Role of Cellular Orientation in Electrical Coupling Between Gastrointestinal Smooth Muscle

Electrical fields produced during depolarization as well as low resistance pathways through gap junctions have been proposed as electrical coupling mechanisms serving to coordinate electrical control activity in gastrointestinal smooth muscle. The differing orientations of the longitudinal and circular muscle layers offer many possible configurations for cells coupled by electrical fields. The boundary element method is used to investigate coupling, with respect to both gap junctions and field effects for ellipsoidal and cylindrical cells. Physiological considerations allow the possibility of aggregates of cells with coordinated electrical activity. The effect of multiple source cells on field coupling is also modeled. Results indicate that even small gap junctional conductances are effective for coupling of smooth muscle and that field coupling is most efficacious when the ellipsoidal cells are coupled side by side and when cylindrical cells are coupled end to end. © 1998 Biomedical Engineering Society. PAC98: 8722Jb, 8710+e, 0260Lj, 8750-a     

Effect of electrical coupling on ionic current and synaptic potential measurements

Recent studies have found electrical coupling to be more ubiquitous than previously thought, and coupling through gap junctions is known to play a crucial role in neuronal function and network output. In particular, current spread through gap junctions may affect the activation of voltage-dependent conductances as well as chemical synaptic release. Using voltage-clamp recordings of two strongly electrically coupled neurons of the lobster stomatogastric ganglion and conductance-based models of these neurons, we identified effects of electrical coupling on the measurement of leak and voltage-gated outward currents, as well as synaptic potentials. Experimental measurements showed that both leak and voltage-gated outward currents are recruited by gap junctions from neurons coupled to the clamped cell. Nevertheless, in spite of the strong coupling between these neurons, the errors made in estimating voltage-gated conductance parameters were relatively minor (<10%). Thus in many cases isolation of coupled neurons may not be required if a small degree of measurement error of the voltage-gated currents or the synaptic potentials is acceptable. Modeling results show, however, that such errors may be as high as 20% if the gap-junction position is near the recording site or as high as 90% when measuring smaller voltage-gated ionic currents. Paradoxically, improved space clamp increases the errors arising from electrical coupling because voltage control across gap junctions is poor for even the highest realistic coupling conductances. Furthermore, the common procedure of leak subtraction can add an extra error to the conductance measurement, the sign of which depends on the maximal conductance.     

Alterations in neuromuscular junction morphology during fast-to-slow transformation of rabbit skeletal muscles

Summary Chronic low frequency stimulation of motor nerves results in transformation of muscle fibre phenotype from fast- to slow-twitch. We examined the light and electron microscopic structure of neuromuscular junctions in normally fast twitch muscles, tibialis anterior and extensor digitorum longus of rabbit after 3 weeks of stimulation to determine whether synaptic structure is also modified during fibre type transformation. Neuromuscular junctions of stimulated and unstimulated (control) tibialis anterior and extensor digitorum longus muscles and unstimulated slow twitch soleus muscle were visualized with rhodamine-conjugated -bungarotoxin. Video light microscopic images of neuromuscular junctions were digitized to allow quantification of their surface areas, perimeters, lengths and widths. Three weeks of stimulation resulted in a decrease in the maximal velocity of muscle fibre shortening and augmentation of mitochondrial volume in fast muscles, demonstrating the efficacy of the stimulation protocol employed in altering muscle fibre phenotype. Neuromuscular junctions of control tibialis anterior and extensor digitorum longus are thin, compact, and continuous, with complex branching patterns. In contrast, those of slow-twitch soleus are thicker and discontinuous. Neuromuscular junctions in control tibialis anterior and extensor digitorum longus are larger than those in soleus. Three weeks of stimulation causes a marked decrease in the size of neuromuscular junctions in tibialis anterior and extensor digitorum longus, as reflected in the significant reduction in neuromuscular junction surface area, length and width. Electron microscopy of these junctions suggests that secondary postsynaptic folds in stimulated muscles are more closely spaced. Also, axon terminals of stimulated muscles appear to contain more densely packed synaptic vesicles and mitochondria than controls. Decreases in neuromuscular junction dimensions can be partly explained by muscle fibre atrophy. However, the decrease in neuromuscular junction size is proportionately greater than that of muscle fibre diameter in both muscles, indicating that factors other than fibre atrophy may contribute to the reduced neuromuscular junction size in stimulated muscles. Neuromuscular junctions of stimulated tibialis anterior and extensor digitorum longus muscles exhibit some features characteristic of normal soleus neuromuscular junctions, indicating structural adaptations consistent with the altered muscle fibre phenotype. On the other hand, neuromuscular junctions of 3 week stimulated tibialis anterior and extensor digitorum longus and their synaptic branches remain as thin and continuous as those of unstimulated controls, suggesting that the transformation of neuromuscular junctions towards a morphology characteristic of slow muscle, is only partial. These results demonstrate that an altered pattern of impulse activity causes significant synaptic remodelling in adult rabbit skeletal muscles.     

