The effects of mercury on the nervous system range from irritability, excitability and parasthesia (numbing of the extremities) at low levels of exposure, to tremors, violent muscle spasms and death in the extreme. While carcinogenicity and mutagenicity (the power to cause mutation) are not commonly associated with mercury exposure, mercury can cross the placental barrier where exposure can lead to spontaneous abortion, congenital malformations and severe neurological defects such as cerebral palsy. Mercury affects the developing fetus by interfering with normal neuronal development; it may also affect cell division during critical stages of formation of the central nervous system.