Myelinated Neurones

In vertebrates, many neurones are myelinated: their axons (and the dendrons of sensory neurones) are wrapped in a myelin sheath.In the peripheral nervous system the sheath is made by Schwann cells. Each Schwann cell wraps itself around the axon many times, so the sheath is made of many layers of Schwann cell membrane. Because membranes are mostly phospholipid, myelin is rich in lipid and is a good electrical insulator. Ions cannot flow across the axon membrane where it is covered.Between neighbouring Schwann cells there are small gaps, about 1 µm wide, where the axon membrane is exposed. These are the nodes of Ranvier, found roughly every 1 mm along the axon. The membrane at the nodes contains a high density of voltage-gated sodium ion channels, so ions can move across the membrane only at these points.This means that action potentials in a myelinated neurone can only occur at the nodes, not along the whole length of the axon. As a result, impulses travel much faster in myelinated neurones (up to about 120 m s⁻¹) than in unmyelinated neurones of the same diameter (often less than 2 m s⁻¹). How the impulse moves from node to node is explained by saltatory conduction.Not all neurones are myelinated. Many invertebrate neurones and some neurones of the mammalian autonomic nervous system are unmyelinated: Schwann cells surround them but do not wrap them in layers.In multiple sclerosis, myelin in the CNS is damaged, so impulses are slowed or blocked, causing problems with movement and sensation.