Monosynaptic connexions of low threshold muscle afferents with hindlimb motoneurones in the turtle spinal cord.

Excitatory postsynaptic potentials (EPSPs) were recorded intracellularly from hindlimb motoneurones of the anaesthetized fresh water turtle. The EPSPs were evoked from low threshold muscle afferents and the amplitudes saturated for stimuli less than two times the nerve threshold. The segmental latencies of these EPSPs, measured from the initial positive peak of the triphasic cord dorsum potential to the onset of the EPSP, ranged from 1.5 to 3.1 ms. The intraspinal conduction time of afferents was estimated by recording afferent volleys in the grey matter along the vertical course of intraspinal afferent fibres. The synaptic delay was estimated by subtracting the latency of the afferent volley at the deepest region of the dorsal horn from the segmental latency of the EPSP (in the range from 1.6 to 2.1 ms) recorded in the same microelectrode track. The average value was 0.99 ms (range: 0.9-1.1 ms), which was close to the known synaptic delay of cold-blooded animals. Therefore, the EPSPs in this range of segmental latencies were regarded as monosynaptic. Taking account of the intraspinal afferent conduction time (0.8 ms on average) and another synaptic delay, the latency for disynaptic transmission would be 2.8 ms or more. Thus, EPSPs having segmental latencies of 1.5-3.1 ms were suggested to be almost all monosynaptic in nature, at least under the present conditions of deep anaesthesia. On the basis of the above criteria for the monosynaptic nature of EPSPs, the pattern of convergence of monosynaptic excitatory inputs from various muscle afferents was investigated. Monosynaptic EPSPs were induced from the homonymous muscle nerve and the nerve innervating the synergist at the same joint. The heteronymous EPSPs were also found between muscles within each group of the anterodorsal musculature and the posteroventral musculature. No monosynaptic connexions were found between anterodorsal and posteroventral muscles except between the muscles innervated by the peroneal and the tibial nerve.