Intracellular recordings from spinal neurons during ‘swimming’ in paralysed amphibian embryos

Abstract

Intracellular microelectrode recordings have been made from probable motoneurons in the spinal cord of Xenopus laevis embryos during fictive 'swimming' in preparations paralysed with the neuromuscular blocking agent tubocurarine. These cells had resting potentials of -50 mV or more. During spontaneous or stimulus-evoked 'swimming' episodes: (a) the cells were tonically excited; the level of tonic synaptic excitation and the conductance increase underlying it were both inversely related to the 'swimming' cycle period; (b) the cells usually fired one spike per cycle in phase with the motor root burst on the same side; spikes did not overshoot zero and were evoked by phasic excitatory synaptic input on each cycle, superimposed on the tonic excitation; (c) in phase with motor root discharge on the opposite side of the body, the cells were hyperpolarized by a chloride-dependent inhibitory postsynaptic potential. The nature of synaptic potentials during 'swimming' was evaluated by means of intracellular current injections. The 'swimming' activity could be controlled by natural stimuli. The results provide clear evidence on the relation of tonic excitation to rhythmic locomotory pattern generation, and indirect evidence for reciprocal inhibitory coupling between antagonistic motor systems.

Excitatory amino acid receptors in Xenopus embryo spinal cord and their role in the activation of swimming.

“…2 and Table 1). These values are very similar to those observed during the tonic excitation which occurs during naturally evoked swimming (Roberts & Kahn, 1982;Soffe & Roberts, 1982 a). Outside these critical concentration ranges NMDA and kainate depolarized motoneurones and increased their conductance.…”

“…Motoneurones fire spikes in phase with the ipsilateral ventral root spikes and receive a mid-cycle i.p.s.p. in phase with the contralateral ventral root spike (Roberts & Kahn, 1982;Soffe & Roberts, 1982b;see Fig. 3A).…”

“…3A). The phasic potentials seen in motoneurones are superimposed upon a tonic level of depolarization about 10-30 mV above the resting potential (Roberts & Kahn, 1982;and Fig. 3A).…”

“…Therefore, changes in the input resistance of motoneurones occurring at mid cycle can reasonably be taken as a measure of the resistance change associated with the tonic excitation. The size of this resistance change decreases throughout a swimming episode and is closely correlated with the cycle period (Roberts & Kahn, 1982). To allow comparisons between swimming episodes, measurements for a particular experiment were always made at a particular cycle period.…”

“…The tonic excitation ofmotoneurones during swimming has an associated resistance change which can be measured by passing constant hyperpolarizing current pulses into the cell before and during swimming (Roberts & Kahn, 1982;Soffe & Roberts, 1982 b). The input resistance of the motoneurone was measured at mid-cycle (see Fig.…”