node is about 2 - 3micro meters long.
because the myelin sheath is tightly wrapped around the neuron, it prevents the movement of ions across the neuron membranes. Thus, movement of ions across the membrane can only occur at the nodes of Ranvier, meaning that the action potential jumps from one node to the next, making conduction much more rapid.
Non myelinated neurons are also associated with schwann cells, but several neurones may be enshrouded in one loosely wrapped Schwann cell. this means that the action potential moves along the neuron in a wave rather than jumping from node to node.
