The crayfish sustaining fibers

SummarySimultaneous intracellular and extracellular 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 estimated 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 compound EPSP are similar to those of retinular cells.2.The SF integrating segment possesses linear I-Vm (Fig. 4) and I-FSPIKE (Fig. 5) characteristics which result in a linear VEPSP-FSPIKE 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 discharge 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 voltage (Table 1). This implies that the pathway between retinular cells and SFs contains neurons capable of a high level of tonic response.4.No direct synaptic interactions have been observed among SFs. The observed spike crosscorrelations reported previously (Glantz and Nudelman 1976) are due to common presynaptic input. Periodic bursting during intense broad-field illumination is due to synchronization of this common excitatory input.5.Inhibition within the excitatory receptive field (Wiersma and Yamaguchi 1967; Glantz 1973) is expressed dramatically as a postexcitation depression, which results from a membrane hyperpolarization. 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 important role that the SF,per se, plays in the determination of its visual properties, particularly its receptive field, response time course and linear input/ output characteristics.