internodesmyelinated

Internodes are the segments of a myelinated axon that lie between adjacent nodes of Ranvier. In the central nervous system, myelin is produced by oligodendrocytes; in the peripheral nervous system, by Schwann cells. The myelin sheath increases membrane resistance and reduces capacitance, enabling rapid electrical signaling along the axon.

Between internodes lie the nodes of Ranvier, gaps where the axonal membrane is exposed and rich in voltage-gated sodium channels. Action potentials leap from node to node in a process called saltatory conduction, greatly increasing conduction velocity compared with continuous conduction in unmyelinated fibers. Internode length, myelin thickness, and axon diameter together determine conduction speed and energy efficiency; g-ratio around 0.6-0.7 is typical.

Developmentally, myelination begins in the fetus and continues postnatally; oligodendrocytes can insulate many internodes on a single axon, whereas a Schwann cell typically forms a single internode segment. Internodes vary in length by neuron type and species; longer internodes tend to support faster conduction but require thicker myelin.

Pathology: demyelination, as seen in multiple sclerosis (CNS) or Guillain-Barré syndrome (PNS), disrupts internodes, slows or blocks conduction, and can lead to neurological deficits. Remyelination may occur but often leaves thinner myelin sheaths and shortened internodes, with partial recovery of function.