Article Figures & Data
Figures
- Fig. 1.
Position and morphology of cerebral motoneurons C15, C16, and C17. A, Schematic diagram of the ventral surface of a cerebral hemiganglion, showing the approximate positions of neurons in the E- and M-clusters used in this study.Filled cells indicate lip motoneurons. B, Drawing of the soma and central processes of C15 filled by intracellular injection of 5(6)-carboxyfluorescin and viewed with a fluorescence microscope. C15 had arborisations in the E- and M-clusters and peripheral axons in the ULAB and AT nerves. C, Morphology of C16 was similar to that of C15, except it had axons in the AT and LLAB nerves. D, Morphology ofC17, which had a single axon in the LLAB.Arrowheads indicate nerves in which there are peripheral axons. ULAB, Upper labial nerve; AT, anterior tentacular nerve; LLAB, lower labial nerve; CBC, cerebral–buccal connective; CPC, cerebral–pedal connective; CPLC, cerebral–pleural connective.
- Fig. 2.
Synaptic connections onto C15. A, Lip motoneurons C15, C16, and C17 were electrically coupled to each other. Negative current injected into any of the three cells (solid bars) produced a smaller hyperpolarization in the other two. B, Firing in the histaminergic mechanoafferent neuron C2 produced slow inhibition in C15. C–E, Spikes in BCIs B18, B19, and B24 produced, respectively, fast one-for-one EPSPs, an I/EPSP, and an IPSP in C15. These responses all persisted in a solution containing high concentrations of divalent cations, which suppresses polysynaptic pathways. Scale bar: vertical = 40 mV except C15 in C–E = 15 mV; horizontal = 1 sec for C, 2 sec for A, 10 sec for B, D, and E.
- Fig. 3.
Input to cerebral lip motoneurons during the CMP in isolated cerebral ganglia. The CMP was driven by injecting constant depolarizing current into C15 (solid bars). A, M-cluster lip motoneurons C11 andC12 were active in alternation. C11 received EPSPs and C12 IPSPs. B, E-cluster lip motoneuronsC16 and C17 both received EPSPs during the CMP and fired at high frequency. C16 and C17 were electrically coupled to C15 (and each other; see Fig.2A) and so were also tonically depolarized throughout the current pulse. Arrows indicate points on the slow record from which the faster time-base records to the right were taken. These show that the PSPs in all neurons were one-for-one with PSPs in the others (examples joined by dashed lines), suggesting a common source. C15 also received phasic excitatory input during the CMP (e.g., open bar inB) that was largely masked by the current-induced spiking but was represented by a rise in the firing frequency and uneven spike amplitudes.
- Fig. 8.
Input to CBIs during the CMP in the isolated cerebral ganglion. A, CBI-1 was weakly, slowly inhibited during the CMP, in phase with the fast IPSPs in C12. B, CBI-2 and CBI-4 received slow excitation during the CMP, which depolarized them above spiking threshold. C, CBI-3 was excited and spiked during the CMP, in phase with the IPSPs in C12 (not shown) and the EPSPs in C4. D, CBI-3 andC4 were electrically coupled, because hyperpolarizing or depolarizing current pulses in either neuron (solid bars) led to a smaller membrane potential change of the same polarity in the other. Action potentials in one neuron gave rise to brief depolarizing potentials in the other without a measurable delay. E, The excitation seen in CBI-3 during a cycle of the CMP induced by firing ofC15 (E1) was effectively mimicked by a depolarizing current pulse into C4 (E2), showing that electrotonic transmission of the chemically mediated excitation and spiking in C4 is sufficient to explain the excitation recorded in CBI-3.
- Fig. 4.
Input to modulatory and sensory neurons during the C15-induced CMP in isolated cerebral ganglia. A, Modulatory neuron C4 received large EPSPs that drove it above threshold during the CMP, whereas its neighbor C5received much smaller EPSPs that were one-for-one with those inC4 (see record to the right, showing input at an expanded time-base). These EPSPs had little effect on the firing rate of C5. B, The sensory mechanoafferentC2 received fast IPSPs during the CMP. The MCC, which has both central and peripheral modulatory roles, was slowly depolarized. There were also a few faster potentials in theMCC associated with the peak of some of the cyclical depolarizations, which were not one-for-one with the PSPs in other neurons (see record to the right). Arrowsindicate points on the slow record from which the faster time-base records to the right were taken.
- Fig. 11.
