Biomedical Engineering Reference
In-Depth Information
of these cells is the presence of multiple endocytotic vesicles, considered as a pos-
sible transport system across the cells. The number of lamellae increases with the
diameter of the fascicle; they number as few as 3-5 in the rat sural nerve (Thomas
and Jones 1967) but as many as up to 15 layers in mammalian nerve trunks (Thomas
and Olsson 1975; Olsson 1990).
A rich content of collagen fibrils is present in the spaces that separate the cell
lamellae (Fig.
6.2
, bottom). The diameters of fibrils are in the range 40-65 nm,
approximately the same as in fibrils of the endoneurium but significantly smaller
than those of the epineurium, where diameters average 80 nm (Thomas and Jones
1967).The sharp gradient in diameter of collagen fibrils along the radial direction,
observed to abruptly change at the outermost perineurial lamellae, has been used
as the point of demarcation between the perineurium and the epineurium (Thomas
and Jones 1967). Collagen fibrils are assembled in interlamellar spaces in a lattice-
like arrangement and are both longitudinally and obliquely oriented to the axis of
the nerve trunk. Elastic fibers, and occasionally fibroblasts and mast cells, are also
observed in the spaces between lamellae (Thomas and Olsson 1975; Olsson 1990).
BM (basal lamina) lines the surfaces of the cylindrical structure of the perineu-
rium, both inside (endoneurial side) and outside (epineurial side). Unlike perineu-
rial cells, perineurial fibroblasts are not encased in a BM. Blood vessels traverse
the perineurium; they connect the network of relatively large blood vessels in the
epineurium with the longitudinally oriented capillary network inside the fascicles
(i.e., in the endoneurium; Thomas and Olsson 1975; Olsson 1990).
A major contribution of the perineurium to nerve function is the maintenance
of a constant chemical composition in the intrafascicular space, and therefore, of a
constant level of the electrical conductivity of the nerve fibers. By forming a wall
around the endoneurial space, it contributes to homeostasis at the site of conduc-
tion even when there are variations in the fluid composition outside it (e.g., during
an inflammatory process). An agent with the capacity to change the electric action
potential is, therefore, usually either unable to diffuse into the endoneurium or can
do so only at a greatly reduced rate of entry (Olsson 1990).
The passive diffusion barrier property of the perineurium primarily depends on
the close contacts between perineurial cells, described above, as well as ensheathing
of these cells each in their own BM. Substances moving into or out of the intrafas-
cicular space must, therefore, always pass through at least one thickness of BM. In
addition, however, perineurial cells contain a wide range of phosphorylating enzymes
and are considered to be well equipped to act as a metabolically active, rather than
simply passive, diffusion barrier. The permeability of the perineurium has generally
increased after crushing or stretching of the nerve or even after administration of a
local anesthetic. Following nerve transection, or other form of trauma, blood ves-
sels in the nerve trunk, including the perineurium, swell and become quite permeable
(edema; Olsson 1990). Increases in vascular permeability largely account for the flow
of exudate into a tubulated gap, generated by transecting the nerve trunk.
A few reversible and irreversible effects of mechanical trauma on perineurial
permeability have been studied. A reversible increase in permeability of a periph-
eral nerve was observed after stretching by 10 %; however, the change became ir-
reversible following stretching to 20 % (Olsson 1990).
