d-tubocurarine and related synthetic drugs (pancuronium, mivacurium, etc) are a class of non-depolarizing, competitive neuromuscular blocking agents. They act by inhibiting the transmission of the signal from the motor neuron innervating your skeletal muscle - they do this by blocking the nicotinic acetylcholine receptor on the skeletal muscle. Because the muscle can no longer respond to the "signal" from the nerve that innervates it, the muscle cannot contract. These curare derivatives are often used by anesthesiologists as muscle relaxants during surgery. The original toxin(tubocurare) was isolated from plants, and used as a paralytic agent to catch prey. However, synthetic compounds were made in order to decrease side effects and shorten onset time and half-life, and these agents are more commonly used in surgery today.