When tissues such as the skin, muscles and joints become inflamed or are injured, pain receptors in those tissues are activated, and electrical pain signals are transmitted from the injured tissues through nerve fibers into the spinal cord. Within the spinal cord, the electrical pain signals are received by a second set of nerve fibers that continue the transmission of the signal up the spinal cord and into the brain. Within the brain, additional nerve fibers transmit the electrical signals to the "pain centers" of the brain where these signals are perceived as pain. Pain receptors are also present in internal, or visceral, organs such as the intestines, uterus, cervix and bladder. These pain receptors also send pain signals via similar pathways to the brain when these organs are inflamed or distended.