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W B Kristan

Publications and source records attributed to W B Kristan.

At least 19 recordsLinked to original sources
PubMed1996

Mapping motor neuron activity to overt behavior in the leech. I. Passive biomechanical properties of the body wall.

As an initial step in constructing a quantitative biomechanical model of the medicinal leech (Hirudo medicinalis), we determined the passive properties of its body wall over the physiological range of dimensions. The major results of this study were: 1. The ellipsoidal cross section of resting leeches is maintained by tonic muscle activation as well as forces inherent in the structure of the body wall (i.e., residual stress). 2. The forces required for longitudinal and circumferential stretch to maximum physiological dimensions were similar in magnitude. Cutting out pieces of body wall did not affect the passive longitudinal or circumferential properties of body wall away from the edges of the cut. 3. The strain (i.e., the percentage change in dimension of different body segments when subject to the same force was identical, despite differences in muscle cross-sections. 4. Serotonin, a known modulator of tension in leech muscles, affected passive forces at all physiological muscle lengths. This suggests that the longitudinal muscle is responsible for at least part of the passive tension of the body wall. 5. We propose a simple viscoelastic model of the body wall. This model captures the dynamics of the passive responses of the leech body wall to imposed step changes in length. Using steady-state passive tensions predicted by the viscoelastic model we estimate the forces required to maintain the leech at any given length over the physiological range.

Animals↗
PubMed1996

An increase in activity of serotonergic Retzius neurones may not be necessary for the consummatory phase of feeding in the leech Hirudo medicinalis.

During the consummatory phase of feeding, in which blood is ingested, medicinal leeches display a characteristic set of behaviours: they extend their jaws, are less responsive to sensory input, produce mucus, relax the body wall and exhibit waves of peristalsis that can run the length of the body. Earlier reports suggested that this pattern of behaviour is orchestrated by serotonin released from Retzius cells in response to the appropriate sensory stimulation of the lip. We have developed a semi-intact preparation in which only the nervous system in the posterior half of the leech was exposed. The front half of the leech was free to explore, bite through and feed until satiated from a blood-filled sausage casing while continuous intracellular and extracellular recordings were made from identified cells and the nerve roots of the exposed segments. Prior to attachment of the animal to the feeding device, the firing frequency of the Retzius cell increased transiently during spontaneous movements or tactile stimuli to its front or posterior end. In contrast, Retzius cell activity decreased after the anterior sucker attached to the membrane of the feeding device at about the time when ingestion was initiated. Increased activity of Leydig cells, which are known to modulate several circuits in the leech, was also associated with exploration. However, unlike that of Retzius cells, the activity of Leydig cells increased significantly following the onset of consumption. These results suggest that increased activity of Retzius cells in midbody ganglia is not a prerequisite for the consummatory phase of feeding and raises questions regarding the role of serotonin in regulating this behaviour.

Animals↗
PubMed1996

Mapping motor neurone activity to overt behaviour in the leech: internal pressures produced during locomotion.

Several behaviour patterns have been studied in the leech at both the kinematic and neuronal levels. However, very little is known about how patterns of motor neurone activity map to actual movements. Internal pressure is an essential biomechanical property in this process, being responsible for producing the rigidity and posture that allow the directed delivery of forces produced by muscle contraction. To obtain a better understanding of the biomechanical processes involved in movement of the leech, we have measured the internal pressure of the animal by placing catheters through the body wall and into the gut of intact animals showing normal patterns of behaviour. Each type of behaviour had a characteristic pressure waveform. The elongation phase of crawling produced a rapid increase in pressure that peaked when midbody segments were maximally elongated. The pressure produced during the contraction phase of crawling depended on the type of crawl, only inchworm crawling producing a second peak. Whole-body shortening in response to a head poke also produced a pressure peak, but it had a faster rise time. Swimming produced the largest pressure, which was marked by a large sustained increase that fluctuated phasically with undulations of the body. Dual pressure recordings using two catheters demonstrated that pressure was not uniform along the length of the leech, indicating that the body cavity is functionally compartmentalised. Injecting fluid into the gut via a recording catheter allowed us to determine the effects of increasing internal volume on pressure. In line with previous predictions made using an abstract biomechanical model of the leech hydroskeleton, we found that an increase in the volume caused a reduction in the pressure. We are in the process of constructing a more realistic biomechanical model of the leech, based on actual data reported elsewhere. The results in this paper will provide key tests for refining these models.

