Flexibility in the patterning and control of axial locomotor networks in lamprey

Abstract

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.

Fig. 1

Fictive swimming activity in dorsal and ventral branches of the ventral root, and in motoneurons projecting out those branches. (A) Schematic showing the dorsal and ventral branches of the ventral roots (dorVR and venVR), the recordings of their spiking activity with suction electrodes, and recordings of intracellular membrane potential from motoneurons with sharp microelectrodes (dorMN and venMN). (B) Individual motoneurons can be identified according to their projection within the branches of the ventral root. (C) Intracellular recording of two motoneurons, both venMNs, in the same spinal segment during fictive swimming induced with d-glutamate. The two venMNs have similar oscillatory waveforms. (D) A pair of intracellular recordings of a dorMN and a venMN. These two motoneurons have somewhat different oscillatory waveforms.

Fig. 2

Comparison of underlying synaptic activity in motoneurons during fictive swimming using cross-correlation of waveforms. (A) Intracellular recording of membrane potentials in two motoneurons projecting out the same ventral branch of the ventral root during fictive swimming (venMNs). The slow swim oscillations in each recording were removed with a digital filter to allow cross-correlation of the underlying fast synaptic activity. Cross-correlations of the two waveforms, excluding regions of spiking, were performed (see Buchanan and Kasicki 1999 for details of method). The boxed region is shown with greater amplification and after filtering in panel B. (B) An amplified view of the boxed region of panel A showing the membrane potential of the two motoneurons after filtering. Similarities in synaptic inputs are apparent. (C) The cross-correlogram of the two venMNs is shown along with a cross-correlogram of two motoneurons projecting out different ventral root branches (thick line). The two similarly-projecting motoneurons had a higher correlation than did the pair of motoneurons projecting in different ventral root branches. (D) Means of the peak cross-correlation coefficients (CCFs) for the 8 pairs of similar motoneurons (projecting out same ventral-root branch) and the 11 dorsal–ventral pairs of motoneurons. The means were significantly different (*P < 0.001; t-test) suggesting that there are separate populations of premotor interneurons for dorMNs and venMNs.

Fig. 3

Examples of spontaneous non-synchronous bursting of the dorsal and ventral branches of one ventral root in the rostral spinal cord in the presence of d-glutamate. (A) Slow modulation of the fast swim rhythm with alternating activity in the ipsilateral dorsal and ventral branches (i.dorVR and i.venVR) of the same spinal segment. (B) Ipsilateral dorsal and ventral branches of the ventral root exhibiting bursting of different durations and phasing. Intracellular recording of an ipsilateral venMN (i.venMN) shows that the activity of the membrane potential matches the bursting of the venVR, and the venMN shows little synaptic input related to the dorVR.

Fig. 4

The effect of serotonin on the synchrony of bursting in the dorsal and ventral branches of the ventral root during fictive swimming. (A) Example of bursting before and after addition of serotonin to the bath. After serotonin, the dorsal and ventral branches were not as well synchronized as in the control. (B) After rectifying and smoothing the bursts of the ventral root, the waveforms of the dorsal and ventral branches were used to create a cross-correlogram. An epoch of 100 s was used for the correlogram. The peak-to-trough CCF was lower after adding the serotonin. (C) A plot of the time course of the fall in the peak-to-trough CCF (i.e., the degree of burst synchrony) after adding serotonin to the bath. (D) In seven preparations, the mean peak-to-trough CCF decreased significantly after adding serotonin (5-HT). (*P < 0.001; paired t-test).

Fig. 5

Summary model of the dorsal/ventral locomotor networks. It is proposed that the locomotor central pattern generator (CPG) comprises dorsal and ventral components, respectively, serving dorsal and ventral myotomal muscles of the lamprey body. Normally, these two components are tightly coupled, but it is proposed that release of an endogenous modulator, perhaps serotonin, can weaken the coupling of the two oscillators, allowing greater flexibility in the activation patterns, perhaps under demands for greater maneuverability during swimming.