Knowledge of anatomy is essential for the practice of regional anesthesia and ultrasound-guided regional anesthesia procedures. This chapter provides a concise overview of the essential functional anatomy necessary for the implementation of traditional and ultrasound-guided regional anesthesia techniques. Figure 1-1 demonstrates the anatomical planes and directions used as a conventional approach throughout the book.
Conventional body planes and directions. Red, sagittal; orange, sagittal paramedian; green, transverse; and purple, coronal or axial.
Anatomy of Peripheral Nerves
The neuron is the basic functional unit responsible for nerve conduction. Neurons are the longest cells in the body, often as long as 1 meter. Most neurons have a limited ability to repair after injury. Advances in the understanding of the neurobiology of nerve regeneration and experimental advances in biotechnology may eventually result in development of the strategies to promote axonal growth and reduce neuronal death.
A typical neuron consists of a cell body (soma) with a large nucleus. The cell body is attached to several branching processes, called dendrites, and a single axon (Figure 1-2). Dendrites receive incoming messages, whereas single axons per neuron conduct outgoing messages. In peripheral nerves, axons are long and slender; they are often referred to as nerve fibers.
Composition of the neuron.
The peripheral nerve is composed of three types of fibers: (1) somatosensory or afferent nerves, (2) motor or efferent nerves, and (3) autonomic nerves. In a peripheral nerve (Figure 1-3), individual axons are enveloped in a loose and delicate connective tissue, the endoneurium. Groups of axons are arranged within a bundle (nerve fascicle) surrounded by the perineurium. The perineurium imparts mechanical strength to the peripheral nerve and functions as a diffusion barrier to the fascicle, isolating the endoneurial space and preserving the ionic milieu of the axon. At each branching point, the perineurium splits with the fascicle. The fascicles, in turn, are embedded in loose connective tissue called the interfascicular epineurium, which contains adipose tissue, fibroblasts, mastocytes, blood vessels, and lymphatics. The outer layer surrounding the nerve is the epineurium, a denser collagenous tissue that protects the nerve. The paraneurium consists of loose connective tissue that holds a stable relationship between adjacent structures filling the space in between them, such as the neurovascular bundles of intermuscular septae. This tissue contributes to the functional mobility of nerves during joint and muscular movement.
Organization of the peripheral nerve.
Of note, the fascicular bundles are not continuous throughout the peripheral nerve but divide and anastomose with one another as frequently as every few millimeters (Figure 1-4...