Animals↗
PubMed1995

Using reflexive behaviors of the medicinal leech to study information processing.

The interneuronal network that produces local bending in the leech is distributed, in the sense that most of the interneurons involved are activated in all forms of local bending, even those in which their outputs would produce inappropriate movements. Such networks have been found to control a number of different behaviors in a variety of animals. This article reviews three issues: the physiological and modeling observations that led to the conclusion that local bending in leeches is controlled by a distributed system; what distributed processing means for this and other behaviors; and why the leech interneuronal network may have evolved to be distributed in the first place.

Animals↗
PubMed1995

The whole-body shortening reflex of the medicinal leech: motor pattern, sensory basis, and interneuronal pathways.

The leech whole-body shortening reflex consist of a rapid contraction of the body elicited by a mechanical stimulus to the anterior of the animal. We used a variety of reduced preparations - semi-intact, body wall, and isolated nerve cord - to begin to elucidate the neural basis of this reflex in the medicinal leech Hirudo medicinalis. The motor pattern of the reflex involved an activation of excitatory motor neurons innervating dorsal and ventral longitudinal muscles (dorsal excitors and ventral excitors respectively), as well as the L cell, a motor neuron innervating both dorsal and ventral longitudinal muscles. The sensory input for the reflex was provided primarily by the T (touch) and P (pressure) types of identified mechanosensory neuron. The S cell network, a set of electrically-coupled interneurons which makes up a 'fast conducting pathway' in the leech nerve cord, was active during shortening and accounted for the shortest-latency excitation of the L cells. Other, parallel, interneuronal pathways contributed to shortening as well. The whole-body shortening reflex was shown to be distinct from the previously described local shortening behavior of the leech in its sensory threshold, motor pattern, and (at least partially) in its interneuronal basis.

Animals↗
PubMed1995

Parallel pathways coordinate crawling in the medicinal leech, Hirudo medicinalis.

Changes in the behavior of crawling leeches were investigated after various kinds of manipulations, including selective transection or inactivation of body parts, as well as partial or complete transection of the central nerve cord, using a frame-by-frame analysis of video tapes of the crawling animals. From these studies, we found that: 1. Leeches made rhythmic crawling cycles even after their suckers were prevented from contacting the substrate by covering them over with glue. Hence, engagement and disengagement of the suckers are not necessary links in the crawling cycle. 2. Cutting the small, medial connective (Faivre's nerve) had no influence on crawling, but contraction during the whole-body shortening reflex was interrupted. Thus two behaviors which use the same motor output (i.e., whole-body shortening and the contraction phase of crawling) are mediated by two different pathways. 3. Cutting all the connectives between two ganglia in the middle of the leech resulted in a loss of coordination between the parts of the animal on either side of the cut. Therefore, temporally coordinated sucker activity must be mediated through these connectives. 4. Pieces of leech bodies produced by complete transection produced rhythmic crawling cycles as long as the pieces included the head or tail plus 2-4 adjacent midbody segments. In all cases, the crawling movements progressed without delays as the movements reached the cut ends. Pieces of animals that included only midbody segments did not produce crawling movements. 5. These results can be explained by a model composed of intersegmental pathways for both elongation and contraction, circuits in the head and tail brains that switch between elongation and contraction, and both ascending and descending inhibitory influences that determine when the cycle switches from elongation to contraction and back again.

Animals↗
PubMed1995

Widespread mechanosensory activation of the serotonergic system of the medicinal leech.

