The central projections of the stretch receptor neurons of crayfish: segmental gradients of synaptic probability and strength.

The 20 stretch receptor neurons (SRs) of the crayfish abdomen send axons into the CNS that then project both to the brain and to the last abdominal ganglion, G6 (Bastiani and Mulloney, 1988). In G6, we recorded intracellularly from different kinds of neurons postsynaptic to SR axons. In a sample of 100 postsynaptic neurons, 59 synapsed with both SR1 and SR2 axons, 19 synapsed only with SR1 axons, and 22 synapsed only with SR2 axons. Most monosynaptic connections in G6 were excitatory and behaved like typical chemical synapses. The EPSPs showed moderate facilitation but could be depressed about 50% by protracted stimulation at 20 Hz or more. In individual postsynaptic neurons, comparisons of synapses made by SRs that originated from different abdominal segments and from each side of the abdomen revealed gradients of probability of synaptic connection and of relative sizes of EPSPs; SRs originating in anterior segments were less likely to synapse with most postsynaptic neurons than were SRs originating in posterior segments, and the EPSPs caused by these anterior SRs tended to be smaller. Similarly, SRs contralateral to the postsynaptic neuron were less likely to make a connection, and the EPSPs they caused tended to be smaller than those caused by ipsilateral SRs. Some local interneurons in G6 had reversed anterior-posterior gradients in EPSP amplitude. Calculations of shape indices for PSPs from SRs originating in different segments and measurements of the maximum shunting by preceding PSPs from other SR axons indicated that neither electrotonic decrement in the postsynaptic neurons nor shunting could account fully for the observed gradients in PSP strength.(ABSTRACT TRUNCATED AT 250 WORDS)