Figure 4. An example of delayed increases in monosynaptic EPSPs from group Ia afferents during cathodal polarization in the MG motor nucleus in the cat.

A, records of EPSPs evoked in an MG motoneuron by near-threshold stimulation of the MG nerve during the first control series. Black trace, average of 20 records. Blue traces, single records illustrating a range of amplitudes of EPSPs, all below threshold for inducing an action potential. B, EPSPs evoked at the beginning of the 2nd period of cathodal polarization (0.2 μA, within 300 μm distance from the tip of the micropipette). Black trace, average of 20 successive EPSPs. Orange traces, a selection of EPSPs illustrating the upper range of amplitudes of the EPSPs; one of which gave rise to an action potential. C, EPSPs evoked at the end of the 2nd period of polarization. Black trace, average of EPSPs evoked during a series of records when only 2 out of 20 stimuli evoked action potentials. Orange traces, EPSPs evoked during a subsequent series of records when 14 out of 20 stimuli were followed by action potentials. During the period of recording, the membrane potential dropped by only 2 mV from ∼60 mV just after the penetration. Calibration pulses 0.2 mV. Action potentials are truncated at the top. The two horizontal dotted lines indicate the original amplitude of the EPSPs and the average amplitude of EPSPs evoked during the last series of DC polarization. D, areas of the early parts of the average EPSPs (measured within 1.9 ms from their onset) during the control period, during 2 periods of cathodal polarization and during two subsequent post-polarization periods, as indicated.