<prompt> During inhibition of SA node by vagal stimulation, which of the following occurs:Acting on K channels resulting in T effluxActing on K channels resulting in T influxActing on Ca2channels resulting in T effluxActing on Ca2 channels resulting in T influx  <response>: Ans: a acting on Kchannels resulting in increased efflux Ref: Ganong, 22nd ed,p. 548, 549Vagal stimulation of SA node causes hyperpolarization by K efflux.Stimulation of parasympathetic vagal fibres to SA node Vagal nerve endings release acetyl choline which T the K conductance of the SA node This is mediated by M, muscarinic receptors which opens a special set of K channels resulting in hyperpolarization and also the slope of prepotential.Stimulation of sympathetic fibres to SA node Membrane potential falls more rapidly and the rate of spontaneous discharge Norepinephrine the intracellular cAMP that facilitates the opening of L long acting calcium channels. This results in increased intracellular calcium which leads to a rapid depolarization and the slope of pre potential. 