Heart Excitor Research Articles

The effects of interactions between intrinsic properties and network parameters on bilateral phasing in a reduced leech heartbeat system

The leech heartbeat central pattern generator (CPG) consists of a network of heart interneurons (HN) that coordinate heart excitor (HE) motor neuron activity via inhibitory chemical synapses. Each segmental pair of HE’s is connected to one another via electrical coupling. Depending on the segment, the pair of motor neurons in the living system is active across a wide range of phase differences from nearly in-phase to anti-phase [1]. Prior efforts to model this complete network have not quantitatively matched all contralateral phase values observed [2]. We have created a reduced network model in Simulink to explore parameters that contribute to these phase differences. This parameter search has been analyzed in MATLAB using the PANDORA toolbox [3]. In our network model, we implemented known neuronal properties and synaptic connections from a single segmental ganglion. The HN’s were modeled as endogenous bursters as previously described with a network cycle period of 9.45 ± 0.11 sec [4]; in previous studies [5], the HE’s were modeled as tonic firers. We varied three network parameters shown in Figure ​Figure11 in this study: phased delay of the right HN synaptic input [ΦSyn], the maximum conductances of the inhibitory synapse [gSyn], and electrical coupling strength [gcoup]. We varied ΦSyn to emulate the variable timing of inhibition onto HE’s in the full network of the living system. In addition, we varied the set of maximum conductances for the HE neurons in a linear fashion from classical HE (tonic firing) to HN (endogenous bursting) values to ascertain the role of intrinsic properties on phase-shifts. Figure 1 Leech heartbeat circuit diagram showing identified CPG heart interneurons (HN) and heart excitor motor neurons (HE). This linear variation of HE properties from tonic firing towards endogenous bursting led to a step-wise phase advance up to ~0.10 for both neurons. In-phase (ΦSyn: 0.0) and anti-phase (0.5) values of ΦSyn produced expected HE phase differences. However, intermediate synaptic delays (0.25, 0.75) caused a reduction in phase differences as the two motor neurons pull towards one another due to their electrical coupling. The amount of phase difference reduction was directly impacted in a nonlinear manner by the intrinsic properties of these HE neurons. Duty cycle (burst duration / cycle period) was not significantly impacted by intrinsic properties. In summary, increases in ΦSyn led to non-monotonic changes in HE motor neuron contralateral phase differences. Our search of parameter space continues to provide a foundation for understanding the mechanisms underlying variable phase differences in neuronal networks and reinforces the importance of interactions between endogenous properties and synaptic connections for producing functional motor patterns. The project described was supported by Vermont Genetics Network through an NIGMS (NIH) Institutional Development Award P20 {type:entrez-nucleotide,attrs:{text:GM103449,term_id:221402118,term_text:GM103449}}GM103449.

Maturation of peptide‐positive synaptic arbors in the medicinal leech requires rhythmic target activity

The formation and refinement of synaptic connections are dependent on the activity that emerges from nascent synaptic connections. Such activity has the effect of regulating the production and release of specific neurotransmitters. To determine the role of activity in regulating the production of peptide-positive synapses, we used antibodies against Phe-Met-Arg-Phe-NH₂ and acetylated α-tubulin as well as intracellular injections of Neurobiotin to examine varicosities belonging to heart excitor (HE) neurons on the heart tubes of medicinal leeches, Hirudo spp. We found that the production of peptide-positive varicosities increased considerably during the last week of embryogenesis, which coincided with the emergence of rhythmic activity of the heart tube. When we compromised central input to HE neurons with bicuculline or by surgical ablation of the central pattern generator during early embryogenesis, we found that activity in the heart tubes and its rhythmicity were greatly diminished. Furthermore, the activity of HE neurons had also lost its rhythmicity and appeared tonic, and production of peptide-positive varicosities was substantially reduced as well. Partial surgical ablations that preserved rhythmic activity in the heart tube while disrupting heart tube innervation by some HE neurons still resulted in peptide-positive varicosity production. Taken together, our results suggest that postsynaptic rhythmic activity of the heart tube is necessary and sufficient for the development and maturation of peptide-positive synapses.

