Peripheral Nerves - Clinical Tree

Study Objectives

1.
In a cross section of a peripheral nerve, be able to identify the different connective tissue sheaths.
2.
Describe the anatomic classification of nerve fibres and its relationship to their function.
3.
Describe the mechanism of myelination and be able to define internode, paranode, and the node of Ranvier.
4.
Describe saltatory conduction and its relationship to myelination.
5.
Describe the microscopic changes in a peripheral nerve after nerve injury and their relevance to functional recovery.
6.
Contrast peripheral versus central nervous system regeneration.

To further understand the clinical importance of peripheral nerves, we suggest previewing the Clinical Panel Boxes in Chapter 12 .

General Features

The peripheral nerves comprise the cranial and spinal nerves linking the brain and spinal cord to the peripheral tissues. The spinal nerves are formed by the union of ventral (anterior) and dorsal (posterior) nerve roots at their points of exit from the vertebral canal ( Fig. 9.1 ). The swelling on each dorsal root is a spinal or dorsal root ganglion . The spinal nerve is relatively short (less than 1 cm) and traverses an intervertebral foramen. On emerging from the foramen, it divides into ventral (anterior) and dorsal (posterior) rami .

Fig. 9.1, Segment of thoracic spinal cord with attached nerve roots. Arrows indicate directions of impulse conduction. Green indicates sympathetic outflow.

The dorsal rami supply the erector spinae muscles and the overlying skin of the trunk. The ventral rami supply the muscles and skin of the side and front of the trunk, including the muscles and skin of the limbs; they also supply sensory fibres to the parietal pleura and parietal peritoneum.

The cervical , brachial , and lumbosacral plexuses are derived from ventral rami, which form the roots of the plexuses. The term root therefore has two different meanings depending on the context. (Details of the plexuses are in standard anatomy texts.)

The neurons contributing to peripheral nerves are partly contained within the central nervous system (CNS) ( Fig. 9.2 ). The cells giving rise to the motor (efferent) nerves to skeletal muscles are multipolar α and γ neurons of similar configuration to the one depicted in Fig. 6.3 ; in the spinal cord they occupy the anterior horn of grey matter. Further details are found in Chapter 10 . The cells giving rise to posterior nerve roots are unipolar neurons whose cell bodies lie in dorsal root ganglia and whose sensory (afferent) central processes enter the posterior horn of grey matter.

Fig. 9.2, Composition and distribution of a cervical spinal nerve. Note: The sympathetic component is not shown. MEP , Motor end plate; MN , multipolar neurons; NMS , neuromuscular spindle.

The spinal nerves contain somatic efferent fibres projecting to the skeletal muscles of the trunk and limbs and somatic afferent fibres from the skin, muscles, and joints. They also carry visceral efferent , autonomic fibres, and some spinal nerves contain visceral afferent fibres as well.

Microscopic Structure of Peripheral Nerves

Fig. 9.3 illustrates the structure of a typical peripheral nerve. It is not possible to designate individual nerve fibres as motor or sensory based on structural features alone.

Fig. 9.3, Transverse section of a nerve trunk. (A) Light microscopy. (B) Electron microscopy.

Peripheral nerves are invested with epineurium , a dense, irregular connective tissue sheath surrounding the fascicles (bundles of nerve fibres) and blood vessels that make up the nerve. Nerve fibres are exchanged between fascicles along the course of the nerve.

Each fascicle is covered by perineurium , which is composed of several layers of pavement epithelium arranged in a distinct lamellar pattern and bonded by tight junctions. Surrounding individual Schwann cells is a network of reticular collagenous fibres, the endoneurium .

Less than half of the nerve fibres are enclosed in myelin sheaths. The remaining, unmyelinated fibres travel in deep gutters along the surface of Schwann cells.

The term nerve fibre is usually used in the context of nerve impulse conduction, where it is equivalent to an axon. A myelinated fibre consists of an axon ensheathed in concentric layers or lamellae of myelin (the original plasma membrane of a Schwann cell). An unmyelinated fibre is embedded in individual nonmyelinating Schwann cells and shares the Schwann cell plasma membrane (neurolemma) with other unmyelinated nerve fibres (axons). This collection of axons and Schwann cells is called a Remak bundle ( Fig. 9.3B ). A basal lamina surrounds both myelinated and nonmyelinated fibres and acts as an important conduit in peripheral nervous system (PNS) repair.

