Pressure on nerves

Anatomy

Peripheral nerves contain both neural and supportive elements. A large multifascicular nerve is composed of a number of different bundles of nerve fibres or fasciculi (Fig. 2.1). These are bound together by the epineurium, a condensation of areolar connective tissue derived from the mesoderm. In humans, the epineurium normally constitutes 30–50% of the total cross-sectional area of the nerve bundle: it contains fibroblasts; collagen (types I and III); variable amounts of fat (possibly to cushion the nerve fibres it surrounds); lymphatic; blood vessels (vasa vasorum); and free nerve endings. In a monofascicular nerve, the epineurium only surrounds the fasciculus and is fused with the perineurium.

Fig 2.1 A peripheral nerve is composed of a number of different fasciculi.

The perineurium surrounds and protects one fascicle. It has two different layers; an outer collagen-rich connective one and an inner epithelial layer of contiguous cells. The perineurium has an important role in maintaining the osmotic milieu and fluid pressure within the endoneurium and also acts as a barrier against chemical and bacterial invasion.¹

The connective tissue of peri- and epineurium possesses blood and lymph vessels – the so-called vasa vasorum.2,3 Also free nociceptive nerve endings which come from the related multifascicular nerve trunks are embedded in the perineurium and epineurium.^4,5

Enclosed in the perineurium is the fasciculus – a bundle of nerve fibres bound together and protected by the endoneurium. The latter consists of long collagen fibres running with the nerve fibres. The fibrous and cellular components of the endoneurium are bathed in endoneural fluid.⁶

The nerve fibres are axons – the distal offshoots of nerve cells (Fig. 2.2). Most axons are surrounded by a myelin sheath formed from the compressed and concentric Schwann cell membranes. Axons range in diameter from 0.2 µm (small non-myelinated nociceptive axons) to 20 µm (large and myelinated efferent motor axons) and in length from 1 to 100 cm. They contain most of the cell volume.

Fig 2.2 Axon, surrounded by myelin sheath.

From central to peripheral, the nervous system can be clinically divided into four zones (Fig. 2.3):

Fig 2.3 Anatomy of the peripheral nervous system: 1, spinal cord; 2, nerve root; 3, nerve plexus and nerve trunk; 4, peripheral nerve branches.

• The spinal cord

• The spinal nerve, which contains fibres belonging to one segment

• In the brachial and sacral area, and distal from the spinal ganglion, the different spinal nerves form a nerve plexus, from which originate the large multifascicular nerve trunks

• Further distally the trunks split into peripheral nerves, with motor, sensory or combined function.

Although the fasciculi, from their exit from the central nervous system to the distal extremity of the nerve, exhibit the general morphology summarized above, the structure and behaviour of the connective supportive elements differ considerably. This may explain the different clinical behaviour of a compressed cord, a compressed nerve root (within or without its dural sleeve), a compressed nerve trunk and a compressed small peripheral nerve.

Roots

The course of the spinal nerve within the spinal canal, from the emergence of the rootlets at the anterior and posterior aspect of the spinal cord to the outer border of the foramen, is called the intraspinal root (Fig. 2.4).⁷ Being inside the meningeal membranes of the spinal cord, the posterior and anterior roots are devoid of the elaborate epineural and perineural membranes that are characteristic of peripheral nerves.

Fig 2.4 Structure of an intraspinal root: 1, spinal cord; 2, ventral root; 3, dorsal root; 4, dorsal ganglion.

Proximally, the rootlets float freely within the cerebrospinal fluid which is the main source of their metabolic needs. In this intrathecal part of the intraspinal root, the rootlets are held together by the endoneurium which is more loosely arranged than is typically seen in peripheral nerves.⁸ Further distally, the nerve root becomes enclosed in the dural sheath – a tubular prolongation of the dura. In this dural investment the nerves do not lie freely but are bound by the arachnoid membrane (Fig. 2.5).^9,10 This area is known as the extrathecal part of the intraspinal nerve root – that length of the root and the dural sleeve between the main dural sac and the exit from the foramen. The extrathecal portion is short in the cervical region but becomes longer with the increasing obliquity of the intraspinal roots in the thoracolumbar and lumbar regions.