Separation of temperature sensitive and temperature insensitive components of the postsynaptic potentials in the frog motoneurons.
Intracellular measurements were made in the in situ spinal cord of the frog at temperatures below 5 degrees C. Responses to volleys in the sciatic nerve, in the descending fibres and in the motor axons were studied. About 30% of the motoneurons responded to sciatic volleys with 1-3 ms segmental latency, which was short enough to assume electrotonic mediation of these responses. Another group of motoneurons responded with 6-8 ms latency, i.e. with the expected delay at chemical synapses at low temperature. Latency distribution of the sciatic-evoked postsynaptic potentials was clearly bimodal in contrast with that found at higher temperatures. Postsynaptic discharges occurred with rather long latency and they were attributed to chemically-mediated excitation. Some of the postsynaptic potentials to descending volleys also occurred with quite short latency, indicating possible electrotonic transmission from supraspinal centres to motoneurons. Latency distribution of the action potentials evoked from the motor axons was bimodal, corresponding to the different, i.e. antidromic and recurrent facilitatory, mechanism of these spikes. Calculated Q10 ratios for the sciatic-evoked reflex discharges and the afferent fibre volleys were about 2.3 and 1.8, respectively. We concluded that cooling helps to separate postsynaptic potentials according to their electrotonic and chemical mediation and that electrotonic excitation does not seem to have a primary role in the generation of postsynaptic discharges initiated by dorsal root volleys in the frog.
D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D003080:Cold Temperature; D004558:Electric Stimulation; D005073:Evoked Potentials, Somatosensory; D008526:Medulla Oblongata / Q000502:physiology; D009046:Motor Neurons / Q000502:physiology*; D011893:Rana esculenta; D011930:Reaction Time / Q000502:physiology; D012018:Reflex / Q000502:physiology; D012584:Sciatic Nerve / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*; D013569:Synapses / Q000502:physiology*; D009435:Synaptic Transmission