Compromise revisited: inhibitory synapse and electrical coupling effects on bilateral phasing in the leech heartbeat system

The leech heartbeat central pattern generator (CPG) consists of a network of heart interneurons (HN) that coordinate heart excitor (HE) motor neuron activity via inhibitory chemical synapses. Each segmental pair of HE’s is connected to one another via electrical coupling. Depending on the segment, the pair of motor neurons in the living system is active across a wide range of phase differences from nearly in-phase to anti-phase [1]. Prior efforts to model this complete network have not quantitatively matched the intersegmental phase differences observed [2]. We have created a reduced network model in Simulink to explore parameters that contribute to these phase differences. In our network model, we implemented known neuronal properties and synaptic connections from a single segmental ganglion as shown in Figure ​Figure1.1. In our initial model run, the HN’s were modeled as endogenous bursters as previously described [3]; the HE’s were modeled as tonic firers. We varied three parameters in this study: phased delay of the right HN synaptic input (ΦSyn) and the maximum conductances of the inhibitory synapse (gSyn) and electrical coupling (gcoup). Figure 1 Leech heartbeat circuit diagram showing identified heart interneurons (HN) of the CPG and their pattern of inhibitory synaptic connections (lines with circles) onto each other and the heart excitor motor neurons (HE). The resistor symbol indicates rectifying ... We found that in this network gSyn must be at least 150 times greater than gcoup in order to obtain 1:1 entrainment of the HE’s with the HN’s. Under conditions with zero ΦSyn, increased gcoup led to increased instantaneous spike frequencies (ISF) and reduced duty cycle, primarily due to a delayed burst beginning. Increasing gSyn alone with zero ΦSyn led to little change in HE phase or duty cycle, but saw increased ISF. With relatively high levels of gSyn (300-600 nS), increasing ΦSyn (0.2-0.5) caused one HE to decrease its duty cycle while the other increased. Higher levels of ΦSyn (0.5-0.8) caused the HE’s to switch their relative duty cycle patterns. With weak gcoup (0.25-0.50 nS) and moderate ΦSyn (0.4-0.6), increasing gSyn led to a reduced HE duty cycle and side-to-side phase difference. The largest phase differences were found when both gSyn and gcoup were relatively strong. In summary, increases in gcoup tended to lead to increased phase differences, while increases in gSyn led to decreased phase differences. Our search of parameter space has provided a foundation for understanding the mechanisms underlying variable phase differences in neuronal networks and reinforced the importance of compromise between synaptic and neuronal properties for producing functional motor patterns.

Targeting a neuropeptide to discrete regions of the motor arborizations of a single neuron

The heart excitor (HE) motor neuron in the leech Hirudo releases acetylcholine (ACh) and a peptide, FMRFamide, to regulate the contractile activity of the heart tube and associated side vessels. Consistent with Dale's principle, it was assumed that both neurotransmitters were localized to all presynaptic varicosities. However, we found discrete peptide-positive and peptide-negative varicosities associated with particular sites of innervation on the heart tube. We produced dual-labeled HE neurons by pressure injecting Neurobiotin into single HE cell bodies and applied anti-FMRFamide antibodies on the same preparations. Consistent with initial expectations, peptide-labeled varicosities were numerous and widely distributed along the heart tube and at one of the three side vessels, the latero-abdominal vessel. Nevertheless, some Neurobiotin-labeled varicosities along the heart tube lacked peptide label entirely. Moreover, there were dense and distinct peptide-negative innervations at the valve junctions of the latero-dorsal and latero-lateral vessels at each segment. Nevertheless, the peptide label was found in HE axons and varicosities that projected distally along the side vessels. Therefore, the more proximal peptide-negative clusters cannot simply be the result of restricted transport or deficient staining of peptide. Rather, we infer that FMRFamide is transported to (or selectively excluded from) discrete locations and that ACh is present in varicosities that lacked peptide. Such targeting of neurotransmitters could be described using a discrete targeting model of synaptic transmission. Compared with Dale's principle, this model may provide a more complete perspective of chemical communication than previously understood.

