Sensory feedback can coordinate the swimming activity of the leech

Abstract

Previous studies showed that sensory feedback from the body wall is important and sometimes critical for generating normal, robust swimming activity in leeches. In this paper, we evaluate the role of sensory feedback in intersegmental coordination using both behavioral and physiological measurements. We severed the ventral nerve cord of leeches in midbody and then made video and in situ extracellular recordings from swimming animals. Our electrophysiological recordings unequivocally demonstrate that active intersegmental coordination occurs in leeches with severed nerve cords, refuting earlier conclusions that sensory feedback cannot coordinate swimming activity. Intersegmental coordination can in fact be achieved by sensory feedback alone, without the intersegmental interactions conveyed by the nerve cord.

Fig. 1.

Measurement of intersegmental phase lags by video recording. The two video frames show side views of a swimming leech from a continuous video recording. Arrival of a trough at midbody segment M7 (A, frame 30) and then midbody segment M14 (B, frame 37). Because the video is recorded at 30 frames/sec, the time interval between the two frames is 7/30 or 0.23 sec. Given that the swim cycle period is 0.50 sec in this swimming episode, the phase lag between M7 and M14 is 166°. [The leech is swimming to the left, and the ruler in the background shows the distance traveled. Pictures here and in other similar figures were captured by a Matrox (Boca Raton, FL) Rainbow Runner video-capturing card and were enhanced with Corel (Ottowa, Canada) Photo-Paint.]

Fig. 2.

In situ recording.A, Two types of preparations used in the experiments. The leech ventral nerve cord is composed of a head ganglion (H), 21 midbody ganglia (M1–M21), and a tail ganglion (T). The median Faivre’s nerve and two lateral connective nerves link the ganglia. Interactions between the nerve cord and the body wall occur via the paired nerve roots that project from each midbody ganglion. The DP nerve, which branches from the posterior nerve root and innervates the dorsal side of the body wall, contains the large axon of the swim motoneuron DE-3.Top, Intact preparation with both the ventral nerve cord and the body wall intact. Bottom, SNC preparation with the ventral nerve cord severed between M10 and M11. B, Recording setup. Leeches were tethered by threads attached to head and tail suckers and suspended in a deep glass dish for physiological recording and videotaping. The lengths of threads tethering the leech were adjusted so that a full body wave could be developed. DP nerve activity was recorded in situ via fine silver hook electrodes. The inset illustrates the detail of a hook electrode. C, Snapshot of an experiment using the setup described above. The oscilloscope in the background displays signals recorded from the implanted electrodes.

Fig. 3.

Swim body waves of a freely swimming leech before and after its nerve cord was severed. Each column shows sequential video frames of the leech in side view. Beads were sewn to the body wall at midbody segments M7 and M14 as markers to facilitate the determination of intersegmental phase relationships. A, Nerve cord intact. A full sinusoid-like swim wave was developed along the leech body, with a crest and a trough passing backward while the leech was swimming forward (anterior is to the left).Frames 1 and 12 show identical profiles, with a cycle period of 11/30 or 0.37 sec. The crest arrives at M7 and M14 in frames 2 and 6, so the crest-to-crest phase lag was [(6.0 − 2.0)/11] * 360° = 131°. Similarly, trough-to-trough phase lag was [(11.0 − 7.6)/11] * 360° = 111° (the trough arrives at M14 betweenframes 7 and 8 and was interpolated asframe 7.6). On average, the phase lag from M7 to M14 was 121°. B, The same leech after its nerve cord was severed. The entire length of the preparation maintains a smooth and strong swim body wave, indicating that the posterior end is active during swimming. More than a full sinusoid-like wave is developed, especially evident in frames 6 and 13 in which two troughs or crests can be observed. Frames 1and 14 are at the same phase angle, so the cycle period is 13/30 or 0.43 sec. Crest-to-crest phase lag is [(8.8 − 4.2)/13] * 360° = 127°, and trough-to-trough phase lag is [(14.8 − 9.4)/13] * 360° = 150°. The average phase lag from M7 to M14 is 139°.

