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Journal of Experimental Biology 107,431-449 (1983)
Published by Company of Biologists 1983

Graded Potentials and Spiking in Single Units of the Oval Organ, a Mechanoreceptor in the Lobster Ventilatory System: I. Characteristics of Dual Afferent Signalling

V.M. PASZTOR ¹ and B.M.H. BUSH ²

¹ Department of Biology, McGill University 1205 Avenue Dr. Penfield, Montreal, Canada H3A 1B1
² Department of Physiology, University of Bristol Park Row, Bristol BS1 5LS

1. The lobster oval organ sends three afferent fibres to the suboesophageal 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.

2. The cell bodies of the sensory units lie centrally amidst the ventilatory motoneurones, and their central branches permeate the same lateral neuropile.

3. 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.

4. 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.

5. 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 6 mm proximal to the confluence of the sensory dendrites). Fibre Z attains the highest firing frequencies (usually around 100s^-1). Fibre Y has intermediate properties.

6. Both the amplitude of the graded potential and the number of spikes per response (in fibres Y and Z) relate linearly to pull amplitude.

7. Estimates of length constant (approximately 10 mm) and specific membrane resistance (100k cm²) are consistent with the hypothesis that the graded potentials spread into the presynaptic terminals with sufficient magnitude to bring about postsynaptic events.

Key words: Mechanoreceptor, afferent signalling, graded potentials, Crustacea

Submitted on March 14, 1983
Accepted on June 24, 1983