Centrally patterned rhythmic activity integrated by a peripheral circuit linking multiple oscillators

The central pattern generator for heartbeat in the medicinal leech, Hirudo generates rhythmic activity conveyed by heart excitor motor neurons in segments 3-18 to coordinate the bilateral tubular hearts and side vessels. We focus on behavior and the influence of previously un-described peripheral nerve circuitry. Extracellular recordings from the valve junction (VJ) where afferent vessels join the heart tube were combined with optical recording of contractions. Action potential bursts at VJs occurred in advance of heart tube and afferent vessel contractions. Transections of nerves were performed to reduce the output of the central pattern generator reaching the heart tube. Muscle contractions persisted but with a less regular rhythm despite normal central pattern generator rhythmicity. With no connections between the central pattern generator and heart tube, a much slower rhythm became manifest. Heart excitor neuron recordings showed that peripheral activity might contribute to the disruption of centrally entrained contractions. In the model presented, peripheral activity would normally modify the activity actually reaching the muscle. We also propose that the fundamental efferent unit is not a single heart excitor neuron, but rather is a functionally defined unit of about three adjacent motor neurons and the peripheral assembly of coupled peripheral oscillators.

Neural control of heartbeat during two antagonistic behaviors: whole body withdrawal and escape swimming in the Mollusk Clione limacina

Two cardioexcitatory and one cardioinhibitory neural groups have been previously identified as the central cardioregulatory system in the pteropod mollusk Clione limacina. We describe in this study one additional element of the central cardioregulatory system, which consists of a large intestinal neuron named Z-cell with a novel effect on the heart activity. Intracellular stimulation of the Z-cell induced only auricle contractions with no effect on the ventricle activity. The Z-cell processes were traced down to the heart, and vigorous branching was found in the auricle tissue. Specific patterns of activity of the Z-cell as well as intestinal heart excitatory and inhibitory neurons were studied during initiation of two behaviors--whole body withdrawal and escape swimming. It was found that initiation of both behaviors was accompanied by activation of Z-cell and intestinal heart excitor neurons. The firing rate of neurons induced by sensory stimuli was sufficient to trigger auricle contractions in the semi-intact preparations. Video analysis of heart activity revealed that auricle indeed was activated during both active and passive avoidance reactions, though the intensity and delay of the activation were different. The possible physiological role of the auricle contractions during antagonistic forms of behavior is discussed.

Developmentally Regulated Tissue-Associated Cues Influence Axon Sprouting and Outgrowth and May Contribute to Target Specificity

The heart circuitry of the medicinal leech (Hirudo medicinalis) is a highly stereotyped circuit in the adult, but selection of the heart tube (HT) as a definitive target by heart excitor (HE) motor neurons during embryogenesis involves redirection of axonal arbors. In the present study we have confirmed the specificity of mature innervation using a retrograde marker and have used a combination of tissue/organ coculture and in situ manipulations to test the ability of HT and body wall to support axon outgrowth compared to CNS associated tissue. We also examined the temporal limits of target influence and the specificity of its action. Embryonic and young juvenile HT and body wall, but not adult HT, support or stimulate marked axon outgrowth from CNS ganglia, including those that would not innervate these tissues in vivo. Outgrowth support/stimulation by young tissue is largely contact based with little or no overt selectivity. Thus, outgrowth-supporting cues are developmentally regulated in the periphery, decreasing in efficacy with age while adult CNS-derived tissues consistently provide effective substrates supporting extensive axon outgrowth and regrowth. The HE motor neuron was very discriminating in that it showed little axon extension onto the HT compared to that of other neurons generally. These studies support a role for bidirectional communication in target selection. We suggest a working hypothesis that the HE motor neuron may initially select HT in response to a hierarchy of outgrowth supporting cues that have very broad influence and subsequently responds to selective signals for slowing or stopping growth and terminating on the functionally appropriate target.

Modulation of swimming speed in the pteropod mollusc, Clione limacina: role of a compartmental serotonergic system.

In locomotory systems, the central pattern generator and motoneuron output must be modulated in order to achieve variability in locomotory speed, particularly when speed changes are important components of different behavior acts. The swimming system of the pteropod mollusc Clione limacina is an excellent model system for investigating such modulation. In particular, a system of central serotonergic neurons has been shown to be intimately involved in regulating output of the locomotory pattern generator and motor system of Clione. There are approximately 27 pairs of serotonin-immunoreactive neurons in the central nervous system of Clione, with about 75% of these identified. The majority of these identified immunoreactive neurons are involved in various aspects of locomotory speed modulation. A symmetrical cluster of pedal serotonergic neurons serves to increase wing contractility without affecting wing-beat frequency or motoneuron activity. Two clusters of cerebral cells produce widespread responses that lead to an increase in pattern generator cycle frequency, recruitment of swim motoneurons, activation of the pedal serotonergic neurons and excitation of the heart excitor neuron. A pair of ventral cerebral neurons provides weak excitatory inputs to the swimming system, and strongly inhibits neurons of the competing whole-body withdrawal network. Overall, the serotonergic system in Clione is compartmentalized so that each subsystem (usually neuron cluster) can act independently or in concert to produce variability in locomotory speed.