The serotonergic system of the medicinal leech comprises a small number of iterated, identified neurons, of which the Retzius (Rz) neurons are major components. Activity in pressure mechanosensory (P) cells sufficient to elicit locomotory and defensive behaviors also excites Rz neurons. We characterized the interactions between P and Rz neurons within the ganglion and at different distances along the nerve cord. 2. Within a ganglion 1) P cells excited both Rz neurons, electrically close to the site of electrical coupling between the Rz neurons; 2) each of the four P cells had similar effects on the Rz neurons; and 3) homologous contralateral P cells shared interneuronal pathways. These data show that P cells provide nearly identical bilateral information onto Rz neurons. 3. Along the nerve cord 1) every P cell excited Rz neurons in ganglia anterior and posterior to the site of stimulation; 2) the signal was carried the entire length of the nerve cord along interneuronal pathways with similar overall (but regionally different) conduction velocities in the two directions; 3) the amplitude of the Rz responses was smaller as the distance to the activated P cell increased; 4) the rate of change of the amplitude along the cord was larger when the signal traveled from front-to-back than in the opposite direction. 4. These data shows that mechanosensory input from any segment could excite Rz neurons along the cord, in proportion to the intensity of the stimulus.

Animals↗
PubMed1994

Cell-cell interactions that modulate neuronal development in the leech.

Mitotic lineage has been found to determine the cellular identity of leech neurons (reviewed in Stent et al., 1992), Int. Rev. Neurobiol. 33:109-133. However, the details of the adult phenotype of many neurons in the central nervous system of the leech have been shown to be shaped by interactions either with other neurons or with non-neuronal tissues in the environment. Four effects of cell-cell interactions will be considered in this article: stimulation of mitosis that generates new neurons, modulation of cell death or axonal retraction, modification of neurotransmitter metabolism, and modification of other physiological properties. In all cases, the interactions that modify development are thought to occur at a location distant from the soma, requiring that signals be transmitted a significant distance from the site of interaction to the metabolic machinery in the soma.

Animals↗
PubMed1993

Segment-specific modulation of the electrophysiological activity of leech Retzius neurons by acetylcholine.

1. The acetylcholine responses of Retzius neurons were electrophysiologically and pharmacologically characterized in situ and in culture. Single-electrode voltage-clamp was used to record currents from leech Retzius neurons from standard segments [Rz(X)] and from reproductive segments [Rz(5,6)]. 2. A 1 s pressure pulse of acetylcholine (ACh) produced a fast inward current followed by a slower outward current in Rz(X) neurons, whereas it produced only an outward current in Rz(5,6) neurons. These segment-specific responses were maintained when the two types of Retzius neurons were isolated in culture for up to 12 days. 3. The inward current of Rz(X) reversed at around -25 mV and was partially carried by Na+. This cationic current desensitized rapidly. The outward current of Rz(X) and Rz(5,6) neurons reversed at around -65 mV and was carried by Cl-. This anionic current desensitized very slowly upon prolonged applications of ACh. 4. The expression of the ACh-induced outward current in Rz(X) was season-dependent and was recorded in a larger proportion of Rz(X) neurons during the summer than during the winter. The expression of the ACh-induced outward current in Rz(5,6) did not show any seasonal pattern. 5. The fast inward current of Rz(X) was also elicited by nicotine; it was blocked by d-tubocurarine, hexamethonium and mecamylamine, but was not affected by alpha-bungarotoxin. The outward current of Rz(X) and Rz(5,6) was also elicited by nicotine and by 4-[N-(3-chlorophenyl)carbamoxyloxy]2-butynyltrimethylammonium chloride (a muscarinic agonist); it was blocked by d-tubocurarine and by alpha-bungarotoxin, but it was not affected by hexamethonium or mecamylamine. 6. The results show that the serotonergic Retzius neurons of the leech could be tonically inhibited by ACh. In addition, the Retzius neurons from standard segments could also be phasically excited by ACh. The receptors responsible for the excitation fit into the classification of neuronal nicotinic receptors, whereas the receptors mediating the inhibition are closer in type to the muscular nicotinic receptor.

(4-(m-Chlorophenylcarbamoyloxy)-2-butynyl)trimethy↗
PubMed1993

Developmental regulation of segment-specific cholinergic receptors on Retzius neurons in the medicinal leech.

