Which fibers are affected earliest in spinal anaesthesia?
**Core Concept**
Spinal anesthesia is a regional anesthetic technique that involves injecting local anesthetic into the cerebrospinal fluid (CSF) surrounding the spinal cord. The effects of spinal anesthesia are primarily due to the blockage of nerve conduction in the spinal cord, leading to sensory, motor, and autonomic blockade.
**Why the Correct Answer is Right**
The earliest fibers affected in spinal anesthesia are the small, unmyelinated C fibers and Aδ fibers, which are responsible for transmitting pain and temperature sensation. These fibers are smaller in diameter and have a lower conduction velocity compared to the larger, myelinated Aβ fibers, which are responsible for transmitting touch and pressure sensation. The early block of C fibers and Aδ fibers results in a loss of pain and temperature sensation, often preceding the block of motor fibers.
**Why Each Wrong Option is Incorrect**
**Option A:** Aβ fibers are large, myelinated fibers responsible for transmitting touch and pressure sensation. They are affected later in spinal anesthesia, not earliest.
**Option B:** Motor fibers, which are responsible for transmitting signals to muscles, are also affected later in spinal anesthesia, not earliest.
**Option C:** Sympathetic fibers, which are responsible for transmitting autonomic signals, are affected early in spinal anesthesia, but not as early as C fibers and Aδ fibers.
**Clinical Pearl / High-Yield Fact**
In spinal anesthesia, the order of blockade is typically C fibers and Aδ fibers (pain and temperature) followed by Aβ fibers (touch and pressure) and then motor fibers. This is because the smaller, unmyelinated fibers are more susceptible to the effects of local anesthetics.
**Correct Answer: C. Aδ fibers**