The principal site of action for neuraxial blockade is believed to be the nerve root. Local anesthetic is injected into CSF (spinal anesthesia) or the epidural space (epidural and caudal anesthesia) and bathes the nerve root in the subarachnoid space or epidural space, respectively. Direct injection of local anesthetic into CSF for spinal anesthesia allows a relatively small dose and volume of local anesthetic to achieve dense sensory and motor blockade. In contrast, the same local anesthetic concentration is achieved within nerve roots only with much larger volumes and quantities of local anesthetic molecules during epidural and caudal anesthesia. Moreover, the injection site (level) for epidural anesthesia must generally be close to the nerve roots that must be anesthetized. Blockade of neural transmission (conduction) in the posterior nerve root fibers interrupts somatic and visceral sensation, whereas blockade of anterior nerve root fibers prevents efferent motor and autonomic outflow. Local anesthetics may also have actions on structures within the spinal cord during epidural and spinal anesthesia.

Somatic Blockade

By interrupting the afferent transmission of painful stimuli and abolishing the efferent impulses responsible for skeletal muscle tone, neuraxial blocks can provide excellent operating conditions. Sensory blockade interrupts both somatic and visceral painful stimuli. The mechanism of action of local anesthetic agents is discussed in Chapter 16. The effect of local anesthetics on nerve fibers varies according to the size and characteristics of the nerve fiber, whether it is myelinated, the length of nerve that is bathed by the local anesthetic, and the concentration of the local anesthetic. Spinal nerve roots contain varying mixtures of these fiber types. Smaller and myelinated fibers are generally more easily blocked than larger and unmyelinated ones. The size and character of the fiber types, and the fact that the concentration of local anesthetic decreases with increasing distance from the level of injection, explains the phenomenon of differential blockade during neuraxial anesthesia.  Differential blockade typically results in sympathetic blockade (judged by temperature sensitivity) that may be two segments or more cephalad than the sensory block (pain, light touch), which, in turn, is usually several segments more cephalad than the motor blockade.