Synaptic connections between nonspiking afferent neurons and motor neurons underlying phase-dependent reflexes in...

1. This paper analyzes the synaptic connections made by nonspiking afferent neurons of the thoracocoxal muscle receptor organ (TCMRO) with basal limb motor neurons in the crayfish. The T fiber, a dynamically sensitive afferent, monosynaptically excites promotor motor neurons. Evidence suggests that both tonic graded chemical transmission and electrical synaptic transmission may be involved, depending on the motor neuron under consideration. 2. In preparations in the active state (spontaneously producing reciprocal motor patterns), the T fiber also inhibits promotor motor neurons in a phase-dependent manner. This inhibitory pathway is probably indirect, because it involves additional synaptic delay. 3. The statically sensitive S fiber also excites promotor motor neurons, but phase-dependent inhibition of promotor motor neurons by the S fiber was not seen. 4. The T fiber excites a subclass of remotor motor neurons (group 1) by a combination of direct chemical input and electrical input. This connection underlies the positive feedback reflex that excites these remotor motor neurons, in a phase-dependent manner, on stretch of the TCMRO during the active state. In inactive preparations, this connection remains subthreshold. 5. Central synaptic outputs of group 1 remotor motor neurons can also inhibit promotor motor neurons. This pathway may contribute to the phase-dependent reflex inhibition of promotor motor neurons that occurs in the active state.