Feeding CPG in Aplysia Directly Controls Two Distinct Outputs of a Compartmentalized Interneuron That Functions as a CPG Element

2007 ◽  
Vol 98 (6) ◽  
pp. 3796-3801 ◽  
Author(s):  
Kosei Sasaki ◽  
Michael R. Due ◽  
Jian Jing ◽  
Klaudiusz R. Weiss
Keyword(s):  
Action Potentials ◽  
Electrical Coupling ◽  
Motor Program ◽  
Pattern Generator ◽  
Synaptic Connections ◽  
Buccal Ganglion ◽  
Full Size ◽  
Program Generation ◽  
Functional Aspects ◽  
Protraction Phase

In the context of motor program generation in Aplysia, we characterize several functional aspects of intraneuronal compartmentalization in an interganglionic interneuron, CBI-5/6. CBI-5/6 was shown previously to have a cerebral compartment (CC) that includes a soma that does not generate full-size action potentials and a buccal compartment (BC) that does. We find that the synaptic connections made by the BC of CBI-5/6 in the buccal ganglion counter the activity of protraction-phase neurons and reinforce the activity of retraction-phase neurons. In buccal motor programs, the BC of CBI-5/6 fires phasically, and its premature activation can phase advance protraction termination and retraction initiation. Thus the BC of CBI-5/6 can act as an element of the central pattern generator (CPG). During protraction, the CC of CBI-5/6 receives direct excitatory inputs from the CPG elements, B34 and B63, and during retraction, it receives antidromically propagating action potentials that originate in the BC of CBI-5/6. Consequently, in its CC, CBI-5/6 receives depolarizing inputs during both protraction and retraction, and these depolarizations can be transmitted via electrical coupling to other neurons. In contrast, in its BC, CBI-5/6 uses spike-dependent synaptic transmission. Thus the CPG directly and differentially controls the program phases in which the two compartments of CBI-5/6 may transmit information to its targets.

Simulator for neural networks and action potentials: description and application

1994 ◽  
Vol 71 (1) ◽  
pp. 294-308 ◽  
Author(s):  
I. Ziv ◽  
D. A. Baxter ◽  
J. H. Byrne
Keyword(s):  
Neural Networks ◽  
Central Pattern Generator ◽  
Action Potentials ◽  
Modular Design ◽  
Second Messenger ◽  
Pattern Generator ◽  
Slow Inward Current ◽  
Synaptic Connections ◽  
Different Types ◽  
Voltage Dependent

1. We describe a simulator for neural networks and action potentials (SNNAP) that can simulate up to 30 neurons, each with up to 30 voltage-dependent conductances, 30 electrical synapses, and 30 multicomponent chemical synapses. Voltage-dependent conductances are described by Hodgkin-Huxley type equations, and the contributions of time-dependent synaptic conductances are described by second-order differential equations. The program also incorporates equations for simulating different types of neural modulation and synaptic plasticity. 2. Parameters, initial conditions, and output options for SNNAP are passed to the program through a number of modular ASCII files. These modules can be modified by commonly available text editors that use a conventional (i.e., character based) interface or by an editor incorporated into SNNAP that uses a graphical interface. The modular design facilitates the incorporation of existing modules into new simulations. Thus libraries can be developed of files describing distinctive cell types and files describing distinctive neural networks. 3. Several different types of neurons with distinct biophysical properties and firing properties were simulated by incorporating different combinations of voltage-dependent Na+, Ca2+, and K+ channels as well as Ca(2+)-activated and Ca(2+)-inactivated channels. Simulated cells included those that respond to depolarization with tonic firing, adaptive firing, or plateau potentials as well as endogenous pacemaker and bursting cells. 4. Several types of simple neural networks were simulated that included feed-forward excitatory and inhibitory chemical synaptic connections, a network of electrically coupled cells, and a network with feedback chemical synaptic connections that simulated rhythmic neural activity. In addition, with the use of the equations describing electrical coupling, current flow in a branched neuron with 18 compartments was simulated. 5. Enhancement of excitability and enhancement of transmitter release, produced by modulatory transmitters, were simulated by second-messenger-induced modulation of K+ currents. A depletion model for synaptic depression was also simulated. 6. We also attempted to simulate the features of a more complicated central pattern generator, inspired by the properties of neurons in the buccal ganglia of Aplysia. Dynamic changes in the activity of this central pattern generator were produced by a second-messenger-induced modulation of a slow inward current in one of the neurons.

