The tubocurarine molecule.

Tubocurarine is an alkaloid found in the bark of a South American vine. It is one of a number of toxic compounds known under the general name of “curare.” Tubocurarine is capable of paralyzing muscle tissue by interfering with the transmission of the nerve impulses which normally control muscle movement. For this reason, it only affects voluntary muscles (not the heart muscle, for example). However, since the diaphragm is a voluntary muscle, it can be fatal by inducing asphyxiation.
Traditionally, curare poisons have been used by South American hunters to coat arrows and darts in order to kill prey. Interestingly, curare does not pass through the digestive tract and so poisoned prey can be eaten without harm! Tubocurarine has also been used as a muscle relaxant during surgery to prevent unwanted movement of the patient, although it has now largely been replaced by synthetic drugs which provide the same effect but more safely.