Scanning electron microscope study of neuromuscular junctions in different muscle fiber types in the zebra finch and rat

Examination by scanning electron microscopy revealed differences between neuromuscular junctions in the muscle fibers of the zebra finch (bird) and rat. The neuromuscular junctions between the anterior and posterior latissimus dorsi muscles of the zebra finch were compared. The junctions of the former, exclusively slow tonic fibers, were small and numerous along the long axis of a single muscle fiber. The synaptic depressions per junction were few. The junctions of the latter, exclusively fast twitch fibers, were large and consisted of more synaptic depressions than the former. Junctional folds were occasionally found in some depressions. The neuromuscular junctions between the extensor digitorum longus and soleus muscles of the rat were also compared. The former consisted almost entirely of fast twitch muscle fibers, whereas the latter consisted of both slow twitch fibers (75%) and fast twitch fibers (25%). The junctions in the extensor digitorum longus muscle were almost all labyrinthine gutters containing exclusively slit-like junctional folds. In the soleus muscle, two types of junctions were observed. One type was similar to that of the extensor digitorum longus muscle; the other was characterized by labyrinthine gutters containing sparse, narrow slit-like and pit-like junctional folds. We suggest from these structural differences of the subneural apparatuses that the junction of the fast twitch muscle is characterized by the subneural apparatus containing numerous slit-like junctional folds, and that of the slow twitch muscle fiber characterized by the apparatus containing sparse, narrow slit-like and pit-like junctional folds.     

Mechanisms underlying fictive feeding in aplysia: coupling between a large neuron with plateau potentials activity and a spiking neuron

The buccal ganglia of Aplysia contain a central pattern generator (CPG) that organizes the rhythmic movements of the radula and buccal mass during feeding. Many of the cellular and synaptic elements of this CPG have been identified and characterized. However, the roles that specific cellular and synaptic properties play in generating patterns of activity are not well understood. To examine these issues, the present study developed computational models of a portion of this CPG and used simulations to investigate processes underlying the initiation of patterned activity. Simulations were done with the SNNAP software package. The simulated network contained two neurons, B31/B32 and B63. The development of the model was guided and constrained by the available current-clamp data that describe the properties of these two protraction-phase interneurons B31/B32 and B63, which are coupled via electrical and chemical synapses. Several configurations of the model were examined. In one configuration, a fast excitatory postsynaptic potential (EPSP) from B63 to B31/B32 was implemented in combination with an endogenous plateau-like potential in B31/B32. In a second configuration, the excitatory synaptic connection from B63 to B31/B32 produced both fast and slow EPSPs in B31/B32 and the plateau-like potential was removed from B31/B32. Simulations indicated that the former configuration (i.e., electrical and fast chemical coupling in combination with a plateau-like potential) gave rise to a circuit that was robust to changes in parameter values and stochastic fluctuations, that closely mimicked empirical observations, and that was extremely sensitive to inputs controlling the onset of a burst. The coupling between the two simulated neurons served to amplify exogenous depolarizations via a positive feedback loop and the subthreshold activation of the plateau-like potential. Once a burst was initiated, the circuit produced the program in an all-or-none fashion. The slow kinetics of the simulated plateau-like potential played important roles in both initiating and maintaining the burst activity. Thus the present study identified cellular and network properties that contribute to the ability of the simulated network to integrate information over an extended period before a decision is made to initiate a burst of activity and suggests that similar mechanisms may operate in the buccal ganglia in initiating feeding movements.     

