The Parameters and Properties of A Group of Electrically Coupled Neurones in the Central Nervous System of A Hydrozoan Jellyfish

Abstract

Swimming of the jellyfish Polyorchis penicillatus is controlled by a network of large, electrically coupled, motor neurones in the inner nervering (Fig. 3). Recordings from pairs of neurones, even at widely separated positions, display essentially the same electrical activity (Fig. 1). Electrical coupling between neurones is via gap-junctions and is very strong, giving the network a space constant of approximately 7.1 mm (Figs. 2, 4, 5). The network acts as a low-pass filter progressively attenuating signals with frequencies greater than 1 Hz (Figs. 4, 6). I/V experiments demonstrate that the neurones show rectifying properties since membrane resistance decreases noticeably with depolarizations greater than about 25 mV (Figs. 7, 8). A number of electrical constants of the network were measured or calculated: r_m = 3.55 MΩ cm⁻¹, R_m = 98 kΩ cm², r_i = 7 MΩ cm⁻¹, R_input = 2.5 MΩ, C_m = 1.52 μF cm⁻². Stimulated action potentials are conducted in the network at approximately 112 cm s⁻¹ while spontaneous action potentials have velocities up to 200 cm s⁻¹. As an action potential propagates from its initiation site its duration decreases from about 30 ms to 8 ms when it reaches the opposite side of the margin. Epithelial impulses, which mediate crumpling, cause large i.p.s.p.s in the motor network that can inhibit swimming for several seconds.