Which of the following is the primary mechanism by which epidural local anesthetic agents produce sensory blockade?

Explanation

## Mechanism of Epidural Anesthesia **Key Point:** Epidural local anesthetics produce sensory blockade primarily by **diffusing across the dura mater and blocking sodium channels in nerve roots** within the epidural space, not by acting on the spinal cord itself. ### Pharmacological Mechanism 1. **Local anesthetic diffusion:** After injection into the epidural space, local anesthetic molecules diffuse through: - The dura mater (outer layer of the meninges) - The nerve sheath (perineurium) - Into the axonal membrane 2. **Sodium channel blockade:** Once inside the axon, local anesthetics bind to and block voltage-gated sodium channels on the inner surface of the nerve cell membrane. 3. **Conduction blockade:** Sodium influx is prevented, preventing depolarization and action potential propagation along sensory, motor, and sympathetic nerve fibers. ### Differential Blockade in Epidural Anesthesia | Fiber Type | Diameter | Myelination | Blockade Order | |-----------|----------|-------------|----------------| | Sympathetic (B) | 3 μm | Myelinated | 1st (blocked first) | | Sensory (A-δ) | 5 μm | Myelinated | 2nd | | Motor (A-α) | 15 μm | Myelinated | 3rd (blocked last) | | Parasympathetic (C) | 1 μm | Unmyelinated | Variable | **Mnemonic:** **SSMM** — Sympathetic → Sensory → Motor → Myelinated (order of blockade) **Clinical Pearl:** This differential blockade explains why epidural anesthesia can produce sensory blockade while preserving some motor function at lower concentrations. **High-Yield:** The site of action is the **nerve root** in the epidural space, NOT the spinal cord itself. This distinguishes epidural from subarachnoid (spinal) anesthesia.