The formation of synapses in reinnervated and cross-reinnervated striated muscle during development

1. A study has been made of the formation of synapses in reinnervated and cross-reinnervated developing striated muscles which normally receive either a focal or distributed innervation, using histological, ultrastructural and electrophysiological techniques.2. The focally innervated mammalian tibialis anterior muscle, denervated soon after birth, was reinnervated at both the original end-plates as well as on the new muscle added during the period of denervation; but not on the muscle present at the time of denervation. Nearly all the synapses which had formed, other than at the original end-plates, disappeared by 6 weeks post-natal.3. The avian anterior latissimus dorsi muscle (ALD), which receives a distributed innervation, was denervated during the first week post-hatched, and became reinnervated both at the original synaptic sites as well as on the new muscle added during the period of denervation; all these synapses were spaced approximately 200 mum apart along the length of individual muscle cells.4. The myofibres of the ALD muscle cross-reinnervated at hatching with the superior brachialis nerve, which contains fast motor axons that normally form a focal innervation, were each focally innervated by a single ;en plaque' terminal; these synapses had the same electrical properties as normal synapses formed by fast motor axons.5. Many of the myofibres of the avian posterior latissimus dorsi (PLD), which normally receive a focal innervation, received a distributed innervation from ;en grappe' terminals when cross-reinnervated with the ALD nerve at hatching.6. It is suggested that during development the nerve type determines the pattern of synapses over an effector; this is achieved by the nerve, after forming the initial synaptic contact, making the rest of the muscle cell membrane refractory to further synapse formation for some distance, this distance being determined by the nerve type.     

Gap junctions between accessory medulla neurons appear to synchronize circadian clock cells of the cockroach Leucophaea maderae

The temporal organization of physiological and behavioral states is controlled by circadian clocks in apparently all eukaryotic organisms. In the cockroach Leucophaea maderae lesion and transplantation studies located the circadian pacemaker in the accessory medulla (AMe). The AMe is densely innervated by gamma-aminobutyric acid (GABA)-immunoreactive and peptidergic neurons, among them the pigment-dispersing factor immunoreactive circadian pacemaker candidates. The large majority of cells of the cockroach AMe spike regularly and synchronously in the gamma frequency range of 25-70 Hz as a result of synaptic and nonsynaptic coupling. Although GABAergic coupling forms assemblies of phase-locked cells, in the absence of synaptic release the cells remain synchronized but fire now at a stable phase difference. To determine whether these coupling mechanisms of AMe neurons, which are independent of synaptic release, are based on electrical synapses between the circadian pacemaker cells the gap-junction blockers halothane, octanol, and carbenoxolone were used in the presence and absence of synaptic transmission. Here, we show that different populations of AMe neurons appear to be coupled by gap junctions to maintain synchrony at a stable phase difference. This synchronization by gap junctions is a prerequisite to phase-locked assembly formation by synaptic interactions and to synchronous gamma-type action potential oscillations within the circadian clock.     

Properties and origin of spikelets in thalamocortical neurones in vitro

Spikelets, or fast prepotentials as they are frequently referred to, are a common feature of the electrophysiology of central neurones and are invariably correlated with the presence of electrotonic coupling via gap junctions. Here we report that in the presence of the metabotropic glutamate receptor agonists, trans-ACPD or DHPG, thalamocortical neurones of the cat dorsal lateral geniculate nucleus maintained in vitro exhibit stereotypical spikelets that possess similar properties to those described in other brain areas. These spikelets were routinely observed in the presence of antagonists of fast chemical synaptic transmission, were resistant to the application of a variety of voltage-dependent Ca(2+) channel blockers but were abolished by tetrodotoxin. In addition, spikelets were reversibly blocked by the putative gap junction blocker carbenoxolone and were nearly always accompanied by dye-coupling. These results indicate that thalamocortical neurones may be electrotonically coupled via gap junctions with spikelets representing attenuated action potentials from adjoining cells. We suggest that the presence of electrotonic communication between thalamocortical neurones would have major implications for the understanding of both physiological (Steriade et al., 1993; Sillito et al., 1994; Alonso et al., 1996; Neuenschwander and Singer, 1996; Weliky and Katz, 1999) and pathological (Steriade and Contreras, 1995; Pinault et al., 1998) synchronised electrical activity in the thalamus.     