Retzius (Rz) neurons in the midbody ganglia of medicinal leeches responded to ACh, applied to their somata, in a manner that depended upon the neuron's segmental location: Rz neurons in ganglia from midbody segments 5 and 6 [Rz(5,6)] hyperpolarized, whereas Rz neurons from all other segments [Rz(X)] depolarized. Midbody segments 5 and 6 are notable because they contain the male and female reproductive organs. Both types of Rz neurons responded to ACh in a complex way, but the initial phase of each response appeared to be nicotinic because nicotinic agonists evoked the responses and nicotinic antagonists blocked them. The reversal potentials of the responses and the effects of changing the internal and external Cl- concentration indicated that the hyperpolarizing response of Rz(5,6) neurons depended upon Cl- whereas the depolarizing response of Rz(X) neurons did not. The segmentally characteristic responses of Rz neurons arose during embryonic development. Removing the reproductive ducts [the peripheral targets of Rz(5,6)] early in embryogenesis caused the Rz(5,6) neurons to depolarize in response to ACh rather than to hyperpolarize. This result indicates that development of the characteristic response of Rz neurons to ACh is strongly influenced by interactions between the neurons and their appropriate target tissues.

Acetylcholine↗
PubMed1992

Mesenchyme of embryonic reproductive ducts directs process outgrowth of Retzius neurons in the medicinal leech.

In the two segments of the medicinal leech (Hirudo medicinalis) that contain the male (segment 5) and the female (segment 6) reproductive ducts, the paired Retzius (Rz) neurons are distinguished by several unique properties. For example, the muscles and glands of the body wall are the primary peripheral targets of Rz neurons in standard segments [Rz(X)], whereas the muscles and glands of the reproductive ducts are the primary peripheral targets of Rz neurons in the two reproductive segments [Rz(5,6)]. In this paper, we show that organogenesis and differentiation, which generate an epithelial tube surrounded by mesenchymal cells, occur in the embryonic reproductive ducts at approximately the time when Rz processes first contact these structures. The growth cones leading one branch of the posterior axon of Rz(5,6) contact the duct mesenchymal cells. Following initiation of this contact, these posterior growth cones enlarge and send out numerous filopodia. Secondarily, growth cones leading the anterior axon of each Rz(5,6) also modify their shapes and trajectories. When embryonic reproductive ducts were transplanted into posterior (nonreproductive) segments, the branch of the posterior Rz axon near the ectopic reproductive tissue produced enlarged growth cones and extended several secondary branches into the mesenchyme of the ectopic tissue. This result suggests that the reproductive mesenchyme is attractive to, and can modify the growth of, all Rz neurons. The behavior of Rz(5,6) growth cones suggests that the reproductive mesenchyme cells provide guidance cues that control the location in which Rz axons elaborate their peripheral arborization and form synapses, and that the mesenchyme may also stimulate the production of a densely branched arbor.

Animals↗
PubMed1992

Monitoring neuronal activity during discrete behaviors: a crawling, swimming and shortening device for tethered leeches.

An apparatus is described which facilities continuous electrophysiological recordings in segmental ganglia of leeches while the animal performs different behavioral patterns: crawling, swimming or shortening. Both the behaviors and neuronal recordings are monitored by a video camera system and can be evaluated simultaneously. The device is well suited to investigate questions of behavioral choice and neuronal decision-making mechanisms in these animals. It can also be modified to study the neural control of behavior in other aquatic animals, such as snails.

Animals↗
PubMed1992

Target influences on the development of leech neurons.

A pair of Retzius neurons is found in each segmental midbody ganglion of the CNS of the leech Hirudo medicinalis. Although all Retzius neurons appear to have the same cell lineage and are indistinguishable from one another through the initial phases of axonogenesis, later in development two pairs of Retzius neurons--those in the segments containing the male and female reproductive ducts--acquire distinctive morphological and physiological characteristics. Experimental manipulations of the reproductive ducts in early embryos have indicated that the outgrowing Retzius axons receive a signal from these peripheral targets that triggers major changes in their developmental program. Such 'end-organ specification' has been shown to contribute to the differentiation of neurons in other nervous systems as well, and the mechanisms underlying such control can be investigated in great detail in the relatively simple, segmented nervous system of the leech.

Animals↗
PubMed1992

Neuronal basis of behavior.

In addition to describing behavior in terms of neuronal properties and interconnections, some studies are using these well defined neuronal circuits to see how the circuits interact, how they develop, and how they are modified by experience, hormones and neuromodulators. The ready availability of computers and computational techniques has helped in some efforts, as have improvements in physiological and morphological techniques. The major insights, however, still come from experiments that ask clear and direct questions. This review highlights some of the promising approaches and suggests some general features of how neuronal circuits produce behavior.

