A variety of types of involuntary muscle activity may be referred to as a “spasm”. Examples include muscle contractions due to abnormal nerve stimulation, or abnormal activity of the muscle itself. A series of spasms or permanent spasms are called a spasmism. A spasm may lead to muscle strains or tears of tendons and ligaments, if the force of the spasm exceeds the tensile strength of the underlying connective tissues, such as with a particularly forceful spasm, or in the case of weakened connective tissues. True hypertonic spasm is caused by malfunctioning feedback nerves, is much more serious, and is permanent unless treated. In this case, the hypertonic muscle tone is excessive and the muscles are unable to relax. A subtype of spasms is colic. This is an episodic pain due to spasms of smooth muscle in a particular organ (e.g. the bile duct). A characteristic of colic is the sensation of having to move about, and the pain may induce nausea or vomiting if severe.