Intrinsic and Extrinsic Modulation of a Single Central Pattern Generating Circuit

2000 ◽  
Vol 84 (3) ◽  
pp. 1186-1193 ◽  
Author(s):  
Peter T. Morgan ◽  
Ray Perrins ◽  
Philip E. Lloyd ◽  
Klaudiusz R. Weiss
Keyword(s):  
Neural Circuits ◽  
Central Pattern Generators ◽  
Motor Program ◽  
Pattern Generator ◽  
Motor Programs ◽  
Appetitive Behavior ◽  
Protraction Phase ◽  
Pattern Generators ◽  
Rhythmic Behaviors ◽  
Appetitive Behaviors

Intrinsic and extrinsic neuromodulation are both thought to be responsible for the flexibility of the neural circuits (central pattern generators) that control rhythmic behaviors. Because the two forms of modulation have been studied in different circuits, it has been difficult to compare them directly. We find that the central pattern generator for biting in Aplysia is modulated both extrinsically and intrinsically. Both forms of modulation increase the frequency of motor programs and shorten the duration of the protraction phase. Extrinsic modulation is mediated by the serotonergic metacerebral cell (MCC) neurons and is mimicked by application of serotonin. Intrinsic modulation is mediated by the cerebral peptide-2 (CP-2) containing CBI-2 interneurons and is mimicked by application of CP-2. Since the effects of CBI-2 and CP-2 occlude each other, the modulatory actions of CBI-2 may be mediated by CP-2 release. Although the effects of intrinsic and extrinsic modulation are similar, the neurons that mediate them are active predominantly at different times, suggesting a specialized role for each system. Metacerebral cell (MCC) activity predominates in the preparatory (appetitive) phase and thus precedes the activation of CBI-2 and biting motor programs. Once the CBI-2s are activated and the biting motor program is initiated, MCC activity declines precipitously. Hence extrinsic modulation prefacilitates biting, whereas intrinsic modulation occurs during biting. Since biting inhibits appetitive behavior, intrinsic modulation cannot be used to prefacilitate biting in the appetitive phase. Thus the sequential use of extrinsic and intrinsic modulation may provide a means for premodulation of biting without the concomitant disruption of appetitive behaviors.

Glutamatergic N2v Cells Are Central Pattern Generator Interneurons of the Lymnaea Feeding System: New Model for Rhythm Generation

1997 ◽  
Vol 78 (6) ◽  
pp. 3396-3407 ◽  
Author(s):  
M. J. Brierley ◽  
M. S. Yeoman ◽  
P. R. Benjamin
Keyword(s):  
Central Pattern Generator ◽  
Synaptic Connection ◽  
Giant Cells ◽  
Pattern Generator ◽  
Synaptic Connections ◽  
Rhythm Generation ◽  
Feeding System ◽  
New Model ◽  
Feeding Cycle ◽  
Protraction Phase

Brierley, M. J., M. S. Yeoman, and P. R. Benjamin. Glutamatergic N2v cells are central pattern generator interneurons of the Lymnaea feeding system: new model for rhythm generation. J. Neurophysiol. 78: 3396–3407, 1997. We aimed to show that the paired N2v (N2 ventral) plateauing cells of the buccal ganglia are important central pattern generator (CPG) interneurons of the Lymnaea feeding system. N2v plateauing is phase-locked to the rest of the CPG network in a slow oscillator (SO)-driven fictive feeding rhythm. The phase of the rhythm is reset by artificially evoked N2v bursts, a characteristic of CPG neurons. N2v cells have extensive input and output synaptic connections with the rest of the CPG network and the modulatory SO cell and cerebral giant cells (CGCs). Synaptic input from the protraction phase interneurons N1M (excitatory), N1L (inhibitory), and SO (inhibitory-excitatory) are likely to contribute to a ramp-shaped prepotential that triggers the N2v plateau. The prepotential has a highly complex waveform due to progressive changes in the amplitude of the component synaptic potentials. Most significant is the facilitation of the excitatory component of the SO → N2v monosynaptic connection. None of the other CPG interneurons has the appropriate input synaptic connections to terminate the N2v plateaus. The modulatory function of acetylcholine (ACh), the transmitter of the SO and N1M/N1Ls, was examined. Focal application of ACh (50-ms pulses) onto the N2v cells reproduced the SO → N2v biphasic synaptic response but also induced long-term plateauing (20–60 s). N2d cells show no endogenous ability to plateau, but this can be induced by focal applications of ACh. The N2v cells inhibit the N3 tonic (N3t) but not the N3 phasic (N3p) CPG interneurons. The N2v → N3t inhibitory synaptic connection is important in timing N3t activity. The N3t cells recover from this inhibition and fire during the swallow phase of the feeding pattern. Feedback N2v inhibition to the SO, N1L protraction phase interneurons prevents them firing during the retraction phase of the feeding cycle. The N2v → N1M synaptic connection was weak and only found in 50% of preparations. A weak N2v → CGC inhibitory connection prevents the CGCs firing during the rasp (N2) phase of the feeding cycle. These data allowed a new model for the Lymnaea feeding CPG to be proposed. This emphasizes that each of the six types of CPG interneuron has a unique set of synaptic connections, all of which contribute to the generation of a full CPG pattern.

