Systematic shifts in the balance of excitation and inhibition coordinate the activity of axial motor pools at different speeds of locomotion

Abstract

An emerging consensus from studies of axial and limb networks is that different premotor populations are required for different speeds of locomotion. An important but unresolved issue is why this occurs. Here, we perform voltage-clamp recordings from axial motoneurons in larval zebrafish during "fictive" swimming to test the idea that systematic differences in the biophysical properties of axial motoneurons are associated with differential tuning in the weight and timing of synaptic drive, which would help explain premotor population shifts. We find that increases in swimming speed are accompanied by increases in excitation preferentially to lower input resistance (Rin) motoneurons, whereas inhibition uniformly increases with speed to all motoneurons regardless of Rin. Additionally, while the timing of rhythmic excitatory drive sharpens within the pool as speed increases, there are shifts in the dominant source of inhibition related to Rin. At slow speeds, anti-phase inhibition is larger throughout the pool. However, as swimming speeds up, inhibition arriving in-phase with local motor activity increases, particularly in higher Rin motoneurons. Thus, in addition to systematic differences in the weight and timing of excitation related to Rin and speed, there are also speed-dependent shifts in the balance of different sources of inhibition, which is most obvious in more excitable motor pools. We conclude that synaptic drive is differentially tuned to the biophysical properties of motoneurons and argue that differences in premotor circuits exist to simplify the coordination of activity within spinal motor pools during changes in locomotor speed.

Keywords: excitation; inhibition; locomotion; motoneurons; recruitment; spinal cord.

Figure 1.

Patterns of motoneuron activity and spike timing. A, Chemically immobilized zebrafish larvae are pinned down to a Sylgard platform and recordings from a spinal motoneuron (Mn) and peripheral motor nerve (MN) are performed following dissection (see Materials and Methods for details). B, Image of a fluorescently filled Mn and its projection into the segmented axial musculature. Note the suction electrode for the MN recording placed in close proximity to the Mn recording site. C, Distribution of input resistance (Rin) values for low-, middle- (mid), and high-Rin pools of motoneurons. D–F, Cell-attached recordings from Mns with progressively greater Rin and simultaneous MN recordings. Expanded traces on the right are taken from regions boxed in gray. Gray lines and arrowheads mark the start of the motor burst cycle, used to quantify spike timing. G–I, Cumulative distributions of the absolute values of spike timing relative to the onset of the MN bursts (at time 0) at slower speeds (gray) and faster speeds (black) for low-Rin (G), middle-Rin (H), and high-Rin Mns (I). Double asterisks indicate a significant difference. See Results for statistical data.

Figure 2.

Changes in excitatory drive to motoneurons as a function of speed and input resistance. A–C, Voltage-clamp recordings of excitatory currents during a bout of fictive swimming (stimulus artifact at asterisk) from motoneurons (Mns) with progressively higher input resistance (Rin). Expanded traces on the right are taken from regions shaded in green. A, Right, the regions assessed for in-phase and anti-phase excitation are shaded in gray. C, The expanded trace has been magnified vertically to more clearly observe rhythmic excitatory current (light green, note color-matched scale bar). D–F, Excitatory currents normalized to phase and averaged in 10 Hz speed bins for low- (D), middle- (E), and high-Rin pools of motoneurons (F). The shaded area represents the SE. G, Comparison of excitation arriving in-phase or anti-phase between the different motor pools at 20–30 Hz (slower swimming). H, As in G, but at 40–50 Hz (faster swimming). I, Log-log plot of peak excitatory current as a function of Mn Rin. Each data point represents an individual preparation. J–L, Regression lines of excitatory current as a function of speed (gain) for Mns within the groups shown to the left in D–F. M, Log-log plot of excitatory gain as a function of Mn Rin. Each data point represents an individual preparation.

Figure 3.

Changes in inhibitory drive to motoneurons (Mns) as a function of speed and input resistance. A–C, Voltage-clamp recordings of inhibitory currents during a bout of fictive swimming (stimulus artifact at asterisk) from Mns with progressively higher input resistance (Rin). Expanded traces on the right are taken from regions shaded in red. A, Right, the regions assessed for in-phase and anti-phase inhibition are shaded in gray. D–F, Inhibitory currents normalized to phase and averaged in 10 Hz speed bins for low- (D), middle- (E), and high-Rin pools of motoneurons (F). G, Comparison of inhibition arriving in-phase or anti-phase between the different motor pools at 20–30 Hz (slower swimming). H, As in G, but at 40–50 Hz (faster swimming). I, Log-log plot of peak inhibitory current as a function of Mn Rin. Each data point represents an individual preparation. J–L, Regression lines of inhibitory current as a function of speed (gain) for Mns within the groups shown to the left in D–F. Note, for low-Rin motoneurons, we were only able to obtain IPSC data from 4 of the 12 cells. M, Log-log plot of inhibitory gain as a function of Mn Rin. Each data point represents an individual preparation.

Figure 4.

Differences in the timing of excitation and inhibition with an increase in speed related to motor group. A–C, Averaged and normalized (to peak) traces of excitation and inhibition overlaid on normalized histograms of spike timing (shown in gray) from low- (A), middle- (B), and high-Rin motoneuron (Mn) groups (C) at slow speeds (left) and fast speeds (right). The shaded area represents the SE. Note histograms of spike timing are normalized as percentages of the total number of spikes recorded in cell-attached mode at that speed range (scale bar in A, right). Values were originally presented in Figure 1, G–I, and are reported in the Results.

Figure 5.

Changes in the timing of excitation and inhibition related to input resistance. A, Half-width of excitation as a function of motoneuron input resistance (Rin) at slow (left) and fast (right) speeds. B, Timing of peak excitation as a function of motoneuron Rin at slow (left) and fast (right) speeds. C, Ratio of in-phase to anti-phase inhibition expressed as a function of Mn Rin at slow (left) and fast (right) speeds. Ratio is calculated from the average level of total current between phase −0.05 to 0.45 (in-phase) and 0.45 to 0.95 (anti-phase).