Nerves: PNS

photo of myelinated axons

The peripheral nervous system (PNS) consists of nerves, which are bundles of nerve fibres, ganglia, which contain not only nerve fibres but also neuronal cell bodies (perikarya) and nerve endings.

In the PNS, nerves contain, in addition to fibres, specialised supporting cells, called Schwann cells. What is their function?

questionWhat are the Nodes of Ranvier? Why are they important for saltatory conduction?

Each Schwann cell wraps its plasma membrane concentrically around the axon, like a layer of tape, to form a segment of myelinsheath about 1mm long. (In reality these layers are much more tightly compacted together than shown here).

The tightly wrapped myelin layers around axons can be seen in the osmium stained section of a peripheral nerve.

photo of peripheral nerve

Now look at the image opposite. This section shows part of a fascicle (bundle of nerve fibres) within a peripheral nerve that has been stained using a technique that stains myelin brown or black. Note that, especially where the fibres have been cut transversely, myelin sheaths can be seen clearly but that they appear empty because this technique does not stain the axons that they surround.

diagram of peripheral nerve

In a typical peripheral nerve such as this there will be several bundles or fascicles of nerve fibres.

The layers of connective tissue have similar names to those found in muscle, epineurium, perineurium and endoneurium.

 

questionWhat are ganglia?

HE photo of peripheral nerve

Look at the H & E (pink) stained section that opposite, and with the help of the labels, see if you can identify the perineurium and endoneurium.