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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.
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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 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.
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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
perineurium....