Figure 7. Increased Firing of SMns are Related to Tonic Activity in the nMLF during Light-evoked swimming.

(A) Paired-whole cell recordings on the MeLc neuron and spinal motoneurons with different responses to MeLc current injection and light stimulus. Activity shown here is elicited by light. Left: a SMn that can be activated by firing the MeLc alone with current injection (supra-threshold pair). Middle: a SMn that cannot be activated by firing the MeLc alone (sub-threshold pair). Right: a PMn that cannot be activated by firing the MeLc alone. Insets are expanded view from the gray shading areas. MeLc spikes are represented as black dots.

(B–D) Whole-cell recording of SMns (B–C) and a PMn (D) with a motor nerve recording during spontaneous and light evoked swimming.

(E–G) Expanded view from the bracketed areas in B–D respectively to show phase relationship of action potentials and swimming bursts. Swim bursts are marked by gray shadings to highlight ‘in-phase firing’, while gray arrowheads mark ‘anti-phase’ firing. The phase of a swim cycle (0: start,1: end) is marked in E.

(H) Polar plots show the phase relationship of action potentials to cyclical swim bursts during light response normalized to the start (0) and end (1) of the swim cycle (example shown in E). Gray shading represents half of the swim cycle. Spike phase distribution of individual cell (dark gray line) and the averaged distribution (black line) are shown. SMns are segregated into groups according to whether their firing pattern was tonic (n= 2,598 spikes from 6 neurons) or rhythmic (n = 435 spikes from 5 neurons). Action potentials of the MeM, MeLc, and MeLr neurons occur throughout the swim cycle consistent with tonic firing behavior (MeM, n = 803 spikes from 5 neurons; MeLc, n = 984 spikes from 4 neurons; MeLr, n = 1071 spikes from 4 neurons).