J Comp Physiol (1983) 150:41%425 Journal of Comparative Physiology. A Spri~ger-Verlag 1983 The Crayfish Sustaining Fibers II. Responses to Illumination, Membrane Properties and Adaptation Mark D. Kirk*, Brian R. Waldrop, and Raymon M. Glantz Department of Biology, Rice University, Houston, Texas 77251, USA Accepted November 8, 1982 Summary. Simultaneous intracellular and extracel- lular recordings were obtained from sustaining fibers (SFs) within the optic lobes of the crayfish compound eye. 1. A step increase in illumination produces a large (~25 mV) compound EPSP with an esti- mated reversal potential of -19 mV (Fig. 3). The visual responses are a significant fraction (~ 50%) of the driving force. The time course and relative amplitudes of the transient and steady-state com- pound EPSP are similar to those of retinular cells. 2. The SF integrating segment possesses linear I-V m (Fig. 4) and I-FsPIKE (Fig. 5) characteristics which result in a linear VEvsv-Fsv~:E function (Fig. 5) for responses to light stimuli. Thus, the transient SF discharge is a faithful reflection of its synaptic input. 3. The profound adaptation of the SF dis- charge to rectangular pulses of illumination (Fig. 5) is primarily due to the adaptation of the SF spike generating mechanism. The steady-state compound EPSP is 83% of the peak transient volt- age (Table 1). This implies that the pathway be- tween retinular cells and SFs contains neurons ca- pable of a high level of tonic response. 4. No direct synaptic interactions have been ob- served among SFs. The observed spike crosscorre- lations reported previously (Glantz and Nudelman 1976) are due to common presynaptic input. Peri- odic bursting during intense broad-field illumina- tion is due to synchronization of this common ex- citatory input. 5. Inhibition within the excitatory receptive field (Wiersma and Yamaguchi 1967; Glantz 1973) * Present address: Department of Psychology, Stanford Uni- versity, Stanford, California 94305, USA Abbreviations: SFs sustaining fibers; EPSPs excitatory post- synaptic potentials is expressed dramatically as a postexcitation de- pression, which results from a membrane hyperpo- larization. The hyperpolarization appears to result in part from a direct postsynaptic inhibition of SFs. 6. When these results are considered along with those reported previously, they indicate the impor- tant role that the SF, per se, plays in the determina- tion of its visual properties, particularly its recep- tive field, response time course and linear input/ output characteristics. Introduction Studies of crustacean visual interneurons have been primarily restricted to extracellular axonal re- cordings from the optic tract (Waterman et al. 1964; Wiersma and Yamaguchi 1967; Glantz 1971; Wiersma et al. 1982) and interganglionic connectives (Prosser 1934; Wiersma and Mill 1965; Wiersma and Roach 1977; Wood and Glantz 1980). Only two cases of recordings from interneurons within the optic lobes of the crusta- cean compound eye have been reported (Erber and Sandeman 1976; Leggett 1976). Recently, we have developed a preparation that permits intracellular recording from interneurons of the crayfish optic lobes (Kirk 1982; Kirk et al. 1982). This series of reports (Kirk etal. 1982; Waldrop et al., in preparation) presents correla- tions in cellular morphology and physiology with the visual receptive fields of the crayfish sustaining fibers (SFs). Crayfish SFs are visual interneurons with large, well defined corneal receptive fields (Wiersma and Yamaguchi 1966). Each SF pos- sesses an axon in the optic tract and a vertical, uniplanar dendritic arborizati0n within the second optic neuropil, or medulla (Waldrop and Glantz