Electrical and mechanical stimulation of a spider slit sensillum: Outward current excites

Summary

• 1. When current is passed between a surface electrode on the lyriform organ of the spider and a reference electrode in the hemolymph, the spike rates of the slit-sensillum sensory cells are modulated. Outward current (surface electrode negative) excites and inward current abolishes spontaneous activity (Fig. 2). This electrical response is the opposite of that reported in other arthropod mechano- and chemoreceptors. It is, however, compatible with a distal site of the spike-initiating region, possibly near the dendrite tip. Such an arrangement in the spider parallels the finding of Rick et al. (1976) that the lymph space surrounding the apical dendrite appears (unlike the situation in the insects examined) to have a high concentration of Na⁺.

• 2. Spikes recorded at the surface of this mechanoreceptor during compression of the slit do not differ appreciably in shape from those elicited by outward current (Fig. 3). Both have a negative leading edge; again, the polarity is the opposite of that measured in most insect epithelial-receptor spikes.

• 3. Responses to electrical and mechanical stimuli can be superimposed (Fig. 4), so that electrical stimuli can be used in behavioral experiments to modulate the response to mechanical input.

• 4. The spike rate elicited by maintained steps of outward current does not decline (Fig. 5). Hence the rapid adaptation to mechanical stimuli is not a property of the spike-initiating process that is driven by imposed current. On the other hand, responses to electrical test stimuli do sample some slowly recovering aftereffect of a period of adaptation to a mechanical stimulus (Fig. 6).

• 5. Although the distributions of capacitance and resistance near these sensilla are unknown, we discuss trial explanations of the negative spikes measured, by qualitative comparison with the volume conductor analyses of Lorente de Nó.