Step 1: Understanding the Concept:
\textit{Rauwolfia serpentina} (Sarpagandha) contains several indole alkaloids. These alkaloids have different pharmacological activities, primarily affecting the cardiovascular and central nervous systems.
Step 2: Detailed Explanation:
1. Reserpine: This is the most significant alkaloid in Rauwolfia. It works by depleting stores of catecholamines (like norepinephrine) from peripheral sympathetic nerve endings and the brain.
2. Antihypertensive Effect: By reducing norepinephrine levels, reserpine decreases sympathetic tone, leading to a fall in blood pressure and heart rate.
3. Others: Ajmaline is primarily an anti-arrhythmic. Vincristine and Colchicine are not found in Rauwolfia; they are obtained from \textit{Vinca} and \textit{Colchicum} respectively.
Step 3: Final Answer:
Reserpine is the specific alkaloid responsible for the traditional use of Rauwolfia as an antihypertensive and tranquilizing agent.