Anatomy and Physiology Related to Peripheral Nerve Stimulation

Abstract

Two factors determine the outcome of peripheral nerve stimulation (PNS), namely the anatomy of the peripheral nerve and its physiologic response. The morphology of peripheral nerves described in this chapter suggests the manner by which a technical interface can deliver its electrical energy to a target nerve. The opportunities and limitations of PNS, reflecting on more than 40 years’ observation, should be clear from this account. The final arbiter, the Hodgkin–Huxley neural tissue response to applied electric fields, is inextricably bound up with the anatomy and physiology of the peripheral nerve.

Keywords

Anatomy, Electric field, Neural interface, Peripheral nerve, Physiology, Stimulator

Outline
Nerve Trunks 723
Fascicular Anatomy 724
Blood Supply of Peripheral Nerves 725
Neural Electrode Interface 726
Conclusion 726
References 726

The mammalian nerve is comprised of a core or axon surrounded by an axolemma that is contained within a complex sheath which varies in diameter and is either dependent or not on the presence of myelin. Myelinated fibers are made up of many laminae called internodes that, depending on the nerve fiber, are interrupted at variable distances. This myelin sheath is contained within a membrane termed the endothelial tube. In unmyelinated fibers these tubes may contain several axons, in comparison with the single axon of a myelinated fiber. Along its length the diameter of an axon may vary from as little as 2.00 to 11.75 μm ( ) (see Fig. 56.1 ).

Nerve fibers, along their length, undergo extensive branching not only in the regions they supply but also within their parent trunks. The result is that the total number of nerve fibers is greater distally than in proximal sections of the nerve trunk ( ). This efficient anatomical disposition enables a single neuron to influence a comparatively large mass of tissue. The corollary to this arrangement is that nociception from an injured branch may, as a result of multiple branching, be referred to undisturbed tissue, and also, by way of the axon reflex in branching axons, be responsible for the algesic substances released in noninjured tissue. Axon reflexes occur in unmyelinated cutaneous nociceptive fibers, which, when stimulated, generate both orthodromic and antidromic impulses in efferent collateral fibers to blood vessels and skin. These aspects have a significant bearing not only on the effects of central neurostimulation but also on peripheral neurostimulation.

The main physiologic types of afferent nerve fibers are sensory fibers varying in diameter from 1.5 to 20 μm, motor fibers with diameters varying from 2 to 20 μm, nociceptors, touch, and temperature fibers at 2–5 μm, preganglionic sympathetic fibers of 1.5–3 μm, and postganglionic sympathetic fibers that are less than 2 μm in diameter (see Table 56.1 ).

Table 56.1

Nerve Conduction Properties

Nerve Fiber	Nerve Fiber (Diameter in μm)	Conduction Velocity (m/s)	Function
Aα	12–20	70–120	Motor, extrafusal muscle fibers, proprioceptors
Aβ	5–12	30–70	Touch, pressure
Aγ	3–6	15–30	Motor, intrafusal muscle fibers
Aδ	2–5	10–30	Nociceptors, touch, temperature
B	1.5–3	3–15	Preganglionic sympathetic fibers
C	<2.0	0.5–2	Postganglionic sympathetic fibers

Nerve Trunks

Nerve trunks consist of fasciculi, which are invested with a thin laminated sheath of perineural cells and collagen. The endoneural tubes investing each nerve fiber are contained within this framework. Other contents are fibroblasts, macrophages, loose mesh-like connective tissue, and longitudinal fibrils of collagen ( ). The Schwann cell provides the main support within the endoneurium. If it envelopes a single axon, the structure is termed a myelinated axon; when it includes many smaller axons, they are known as unmyelinated axons. The fasciculi within a nerve trunk are surrounded by loose areolar tissue termed the epineurium, which also encloses the nerve trunk.

Fascicular Anatomy

As a result of a lifetime spent studying the microanatomy of peripheral nerves, gave us an accurate picture of the fascicular anatomy. This anatomy has a great bearing on peripheral nerve stimulation and electrode design. The disposition of fasciculi is not in parallel groups, but rather they repeatedly divide, unite, and redivide to form extensive plexuses that occur throughout the length of a nerve. This arrangement continues to the terminal branches, such as digital nerves. Fasciculi vary in size from 0.04 to 2 mm, but occasionally, as in the sciatic nerve, maybe as large as 4 mm. Where a nerve crosses a joint the fasciculi are more numerous and, for example in the median and ulnar nerves, fewer fasciculi are found proximal to the elbow when compared to the number of fasciculi of the nerves in the forearm. Further variability is found in some areas where the nerve fibers are contained within a single fasciculus for a short distance, e.g., for the ulnar nerve behind the medial epicondyle, the radial nerve in the spiral groove, the axillary nerve behind the shoulder, and the common peroneal nerve in the lower thigh ( ). See Fig. 56.2 .