Description of the structural control systems of ovum transport in the quail oviduct

Some electronmicroscropic and light microscopic aspects of the quail oviduct have been studied in relation to ovum transport. Previously it has been shown that in this smooth muscle there exist spontaneous coordinated electrical and mechanical activity which suggests a good electrical and mechanical coupling of the muscle cells. The structural basis for this coupling is not known. The majority of muscle cell contacts observed were simple appositions and intermediate junctions. Less numerous were attachments of the interdigitation type. No nexuses or tight junctions were seen. Mechanical stretching or contraction of cells induced with 10^-4 M carbachol did not affect the contacts. The fine structure of the muscle cells did not differ from that described for other smooth muscles. Electronmicroscopically the muscle cell bundles could not be distinguished into separate layers, in the light microscope the cell bundles were spirally arranged. Stretching of the oviductal strips to the length to which the ovum stretches the muscular wall during ovum transport caused re-orientation of the muscle cell bundles. One-directional stretching turned the axes of the muscle cells and the collagen bundles parallel, while stretching in two direction made the tissue look like a network. The re-orientation of muscle cell bundles may be of importance in producing forces in the muscular tunic during ovum transport. The nerve supply to the muscle cells was negligible. These and previous results show that structurally the muscular wall of the quail oviduct is a dynamic unity in which the ovum via stretch induces the electrical and mechanical activity throughout the tissue. Innervation may play a minor role in controlling the contractions.     

Neuromuscular system of the flexible arm of the octopus: physiological characterization

The octopus arm is an outstanding example of an efficient boneless and highly flexible appendage. We have begun characterizing the neuromuscular system of the octopus arm in both innervated muscle preparations and dissociated muscle cells. Functionally antagonistic longitudinal and transverse muscle fibers showed no differences in membrane properties and mode of innervation. The muscle cells are excitable but have a broad range of linear membrane properties. They are electrotonically very compact so that localized synaptic inputs can control the membrane potential of the entire muscle cell. Three distinct excitatory neuronal inputs to each arm muscle cell were identified; their reversal potentials were extrapolated to be about -10 mV. These appear to be cholinergic as they are blocked by hexamethonium, D-tubocurarine, and atropine. Two inputs have low quantal amplitude (1-7 mV) and slow rise times (4-15 ms), whereas the third has a large size (5-25 mV) and fast rise time (2-4 ms). This large synaptic input is most likely due to exceptionally large quantal events. The probability of release is rather low, suggesting a stochastic activation of muscle cells. All inputs demonstrated a modest activity-dependent plasticity typical of fast neuromuscular systems. The pre- and postsynaptic properties suggest a rather direct relation between neuronal activity and muscle action. The lack of significant electrical coupling between muscle fibers and the indications for the small size of the motor units suggest that the neuromuscular system of the octopus arm has evolved to ensure a high level of precise localization in the neural control of arm function.     

Mechanisms of physiological and epileptic HFO generation

High frequency oscillations (HFO) have a variety of characteristics: band-limited or broad-band, transient burst-like phenomenon or steady-state. HFOs may be encountered under physiological or under pathological conditions (pHFO). Here we review the underlying mechanisms of oscillations, at the level of cells and networks, investigated in a variety of experimental in vitro and in vivo models. Diverse mechanisms are described, from intrinsic membrane oscillations to network processes involving different types of synaptic interactions, gap junctions and ephaptic coupling. HFOs with similar frequency ranges can differ considerably in their physiological mechanisms. The fact that in most cases the combination of intrinsic neuronal membrane oscillations and synaptic circuits are necessary to sustain network oscillations is emphasized. Evidence for pathological HFOs, particularly fast ripples, in experimental models of epilepsy and in human epileptic patients is scrutinized. The underlying mechanisms of fast ripples are examined both in the light of animal observations, in vivo and in vitro, and in epileptic patients, with emphasis on single cell dynamics. Experimental observations and computational modeling have led to hypotheses for these mechanisms, several of which are considered here, namely the role of out-of-phase firing in neuronal clusters, the importance of strong excitatory AMPA-synaptic currents and recurrent inhibitory connectivity in combination with the fast time scales of IPSPs, ephaptic coupling and the contribution of interneuronal coupling through gap junctions. The statistical behaviour of fast ripple events can provide useful information on the underlying mechanism and can help to further improve classification of the diverse forms of HFOs.     

