Graded potentials and spiking in single units of the oval organ, a mechanoreceptor in the lobster ventilatory system. I: Characteristics of dual afferent signalling

The lobster oval organ sends three afferent fibres to the sub-oesophageal ganglion: X (mean diameter 41 μm), Y (32 μm) and Z (22 μm). The distance between the oval organ and the ganglion is 10–15 mm in animals of approximately 10 cm carapace length. The cell bodies of the sensory units lie centrally amidst the ventilatory motoneurones, and their central branches permeate the same lateral neuropile. Intracellular recordings from each fibre show that the afferent response to stretch has two components: overshooting, regenerative, TTX-sensitive impulses, and an underlying graded depolarization. The analogue signal has the characteristics of a receptor potential: a dynamic response with rapidly depolarizing initial component, some adaptive fall leading to a maintained static plateau, and a post-release hyperpolarizing undershoot. The two signals are not equally represented in the response of the three afferents. Fibre X has the largest graded potentials (up to 30 mV in recordings 6mm proximal to the confluence of the sensory dendrites). Fibre Z attains the highest firing frequencies (usually around 100 s−1). Fibre Y has intermediate properties. Both the amplitude of the graded potential and the number of spikes per response (in fibres Y and Z) relate linearly to pull amplitude. Estimates of length constant (approximately 10 mm) and specific membrane resistance (l00 kΩcm2) are consistent with the hypothesis that the graded potentials spread into the presynaptic terminals with sufficient magnitude to bring about postsynaptic events.