Central generation of grooming motor patterns and interlimb coordination in locusts

Abstract

Coordinated bursts of leg motoneuron activity were evoked in locusts with deefferented legs by tactile stimulation of sites that evoke grooming behavior. This suggests that insect thoracic ganglia contain central pattern generators for directed leg movements. Motoneuron recordings were made from metathoracic and mesothoracic nerves, after eliminating all leg motor innervation, as well as all input from the brain, subesophageal ganglion, and prothoracic ganglion. Strong, brief trochanteral levator motoneuron bursts occurred, together with silence of the slow and fast trochanteral depressor motoneurons and activation of the common inhibitor motoneuron. The metathoracic slow tibial extensor motoneuron was active in a pattern distinct from its activity during walking or during rhythms evoked by the muscarinic agonist pilocarpine. Preparations in which the metathoracic ganglion was isolated from all other ganglia could still produce fictive motor patterns in response to tactile stimulation of metathoracic locations. Bursts of trochanteral levator and depressor motoneurons were clearly coordinated between the left and right metathoracic hemiganglia and also between the mesothoracic and the ipsilateral metathoracic ganglia. These data provide clear evidence for centrally generated interlimb coordination in an insect.

Fig. 1.

The mesothoracic–abdominal preparation with all leg motor innervation severed. A, Schematic illustration of the dissected mesothoracic and metathoracic nerves on one side, viewed from above. The only intact nerve branches were several thin, anterior sensory branches of metathoracic nerve 3B, which provide mechanosensory innervation of the hindleg coxa, and the thick, lateral branch of metathoracic nerve 6, which provides mechanosensory (and auditory) innervation of the ear. B, Sensory responses of nerve branches innervating the hindleg coxa and ear. In each case, the nerve branch was severed, and the distal part was recorded with a suction electrode. Each nerve burst corresponds to one discrete stroke of the fire-polished tip of a glass micropipette (5 strokes in1; 6 strokes in 2; 5 strokes in3). 1, Metathoracic nerve 3B recording; the ventral hindleg coxa was stimulated. 2, Metathoracic nerve 5A recording; the ventral hindleg coxa was stimulated.3, Metathoracic nerve 6 recording; the ear was stimulated. The recording in 3 was performed in a fifth instar animal in which nerve 6 showed no response to sound.

Fig. 2.

Fictive motor pattern evoked by tactile stimulation of the posterior abdomen in a mesothoracic–abdominal preparation. Each trace is a suction electrode recording from the proximal part of a severed metathoracic nerve branch; all nerves were recorded ipsilateral to the stimulated location. A shows the motor pattern at a slow time scale; B shows the initial part of the motor pattern, indicated by a thick,horizontal bar, at a faster time scale.Up and down arrows indicate the beginning and end of tactile stimulation. Tactile stimulation consisted of continuous rubbing at 1–4 Hz with the tip of a fine paintbrush; there was no correspondence between the frequency of rubbing and the frequency of motoneuron bursts. Motoneurons are identified by the relative sizes of their extracellularly recorded action potentials (see Materials and Methods). The first large burst of trochanteral levator motoneurons began 1.1 sec after the onset of tactile stimulation.Troch Lev, Trochanteral levator motoneurons;SETi, slow tibial extensor motoneuron;D_s, slow trochanteral depressor motoneuron; D_f, fast trochanteral depressor motoneuron; CI, common inhibitor motoneuron. Note that coordinated hindleg motoneuron bursts were evoked by tactile stimulation of the posterior abdomen in the absence of leg movement.

Fig. 3.

Fictive motor pattern evoked by tactile stimulation of the ventral hindleg coxa in a mesothoracic–abdominal preparation. A shows the motor pattern at a slow time scale; B shows part of the motor pattern at a faster time scale. The first large burst of trochanteral levator motoneurons began 32.5 sec after the onset of tactile stimulation. These recordings are from the same preparation as shown in Figure 2; conventions are as in Figure 2. Note that coordinated hindleg motoneuron bursts were evoked by tactile stimulation of the ventral hindleg coxa in the absence of leg movement.

Fig. 4.