New role for astroglia in learning: Formation of muscle memory

Muscle memory can be described as gradual adaptation of muscles over a period of time to perform a new movement or action. Its precise mechanism is unknown; however, it is now known that when a motor skill is learned it leads to significant brain activity. Astrocytes are the most abundant glial cell types in the CNS that play an associative active role with neurons in learning and memory. They are interconnected to neurons via gap junctions forming astroglial network for fast communication and synchronization. We hypothesize that astroglial cells play main role in the formation of muscle memory and evaluate it by the experimental evidence published so far that indicates role of astroglia on various cellular and molecular aspects of muscle memory. The basis of our hypothesis is the fact that during training or motor learning period, neuronal output data related to learning lead to certain specific pattern for stimulating target muscles over a period of time and partly these data are stored in astroglial network. This stored data fine tune glial parameters that affect synaptic space and neuronal output used to perform rapid motor actions. For the validation of our hypothesis, we have generated a computational model for a section of neural pathway with astroglial network and have shown that the astroglial network by using inhibitory and stimulatory neurotransmitters can generate certain patterns, modulate and balance synaptic space across the neural pathway during acquisition of muscle memory.     

New role for astroglia in learning: Formation of muscle memory

Muscle memory can be described as gradual adaptation of muscles over a period of time to perform a new movement or action. Its precise mechanism is unknown; however, it is now known that when a motor skill is learned it leads to significant brain activity. Astrocytes are the most abundant glial cell types in the CNS that play an associative active role with neurons in learning and memory. They are interconnected to neurons via gap junctions forming astroglial network for fast communication and synchronization. We hypothesize that astroglial cells play main role in the formation of muscle memory and evaluate it by the experimental evidence published so far that indicates role of astroglia on various cellular and molecular aspects of muscle memory. The basis of our hypothesis is the fact that during training or motor learning period, neuronal output data related to learning lead to certain specific pattern for stimulating target muscles over a period of time and partly these data are stored in astroglial network. This stored data fine tune glial parameters that affect synaptic space and neuronal output used to perform rapid motor actions. For the validation of our hypothesis, we have generated a computational model for a section of neural pathway with astroglial network and have shown that the astroglial network by using inhibitory and stimulatory neurotransmitters can generate certain patterns, modulate and balance synaptic space across the neural pathway during acquisition of muscle memory.     

Computational model of electrically coupled, intrinsically distinct pacemaker neurons

Electrical coupling between neurons with similar properties is often studied. Nonetheless, the role of electrical coupling between neurons with widely different intrinsic properties also occurs, but is less well understood. Inspired by the pacemaker group of the crustacean pyloric network, we developed a multicompartment, conductance-based model of a small network of intrinsically distinct, electrically coupled neurons. In the pyloric network, a small intrinsically bursting neuron, through gap junctions, drives 2 larger, tonically spiking neurons to reliably burst in-phase with it. Each model neuron has 2 compartments, one responsible for spike generation and the other for producing a slow, large-amplitude oscillation. We illustrate how these compartments interact and determine the dynamics of the model neurons. Our model captures the dynamic oscillation range measured from the isolated and coupled biological neurons. At the network level, we explore the range of coupling strengths for which synchronous bursting oscillations are possible. The spatial segregation of ionic currents significantly enhances the ability of the 2 neurons to burst synchronously, and the oscillation range of the model pacemaker network depends not only on the strength of the electrical synapse but also on the identity of the neuron receiving inputs. We also compare the activity of the electrically coupled, distinct neurons with that of a network of coupled identical bursting neurons. For small to moderate coupling strengths, the network of identical elements, when receiving asymmetrical inputs, can have a smaller dynamic range of oscillation than that of its constituent neurons in isolation.     

Effects of RHC-80267, an inhibitor of diacylglycerol lipase, on excitation of circular smooth muscle of the guinea-pig gastric antrum.

