1. DESCRIPTION

Cerebrospinal nerves leave the brain and spinal cord to innervate peripheral issues. Most these nerves are mixed, i.e. they contain both myelinated and unmyelinated nerve fibres. Nevertheless the myelinated nerve fibres are more abundant. The whole nerve is a collection of nerve bundles (Fig. 1) covered by the epineurium. The epineurium contains a dense connective tissue (continuation of the dura mater from the CNS). A surface of nerve bundles is encapsulated by several layers of flattened cells constituting the perineurium. The perineurium is an analogue of the arachnoid membrane of the CNS - perineural cells are sealed with zonulae occludentes (tight junctions) and they border each nerve bundle as a barrier from outside. Every nerve bundle contains a different number of nerve fibres. Inside of the bundle the nerve fibres are surrounded by the endoneurium - a thin layer of loose connective tissue (analogue of the pia mater from the CNS). The endoneurium contains blood capillaries with the blood-brain barrier.
The cerebrospinal nerve consists of mainly myelinated nerve fibres; the vegetative nerve contains mainly unmyelinated nerve fibres.

Fig. 1. 3D reconstruction of the peripheral nerve. The epineurium (brown) and perineurium (grey) is cut to show a wavy course of nerve firbes (yelow) inside of bundles. Connective tissue fibres in the epineurium are also wavy, which provides the nerve some elasticity. The blood vessels (red) and adipose tissue (white) from the epineurium enter between the primary bundles. Around the entire nerve, the paraneurium (green), a layer of motile loose connective tissue (green) can occur. A lower image shows how a transverse section of a nerve bundle looks at light microscopy.

Author: R.V. Krstic; colours: J. Mokrý

A nerve fibre consists of a nervous cell processus, axon, and of its coverings. In the PNS, the axon is covered by the Schwann cells. The axon of the myelinated nerve fibre is ensheathed by concentric lamelae of the Schwann cells that form the myelin sheath. The cell body and nucleus of the Schwann cell is located at the periphery. Each Schwann cell covers a short segment of one axon. The next segment of the same axon is covered by the myelin from the next Schwann cell. The segment of the myelin formed by the single Schwann cell and separated by Ranvier nodes is called the internodium.

In the vegetative nerve, the unmyelinated nerve fibres prevail; these consist of the axon covered by the  Schwann sheath. Many axons are covered by the single Schwann cell; in many the mesaxon appears although few with a naked surface do not form the mesaxon. The unmyelinated nerve fibres are not segmented - there are no nodes of Ranvier. Their surface is covered by the basal lamina and the endoneurium. The entire bundle is covered the perineurium. As autonomic axons are of small sizes the bodies and nuclei of Schwann cells at light microscopy are closely apposed.