The CMP was restricted to the ipsilateral half of the cerebral ganglion, but contralateral C15s received synchronous input during BMPs. A, Constant depolarizing current injected into the right C15 (rC15) produced a CMP, demonstrated by input to the ipsilateral C4 (rC4). However, no input was seen in contralateral neurons, in this case represented by the left C15 (lC15). This suggests that there is a CPG in each cerebral hemiganglion that can operate independently.B, The lC15 andrC15 received synchronous excitatory input during a BMP driven by CBI-2. Thus, activity in the two hemiganglionic cerebral CPGs will be synchronized by buccal input during BMPs. Note that the occasional slow inhibitory input (arrowheads) was also synchronous.
- Fig. 5.
Feedback to the buccal ganglion during the CMP. A, A time-expanded record of the start of one cycle of a CMP. A series of extracellularly recorded spikes in theCBC were one-for-one with the IPSPs in C12(examples joined by lines). Because these were not attributable to spiking in any of the identified CBIs (see Results), these are the result of activity in an as yet uncharacterized cerebral-to-buccal neuron. There was also a general increase in the frequency of spikes in the CBC (examples atarrowheads), possibly because of spiking in CBI-2, 3, and 4 (Fig. 8). B, A time-expanded record of the start of one cycle of a CMP. Facilitating IPSPs in an unidentified neuron (b MN) in the ventral motoneuron cluster of the buccal ganglion were one-for-one with IPSPs in C12, suggesting a common source, probably the unidentified cerebral-to-buccal neuron. In both cases, the CMP was driven by constant current injected into C15 (not shown). Scale bar = 40 mV for C12, 10 mV forb MN, and 250 msec for A, 500 msec forB.
- Fig. 9.
C15 initiated single cycles of a BMP, possibly via excitation of CBI-2 and CBI-4. A, Constant current injected into C15 (solid bar) resulted in a single cycle of a BMP (gray bar), consisting of strong multiphasic motor output in bn2 and firing in buccal neuron B4. This is followed by repeating cycles of a CMP (open bar) with input toC12, but only weak output in bn2 and no firing inB4. B, With CBI-2 hyperpolarized (B1) or at its resting potential (B2), a 10 sec pulse of depolarizing current into C15 (solid bars) resulted in a single cycle of a CMP with slow excitation inCBI-2, fast IPSPs in C12 and no output inbn2. B3, When CBI-2 was depolarized by 4 mV, it fired at high frequency during the CMP and a single cycle of a BMP was evoked, represented by multiphasic output in bn2 and feedback to CBI-2, C12, and C15. C, WithCBI-4 hyperpolarized (C1) or at its resting potential (C2), a 10 sec pulse of depolarizing current intoC15 (solid bars) resulted in a single cycle of a CMP with slow excitation in CBI-4, fast IPSPs inC12, and weak activity in bn2. C3, WhenCBI-4 was depolarized by 5 mV, it fired at high frequency during the CMP and a single cycle of a BMP was evoked, represented by multiphasic output in bn2 and feedback to C12 andC15. Membrane potential of CBI-2 andCBI-4 is indicated at the right side of each trace.
- Fig. 6.
A CMP can be driven by high-frequency stimulation of the CBC. A, C15, C4, and C12 all receive rhythmic input during CBC stimulation (open bar). The faster time-base records to the right show examples of the input to each neuron during the periods indicated by thenumbered arrowheads on the slow record. The briefdownward deflections represent CBC stimulus artifacts.B, In another preparation, stimulation of the CBC again resulted in rhythmic input, this time recorded in C15, C4, and C16/C17. In this case, C15 was hyperpolarized by 11 mV, which prevented any spiking during the middle three cycles of the CMP, showing that spiking in C15 was not necessary for this CMP. Note that this CMP had a similar cycle period to the CMP driven by C15 and that all of the neurons received similar inputs in both programs (compare with Figs. 3, 4).
- Fig. 7.
Properties of the PSPs evoked in cerebral neurons in response to CBC stimulation. A, The latency of the PSPs (in this case the EPSP in C15) was not affected by increasing the stimulation intensity (top three traces) or frequency (bottom two traces), indicating that the PSPs were probably attributable to a direct, monosynaptic connection from a cerebral-to-buccal neuron. The timing of CBC stimulation is indicated by the downward artifact at the arrowhead. B1, The EPSPs in C15 were classical chemical EPSPs, which increased in amplitude with increasing negative current (current injected at each arrowhead). B2, HyperpolarizingC15 (during solid bar) beyond a threshold level completely abolished the EPSPs in C15, as well as the PSPs recorded in other cerebral neurons, in this case C11 andC12. This suggests that C15 may be electrically coupled to the cerebral-to-buccal neuron responsible for the PSPs (see Results for details).
- Fig. 10.
