Synaptic delay: its effect on information transmission in the crayfish caudal photoreceptor system

Abstract

Upon observation of the delay time associated with synaptic transmission 2, it might be suspected that the chemical synapse acts as a noisy element and thus limits the amount of information per unit time which it can transmit. However, it is not the synaptic delay per se which constitutes noise, but rather the random variations in delay. That such fluctuations do occur can be seen in the study by Katz and Miledi 5 in which they found neuromuscular synaptic delays which fluctuated from about 0.5-2.5 msec. What Katz and Miledi measured as synaptic delay was the time interval between the preand postsynaptic membrane currents. In the present work, the term synaptic delay will indicate the total delay caused by the interposition of a synapse; this includes time lags due to transmitter release and diffusion, axonal thinning of the synaptic endings acting to slow conduction time, postsynaptic action of the transmitter, and the buildup of the postsynaptic potential towards threshold. Thus, synaptic delay as is defined here should be longer and have a greater variance than that measured by Katz and Miledi. In the present paper, experiments will be presented which determine the fluctuations in the transmission delay caused by a synaptic link located in the subesophageal ganglion of the crayfish; here, the two fibers carrying light-relative information from the caudal photoreceptor in the sixth abdominal ganglion are linked to secondary fibers in the circumesophageal commissures. After removal of all appendages, the crayfish, Procambarus clarkii (Girard), were pinned ventral side up onto a shaped paraffin dish. Two silver wire recording electrodes were applied 2 cm apart: one on the thoracic nerve cord midway between the subesophageal and first thoracic ganglia, and the other on one of the circumesophageal commissures. The exposed nerve was bathed in Van Harreveld's solution and