Animals↗
PubMed1992

Analysis and modeling of the multisegmental coordination of shortening behavior in the medicinal leech. I. Motor output pattern.

1. To understand how a multisegmental animal coordinates motor activity over more than one segment, we studied shortening behavior in the medicinal leech, in which several segments contract longitudinally in response to a moderately strong mechanical stimulus. 2. We first demonstrated that the neuronal activity responsible for shortening behavior occurred in semi-intact and isolated nerve cord preparations, and then characterized the responses of motor neurons in isolated preparations. The motor output during shortening was simultaneous excitation of motor neurons innervating dorsal longitudinal muscle and of motor neurons innervating ventral longitudinal muscle. 3. The stronger the stimulus, the more segments produced the shortening motor output, with the segments nearest the stimulus recruited first. 4. Although the shortening response was produced in several segments near the site of stimulation, it was never produced in the stimulated segment, where the local bending motor output pattern was produced. The motor pattern suggests that shortening, initially considered a very simple behavior, requires the involvement of at least few segmentally iterated interneurons.

Acetates↗
PubMed1992

Analysis and modeling of the multisegmental coordination of shortening behavior in the medicinal leech. II. Role of identified interneurons.

1. Mechanical stimulation of the leech, Hirudo medicinalis, elicits withdrawal behavior that has two components: local bending in the segment stimulated and shortening in outlying segments. Local bending is characterized by excitation of longitudinal muscle on one side of the segment and inhibition on the other side. In shortening, all longitudinal muscles are excited. We wished to understand how these distinct motor patterns are produced by a nervous system with segmentally iterated neurons, a configuration that places some limitations on the complexity of connection patterns. 2. We searched for neurons in the segmental nervous system that subserved shortening behavior, expecting to find at least one interneuron in each segment that was involved in shortening behavior exclusively. We found instead that all interneurons involved in shortening are also involved in local bending, and no individual interneuron can completely account for shortening. 3. The motor output caused by individual identified interneurons is not entirely consistent with the shortening motor output pattern. For instance, one interneuron, cell 115, has the same pattern of motor effects from segment to segment, causing excitation of dorsal excitatory motor neurons and inhibition of ventral excitatory motor neurons. These effects would cause dorsal local bending, not shortening, in a few segments. Only one interneuron, cell 125, has motor effects that would cause shortening. 4. Individual interneurons were hyperpolarized while single sensory cells were stimulated, to quantify the contributions of individual interneurons to the observed motor pattern. Interneurons 115 and 125, and the inhibitory motor neuron, cell 1, were found to have significant roles in producing the shortening motor output. 5. A quantitative estimate of the role of each interneuron type showed that the identified interneurons account for most of the excitation of dorsal motor neurons, but for very little of the excitation of ventral motor neurons. This predicts that at least one additional interneuron type remains to be identified, one that would provide excitation to ventral motor neurons in several segments. 6. A back-propagation trained neural network model was constructed to predict the connections of the as yet unidentified interneurons. To match the known properties of interneurons, it was necessary to include a segmental similarity constraint in the training algorithm for segmentally iterated model neurons. The modeled networks predicted that there are at least two kinds of interneurons yet to be found. Also, the modeling showed that interneurons can have input and output patterns that differ very little from segment to segment but yet produce major differences in the motor output.

Action Potentials↗
PubMed1991

Two forms of sensitization of the local bending reflex of the medicinal leech.

Sensitization of the local bending reflex of the medicinal leech Hirudo medicinalis was studied in a semi-intact preparation in which behavioral and electrophysiological recordings were made simultaneously. 1. Sensitization of local bending could be produced in two ways: by repeated stimulation of the mechanoreceptor sensitive to pressure (the P cell), and by stimulation of the mechanoreceptor sensitive to noxious stimuli (the N cell). 2. Both forms of sensitization produced a central neuronal change, measured as an increase in the number of stimulus-evoked action potentials in cell 3 (an excitor of dorsal longitudinal muscles). 3. Intracellular stimulation of serotonin-containing neurons 21 and 61 mimicked the sensitizing stimuli, but stimulation of the Retzius cell, which also contains serotonin, did not. 4. Stimulation of the Leydig cell, which releases octopamine, decreased the strength of local bending.

Action Potentials↗