<prompt> Muscle rigidity by fentanyl is due to which receptor:Meu mKappa kDelta dSigma s  <response>: Ans. a Meu mRef.: Millers Anaesthesia 7th ed. 914Opioid receptors are of four variety - mu m, Kappa k, delta d sigma sReceptorClinical effectAgonist m2Muscle rigidityPhysical dependenceSupraspinal analgesiaRespiratory depressionFentanylMorphineb- EndorphinMu m m2Kappa kSedationSpinal analgesiaMorphineNalbuphineDynorphin endogenous opioidButorphanol oxycodoneDelta dAnalgesiaBehavioralEpileptogenicLeu - Enkephalinb- EndorphinSigma sDysphoriaHallucinationRespiratory stimulationPentazocineNalorphineKetamine 