The crayfish Sustaining fibers (SFs) are used to analyze the functional organization of the second optic lobe, the Medulla. The SFs are tonic ON visual interneurons whose dendrites lie in a single plane in the Medullary neuropile, and whose axons project via the optic tract to the supraesophageal ganglion, or brain. The SF passive properties are remarkable linear. In the region of the primary neurite generally recorded from, these neurons show linear i-V, i-f and V-f functions. The SF dendrites appear to be entirely passive integrators of their synaptic inputs.
The light-driven synaptic input to SFs is delivered by a retinotopic columnar array. The EPSP in response to a light stimulus is large (20-40 mV), and consists of two phases. The transient phase has a reversal potential approximately 50 mV above resting potential. The steady-state phase has a reversal potential about 37 mV above rest, and is associated with a steady-state input conductance which has a peak of about 120% of the resting value. A light-OFF hyperpolarization has a reversal potential 5 mV below rest and is associated with a conductance increase.
Cable modelling of SFs revealed that the dendrites are electrotonically compact, about 0.5(lamda) in total width. Passive voltage attenuations in the dendrites are very small (average 6% proximal to distal, 35% distal to proximal). The synaptic parameters (reversal potential and conductance change) are used in conjunction with the model to accurately predict the EPSP voltage.
A class of local, non-spiking amacrine (axon-less) cells is described. They are morphologically similar to the SF dendritic trees but with two distinct planes of dendrites and no axons. These neurons respond to light with a response like the SF EPSP but produce no action potentials. The amacrine cells strongly inhibit SF excitatory inputs without directly synapsing on the SFs. Similarities between amacrine cell properties and SF surround inhibition lead to the conclusion that the amacrine cells are directly involved in the formation of SF surround inhibition, and act on the columnar input pathway of the SFs.