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Fig. 1. Line drawing of the dorsal surface of the cerebropleural and pedal ganglia indicating locations of identified neuronal somata of networks for feeding (inset: MCG, PCP, PSE, PCT, 2 I1s, and 3 I2s), escape swimming (inset: A1, As1-4, A10, A3, and A-ci1), and locomotion (G neuron cluster). Filled circles: serotonin immunoreactive somata. Whereas bilaterally symmetrical, somata are only shown unilaterally for convenience. Cell abbreviations: MCG, metacerebral giant neuron; PCP, phasic paracerebral interneuron; PSE, polysynaptic excitor of the PCP; PCT, tonic paracerebral interneuron; I1, Interneuron 1; and I2, Interneuron 2. Nerve abbreviations, cerebropleural ganglion: BWN, body wall nerve; sBWN, small body wall nerve; CBC, cerebrobuccal connective; aCPC, anterior cerebropedal connective; pCPC, posterior cerebropedal connective; CVC, cerebrovisceral connective; MN, mouth nerve; OVN, oral veil nerve; RN, rhinophore nerve; SCC, subcerebral commissure; and TN, tentacle nerve. Pedal ganglion: aLBWN, anterior lateral body wall nerve; pLBWN, posterior lateral body wall nerve; PC, pedal commissure; pPC, parapedal commissure; aPN, anterior pedal nerve; mPN, medial pedal nerve; and pPN, posterior pedal nerve.
- Figure 1 shows the relative positions of the neuron cell bodies.
- Known premotor neurons that either compose the central pattern generator for escape swimming or receive outputs from it are found on the dorsal cerebral region of the cerebropleural ganglion, in a group called the A cluster (Fig. 1).
- Feeding behavior is inhibited during swim episodes in part by spike activity in the swim neuron A1, which was previously shown to activate a strong polysynaptic inhibitory pathway to the PCp feeding command neurons (Jing and Gillette 1995) (Fig. 1).
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Fig. 2. Morphology, activity, and connectivity of the commissural interneuron A-ci1. A: morphology (from 4 preparations). Axon innervates neuropil in the cerebral lobes of both sides of the brain but does not exit the ganglion. B: activity during a fictive swim episode induced by BWN stimulation (bar). Sustained spontaneous activity followed the swim. A-ci1 burst onsets during the swim preceded those of A1 and As2/3, presumably resulting from the continuous activity of A10 during the burst cycle and less effective inhibition during ventral flexion. C: synaptic connections. Swim interneurons A1 and A10 excited A-ci1 in normal saline. Calibration bar: vertical, 40 mV; horizontal, 4 s.
- An important output of the CPG is a commissural interneuron of the A cluster, A-ci1 (soma diam 40-65 µm; Jing and Gillette 1999), whose single axon crosses to the contralateral cerebral lobe and ramifies in the neuropil of both sides (Fig. 2A).
- A-ci1 was rhythmically active during the dorsal flexion phase of escape swimming (Fig. 2B; n = 18).
- A-ci1 was excited by driven activity in the ipsilateral A1 and A10 (Fig. 2C; n = 8).
- Fig. 2B and Jing and Gillette 1999) induced spontaneous slow bursting activity of the cell at 3 Hz, which lasted for minutes and coincided with induction of similar slow and spontaneous rhythmic activity in the MCGs (Fig. 8).
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Fig. 3. Connections from the swim interneurons A1/A10 to the feeding command neurons PCPs and PSE, and to I1 and I2. A: A1 stimulation induced 2 cycles of inhibition in the PCPs and PSE in this preparation. B: A10 induced simultaneous barrages of IPSPs in PCP and I2. A10 activity (B1) was more effective than A1 (B2) in inhibiting PCPs and I2 (see RESULTS). Common IPSPs in PCp and I2 (dots in B3) from I1 activity (cf. Fig. 13) are observed in the expanded record of B2 (between arrows). C: A10 induced EPSPs in I1. Bars under each trace indicate the duration of current injection. All recordings were made in normal saline. Calibration bar: vertical, 40 mV for top 1st record in A, and top 2 records in B and C, 8 mV for bottom records, except for B3 (both records), 2 mV; horizontal, 3 s, except for B3, 0.5 s.
- The inhibition caused by A1, A10, and A-ci1 was somewhat stronger than that caused by A3 (Figs. 3-6).
- Prolonged stimulation of A1 or A10 (>10 s) often induced cyclic barrages of IPSPs in the PCPs and PSEs (Fig. 3A; for A1, n = 8 of 18; for A10, n = 12 of 15), consistent with the abilities of those neurons to drive the swim motor program.
- Discharge of A1/A10 caused moderate depolarization in I1s that occasionally induced spiking (Fig. 3C), often followed by weak inhibition.
- Both A1 and A10 strongly inhibited the I2s (n = 5) and the inhibitory connections were as strong or even stronger than those from A1/A10 to the PCPs (Fig. 3B).
- The inhibition of the I2s was largely caused by monosynaptic IPSPs from I1s, simultaneous with inhibition in the feeding command neurons (Fig. 3B); this observation provides functional significance to a negative feedback connection from the I1s to the I2s reported earlier but not understood (London and Gillette 1984) (see DISCUSSION).
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Fig. 4. Inhibition of PCP and PSE by the swim interneuron A3 in normal saline. A3 stimulation induced similar IPSPs from common presynaptic inhibitors of the feeding command cells. Calibration bar: vertical, 40 mV for top record, 8 mV for bottom records; horizontal, 3 s.
