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Biomedical subjects

G S Stent

Publications and source records attributed to G S Stent.

12 recordsLinked to original sources
PubMed1979

Neural control of heartbeat in the leech and in some other invertebrates.

The heartbeat of the leech Hirudo consists of the contractile rhythm of the circular muscles in the wall of a bilateral pair of celomic sinuses, the heart tubes, that run the length of the leech body. The constriction cycles of the segmental heart-tube sections are coordinated so that on one body side they constrict in a rear-to-front progression (peristalsis), while on the other side they constrict nearly in concert (nonperistalsis). Spontaneous right-left reciprocal transitions between peristaltic and nonperistaltic coordination modes occur every few dozen heartbeat cycles. The constriction of each segmental heart-tube section is controlled via excitatory synapses by a rhythmically active heart motor neuron, or HE cell, of which 17 bilateral pairs are iterated in segmental ganglia of the ventral nerve cord. The activity rhythm of the HE cell ensemble is in turn controlled via inhibitory synapses by a rhythmically active heart interneuron, the HN cell, of which seven bilateral pairs are iterated in the rostral segmental ganglia. The HN heart interneuron owes its activity rhythm to an endogenous polarization cycle, and the cycles of all members of the HN cell ensemble are locked into an appropriate phase relation thanks to their mutual interconnection via excitatory and inhibitory synaptic connections. The observed activity pattern and identified synaptic connections of HE cells and HN cells can account not only for the generation of the two bilaterally asymmetric heartbeat coordination modes but also for the right-left coordination mode transitions. In contrast to the heartbeat of Hirudo, the beat of the single-chambered heart of the lobsters Panulirus and Homarus is controlled by a set of nine rhythmically active neurons that make up the cardiac ganglion. Of these, five larger cells are heart motor neurons that innervate the heart muscle fibers via excitatory synapses. The remaining four smaller neurons of the cardiac ganglion are interneurons that provide excitatory input to each other and to the heart motor neurons. Although all the neurons of the cardiac ganglion appear capable of producing their own endogenous polarization rhythm, it is currently believed that one of the interneurons acts as a pacemaker for the whole ensemble of interneurons and motor neurons. The beat of the two-chambered heart of the marine snail Aplysia is generated by yet an entirely different mechanism. Here, the basic contractile rhythm of the heart is due to an endogenous polarization cycle of the heart muscle fibers. That myogenic rhythm is controlled and modulated by a set of cardiovascular motor neurons located in the abdominal ganglion, some of which make excitatory and others of which make inhibitory connections with the heart muscle fibers. The activity of these cardiovascular motor neurons is controlled by three types of heart interneurons via both inhibitory and excitatory connections. The interneurons are in turn interconnected in a manner that prevents the simultaneous activation of antagonistic cardiac motor acts...

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PubMed1978

Cell lineage analysis by intracellular injection of a tracer enzyme.

Cell lineages during development of leeches can be ascertained by injection of horseradish peroxidase as a tracer into identified cells at early stages of embryogenesis. The injected embryos continue their normal development, in the course of which horseradish peroxidase is passed on in catalytically active form to the descendants of the injected cell. The distribution of the tracer enzyme and hence of the progeny of the injected cell can then be observed at a later stage of development by staining the preparation for horseradish peroxidase.

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PubMed1978

Neuronal generation of the leech swimming movement.

The swimming movement of the leech is produced by an ensemble of bilaterally symmetric, rhythmically active pairs of motor neurons present in each segmental ganglion of the ventral nerve cord. These motor neurons innervate the longitudinal muscles in dorsal or ventral sectors of the segmental body wall. Their duty cycles are phase-locked in a manner such that the dorsal and ventral body wall sectors of any given segment undergo an antiphasic contractile rhythm and that the contractile rhythms of different segments form a rostrocaudal phase progression. This activity rhythm is imposed on the motor neurons by a central swim oscillator, of which four bilaterally symmetric pairs of interneurons present in each segmental ganglion appear to constitute the major component. These interneurons are linked intra- and intersegmentally via inhibitory connections to form a segmentally iterated and inter-segmentally concatenated cyclic neuronal network. The network appears to owe its oscillatory activity pattern to the mechanism of recurrent cyclic inhibition.

