In the
peripheral nervous system (PNS)
axons can be either myelinated or unmyelinated. Myelination refers to the insulation of an axon with concentric surrounding layers of
lipid membrane (
myelin) produced by
Schwann cells. These layers are generally uniform and continuous, but due to imperfect nature of the process by which
Schwann cells wrap the nerve axon, this wrapping process can sometimes leave behind small pockets of residual
cytoplasm displaced to the periphery during the formation of the myelin sheath. These pockets, or "incisures", can subdivide the myelinated axon into irregular portions. These staggered clefts also provide communication channels between layers by connecting the outer collar of cytoplasm of the Schwann cell to the deepest layer of myelin sheath. Primary incisures appear
ab initio in myelination and always extend across the whole radial thickness of the myelin sheath but initially around only part of its circumference. Secondary incisures appear later, in regions of a compact myelin sheath, initially traversing only part of its radial thickness but commonly occupying its whole circumference. == References ==