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The locust frontal ganglion: a central pattern generator network controlling foregut rhythmic motor patterns

Amir Ayali^*, Yael Zilberstein and Netta Cohen

Department of Zoology, Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel

View larger version (92K): [in a new window]   Fig. 1. (A) A sectional diagram of the locust head showing the relative position of the frontal ganglion (FG). CC, corpora cardiaca; HG, hypocerebral ganglion; CA, corpora allata; SOG, suboesophageal ganglion. (Bi, Bii) Schematic drawing of the FG, the nerves leaving it and the muscles they innervate (Bii shows an area more posterior to Bi, after Allum, 1973). RN, recurrent nerve; PPN, posterior pharyngeal nerve; MPN, median pharyngeal nerve; APN, anterior pharyngeal nerve; FC, frontal connective; I, II and III, first, second and third branch of the FC; 1 and 3, muscles of the labrum; 33 and 34, first and second anterior dilators of cibarium; 35, 36 and 37, first, second and third dorsal dilators of pharynx; 39 and 40, first and second lateral dilators of pharynx; 43 second ventral dilator of pharynx. (C) A frontal section through the center of a whole-mount Hematoxylin and Eosin-stained FG. The neuronal cell bodies are in the peripheral zone, surrounding the neuropil (marked by a dashed line). The coarse neuropil in the central core of the ganglion (marked by a dotted line) can be distinguished from the surrounding finer neuropil. The RN and one of each of the paired nerves are visible and marked by arrows. Scale bar, 50 mm.

View larger version (39K): [in a new window]   Fig. 2. FG motor output in vivo can be correlated to foregut movement and to specific gut dilators. (A) Extracellular recording from the recurrent nerve (RN) and the simultaneous output of a force transducer (FT) connected to the esophagus wall in a fully intact locust. (B) Simultaneous extracellular recordings from the posterior pharyngeal nerve (PPN), and from the third dorsal dilator of the pharynx (37) in vivo. Inset on left shows 5 overlaid sweeps triggered by the large unit in the PPN trace, demonstrating 1:1 relationship between this unit and the muscle activity.

View larger version (60K): [in a new window]   Fig. 3. Emergence of the FG rhythm in vitro. A continuous frontal connective recording in a ganglion totally isolated from all descending and sensory inputs, dissected out from a locust with very full crop and gut. (i-iv) 15, 45, 60 and 75 min, respectively, after dissection.

View larger version (47K): [in a new window]   Fig. 4. (A) The effect of haemolymph collected from a fifth instar larva with a very full crop, on an ongoing FG rhythm in vitro, as seen in a frontal connective extracellular recording. (B) As in A; haemolymph collected from a non-feeding premolt fifth instar larvae. (C) As in A and B; haemolymph collected from a fifth instar larvae just after the initiation of feeding. In all panels, (i) is control; (ii) haemolymph application; (iii) wash. Scale bars, 10 s.

View larger version (36K): [in a new window]   Fig. 5. (A) Simultaneous extracellular recording from three of the FG efferent nerves (see Fig. 1) in vitro. Data demonstrate spontaneous rhythmic bursting activity in all the recorded nerves. (B) The area marked by the bar in A is amplified to reveal a three-phase rhythmic pattern of the different members of the FG pattern generator (Bi). The graph in Bii shows burst profiles of the activity recorded on the three nerves in the same time window as in Bi. The cumulative peak amplitudes are plotted as a function of time (see Materials and methods). Six consecutive sweeps as in Bi were normalized and overlaid. Different bursts display similar profiles. Different nerves have distinct profiles and exhibit clear time lags in their onset times, burst end times, and recovery profile. Quantitative parameterization of the burst is performed by linear fits to different phases of the bursts (corresponding to normalized rates) and extrapolation of the crossover times between these linear regimes (marked by dotted lines). These analyses are performed on multi-burst average profiles, and are only schematically shown in the figure.

View larger version (42K): [in a new window]   Fig. 6. (A) An example of a simultaneous extracellular recording from three of the FG efferent nerves, in which an additional early unit participated in the PPN burst (large spikes). The time window shown corresponds to that in Fig. 5B. (B) Here, FC nerve and PPN bursts commence together (compare to Fig. 5Bii). The early PPN activity is dominated by a single unit generating 2-4 high-amplitude spikes. Other PPN units appear to be unaffected by this early unit (see, for instance, the return to low spiking rate between the early PPN activity and the main burst).

View larger version (20K): [in a new window]   Fig. 7. Burst profile representations, averaged over 15 experiments (chosen from those in which the PPN did not include activity of an early unit). The mean cumulative amplitude burst profiles were calculated for each recording. (Ai) Mean cumulative amplitude burst profiles averaged over all preparations. (Aii) Magnification of the burst time window. (Bi, Bii) The first derivative of the above, normalized to yield burst energy densities. Again, the left and right columns differ only by the horizontal (time) domain. Note the strong pre-burst PPN inhibition and the post-burst inhibition in all three nerves. (C) Time-lag diagram calculated by linear extrapolation of burst onset and termination times in cumulative amplitude traces from each of the recordings. The diagram summarizes burst temporal progression in the three nerves (means ± S.D.). N=15, 14 and 5 for FC, MPN and PPN, respectively.

View larger version (43K): [in a new window]   Fig. 8. Simultaneous extracellular recording from the FC nerve, and intracellular recording from a FG neuron. (A) and (B) are from two different preparations. The neuron recorded in A demonstrates rhythmic bursting, which coincide with the large action potentials on the FC recording. The neuron in B shows strong rhythmic inhibition during the FC burst.