In small segments of circular smooth muscle isolated from the guinea-pig gastric antrum, the effects of RHC-80267, an inhibitor of diacylglycerol lipase, were investigated both on regenerative slow potentials (either occurring spontaneously or as the result of a depolarizing intracellular current injection) and on the actions of acetylcholine (ACh). As diacylglycerol is a known activator of protein kinase C (PKC), it would therefore be expected that RHC-80267 would activate PKC indirectly. In circular smooth muscle bundles, spontaneously generating slow potentials recorded simultaneously from two given cells were synchronized, indicating that these two cells were electrically coupled. RHC-80267 (0.3-1 microM) increased the frequency of slow potential generation, with no alteration to the amplitude of either the slow potentials or the resting membrane potential. Synchronous electrical activity in a given pair of cells was also unchanged by RHC-80267, indicating that intercellular electrical coupling was not altered. The input resistance of smooth muscle cells calculated from the amplitude of electrotonic potentials produced by injection of current was not significantly altered by RHC-80267. The refractory period for the generation of slow potentials evoked by depolarizing stimuli was about 8 s, and it was decreased to about 5 s by RHC-80267, with no significant alteration to the amplitude of spontaneous or evoked slow potentials. ACh (0.5 microM) depolarized the membrane by about 5 mV and increased the amplitude and frequency of slow potentials. The actions of ACh on the frequency of slow potentials were enhanced by RHC-80267, with no alteration to the amplitudes of both the ACh-induced depolarization and slow potentials. These results support the idea that PKC is involved in determining the frequency of slow potentials, by shortening the refractory period for excitation of gastric smooth muscle cells.     

Electrical coupling: novel mechanism for sleep-wake control

STUDY OBJECTIVES: Recent evidence suggests that certain anesthetic agents decrease electrical coupling, whereas the stimulant modafinil appears to increase electrical coupling. We investigated the potential role of electrical coupling in 2 reticular activating system sites, the subcoeruleus nucleus and in the pedunculopontine nucleus, which has been implicated in the modulation of arousal via ascending cholinergic activation of intralaminar thalamus and descending activation of the subcoeruleus nucleus to generate some of the signs of rapid eye movement sleep. DESIGN: We used 6- to 30-day-old rat pups to obtain brainstem slices to perform whole-cell patch-clamp recordings. MEASUREMENTS AND RESULTS: Recordings from single cells revealed the presence of spikelets, manifestations of action potentials in coupled cells, and of dye coupling of neurons in the pedunculopontine nucleus. Recordings in pairs of pedunculopontine nucleus and subcoeruleus nucleus neurons revealed that some of these were electrically coupled with coupling coefficients of approximately 2%. After blockade of fast synaptic transmission, the cholinergic agonist carbachol was found to induce rhythmic activity in pedunculopontine nucleus and subcoeruleus nucleus neurons, an effect eliminated by the gap junction blockers carbenoxolone or mefloquine. The stimulant modafinil was found to decrease resistance in neurons in the pedunculopontine nucleus and subcoeruleus nucleus after fast synaptic blockade, indicating that the effect may be due to increased coupling. CONCLUSIONS: The finding of electrical coupling in specific reticular activating system cell groups supports the concept that this underlying process behind specific neurotransmitter interactions modulates ensemble activity across cell populations to promote changes in sleep-wake state.     

The extent and strength of electrical coupling between inferior olivary neurons is heterogeneous

Gap junctions constitute the only form of synaptic communication between neurons in the inferior olive (IO), which gives rise to the climbing fibers innervating the cerebellar cortex. Although its exact functional role remains undetermined, electrical coupling was shown to be necessary for the transient formation of functional compartments of IO neurons and to underlie the precise timing of climbing fibers required for cerebellar learning. So far, most functional considerations assume the existence of a network of permanently and homogeneously coupled IO neurons. Contrasting this notion, our results indicate that coupling within the IO is highly variable. By combining tracer-coupling analysis and paired electrophysiological recordings, we found that individual IO neurons could be coupled to a highly variable number of neighboring neurons. Furthermore, a given neuron could be coupled at remarkably different strengths with each of its partners. Freeze-fracture analysis of IO glomeruli revealed the close proximity of glutamatergic postsynaptic densities to connexin 36-containing gap junctions, at distances comparable to separations between chemical transmitting domains and gap junctions in goldfish mixed contacts, where electrical coupling was shown to be modulated by the activity of glutamatergic synapses. On the basis of structural and molecular similarities with goldfish mixed synapses, we speculate that, rather than being hardwired, variations in coupling could result from glomerulus-specific long-term modulation of gap junctions. This striking heterogeneity of coupling might act to finely influence the synchronization of IO neurons, adding an unexpected degree of complexity to olivary networks.     