Different Roles of Neurons B63 and B34 That Are Active During the Protraction Phase of Buccal Motor Programs in Aplysia californica

1997 ◽  
Vol 78 (3) ◽  
pp. 1305-1319 ◽  
Author(s):  
Itay Hurwitz ◽  
Irving Kupfermann ◽  
Abraham J. Susswein
Keyword(s):  
Motor Neurons ◽  
Aplysia Californica ◽  
Motor Program ◽  
Excitatory Postsynaptic Potential ◽  
Strongly Correlated ◽  
Excitatory Effect ◽  
Buccal Ganglion ◽  
Motor Programs ◽  
Pattern Characteristic ◽  
Protraction Phase

Hurwitz, Itay, Irving Kupfermann, and Abraham J. Susswein. Different roles of neurons B63 and B34 that are active during the protraction phase of buccal motor programs in Aplysia californica. J. Neurophysiol. 78: 1305–1319, 1997. The buccal ganglion of Aplysia contains a central pattern generator (CPG) that organizes sequences of radula protraction and retraction during food ingestion and egestion. Neurons B63 and B34 have access to, or are elements of, the CPG. Both neurons are depolarized along with B31/B32 during the protraction phase of buccal motor programs. Both cells excite the contralateral B31/B32 neurons and inhibit B64 and other neurons active during the retraction phase. B63 and B34 also both have an axon exiting the buccal ganglia via the contralateral cerebrobuccal connective. Despite their similarities, B63 and B34 differ in a number of properties, which reflects their different functions. B63 fires during both ingestion and egestion-like buccal motor programs, whereas B34 fires only during egestion-like programs. The bilateral B63 neurons, along with the bilateral B31 and B32 neurons, act as a single functional unit. Sufficient depolarization of any of these neurons activates them all and initiates a buccal motor program. B63 is electrically coupled to both the ipsilateral and the contralateral B31/B32 neurons but monosynaptically excites the contralateral neurons with a mixed electrical and chemical excitatory postsynaptic potential (EPSP). Positive feedback caused by electrical and chemical EPSPs between B63 and B31/B32 contributes to the sustained depolarization in B31/B32 and the firing of B63 during the protraction phase of a buccal motor program. B34 is excited during the protraction phase of all buccal motor programs, but, unlike B63, it does not always reach firing threshold. The neuron fires in response to current injection only after it is depolarized for 1–2 s or after preceding buccal motor programs in which it is depolarized. Firing of B34 produces facilitating EPSPs in the contralateral B31/B32 and B63 neurons and can initiate a buccal motor program. Firing in B34 is strongly correlated with firing in the B61/B62 motor neurons, which innervate the muscle (I2) responsible for much of protraction. B34 monosynaptically excites these motor neurons. B34 firing is also correlated with firing in motor neuron B8 during the protraction phase of a buccal motor program. B8 innervates the I4 radula closer muscle, which in egestion movements is active during protraction and in ingestion movements is active during retraction. B34 has a mixed, but predominantly excitatory, effect on B8 via a slow conductance-decrease EPSP. Thus firing in B34 leads to amplification of radula protraction that is coupled with radula closing, a pattern characteristic of egestion.

scholarly journals Phase-Locked Coordination Between Two Rhythmically Active Feeding Structures in the Mollusk Clione limacina. I. Motor Neurons