Cerebral serotonergic neurons reciprocally modulate swim and withdrawal neural networks in the mollusk Clione limacina

1. A pair of serotonin-immunoreactive neurons has been identified in the cerebral ganglia of the pteropod mollusk Clione limacina, which produce coordinated, excitatory/inhibitory effects on neurons controlling two incompatible behaviors, swimming and whole body withdrawal. These cells were designated cerebral serotonergic ventral (Cr-SV) neurons. 2. Activation of Cr-SV neurons produces a prominent inhibition of the pleural withdrawal neurons, which have been previously shown to induce whole body withdrawal in Clione. In addition, the cerebral neurons produce weak excitatory inputs to swim motor neurons, pedal serotonergic neurons involved in the peripheral modulation of swimming, and to the serotonergic heart excitor neuron. 3. Inhibitory and excitatory effects appear to be produced by serotonin because they are mimicked by exogenous serotonin and are blocked by the serotonin antagonist mianserin. 4. All serotonergic neurons identified thus far in the CNS of Clione appear to function in a coordinated manner, altering a variety of neural centers all directed toward the activation of swimming behavior.

An identified cerebral interneuron initiates different elements of prey capture behavior in the pteropod mollusc,Clione limacina

The prey capture phase of feeding behavior in the pteropod molluscClione limacina consists of an explosive extrusion of buccal cones, specialized oral appendages which are used to catch the prey, and significant acceleration of swimming. Several groups of neurons which control different components of prey capture behavior inClione have been previously identified in the CNS. However, the question of their coordination in order to develop a normal behavioral reaction still remains open. We describe here a cerebral interneuron which has wide-spread excitatory and inhibitory effects on a number of neurons in the cerebral and pedal ganglia, directed toward the initiation of prey capture behavior inClione. This bilaterally symmetrical neuron, designated Cr-PC (Cerebral interneuron initiating Prey Capture), produced monosynaptic activation of Cr-A motoneurons, which control buccal cone extrusion, and inhibition of Cr-B and Cr-L motoneurons, whose spike activities maintain buccal cones in a withdrawn position inside the head in non-feeding animals. In addition, Cr-PC produced monosynaptic activation of a number of swim motoneurons and interneurons of the swim central pattern generator (CPG) in the pedal ganglia, pedal serotonergic Pd-SW neurons involved in a peripheral modulation of swimming and the serotonergic Heart Excitor neuron.

Sprouting and connectivity of embryonic leech heart excitor (HE) motor neurons in the absence of their peripheral target.

The rhythmic pumping of the hearts in the medicinal leech, Hirudo medicinalis, is neurogenic and mediated by a defined circuit involving identified interneurons in a central pattern generator (CPG) and segmentally iterated motor neurons that drive the heart muscle. During early embryogenesis, presumptive heart excitor (HE) motor neurons extend many axon branches into the body wall; they later innervate the heart while retracting the supernumerary peripheral axons, and only much later in development receive synaptic input from the central pattern generator (Jellies, Kopp and Bledsoe (1992) J. Exp. Biol., 170, 71-92.). In this study, HE motor neurons were deprived of an early interaction with the heart by surgical ablation of a circumscribed portion of body wall including the heart primordium. Anatomical and electrophysiological data were obtained using intracellular techniques to examine the hypothesis that peripheral interactions with the developing heart provide instructive cues for the final differentiation of these neurons. Target-deprived HE motor neurons continued to extend multiple axons in ventral, lateral and dorsal body wall throughout late embryonic and into postembryonic stages and they extended anomalous axons within the CNS. This resembles the early embryonic growth of HE motor neurons before heart tube differentiation. Furthermore, HE motor neurons deprived of heart contact exhibited tonic activity similar to the situation during early development before they are contacted by the CPG interneurons. In contrast, sham-operated and contralateral HE motor neurons oscillated normally. These results suggest that heart tube contact is specifically required for at least some aspects of HE development and provide a framework in which to identify cell-cell interactions that are involved in matching neurons and targets to generate behaviorally relevant neural circuits.