- The IPSP barrage in the ventral flexion phase may be driven partly by A3 (Fig. 4), whose swim activity coincides with the transition from dorsal to ventral flexion (Jing and Gillette 1995, 1999).
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Fig. 5. Activation of a single A10 suppresses fictive feeding in the isolated CNS. Driving the PCP induced rhythmic feeding motor output monitored in cerebrobuccal connectives (CBC) and buccal motor root 3 (R3). Driving A10 caused inhibition in the PCP and halted cyclic feeding bursts in R3. Intense spiking in PCP and the feeding motor program in R3 resumed on termination of A10 spike activity.
- However, spike activity in a single A10 comparable to its firing rate during swim episodes (~16 Hz) significantly inhibited driven activity in a PCP and suppressed feeding motor output driven by the feeding command neuron (Fig. 5; n = 4).
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Fig. 6. Connections from A-ci1 to PCP, PSE, I1, and I2 recorded in normal saline. A: PCP and PSE received common IPSPs from A-ci1 activity. B: A-ci1 inhibited I2, but excited I1. Spikes were clipped in the I1 record. Calibration bar: vertical, 40 mV for all top records, 4 mV for bottom records, except for the I1 record in B, 8 mV; horizontal, 3 s.
- Powerful inhibitory effects were also exerted on the PCPs and PSEs by A-ci1, the output neuron of the swim CPG (Fig. 6A; n = 9).
- In contrast, A-ci1 was quite strong in excitation of the I1s (Fig. 6B).
- A-ci1 strongly inhibited the I2s (Fig. 6B; n = 3).
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Fig. 7. Slow excitation of PCPs, I1 and I2 by As1-4. As2/3 caused discrete IPSPs riding on a slow EPSP in PCP, but provided only slow excitation to the likely sources of the IPSPs, I2 (A) and I1 (B). A and B are from 2 different preparations. Calibration bar: vertical, 40 mV for top records, 4 mV middle, 8 mV bottom; horizontal, 3 s. Spikes were clipped in records of I2 and I1. Recordings were made in normal saline.
- In contrast to the inhibition of the feeding command neurons caused by the pattern-generating swim neurons, the effects of discharging the serotonergic As1-4 neurons on the PCP and PSE were predominantly long-lasting depolarization that occasionally induced spiking (Fig. 7; n = 14).
- As1-4, and in particular As2/3, also excited both the I2s (n = 5) and the I1s (Fig. 7; n = 3).
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Fig. 8. Excitation of MCGs by As2/3 activity. Driving As2/3 with current injection (bar) excited the contralateral cell (c-MCG) more than the ipsilateral (i-MCG). Spontaneous, slow bursting As2/3 activity followed intracellular stimulation and spiking, and coincided with increased activity in the MCGs. Calibration bar: vertical, 40 mV; horizontal, 4 s. Recordings in normal saline.
- Direct stimulation of As2/3 (Fig. 8) or indirect stimulation by nerve shock (cf.
- Fig. 2B and Jing and Gillette 1999) induced spontaneous slow bursting activity of the cell at 3 Hz, which lasted for minutes and coincided with induction of similar slow and spontaneous rhythmic activity in the MCGs (Fig. 8).
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Fig. 9. As1-4 more strongly excited the contralateral than ipsilateral MCG. Recordings in high-divalent saline. A: connections from As2 and As3 in the same preparation to the contralateral MCG (c-MCG) had two components; a fast unitary EPSP following presynaptic spikes one-for-one and a slow EPSP lasting >30 s. One of the As2-3 pair also excited the ipsilateral MCG (i-MCG) but with lesser intensity (A1). B: As1 excitation of the contralateral MCG. A fast EPSP in the c-MCG was not distinguishable. Calibration bar: vertical, 40 mV for top record, 4 mV for bottom records; horizontal, 3 s.
- In high-divalent saline both fast and slow excitatory connections from As1-4 to the contralateral MCG remained (Fig. 9).
- The fast components of the EPSPs from As1 and As4 to the MCG (n = 4) could not usually be resolved (Fig. 9B), presumably because the EPSPs were too small and/or distant from the recording site; their slow EPSPs were also smaller, ranging from 0.7 to 1.5 mV.
- The As2/3 connections to the ipsilateral MCG in the normal saline were variable and decremented markedly with repeated tests; only a weak slow EPSP ~0.6 mV from one of the As2/3 pair remained in high-divalent saline (Fig. 9A1; n = 3).
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Fig. 10. As2-3 excited both ipsilateral and contralateral neurons of the cluster adjacent to the MCG. All recordings were from the same preparation in high-divalent saline. A: As2 and As3 excited an ipsilateral MCG neighbor (i-Mn) with both fast unitary EPSPs and a slow depolarizing component lasting >30 s. B: As2 similarly excited a contralateral MCG neighbor (c-Mn1) that was weakly electrically coupled to As2 but did not excite an adjacent cell, c-Mn2, to which it was not electrically coupled. A third cell, c-Mn3, also excited by As2 and i-Mn were both electrically coupled to As2 (not shown). Bottom: positions of the cells recorded in the experiment (dorsal view). Calibration bar: vertical, 40 mV for top record, 4 mV for bottom records; horizontal, 3 s.
- The cells of this cluster that we sampled received chemical excitation from As2/3 (Fig. 10; n = 9 pairs in 3 preparations) like the MCG.
- Other neurons adjacent to the MCG, but not electrically coupled to As2/3, were not excited by As2/3 (Fig. 10B).