2002 ◽  
Vol 87 (6) ◽  
pp. 2996-3005 ◽  
Author(s):  
Aleksey Y. Malyshev ◽  
Tigran P. Norekian
Keyword(s):  
Central Pattern Generator ◽  
Motor Neurons ◽  
Electrical Coupling ◽  
Pattern Generator ◽  
Dependent Manner ◽  
Synaptic Connections ◽  
Buccal Ganglia ◽  
Active Feeding ◽  
Clione Limacina ◽  
Specific Phase

Coordination between different motor centers is essential for the orderly production of all complex behaviors, in both vertebrates and invertebrates. The current study revealed that rhythmic activities of two feeding structures of the pteropod mollusk Clione limacina, radula and hooks, which are used to extract the prey from its shell, are highly coordinated in a phase-dependent manner. Hook protraction always coincided with radula retraction, while hook retraction coincided with radula protraction. Thus hooks and radula were always moving in the opposite phases, taking turns grabbing and pulling the prey tissue out of the shell. Identified buccal ganglia motor neurons controlling radula and hooks protraction and retraction were rhythmically active in the same phase-dependent manner. Hook protractor motor neurons were active in the same phase with radula retractor motor neurons, while hook retractor motor neurons burst in phase with radula protractor motor neurons. One of the main mechanisms underlying the phase-locked coordination was electrical coupling between hook protractor and radula retractor motor neurons. In addition, reciprocal inhibitory synaptic connections were found between hook protractor and radula protractor motor neurons. These electrical and inhibitory synaptic connections ensure that rhythmically active hooks and radula controlling motor neurons are coordinated in the specific phase-dependent manner described above. The possible existence of a single multifunctional central pattern generator for both radula and hook motor centers is discussed.

B64, a newly identified central pattern generator element producing a phase switch from protraction to retraction in buccal motor programs of Aplysia californica

1996 ◽  
Vol 75 (4) ◽  
pp. 1327-1344 ◽  
Author(s):  
I. Hurwitz ◽  
A. J. Susswein
Keyword(s):  
Phase Shift ◽  
Central Pattern Generator ◽  
Motor Program ◽  
Pattern Generator ◽  
Synaptic Activity ◽  
Excitatory Postsynaptic Potential ◽  
Motor Programs ◽  
Identified Neuron ◽  
Protraction Phase ◽  
Two Phases

1. Buccal motor programs in Aplysia are characterized by two phases of activity, which represent protraction and retraction of the radula in intact animals. The shift from protraction to retraction is caused by synaptic activity inhibiting neurons that are active during protraction and exciting neurons that are active during retraction. 2. B64, a newly identified neuron present bilaterally in the buccal ganglia, is partially responsible for the phase shift. Stimulating a single B64 causes bilateral inhibition of neurons B31/B32 and other neurons active during protraction and cause bilateral excitation of neurons B4/B5 and other neurons active during retraction. B64 is active throughout retraction. The amplitude and waveforms of the synaptic potentials caused by firing B64 are similar, but not identical, to those seen during retraction. 3. Some of the effects of B64 on B31/B32 and on B4/B5 are monosynaptic, as shown by their maintained presence in high divalent cation seawater, which blocks polysynaptic activity. 4. A brief depolarization of B64 leads to a long-lasting depolarization and firing. The ability of B64 to respond in this way is at least partially caused by an endogenous plateau potential, as this property is still seen after synaptic transmission is blocked. 5. Hyperpolarization of B64 bilaterally and preventing the somata from firing unmasks a large excitatory postsynaptic potential in B64. This procedure does not block the shift from protraction to retraction, indicating that spiking in the B64 somata is not necessary for the phase shift. 6. The firing pattern and synaptic connections of B64 are consistent with the hypothesis that the neuron is part of a central pattern generator underlying buccal motor programs. B64 is monosynaptically inhibited by neurons that are active along with B31/B32, which are responsible for producing the protraction phase of a buccal motor program. During the later portion of the protraction phase B64 is excited. In addition, firing B64 can phase advance and phase delay buccal motor programs. 7. Regulating the firing of B64 can regulate the expression of buccal motor programs. Stimulation of B64 at frequencies of 0.5-1.0 Hz leads to complete inhibition of buccal motor programs, whereas steady-state depolarization of B64 can lead to repetitive bursts of activity.

scholarly journals Interneuronal and Peptidergic Control of Motor Pattern Switching in Aplysia

