Chapter 3

It is often said that the practice of regional anesthesia is the practice of applied anatomy. Indeed, the practice of regional anesthesia is inconceivable without a sound knowledge of the basic anatomic facts that pertain to the individual anesthesia techniques. However, just as surgeons rely on surgical anatomy or pathologists rely on pathologic anatomy, the anatomic information necessary for the practice of regional anesthesia must be specific to this application. In the past, many new nerve block techniques and “me-too” approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather then on functional anatomy. Ultimately, many of these techniques have only introduced unnecessary confusion in the field and been of negligible relevance to clinical practice. Indeed, once the anatomic layers and tissues sheets are dissected, the fully exposed nerve structures are almost irrelevant to the practice of regional anesthesia. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depends on the interplay between neurologic structures and the neighboring tissues where local anesthetic pools and accumulates, rather than on the mere anatomic organization of the nerves and plexuses. However, much research by regional anesthesiologists has been done in the past 10–15 years on this subject, and many myths of the past have been dispelled. The reader should note that specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. The purpose of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia. The reader is referred to Figure 3–1 for an easier orientation of the body planes discussed throughout the book.

###### Fig. 3-1

Conventional body planes.

All peripheral nerves are similar in structure. The neuron is the basic functional neuronal unit responsible for the conduction of nerve impulses. Neurons are the longest cells in the body, many reaching a meter in length. Most neurons are incapable of dividing under normal circumstances and have a very limited ability to repair themselves after injury. A typical neuron consists of a cell body (soma) that contains a large nucleus. The cell body is attached to several branching processes, called dendrites, and a single axon. Dendrites receive incoming messages; axons conduct outgoing messages. Axons vary in length, and there is one only per neuron. In peripheral nerves, axons are very long and slender. They are also called nerve fibers. The peripheral nerve (PN) is composed of three parts: (1) somatosensory or afferent neurons, (2) motor or efferent neurons, and (3) autonomic neurons.

Individual nerve fibers bind together, somewhat like individual wires in an electric cable (Figure 3–2). In a peripheral nerve, individual axons are enveloped in a loose connective tissue, the endoneurium. The endoneurium is a delicate layer of connective tissue around each nerve that is embedded within the ...

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