Dead bodies are at first limp. Several hours after death, the skeletal muscles undergo a partial contraction that fixes the joints. This condition, known as rigor mortis, may continue for 72 hours or more. When neurons signal living muscle fibers to contract, they do so with a neurotransmitter that is received at the surface of the muscle fiber. The signal makes the fiber open calcium ion channels, and it is the calcium that causes the contraction. The muscle then removes the calcium in two ways: it stores some in its mitochondria, and it pumps out the rest. When a body dies, stored calcium leaks and calcium pumps no longer function. The excess calcium causes the actin and myosin filaments of the muscle fibers to remain linked, stiffening the whole body until the muscles begin to decompose.