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PubMed1978

Neural circuits for generating rhythmic movements.

Inasmuch as the identified neural circuits discussed in this review pertain only to the nervous systems of two invertebrate species, one may ask whether or not these findings are generally applicable to central nervous oscillators that generate rhythmic movements in animals of other species and phyla, particularly in the vertebrates. This question is not easy to answer at this time, because detailed cellular network analyses thus far have been possible only in a very few neurophysiologically favorable preparations, such as those presented by the cardiac and stomatogastric ganglia of the lobster and the segmental ganglion of the leech. Nevertheless it is significant that the mechanisms according to which these invertebrate circuits are now thought to generate their oscillations--endogenous rhythmic polarization, reciprocal inhibition, and recurrent cyclic inhibition--were all first proposed to account for generation of rhythmic movements in vertebrate animals (7-9, 51, 71, 79). Moreover, the pattern of motor neuron activity in rhythmic movements of vertebrates is not necessarily more complex than the corresponding pattern in analogous movements of invertebrates. Therefore, the very much greater number of neurons in the central nervous system of vertebrates does not necessarily imply a greater complexity of the central oscillators that generate their rhythmic movements; it may only place greater obstacles in the way of identifying the underlying neuronal circuitry. In any case, it is worthy of note that the current list of fundamentally different and theoretically plausible types of neuronal oscillators is not only quite short but also of long standing. Thus, on these grounds, it seems reasonable to expect that the identified circuits discussed here will prove to be of general applicability to the generation of rhythmic movements in the whole animal kingdom.

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PubMed1978

Neuronal control of swimming in the medicinal leech. IV. Identification of a network of oscillatory interneurones.

Four oscillatory interneurones that appear to be the principal components of the central swim oscillator of Hirudo medicinalis have been identified on each side of the segmental ganglia of the ventral nerve cord. During 'swimming' episodes of an isolated nerve cord preparation each interneurone undergoes a polarization rhythm that is phase-locked with the impulse burst rhythm of the motor neurones known to drive the swimming movement. Passage of current into any of the interneurones can shift the phase of the swim rhythm. One of the interneurones projects its axon rearward to posterior ganglia and the other three project their axons frontward to anterior ganglia. The oscillatory interneurones are connected both intra- and interganglionically to form a topologically complex intersegmental network of concatenated ring circuits that possess the feature of recurrent cyclic inhibition. Theoretical analysis and electronic analogue models show that the network is inherently oscillatory and can produce both a cycle period and intra- and intersegmental phase relations of its elements that are appropriate for generating the body wave of the swimming movement.

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PubMed1978

Neuronal control of swimming in the medicinal leech. V. Connexions between the oscillatory interneurones and the motor neurones.

A network of intra- and intersegmental synaptic connexions has been identified in the ventral nerve cord of the leech that links the set of oscillatory interneurones of the central swim oscillator to the motor neurones commanding the swimming rhythm. Excitatory connexions lead from oscillatory interneurones to both excitatory and inhibitory motor neurones, whereas inhibitory connexions lead from oscillatory interneurones to only the inhibitory motor neurones. Connexions leading from a motor neurone back to the oscillatory interneurones were found in only one exceptional case, an inhibitory motor neurone previously known to have access to the central swim oscillator. This network of identified connexions can account reasonably well for the mechanism by which the oscillatory interneurones drive their follower motor neurones into the phasic activity pattern characteristic of the swimming movement.

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PubMed1976

An oscillatory neuronal circuit generating a locomotory rhythm.

A quartet of interconnected interneurons whose periodic activity appears to generate the traveling body wave of the swimming leech has been identified on each side of segmental ganglia of the ventral nerve cord of Hirudo medicinalis. Theoretical analysis and electronic analog models of the identified intra- and interganglionic synaptic connections of the segmentally iterated interneurons showed that they form an oscillatory network with cycle period and intra-and intersegmental phase relations appropriate for the swimming movement.

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