Fig. 4.

Swim body waves of anterior and posterior half-leeches. A, Eleven continuous frames captured from an anterior half-leech. Although the anterior half-leech seemed to be simply flexing its body up and down, a traveling wave might still be present (frames 6–10). Because the shape was only slightly nonuniform along the half-leech, we infer that phase lags between segments were very small. The camera was fixed during filming; hence, the vertical alignment of the leech silhouettes demonstrates that the anterior half did not progress forward. B, Ten consecutive frames captured from a posterior half-leech. A traveling wave is obvious, and sometimes a crest and a trough can be simultaneously observed in the same profiles (frames 1, 2, 6, 7,9, and 10), as in an intact leech. Although the amplitude of its swim wave is less than that of the anterior half, the posterior half travels forward approximately one-third of its body length in this swim cycle (indicated by thebroken line). In both columns, anterior is to the left.

Fig. 5.

Swimming undulations in tethered leeches.A, Intact preparations. B, SNC preparations. Amplitudes are larger and appear somewhat distorted compared with freely swimming leeches (Fig. 3). More than one full wavelength is present, even in the intact preparations (anterior is to the left).

Fig. 6.

In situ extracellular recordings from tethered swimming leeches. Nerve impulses recorded from the DP nerves were generated by motoneurons DE-3, which command dorsal longitudinal muscle contraction during swimming. DE-3 neurons generate one burst of impulses per swim cycle; relative timing of DE-3 bursting in different segments was used to measure intersegmental phase relationship. A, An intact preparation (left) and an SNC preparation (right; different animal). B, A sample record from the intact leech. DE-3 activity in M7 and M14 was phase-locked, with a phase lag of less than one-third of a cycle. C, A sample record from the SNC leech. DE-3 activity in M7 and M14 was again phase-locked after nerve cord transection, but the phase lag between them is now approximately one-half of a swim cycle. [DP(7), DP nerve from M7; DP(14), DP nerve from M14. In theDP(14) trace in B and bothtraces in C, the largest spikes were identified as T-cell impulses by their large size and their activation by light touch.]

Fig. 7.

Intersegmental phase lags between M7 and M14 in intact, SNC, and isolated nerve cord preparations. In each plot, a counterclockwise 360° circle represents the swim cycle, 0/360° is the midpoint of DP(7) bursting, and the instantaneous phase lags between M7 and M14 are plotted as a circular histogram. The length of the filled wedgesrepresents

Fig. 8.

Instantaneous phase lags in intact, SNC, and isolated nerve cord preparations. Instantaneous phase lags between M7 and M14, measured for each individual swim cycle, are plotted against cycle number. A, An intact preparation (Fig.7A, #3). Although there is some fluctuation, the phase lags are relatively stable within the range of 80–130° during the whole swim episode. B, An SNC preparation (Fig. 7A, #1). Large fluctuations can be seen all through the swim episode, with phase lags as low as 60° and as high as 180°. C, An isolated nerve cord preparation (Fig. 7A,#4). Variance in cycle period is approximately the same as that of the intact preparation shown in A. Phase lags were obtained from DP records in tethered animals. the number of swim cycles that fall into the corresponding phase bin. An asterisk indicates the mean value of the histogram. Numbers under each plot are the mean ± SD; the total number of swim cycles included in the plots is inparentheses. A, Data from individual preparations. Each preparation is represented by its best swim episode(s). For two preparations (#3, #4of intact and SNC), DP nerve activity was recorded in both intact and SNC conditions. For others, different animals were used for these two conditions. Isolated nerve cord data are from five additional leech preparations. B, Pooled data. Data from all preparations of the same category are pooled in one plot. Phase lags were obtained from DP records in tethered animals.