The specific ionic conductances and the ionic movements across the motoneuronal membrane that produce the inhibitory post‐synaptic potential

Spatial synaptic distribution of recurrent and group Ia inhibitory systems in cat spinal motoneurones

“…The observation that group La and recurrent IPSPs show similar responses to current injection both with and without relatively large changes in intracellular C1-concentration (Coombs et al 1955;Araki et al 1961; and present results), taken together with the present observation that relatively small Cl-concentration changes at the motoneurone soma cause variable degrees of differential reversal of the same IPSPs, seem best explained by postulating that the synaptic terminals of Renshaw interneurones are located largely on the proximal motoneurone dendrites while the terminals of the group la inhibitory interneurones are located both directly on, and very close to, the cell soma. The reasons for drawing this conclusion and possible alternatives will be taken up in the Discussion.…”

“…In this case, the records were obtained about 10 min after penetration of the cell with a KCl filled micropipette and the first set of records ('no current') shows that by this time both IPSPs were reversed by diffusion of Cl-from the electrode tip. Hyperpolarizing current enhanced the reversed IPSPs and, as was rather common after passing large anodal currents, sudden removal of the current flow ('current off') led to depolarization of the cell (fourth set of records, membrane potential decreased to about -45 mV; see Coombs et al 1955). At this membrane potential both IPSPs were hyperpolarizing but thereafter, as the membrane potential returned to its original value of -70 mV.…”

“…Finally, removing the transmembrane current left the cell at approximately the original resting membrane potential (about -58 mV) and the recurrent IPSP returned to a hyperpolarizing transient while the early portion of the group Ia IPSP was still reversed, presumably due to Cl-ions injected into the cell during passage of the hyperpolarizing current. The diphasic reversal evident with the group I a IPSP in this cell (especially evident in the second and third 'no current' records) has been seen rather often, particularly in those cases in which Cl-injection has been limited (see Coombs et al 1955).…”

“…Parallel reversal of IPSPs with citrate electrodes The reversal potentials for group I a and recurrent IPSPs also have been compared using a non-reversing anion solution in the micropipettes (see Coombs et al 1955). The records in Fig.…”

“…1955;Curtis & Eccles, 1959), and on the fact that the falling phase of the group I a IPSP closely resembles the time course of the membrane response obtained by injecting a constant current pulse into the cell soma (Curtis & Eccles 1959). A similar location has been assumed for the synaptic terminals of Renshaw interneurones which are responsible for the recurrent IPSP in motoneurones (Eccles, Fatt & Koketsu, 1954), since the effect of intracellular current and ion injection on the recurrent IPSP has been described to be about the same as that seen with the group Ia IPSP (Coombs et al 1955;Araki et al 1961).…”