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The practice of regional anesthesia is inconceivable without sound knowledge of the functional regional anesthesia anatomy. Just as surgical technique relies on surgical anatomy or pathology leans on pathologic anatomy, the anatomic information necessary for the practice of regional anesthesia must be specific to this application. In the past, many nerve block techniques and approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather than on functional anatomy. However, once the anatomic layers and tissue sheets are dissected, the anatomy of nerve structures without the tissue sheaths around them is of little relevance to the clinical practice of regional anesthesia. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depend 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. Much research on functional regional anesthesia, a term introduced by Dr. Jerry Vloka in the 1990s, has contributed to better understanding of the anatomy of regional nerve blockade. Moreover, the introduction of ultrasound in the practice of regional anesthesia has further clarified the relationship of the needle and the nerve and the dynamics of local anesthetic spread.
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The goal of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia; more specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. The reader is referred to Figure 3–1 for an easier orientation of the body planes discussed throughout the book.
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ANATOMY OF PERIPHERAL NERVES
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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 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 long and slender. They are also called nerve fibers. The peripheral nerve is composed of three parts: (1) somatosensory or afferent neurons, (2) motor or efferent neurons, and (3) autonomic neurons.
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Individual nerve fibers bind together, somewhat like individual wires in an electric cable (Figure 3–2). In the peripheral nerve, individual axons are eveloped by the endoneurium, which is a delicate layer of loose connective tissue around each axon. Groups of axons are closely associated within a bundle called a ...