Atropine is a pharmaceutical derivative of the nightshade family of plants. Its primary effect is to suppress the nerves that result in the relaxation of smooth muscle, heart muscle and glands. It is commonly used during a heart attack to prevent the heart from slowing during the attack, which could result in a lack of blood oxygen to the rest of the body.
As atropine also blocks the action of acetycholine (which controls the autonomic relaxation functions), it is often used in the treatment of organophosphate poisoning as organophosphates also bind to the same receptors.
In addition, although it is often used to treat an extremely low heart rate, atropine can occasionally have the opposite effect as intended and slow the heart even further.