Serotonergic modulation of swimming speed in the pteropod mollusc Clione limacina. III. Cerebral neurons.

Swim acceleration in Clione limacina can occur via central inputs to pattern generator interneurons and motor neurons and through peripheral inputs to the swim musculature. In the previous paper, peripheral modulation of the swim muscles was shown to increase wing contractility. In the present paper, central inputs are described that trigger an increase in swim frequency and an increase in motor neuron activity. In dissected preparations, spontaneous acceleration from slow to fast swimming included an increase in the cycle frequency, a baseline depolarization in the swim interneurons and an increase in the intensity of motoneuron firing. Similar effects could be elicited by bath application of 10(-5) mol l-1 serotonin. Two clusters of cerebral serotonin-immunoreactive interneurons were found to produce acceleration of swimming accompanied by changes in neuronal activity. Posterior cluster neurons triggered an increase in swim frequency, depolarization of the swim interneurons, an increase in general excitor motoneuron activity and activation of type 12 interneurons and pedal peripheral modulatory neurons. Cells from the anterior cerebral cluster also increased swim frequency, increased activity in the swim motoneurons and activated type 12 interneurons, pedal peripheral modulatory neurons and the heart excitor neuron. The time course of action of the anterior cluster neurons did not greatly outlast the duration of spike activity, while that of the posterior cluster neurons typically outlasted burst duration. It appears that the two discrete clusters of serotonin-immunoreactive neurons have similar, but not identical, effects on swim neurons, raising the possibility that the two serotonergic cell groups modulate the same target cells through different cellular mechanisms.

Serotonergic Modulation of Swimming Speed in the Pteropod Mollusc Clione Limacina:I. Serotonin Immunoreactivity in the Central Nervous System and Wings

Serotonin-immunoreactive somata in the pteropod mollusc Clione limacina were restricted to the cerebral and pedal ganglia. 10-14 pairs of cells were consistently found in the cerebral ganglia, including one large pair that had soma positions and axon branching patterns reminiscent of those of the metacerebral cells of other molluscs. Two clusters of somata were found on the midline near the cerebral commissure, one on the anterior-lateral margin and one posterior-laterally. A distinct paired cluster of up to nine somata was found on the dorso-lateral margin of the pedal ganglia, near the emergence of the pedal commissure. Up to five of these cells innervated the ipsilateral wing via the wing nerve. Dye-fills of these cells showed that they branch repeatedly in the ipsilateral wing and innervate the swim musculature. Double-labelling experiments indicated that the filled neurons were also serotonin-immunoreactive. Neurobiotin fills that were processed for electron microscopy revealed two types of terminals associated with the swim musculature: direct contacts and reactive terminals adjacent to non-labelled presynaptic terminals. Additional immunoreactive neurons in the pedal ganglia included the asymmetrical heart excitor neuron of the left pedal ganglion and up to nine ventral somata.

Development of segment- and target-related neuronal identity in the medicinal leech.

The rhythmic pumping of the paired heart tubes in the medicinal leech Hirudo medicinalis offers an excellent system for studying the development of a simple behavior in terms of its neuronal and muscular components. The present experiments examined the development of identified heart excitor (HE) motor neurons during normal embryogenesis. Using intracellular impalements and dye-filling, we found that the HE motor neurons could be identified at an early stage of development and that they initially elaborated axonal arborizations in inappropriate target fields in the ventral body wall. These inappropriate projections were retracted as those at the appropriate target (developing heart tube muscle) extended. This remodelling occurred at least 4 days before the HEs acquired the adult phenotype of being driven to fire action potentials in a rhythmic pattern. Although the HEs exhibited centrally driven rhythmic oscillations late in embryogenesis, at earlier stages they exhibited largely a tonic discharge interrupted by bursts of inhibitory potentials in a periodic, but not a rhythmic, pattern. We also found what appeared to be non-rhythmic HE homologs in anterior and posterior segments where HE neurons have not been previously described. These homologs may project along similarly patterned guidance cues early in development, since they are at first indistinguishable from the definitive HEs, but they continued to elaborate both lateral and medial body wall projections over the same period that definitive HEs were expanding their arborizations over the developing heart tube and retracting their body wall projections. In both adult and embryonic leeches the homologs exhibited mostly tonic activity that was interrupted by pronounced, but non-rhythmic, hyperpolarizing postsynaptic potentials. Thus, there appears to be early segmental specification directing the final phenotype of the iterated neuron that, in most segments, becomes the HE motor neuron.

