Mechanisms of Nerve Impulse Initiation in a Pressure Receptor (Lorenzinian Ampulla)

Abstract

ELASMOBRANCHS have a well-developed system of large tubular sense organs, the Lorenzinian ampullæ^1–4. In the dogfish, each ampulla consists of a bulb-like swelling, about 0.7 mm. in diameter, located deeply in the head of the fish, and a jelly-filled tube, several mm. long, that opens in a skin pore (see inset of Fig. 1). The ampulla is innervated by a bundle of 4–8 nerve fibres that end inside the swelling. Since the findings of Sand⁵ and Hensel⁶ that the stationary frequency of impulses discharged by the Lorenzinian ampulla has a high temperature coefficient (Q ₁₀ = 2–3), the organ has been generally regarded as a thermo-receptor. But at that time, it was not yet known that not only thermo-receptors but also mechano-receptors may have such a high temperature coefficient without losing their mechano-receptor specificness^7,8. The general morphology of the Lorenzinian ampulla suggests a mechano- rather than, a thermo-receptor function. In any event, its deep location in the head, and the relatively low thermal conductivity of the jelly of its tube, argue strongly against its being a thermo-receptor. In fact, Murray⁹ has recently shown in in situ experiments that probing of the ampulla pore with a bristle causes a brief change in the frequency of the nerve impulses discharged. In this communication evidence will be given that the ampulla is sensitive to pressure; it produces generator potentials which increase with the internal pressure of the ampulla.