Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Anatomy and Physiology Related to Peripheral Nerve Stimulation
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 fiber anatomy.
Redrawn with permission from Kline, D.G., Hudson, A.R., 1995. Nerve Injuries: Operative Results for Major Nerve Injuries, Entrapments, and Tumors, W.B. Saunders, Philadelphia, p. 3. Copyright (1995) Elsevier.
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.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here