Direct excitatory interactions between spinal motoneurones of the cat.

Abstract

1. Ninety-seven spinal motoneurones were identified by their antidromic invasion following stimulation of the muscle nerve and submitted to a series of four tests to reveal a possible direct excitation between motoneurones. 2. Threshold differentiation, refractoriness, hyperpolarization and collision revealed antidromically induced depolarizations in fourteen of the ninety-seven tested motoneurones. 3. The parameters of the antidromically induced depolarizations indicate a short latency, a low amplitude and independence with regard to the membrane polarization. 4. It is concluded that the antidromically induced depolarizations reached the impaled motoneurone via a route other than its own axon. 5. The mechanism may involve either electrotonic interactions between neighbouring motoneurones or excitatory recurrent collaterals between synergist motoneurones.

Postnatal changes in motoneurone electrotonic coupling studied in the in vitro rat lumbar spinal cord.

“…8B for P1-4 agrees well with the range of 0-8 to 2-7 mV reported by Fulton et al (1980) and that of 0 5 to 3 mV for P9 to P14 of Karamian et al (1988). The SLD amplitude in neonatal motoneurones was larger than the < 1 mV seen in cat motoneurones (Gogan et al 1977), but smaller than the mean of 1-48 mV reported for mature mesencephalic neurones (Baker & Llina's, 1971). Although the duration of the action potential in coupled cells is a significant factor in determining SLD amplitude (Llina's & Yarom, 1981), this parameter did not change significantly with development in neonatal motoneurones, nor did the VR spike amplitude (R. Navarrete & K. Walton, unpublished results).…”

“…2A, inset), but was quite short compared to that of the recurrent IPSP (> 2-0 ms). In this preparation the latency alone is indicative that the subthreshold response was due to activation of an electrical synapse (Fulton et al 1980 Baker & Llinas, 1971;Gogan et al 1977;Shapovalov, Shiriaev & Tamarova, 1979;Fulton et al 1980;Taylor & Dudek, 1982;Connors et al 1983;Karamian, Kozhanov & Chmykhova, 1988): (1) had a short latency compared to that for monosynaptic chemical synaptic potentials, (2) showed a graded response to increase stimulus intensity, (3) were resistant to high frequency (50 Hz) stimulation, (4) were resistant to collision with an action potential conducted from the soma, (5) were insensitive to changes in membrane potential, (6) were insensitive to removal of Ca2+ from, or addition of calcium channel blockers to, the bathing solution.…”

Stochastic Resonance in the Motor System: Effects of Noise on the Monosynaptic Reflex Pathway of the Cat Spinal Cord

“…Furthermore, because the proportion of possible direct excitatory connections between motoneurons is low (Gogan et al 1977) and there is controversy that there are electrical connections between neighboring motoneurons in adult cats (Hinckley and Ziskind-Conhaim 2006), we have not considered couplings between them. Moreover, we do not expect that our model yield qualitatively different results after including either electrical or excitatory connection between motoneurons.…”

Mixed synapses discovered and mapped throughout mammalian spinal cord.

“…Likewise, the intercellular geometry (38) at mixed synapses appears to preclude a dominant role for these extremely small gap junctions in conventional orthodromic electrical transmission in mammals (ref. 37 Although the electrical effects of mixed synapses have yet to be detected in adult mammalian spinal cord neurons using current electrophysiological methods (15,17,18), the presence of several small gap junctions at mixed synapses may, nevertheless, produce profound effects on the electrical and metabolic properties of coupled cells: (i) summation of electrical and chemical transmission would increase the amplitude of postsynaptic depolarization, thereby improving synaptic efficacy at excitatory synapses (37); (ii) retrograde electrical activation could potentiate neurotransmission in a particular subset of excitatory synapses, thereby providing a mechanism for recruitment of that class of excitatory fibers; (iii) retrograde activation of inhibitory terminals could provide a mechanism for negativefeedback inhibition; and (iv) direct transfer of metabolites and second messengers could be involved in regulation of synaptic efficacy (37) and/or defining functional groups of neurons [e.g., defining "pattern generator" circuits (40)]. …”