C15 received synaptic input during various BMPs. A, Constant depolarizing current injected intoCBI-2 (solid bar) resulted in an ingestive-like BMP during which buccal neurons B8 and B4 fired together. During this program, C15 was strongly excited just before the strongest activity in these neurons. B, During an egestive-like BMP induced by brief 10 Hz electrical stimulation of the radula nerve (during open bar), B8 fires beforeB4. In this program, C15 was active at the same time as B8 and before spiking in B4. C, During an egestive-like BMP induced by continuous 2.5 Hz stimulation of the esophageal nerve (open bar), C15 is again active with B8, and before B4. After the termination of nerve stimulation, there is a single cycle of a BMP that shows similar phase relations to the ingestive-like program shown in A. D, During a BMP induced by exciting CBI-4 (solid bar), C15 was again active in phase with B8and before activity in B4. In this program, there was a second burst of spikes in B8 that coincided with inhibition in C15 (arrowheads). For each BMP, the excitatory input to C15 consisted of a barrage of fast EPSPs. Buccal motor output was also monitored from bn2 in each case.
- Fig. 13.
Input to cerebral neurons during CBI-1- and CBI-4-driven BMPs could be altered by changing the membrane potential of C15. A1, With all neurons at their resting potentials, stimulation of CBI-1 (solid bar) resulted in a single cycle of a BMP with organized firing in bn2, and input to cerebral neurons C12 and C15. The input to C12 included a barrage of fast IPSPs that was coincident with the peak depolarization of C15 (arrowhead).A2, With C15 hyperpolarized, the output ofbn2 remained the same but the fast IPSPs were no longer present in C12. Note that even with C15hyperpolarized C12 still received some synaptic input during the BMPs. This presumably represents the direct feedback from the buccal CPG, via BCIs. B1, With all neurons at their resting potentials, stimulation of CBI-4 (solid bar) resulted in repeating cycles of a BMP with organized firing inbn2 and feedback to C12 and C15. No large IPSPs are present in C12. B2, C15 is depolarized by 5 mV, so that it fired tonically at 5 Hz, below the threshold for a CMP. During the BMP driven by CBI-4, the input to C12now includes a brief burst of 2–4 large IPSPs in each cycle (arrowheads), coincident with the maximum firing rate inC15. When C15 was depolarized by a further 3 mV, more IPSPs were observed in C12 (6–25 per cycle; not shown).
- Fig. 12.
Input received by cerebral neurons during the CBI-2-driven BMP was not affected by altering the membrane potential of C15. A, With all neurons at their resting potentials, stimulation of CBI-2 (solid bar) resulted in a BMP with organized firing in bn2 and input to cerebral neurons C12 and C15. The input toC12 includes a barrage of fast IPSPs (at thearrowheads) that coincides with excitation in C15. B, With C15 hyperpolarized by 40 mV, it no longer fires during the CBI-2-driven BMP, although it still receives phasic excitation. Under these conditions, C12 still receives IPSPs during the BMP. C1, Time-expanded records from C12 showing the start and the end of the second barrage of IPSPs from A. Both small (arrow 1) and large IPSPs (arrow 2) can be seen. C2, The equivalent record from B. The appearance and frequency of the IPSPs inC12 are not affected when C15 is hyperpolarized.
- Fig. 14.
C15 could not drive a CMP during the egestive-like BMP induced by esophageal nerve stimulation. In the quiescent state, injecting C15 with a 10 sec pulse of either 8 or 4 nA depolarizing current (filled bars) initiated a cycle of the CMP, shown by the barrage of IPSPs inC12 (arrowheads 1 and 2). A BMP was then driven by 2 Hz esophageal nerve stimulation (during open bar). C15 was excited during this program, at the same time as B8, and just before B4 (compare Fig. 11, and this synaptic drive is visible just before and after the current pulse). Depolarizing current (8 nA, previously suprathreshold for the CMP) injected into C15 at the phase in which it received synaptic excitation could not drive a CMP, shown by the lack of large IPSPs in C12 (arrowhead 3). Soon after the end of the BMP, C15 could again drive the CMP (4 nA,arrowhead 4).
- Fig. 15.
Summary diagram of the interactions between buccal and cerebral CPGs. A, Previously, it was thought that cerebral motoneurons that innervate the lips were controlled during feeding-related behaviors by direct feedback from one or more buccal CPGs (1). B, We now propose that there is a CPG in each cerebral hemiganglion that interact in variable ways with the buccal CPGs (2a, 2b). Each CPG also makes direct connections onto motoneurons in the other ganglion (1a, 1b). In both ganglia, arrows also indicate that there is feedback into the CPGs from some motoneurons (buccal: B31/32, Hurwitz et al., 1994; cerebral: C15, this study). The situation in B allows greater flexibility than in A, because the output of the whole system can be altered by changing the strength or type of interaction at any of these points. Note that in both parts other known specific cerebral influences over buccal circuitry, such as the CBIs and MCC, are omitted for clarity.
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