Excitation and inhibition of the heart of the snail, Lymnaea, by non-FMRFamidergic motoneurons

1. The present paper extends the model of neuronal control of the Lymnaea heart by the use of intracellular recording techniques to identify further types of cardioactive neurons in the CNS that, like the previously described E heart excitor (Ehe) cells, influence the myogenic heartbeat. 2. Four new types of neuron that act on the heart are described. These are excitatory Hhe and She cells (H and S heart excitors) and the inhibitory Khi cell (K heart inhibitor). The fourth class, tonus pericardium excitor (Tpe), modulates the heart by action on pericardial tissue. 3. Pharmacologic, electrophysiological, and anatomic evidence is presented that shows that these cells are motoneurons, innervating heart muscle fibers directly: blocking central chemical synapses failed to prevent the actions of the neurons on the heart; simultaneous intracellular recordings showed unitary EJPs in heart muscle after 1:1 and with constant delay from evoked neuronal action potentials; intracellular injection of the dye Lucifer yellow showed all cells had axonal branches entering the intestinal nerve (which innervates the heart). 4. The use of selective antagonists to 5-hydroxytryptamine (5-HT) (cinanserin), dopamine (ergonovine), and acetylcholine (alpha-bungarotoxin) provided evidence that the actions of She and Hhe cells are mediated by 5-HT, whereas those of the Khi cell are mediated by acetylcholine. 5. A cyclically active network of three interneuronal inputs acting on the heart motoneurons is described. 6. One of these, input 3, is responsible for periodic excitation of the heart via its effects on the Hhe cells.

Regulation of heartbeat in Lymnaea by motoneurons containing FMRFamide-like peptides

1. Intracellular stimulation of single neurons in the Lymnaea CNS was carried out to identify heart motoneurons. 2. Two of the identified motoneurons, the E heart excitor (Ehe) cells, were shown to contain Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides by immunocytochemical staining of dye-marked neurons and by radioimmunoassay (RIA) applied to extracts of single dissected cells. 3. Bursts of spikes in the Ehe cells increased heart rate, beat amplitude and muscle tonus. This response was mimicked by perfusion of exogenous FMRFamide at low concentration (10(-6) to 5 x 10(-8) M) through the interior of the intact heart. 4. Application of selective antagonists to 5-hydroxytryptamine (5-HT) and dopamine failed to block Ehe cardiac effects. 5. Detailed evidence that the Ehe cells were heart motoneurons was obtained. 1) Anatomic mapping using the dye Lucifer yellow showed Ehe cells had peripheral projections restricted mainly to the intestinal nerve, the only nerve known to innervate the heart. 2) Perfusion of the CNS with a saline containing Co2+ blocked central chemical synapses but did not affect activity of Ehe cells on the heart. 3) Simultaneous intracellular recordings from Ehe cells and auricle muscle fibers showed unitary excitatory junction potentials following with constant latency from spikes in Ehe cells. 6. The present study elucidates the role of FMRFamide in cardioregulation and provides the first evidence that it acts as an excitatory neurotransmitter on the snail heart.

Characterization of cardiac innervation in the nudibranch, Archidoris montereyensis

The heart of the nudibranch mollusc Archidoris montereyensis is regulated by a small number of powerful effector neurons located in the right pleural and visceral ganglia. Two identifiable neurons in the pleural ganglion, a heart excitor (PlHE) and a heart inhibitor (PlHI), are especially important regulators of cardiac function in that low levels of spontaneous activity in either cell significantly alters the amplitude and rate of heart contractions. These neurons have extensive dendritic arbors within the right pleural ganglion and branching axonal processes within the visceral ganglion. The visceral ganglion also contains a heart excitor neuron (VHE) and at least two heart inhibitor neurons (VHI cells), but their influence on cardiac activity is weaker than that of the pleural ganglion cells. All of these heart effector cells appear to be motor neurons with axons that terminate predominantly in the atrio-ventricular valve region of the heart via the pericardial nerve. The simplicity and strength of these neuronal connections to the heart of Archidoris make this a favorable preparation for studies of cardiac regulation.