Myelin Formation

The Schwann cell is the myelinating cell of the PNS. Schwann cells form a continuous chain along nerve fibres in the PNS; in myelinated fibres an individual Schwann cell may be responsible for the myelination of 0.3 to 1 mm of the length of an axon. Modified Schwann cells form satellite cells in dorsal root ganglia and in autonomic ganglia and form teloglia ( Fig. 10.3 ) at the myoneural junction.

Whether or not a given axon becomes myelinated is determined by the axon itself. If an axon is to be myelinated, it receives the simultaneous attention of a sequence of Schwann cells along its length. Each one encircles the axon completely, creating a ‘mesentery’ of plasma membrane, the mesaxon ( Fig. 9.4 ). The mesaxon is displaced progressively, being rotated around the axon. Successive layers of plasma membrane come into apposition to form the major and minor dense lines of the myelin sheath ( Fig. 9.4 ) and the cytoplasm is ‘squeezed out’. Paranodal pockets of cytoplasm persist at the ends of the myelin segments, on each side of the nodes of Ranvier , or in the gap between the ends (paranodes) of adjacent Schwann cells.

Fig. 9.4, Myelination in the peripheral nervous system. Arrows indicate movement of a flange of Schwann cell cytoplasm.

Myelin Expedites Conduction

In unmyelinated fibres, impulse conduction is continuous (uninterrupted) along the axon. Its average speed is only 2 m/s. In myelinated fibres, excitable membrane is confined to the nodes of Ranvier because myelin acts as an electrical insulator. Impulse conduction is called saltatory (‘jumping’) because it leaps from node to node. The speed of conduction is much faster along myelinated fibres, with a maximum of 120 m/s. The number of impulses that can be conducted per second by myelinated fibres is also much greater than that by unmyelinated fibres.

The larger the myelinated fibre, the more rapid the conduction, because larger fibres have longer internodal segments and the nerve impulses take longer ‘strides’ between nodes. A ‘rule of six’ can be used to express the ratio between size and speed: a fibre of 10 μm external diameter (i.e. including myelin) will conduct at 60 m/s, one of 15 μm will conduct at 90 m/s, and so on.

In physiologic recordings, peripheral nerve fibres are classified in accordance with conduction velocities and other criteria. Motor fibres are classified into the groups A, B, and C in descending order. Sensory fibres are classified into types I to IV, also in descending order. In practice there is some interchange of usage: for example, unmyelinated sensory fibres are usually called C fibres rather than type IV fibres. A commonly used descriptor of the axon myelin relationship is the axon to fibre ratio or the g-ratio ; this provides information of sheath thickness and relative internodal length. Suboptimal g-ratios have been implicated in several pathological conditions.

Details of diameters and sources of peripheral nerve fibres are given in Tables 9.1 and 9.2 .

Table 9.1

Classification of Peripheral Nerve Fibres.

Nerve Type	Number	Letter	Diameter (μm)	Conduction Velocity (m/s)
Myelinated
Large	I	Aα	12–20	70–120
Medium	II	Aβ	6–12	35–70
Small	III	Aγ	3–6	10–40
Small	–	Aδ	2–5	5–35
Unmyelinated	IV	C	0.2–1.5	0.5–2

Table 9.2

Locations of Peripheral Nerve Fibre Types.

Fibre Type	Origin
Sensory
Ia	Muscle spindle annulospiral endings
Ib	Golgi tendon organs
II (Aβ)	Muscle spindle flower spray endings; touch or pressure receptors in skin and elsewhere
III (Aδ)	Follicular endings; fast pain and thermal receptors
IV (C)	Slow pain, itch, touch receptors
Motor
Aα	α motor neurons supplying extrafusal muscle fibres
Aγ	γ motor neurons supplying intrafusal muscle fibres

The electron micrograph in Fig. 9.5 illustrates a myelinated peripheral nerve fibre with attendant Schwann cell. Fig. 9.6 illustrates a group of unmyelinated fibres embedded in the cytoplasm of a (Remak) Schwann cell. Fig. 9.7 illustrates a nodal region along an axon within the CNS.