Fictive motor pattern evoked by tactile stimulation of the ear in a mesothoracic–abdominal preparation. Tactile stimulation began before the period illustrated and continued throughout this period. The large burst of trochanteral levator motoneurons began 10.4 sec after the onset of tactile stimulation. Conventions are as in Figure 2. Note that the coordinated hindleg motoneuron burst was evoked by tactile stimulation of the ear in the absence of leg movement.

Fig. 5.

Fictive motor pattern evoked by tactile stimulation of the anterior hindleg coxa in a mesothoracic–abdominal preparation. Each trace is a suction electrode recording from the proximal end of a cut mesothoracic nerve branch; all nerves were recorded ipsilateral to the stimulated location. A shows the motor pattern at a slow time scale; B shows part of the motor pattern at a faster time scale. Tactile stimulation began before the period illustrated. The first large burst of trochanteral levator motoneurons began 27.6 sec after the onset of tactile stimulation. Other conventions are as in Figure 2. Note that coordinated middle leg motoneuron bursts were evoked by tactile stimulation of the anterior hindleg coxa in the absence of leg movement.

Fig. 6.

Intracellular motoneuron recordings during tactually elicited fictive motor patterns in mesothoracic–abdominal preparations. A, B, Metathoracic SETi intracellular recording and simultaneous ipsilateral metathoracic nerve 3B recording. SETi spikes can be seen in both the intracellular and the extracellular recording. A, SETi activity during a weak trochanteral levator burst; the posterior abdomen was stimulated.B, SETi activity during a strong trochanteral levator burst; the posterior abdomen was stimulated. A andB are from the same preparation. C,D, Intracellular recording from a metathoracic trochanteral levator motoneuron with an axon in nerve 3B (small unit).C, Activity of the trochanteral levator during a weak, spontaneous levator burst. D, Activity of the levator during two strong levator bursts evoked by stimulation of the posterior abdomen.

Fig. 7.

Spontaneous and tactually elicited fictive motor patterns evoked in a metathoracic-alone preparation. The connectives were severed just anterior to and just posterior to the metathoracic ganglion. A, Spontaneous motor pattern.B, Motor pattern during tactile stimulation of the ear. Tactile stimulation began before the period illustrated and continued throughout this period. The large levator burst began 21.3 sec after the onset of tactile stimulation. C, Motor pattern during tactile stimulation of the ventral hindleg coxa. The first large levator burst began 3.2 sec after the onset of tactile stimulation. Conventions are as in Figure 2.

Fig. 8.

Coordination between left and right metathoracic motoneuron bursts during fictive motor patterns in a mesothoracic–abdominal preparation. A, Spontaneous motor pattern. B, Examples of the most common type of coordination during tactile stimulation of the left ear. The first large levator burst began 0.9 sec after the onset of tactile stimulation. C, Example of synchronous left and right levator bursts during tactile stimulation of the right ear. Tactile stimulation began before the period illustrated and continued throughout this period. The first large levator burst began 10.4 sec after the onset of tactile stimulation. L andR indicate left and right trochanteral levator bursts, respectively, with simultaneous activation of the contralateral trochanteral depressors. Asterisk indicates synchronous levator bursts. Other conventions are as in Figure 2.

Fig. 9.

Coordination between mesothoracic and ipsilateral metathoracic motoneuron bursts during fictive motor patterns in mesothoracic–abdominal preparations. A,B, Examples of one type of coordination during spontaneous motor patterns. C, Examples of another type of coordination (the most common type) during tactile stimulation of the posterior abdomen. Mesothoracic and metathoracic trochanteral levator bursts are indicated by 1 and 2, respectively; in each case, the trochanteral depressors in the ipsilateral adjacent ganglion were simultaneously activated. Tactile stimulation began before the period illustrated and continued throughout this period. The first large levator burst began 18.9 sec after the onset of tactile stimulation. D, Examples of a third type of coordination during tactile stimulation of the left anterior hindleg coxa. The first D_f burst began 0.4 sec after the onset of tactile stimulation. A,B, and D are from the same preparation.Meso, Mesothoracic. Other conventions are as in Figure2.