Neural Control of Heart Beat in the Pteropod Mollusc Clione Limacina: Coordination of Circulatory and Locomotor Systems

ABSTRACT In the intact pteropod mollusc Clione limacina a correlation between heart beat and locomotor activity has been found. In freely swimming Clione, pauses in locomotor activity arose spontaneously with intervals of a few minutes. During these pauses, the heart stopped beating. A link between the heart and locomotor activity was also observed during defensive reactions: mechanical stimulation of the head resulted in the termination of both locomotion and heart beating, while stimulation of the tail accelerated both the locomotor and the heart rhythms. After transection of the medial abdominal nerve supplying the heart, the link between heart rate and locomotor activity disappeared. The heart rhythm accelerated during hunting behaviour of Clione. Four efferent neurones controlling heart beat were found: one heart excitor (HE) in the left pedal ganglion, and three heart inhibitors (HI) in the left abdominal ganglion. The HE exerted an inhibitory action upon the HIs. Spontaneous or reflex activation of the locomotor generator was accompanied by excitation of the HE and by inhibition of the HIs, while spontaneous or reflex inhibition of the locomotor generator was accompanied by inhibition of the HE and by excitation of the HIs. These effects were due, at least partly, to the direct action of the neurones of the locomotor generator upon the HE and HIs.

Ganglionic circulation and its effects on neurons controlling cardiovascular functions in Aplysia californica.

The abdominal ganglion of the mollusk Aplysia californica receives most of its blood supply through a small caudal artery that branches off the anterior aorta near its junction with the heart. Injection of an ink/gelatin mixture into the caudal artery revealed a consistent pattern of arterial branching within the ganglion and a general proximity of larger vessels to identified neurons controlling circulation in this animal. This morphological arrangement was particularly evident for the heart excitor interneuron, cell L10, which lies next to the caudal artery near its entry into the ganglion. In electrophysiological experiments, L10 was excited when blood flow or oxygen tension within the ganglion was reduced. This effect was expressed as a gradual increase in impulse frequency of L10 and conversion from tonic to bursting mode of spike discharge. L10 follower cells in the RB and LD neuron clusters were affected synaptically by the changes in L10 activity, while other follower cells (L3 and RD neurons) responded independently of L10's synaptic influence. The neurosecretory white cells (R3 to R14) that innervate the major arteries and pericardial tissues were also excited when ganglionic circulation was interrupted. In innervated preparations of the heart and respiratory organs, decreased circulation through the abdominal ganglion stimulated a transient increase in the rate and amplitude of respiratory (gill) pumping and pericardial contractions and caused a sustained increase in activity of the heart. Both responses increase cardiac output and both appear to involve a direct influence of ganglionic circulation on interneurons controlling the gill and heart. These results indicate that the cell-specific patterns of excitation and inhibition caused by fluctuations in ganglionic circulation may be important factors for maintaining circulatory homeostasis in this animal.

Neural Control of Heart Beat in the African Giant Snail, Achatina Fulica Férussac: I. Identification of the Heart Regulatory Neurones

ABSTRACT Seven heart regulatory neurones (PON, TAN, TAN-2, TAN-3, d-RCDN, d-LCDN and VG1) were identified in the central nervous system of the African giant snail, Achatina fulica Férussac. Among these neurones, the periodically oscillating neurone (PON) was the most effective heart excitor, producing heart excitation at rather low firing frequencies. The tonically autoactive neurones (TAN, TAN-2 and TAN-3) were tonically firing neurones and their spontaneous activity was found to produce tonic heart excitation which supplemented the myogenic heart activity. There was some evidence that two cerebral ganglion cells (the dorsal right and left cerebral distinct neurones, d-RCDN and d-LCDN) were also likely to be heart excitors although the direct connection to the heart was somewhat doubtful in some specimens. No direct inhibitory neurone was found, but the high firing frequency of the visceral ganglion neurone (VG1) usually produced heart inhibition.