First fibres to be blocked in spinal anaesthesia is:
## **Core Concept**
Spinal anesthesia involves injecting a local anesthetic into the cerebrospinal fluid (CSF) to block nerve transmission in the spinal cord. The effect of spinal anesthesia on nerve fibers depends on the size and type of nerve fibers. The order of blockade is generally based on the size of the nerve fibers.
## **Why the Correct Answer is Right**
The correct answer, , involves understanding that the first fibers to be blocked in spinal anesthesia are the **smallest** nerve fibers. These are typically the **B fibers** and **C fibers**, which are involved in the transmission of autonomic functions and pain. B fibers are preganglionic autonomic fibers, and C fibers are small, unmyelinated fibers responsible for transmitting pain and temperature sensations. These fibers have a lower threshold for blockade by local anesthetics.
## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option is incorrect because involves motor functions which are blocked later in the sequence of spinal anesthesia. Motor fibers are larger and more myelinated, making them more resistant to the effects of local anesthetics initially.
* **Option B:** - This option is incorrect because are larger than B and C fibers and are involved in proprioception and motor functions. These are not the first to be blocked.
* **Option D:** - This option is incorrect because are the largest and most myelinated fibers, responsible for motor functions and proprioception. These are among the last to be affected by spinal anesthesia.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the sequence of blockade in spinal anesthesia follows the size of the nerve fibers: autonomic (B fibers) > pain and temperature (C fibers) > motor. This sequence can be remembered as **"B-C-A"** (B fibers, then C fibers, and finally A fibers). Clinically, this means that patients may first experience sympathetic blockade (e.g., hypotension), followed by sensory blockade (e.g., loss of pain sensation), and finally motor blockade.
## **Correct Answer:** .