Compare the sustained contraction of muscle cells in rigor mortis with that whic

In the case of rigor mortis, the ATP supply is readily depleted because the cells are in various stages of dying. Low levels of ATP are associated with the inability of the cross-bridges to disassociate. The cells accumulate cross-bridges over time until ATP is depleted from the cell.
In the case of tetanospasmin, the release of inhibitory neurotransmitters is prevented. As a result, the signal to contract is persistent. The continuous stimulation of contraction does not allow sufficient numbers of calcium ions to be moved back into the sarcoplasmic reticulum. As a result, the consistent levels of calcium in cytosol keep the muscle cells in a chronic state of contraction.