The receptor potential and its functional relationship to the nerve impulse analysed in a sense organ by means of thermal and electrical stimuli

Abstract

1. The receptor potential (RP) which occurs at the level of tuberous organs in the mormyrid fish (Gnathonemus petersii andG. stanleyanus) was investigated using simultaneous thermal and electrical stimuli (Part I). Its functional relationship to the propagated potential (PP) in the afferent nerve fiber was analysed (Part II). 2. Two types of spontaneously occurring potentials were recorded extracellularly at the level of these organs: 1) small (100 μV) slow potentials of variable amplitude and 2) large (several mV) diphasic initially positive spike-like potentials (RP) with constant amplitude and duration. The behaviour of the two types of potential to thermal and electrical stimuli indicates that the slow potential is the generator potential of the RP. 3. The spontaneous as well as the electrically provoked RP obeys the “all or none” law. In contrast, the RP as well as its generator potential, both are graduable with combined electrical and thermal stimuli. 4. The amplitude of the RP diminishes with decreasing temperature, whereas its duration remains constant. Tested with paired electrical stimuli, the RP does not show any unresponsive period; also, the RP can be driven by repetitive stimulation to several thousands cycles/s. Thus, the RP does not show the characteristics of a nerve-spike. 5. Simultaneous recordings of RP and correlated propagated potentials (PP) show that the RP is a local phenomenon arising at the sensory cell membrane. 6. Without being the generator potential, the RP is indispensable for the advent of the nerve impulse; the RP and PP are strictly phase related. 7. The generation of the nerve impulse is not related to the amplitude of the RP. 8. Considering the morphological features of the junction between the sensory cells and the afferent nerve fiber, it is suggested that the RP provokes the release of transmitter substances present in the presynaptic area, which in turn depolarizes the post-junctional membrane.