Strychnine

• glycine receptor antagonist, meaning that it acts on the spinal cord and brain and messes with your ability to control the movements of your muscles

• has a LD50 of 1 mg/kg, meaning that less than a teaspoon of it will kill you dead
  
• causes violent convulsions (muscle spasms) with death resulting from either asphyxiation (being deprived of oxygen since you can't breath properly) or exhaustion (extreme fatigue due to the convulsions)
• present in several plants including the very obviously named strychnine tree (Strychnos nux vomica)
• in lower doses, enhances the ability of lab animals to learn how to navigate through a maze and to discriminate between different visual stimuli (i.e. it enhances their mental functions)
  

http://en.wikipedia.org/wiki/Strychnine