The site of action of local anaesthetic in epidural anesthesia is
**Core Concept**
The site of action of local anaesthetics in epidural anesthesia involves blocking the conduction of nerve impulses in the spinal nerves, thereby preventing the transmission of pain signals to the brain. This is achieved by inhibiting the sodium channels in the nerve cell membrane, leading to a reduction in excitability and ultimately, a loss of sensation.
**Why the Correct Answer is Right**
The correct answer involves the action of local anaesthetics on the spinal nerves, specifically the dorsal horn of the spinal cord, where the first-order neurons responsible for transmitting pain signals are located. Local anaesthetics, such as lidocaine, bind to the sodium channels in the nerve membrane, preventing the influx of sodium ions and thereby blocking the generation and conduction of action potentials. This results in a loss of pain and sensory perception in the affected area.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it refers to the site of action of local anaesthetics in general, rather than specifically in epidural anesthesia. While local anaesthetics do act on the nerve cell membrane, the site of action in epidural anesthesia is more specific to the spinal nerves.
**Option B:** This option is incorrect because it refers to the site of action of opioids, which are used in epidural anesthesia for pain relief, but do not involve the same mechanism as local anaesthetics.
**Option D:** This option is incorrect because it refers to the site of action of general anaesthetics, which involve a different mechanism of action and are not specific to epidural anesthesia.
**Clinical Pearl / High-Yield Fact**
In epidural anesthesia, the dose and concentration of local anaesthetic required to achieve effective analgesia can vary depending on the individual patient and the specific procedure being performed. A general rule of thumb is to use a dose of 1-2 mL of 1-2% lidocaine per segment of the spinal cord to be anesthetized.
**Correct Answer: C.**