2002 ◽  
Vol 87 (1) ◽  
pp. 49-61 ◽  
Author(s):  
Peter T. Morgan ◽  
Jian Jing ◽  
Ferdinand S. Vilim ◽  
Klaudiusz R. Weiss
Keyword(s):  
Motor Pattern ◽  
Electrical Coupling ◽  
Motor Program ◽  
Pattern Generator ◽  
Higher Order ◽  
Pattern Selection ◽  
Motor Programs ◽  
Additional Support ◽  
Selection Of ◽  
Stimulation Of

It has been proposed that a choice of specific behaviors can be mediated either by activation of behavior-specific higher order neurons or by distinct combinations of such neurons in different behaviors. We examined the role that two higher order neurons, CBI-2 and CBI-3, play in the selection of motor programs that correspond to ingestion and egestion, two stimulus-dependent behaviors that are generated by a single central pattern generator (CPG) of Aplysia. We found that CBI-2 could evoke either ingestive, egestive, or ambiguous motor programs depending on the regime of stimulation. When CBI-2 recruited CBI-3 firing via electrical coupling, the motor program tended to be ingestive. In the absence of CBI-3 activation, the program was usually egestive. When CBI-2 was stimulated to produce ingestive programs, hyperpolarization of CBI-3 converted the programs to egestive or ambiguous. When CBI-2 was stimulated to produce egestive or ambiguous programs, co-stimulation of CBI-3 converted them into ingestive. These findings are consistent with the idea that combinatorial commands are responsible for the choice of specific behaviors. Additional support for this view comes from the observations that appropriate stimulus conditions exist both for activation of CBI-2 together with CBI-3, and for activation of CBI-2 without a concomitant activation of CBI-3. The ability of CBI-3 to convert egestive and ambiguous programs into ingestive ones was mimicked by application of APGWamide, a neuropeptide that we have detected in CBI-3 by immunostaining. Thus combinatorial actions of higher order neurons that underlie pattern selection may involve the use of modulators released by specific higher order neurons.

Control of feeding motor output by paracerebral neurons in brain of Pleurobranchaea californica

1982 ◽  
Vol 47 (5) ◽  
pp. 885-908 ◽  
Author(s):  
R. Gillette ◽  
M. P. Kovac ◽  
W. J. Davis
Keyword(s):  
Feeding Behavior ◽  
Action Potentials ◽  
Tactile Stimulation ◽  
Natural Food ◽  
Buccal Ganglion ◽  
Pleurobranchaea Californica ◽  
Feeding Rhythm ◽  
Intracellular Stimulation ◽  
Motor Rhythm ◽  
Stimulation Of

1. A population of interneurons that control feeding behavior in the mollusk Pleurobranchaea has been analyzed by dye injection and intracellular stimulation/recording in whole animals and reduced preparations. The population consists of 12-16 somata distributed in two bilaterally symmetrical groups on the anterior edge of the cerebropleural ganglion (brain). On the basis of their position adjacent to the cerebral lobes, these cells have been named paracerebral neurons (PCNs). This study concerns pme subset pf [MCs. the large, phasic ones, which have the strongest effect on the feeding rhythm (21). 2. Each PCN sends a descending axon via the ipsilateral cerebrobuccal connective to the buccal ganglion. Axon branches have not been detected in other brain or buccal nerves and hence the PCNs appear to be interneurons. 3. In whole-animal preparations, tonic intracellular depolarization of the PNCs causes them to discharge cyclic bursts of action potentials interrupted by a characteristic hyperpolarization. In all specimens that exhibit feeding behavior, the interburst hyperpolarization is invariably accompanied by radula closure and the beginning of proboscis retraction (the "bite"). No other behavorial effect of PCN stimulation has been observed. 4. In whole-animal preparations, the PCNs are excited by food and tactile stimulation of the oral veil, rhinophores, and tentacles. When such stimuli induce feeding the PCNs discharge in the same bursting pattern seen during tonic PCN depolarization, with the cyclic interburst hyperpolarization phase locked to the bit. When specimens egest an unpalatable object by cyclic buccal movements, however, the PCNs are silent. The PCNs therefore exhibit properties expected of behaviorally specific "command" neurons for feeding. 5. Silencing one or two PCNs by hyperpolarization may weaken but does not prevent feeding induced by natural food stimuli. Single PCNs therefore can be sufficient but are not necessary to induction of feeding behavior. Instead the PCNs presumably operate as a population to control feeding. 6. In isolated nervous system preparations tonic extracellular stimulation of the stomatogastric nerve of the buccal ganglion elicits a cyclic motor rhythm that is similar in general features to the PNC-induced motor rhythm. Bursts of PCN action potentials intercalated at the normal phase position in this cycle intensify the buccal rhythm. Bursts of PCN impulses intercalated at abnormal phase positions reset the buccal rhythm. The PCNs, therefore, also exhibit properties expected of pattern-generator elements and/or coordinating neurons for the buccal rhythm. 7. The PCNs are recruited into activity when the buccal motor rhythm is elicited by stomatogastric nerve stimulation or stimulation of the reidentifiable ventral white cell. The functional synergy between the PCNs and the buccal rhythm is therefore reciprocal. 8...

