Schwann Cells
Structure
Schwann Cells are neuroglial cells which are in close association with neurones, and whose plasma membrane spirals round and round the axon or dendron of the neurone to form a myelin sheath.
Myelin Sheaths
This myelin sheath is largely made up of lipids together with some proteins. Not all axons have, or need, myelin sheaths - some invertebrate animals, such as earthworms, have no myelin sheaths around any of their neurones. In humans, however, about one third of motor and sensory neurones are myelinated.
What is the function of Schwann Cells?
The myelin sheath around a neurone affects the speed of conduction of the nerve impulse - action potentials can travel faster through myelinated neurones. The small, uncovered areas between Schwann cells are called nodes of Ranvier. An action potential occuring at one node of Ranvier sets up a local circuit which depolarises the membrane at the next node of Ranvier, but has no effect on the membrane in between. The action potential therefore jumps from one node to the next in a process known as saltatory conduction. Schwann cells are also thought to be involved in the repair of damaged axons.