<prompt> Muscular timidity caused by opioids is due to the agonist effect on which receptorMuKappaDeltaSigma  <response>: Refer Miller 7ep781 Truncal rigidity caused by highly lipid soluble opioids like Fentanyl is supraspinal in origin. It is mainly caused by stimulation of mu receptors whereas kappa and delta receptors tend to reduce the rigidity 