scholarly journals Release of a single neurotransmitter from an identified interneuron coherently affects motor output on multiple time scales

2013 ◽  
Vol 109 (9) ◽  
pp. 2327-2334 ◽  
Author(s):  
Andrew M. Dacks ◽  
Klaudiusz R. Weiss
Keyword(s):  
Time Scales ◽  
Motor Program ◽  
Gaba Release ◽  
Motor Output ◽  
Multiple Time Scales ◽  
Multiple Time ◽  
Intrinsic Properties ◽  
Buccal Ganglion ◽  
Motor Programs ◽  
Pattern Characteristic

Neurotransmitters can have diverse effects that occur over multiple time scales often making the consequences of neurotransmission difficult to predict. To explore the consequences of this diversity, we used the buccal ganglion of Aplysia to examine the effects of GABA release by a single interneuron, B40, on the intrinsic properties and motor output of the radula closure neuron B8. B40 induces a picrotoxin-sensitive fast IPSP lasting milliseconds in B8 and a slow EPSP lasting seconds. We found that the excitatory effects of this slow EPSP are also mediated by GABA. Together, these two GABAergic actions structure B8 firing in a pattern characteristic of ingestive programs. Furthermore, we found that repeated B40 stimulation induces a persistent increase in B8 excitability that was occluded in the presence of the GABA B receptor agonist baclofen, suggesting that GABA affects B8 excitability over multiple time scales. The phasing of B8 activity during the feeding motor programs determines the nature of the behavior elicited during that motor program. The persistent increase in B8 excitability induced by B40 biased the activity of B8 during feeding motor programs causing the motor programs to become more ingestive in nature. Thus, a single transmitter released from a single interneuron can have consequences for motor output that are expressed over multiple time scales. Importantly, despite the differences in their signs and temporal characteristics, the three actions of B40 are coherent in that they promote B8 firing patterns that are characteristic of ingestive motor outputs.

scholarly journals Propagation of Action Potentials and the Structure of the Nexus in Cardiac Muscle

1965 ◽  
Vol 48 (5) ◽  
pp. 797-823 ◽  
Author(s):  
L. Barr ◽  
M. M. Dewey ◽  
W. Berger
Keyword(s):  
Guinea Pig ◽  
Cardiac Muscle ◽  
Action Potentials ◽  
Structural Changes ◽  
Sucrose Solution ◽  
Electrical Coupling ◽  
Intercalated Discs ◽  
Sucrose Gap ◽  
Specialized Structure ◽  
Frog Atrium

The hypothesis that the nexus is a specialized structure allowing current flow between cell interiors is corroborated by concomitant structural changes of the nexus and changes of electrical coupling between cells due to soaking in solutions of abnormal tonicity. Fusiform frog atrial fibers are interconnected by nexuses. The nexuses, desmosomes, and regions of myofibrillar attachment of this muscle are not associated in a manner similar to intercalated discs of guinea pig cardiac muscle. Indeed, nexuses occur wherever cell membranes are closely apposed. Action potentials of frog atrial bundles detected extracellularly across a sucrose gap change from monophasic to diphasic when the gap is shunted by a resistor. This indicates that action potentials are transmitted across the gap when sufficient excitatory current is allowed to flow across the gap. When the sucrose solution in the gap is made hypertonic, propagation past the gap is blocked and the resistance between the cells in the gap increases. Electron micrographs demonstrate that the nexuses of frog atrium and guinea pig ventricle are ruptured by hypertonic solutions.

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