Figure 5. Disynaptic excitation from group II afferents in a commissural interneurone that was monosynaptically excited from the MLF.

A and B, EPSPs evoked by stimulation of ipsilateral group II afferents and from the MLF. Upper (averaged) records are from a commissural interneurone; lower records are from the cord dorsum. The vertical dotted lines in A indicate the onset of the afferent volley and of the temporally facilitated EPSP following the 2nd stimulus applied to the ipsilateral quadriceps nerve. The vertical dotted lines in B indicate the first components of the MLF volley and the onset of the EPSP following this volley. C and D show one limitation of the test for temporal facilitation of group II synaptic actions. Upper traces show PSPs evoked by stimulation of group II afferents. The grey trace in C shows the monosynaptic extracellular field potential from group II afferents; the EPSP was considered to be evoked monosynaptically since its onset had the same latency as the field potential (2.75 ms). The arrow indicates where the monosynaptic EPSP was expected to occur following the second stimulus. It might have failed to appear either because it was masked by the IPSP, or because of strong presynaptic inhibition of transmission from group II afferents following the first stimulus.