ANSWER: (D) Is a graded changeREF: Textbook of Medical Physiology by Khurana page 1032-1033, Physiology - Prep Manual for Undergraduates, 4th Edition by Vijaya D Joshi page 562, Ganongs 22nd ed p. 123-125When a stimulus excites the receptor, it changes the potential across the membrane of the receptors. This change in the potential is called receptor or generator potential.The change in membrane potential in a receptor is caused by a change in permeability of membrane of the unmylelinated terminals to Na+. The resultant influx of Na+ causes development of generator or receptor potential. Since the receptor potential may generate the action potential it is also called as generator potential.The larger the receptor potential, the greater the frequency of action potential generated in the afferent neuron. A larger receptor potential can not bring about a larger action potential (because of all or none phenomenon), but it can induce more rapid firing of action potential.DIFFERENCE BETWEEN RECEPTOR POTENTIAL AND ACTION POTENTIAL:Receptor potentialAction potentialReceptor potential is a graded response i.e. amplitude of receptor potential increases with increase velocity of stimulus application and increase strength of stimulusAction potential obeys all or none law i.e. further increase in stimulus above threshold do not bring any change in amplitudeCan be added together if second stimulus arrives before the first stimulus is overCan not be added togetherHas no refractory periodHas a refractory period of 1 msMostly it is local and can not be propagatedIt can be propagated without loss in the amplitude along the nerve fibreDuration is greater (approximately 5-10 ms)Duration is small (approximately 1-2 ms)