Chemically and electrically coupled interneurons mediate respiratory pumping in Aplysia

1. Respiratory pumping in Aplysia consists of synchronous, brief contractions of the mantle organs that can occur spontaneously and also can be triggered by tactile or noxious stimulation. It has been shown previously to be driven in part by a cluster of electrically coupled interneurons, called the L25 cells, located in the left hemiabdominal ganglion. This paper describes a second class of interneurons, the R25 cells, that also plays an important role in the control of respiratory pumping. 2. The R25 cells are a cluster of approximately 14 interneurons located in the right hemiabdominal ganglion, roughly symmetrical to the L25 cells. The R25 cells are electrically coupled to each other and to the L25 cells. Some R25 and L25 cells also produce chemically mediated fast excitation and slow inhibition of other neurons in the R25/L25 network. 3. Three lines of evidence demonstrate that the R25 cells play a role in mediating respiratory pumping: 1) They fire in a synchronous burst each time the behavior occurs. 2) They make direct chemical synaptic connections to motoneurons that mediate the behavior; and 3) Firing a single R25 cell can trigger the entire behavior, by recruiting synchronous bursts of activity in all of the other R25 and L25 neurons. Individual R25 and L25 cells can act both as trigger cells (exciting the other interneurons) and as relay cells (projecting directly to motoneurons). 4. Burst initiation within the R25/L25 network appears to have two phases: 1) There is an initial phase when the R25 and L25 cells fire at a relatively low frequency. This phase can be driven either by endogenous pacemaker activity of the R25/L25 cells or by afferent synaptic input from sensory pathways; and 2) The late, high-frequency phase of the burst results largely from reverberation within the network, as activity in each cell contributes positive feedback via the excitatory chemical and electrical connections between R25 and L25 cells. 5. Synchronization of the different motor outputs that make up the pumping behavior is achieved by three mechanisms: 1) When pumping occurs spontaneously, the electrical coupling between the cells of the R25/L25 network ensures that these cells will all be near spike threshold at the end of each interburst period.(ABSTRACT TRUNCATED AT 400 WORDS)     

The interstitial cells of Cajal and a gastroenteric pacemaker system

In spite of a claim by Kobayashi (1990) that they do not correspond to the cells originally depicted by CAJAL, a particular category of fibroblast-like cells have been identified in the gut by electron microscopy (Faussone-Pellegrini, 1977; Thuneberg, 1980) and by immunohistochemistry for Kit protein (Maeda et al., 1992) under the term of the "interstitial cells of Cajal (ICC)". Generating electrical slow waves, the ICC are intercalated between the intramural neurons and the effector smooth muscular cells, to form a gastroenteric pacemaker system. ICC at the level of the myenteric plexus (IC-MY) are multipolar cells forming a reticular network. The network of IC-MY which is believed to be the origin of electrical slow waves is morphologically independent from but associated with the myenteric plexus. On the other hand, intramuscular ICC (IC-IM) usually have spindle-shaped contours arranged in parallel with the bulk smooth muscle cells. Associated with nerve bundles and blood vessels, the IC-IM possess receptors for neurotransmitters and such circulating hormones as cholecystokinin, suggesting their roles in neuromuscular and hormone-muscular transmissions. In addition, gap junctions connect the IC-MY and IC-IM, thereby realizing the electrically synchronized integrity of ICC as a pacemaker system in the gut. The smooth muscle cells are also coupled with ICC via gap junctions, and the functional unit thus formed enables rhythmically synchronized contractions and relaxations. It has recently been found that a lack of Kit-expressing cells may induce hyper-contractility of the tunica muscularis in vitro, whereas a decrease in Kit expression within the muscle wall causes dysmotility-like symptoms in vivo. The pacemaker system in the gut thus seems to play a critical role in the maintenance of both moderate and normal motility of the digestive tract. A loss of Kit positive cells has been detected in several diseases with an impaired motor activity, including diabetic gastroenteropathy. Pathogenesis of these diseases is thought to be accounted for by impaired slow waves and neuromuscular transmissions; a pacemaker disorder may possibly induce a dysmotility-like symptom called 'gastroenteric arrhythmia'. A knowledge of the structure and function of the ICC and the pacemaker system provides a basis for clarifying the normal mechanism and the pathophysiology of motility in the digestive tract.