Biomedical subjects

A P Kramer

Publications and source records attributed to A P Kramer.

10 recordsLinked to original sources

PubMed1985

Developmental neural kinship groups in the leech.

We have traced the developmental origins of various CNS neurons and glial cells of a leech to 10 clonally related groups of cells, the bilaterally paired M, N, O, P, and Q kinship groups. Each kinship group is descended from one of 10 identifiable blastomeres of the early embryo, the teloblasts. Of the approximately 200 neurons in each side of a segmental ganglion, 130 to 160 are in the ipsilateral N, 20 to 50 in the O, 8 to 12 in the P, 6 to 9 in the Q, and 3 to 6 in the M kinship group. A given identified neuron or glial cell was invariably found to belong to a particular kinship group, indicating that in leech development neuronal lineage is highly stereotyped. But cells of related function and morphology do not necessarily belong to the same neuronal kinship group: of the mechanosensory neurons, the T and N neurons belong to the N, the Pv neuron belongs to the P and the PD neuron belongs to the O kinship group. Similarly, glial cells arise from all four ectodermal teloblasts. Conversely, neurons within a kinship group are not obviously related in structure or function: the N kinship group includes sensory, motor, and effector neurons and interneurons: the O and P kinship groups each include sensory neurons and interneurons; both the P and Q groups contain representatives of three distinct morphological classes of interneurons. Consequently, in early development, the determinants of neuronal identity in the leech CNS are not segregated in any obvious thematic way in the cleavages that give rise to the five bilateral pairs of teloblasts. Rather, the neural kinship groups may be merely the evolutionary vestige of a primordial distributed nervous system, each quadrant of which was derived from one teloblast.

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PubMed1985

Developmental arborization of sensory neurons in the leech Haementeria ghilianii. I. Origin of natural variations in the branching pattern.

The overall sizes, contours, and positions of the receptive fields maintained by different individual cells of the T, P, and N types of mechanosensory neurons in the segmental skin of the leech Haementeria ghilianii are not subject to wide variation. However, the locations and contours of the boundaries which separate the various compartments of the sensory field, namely, the major and minor fields, as well as their component subfields, do vary significantly. These variations are reflected in differences in the detailed pattern of arborization of the mechanosensory axon branches that innervate different parts of the receptive field. The appreciable variation in the kinetics of embryonic outgrowth of sensory axon branches, in conjunction with a mechanism of neuronal self-avoidance, is a probable source of this variability in adult receptive field structure. Thus, establishment of these sensory field components would seem to entail a first-come-first-served territorial exclusion between different axon branches extended by the same neuron.

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PubMed1985

Developmental arborization of sensory neurons in the leech Haementeria ghilianii. II. Experimentally induced variations in the branching pattern.

The sharp, nonoverlapping boundaries of the major and minor receptive fields of the mechanosensory neuron Pv of the leech, as well as the mutual exclusion during embryonic development of growing axon branches belonging to the same Pv cell, have suggested that peripheral axon arborization of these neurons is constrained by a process of neuronal self-avoidance. To provide a direct experimental test of this proposal, the development of the major and minor receptive fields of the Pv neuron was studied in embryos of the leech, Haementeria ghilianii, after surgically preventing or delaying the outgrowth of the axon branches which establish only a minor or only the major field of that neuron. As predicted by the proposal of self-avoidance, interference with the outgrowth of a minor field axon branch resulted in the spread of the major field axon branch into what is normally minor field territory. Conversely, similar interference with the establishment of the major field resulted in the spread of the minor field axon branches into what is normally major field territory. The findings presented here indicate that neuronal self-avoidance does play a significant role in the development of mechanosensory receptive field structure but suggest also that the detailed pattern of arborization of the sensory axons is guided by prespecified pathways of only ephemeral availability or recognizability.

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PubMed1984

Crayfish escape behavior: production of tailflips without giant fiber activity.

The giant interneurons of the crayfish nerve cord are well-known mediators of fast tail flexions, "tailflips," that propel animals through the water away from danger. More recent studies have revealed an additional nongiant generator of tailflips. In contrast to giant tailflips, which are stereotyped, nongiant tailflips have variable form. The operating principles and portions of the neural circuitry governing nongiant tailflips were here investigated. Whereas fast flexor motor neurons (FFs) receive excitatory postsynaptic potentials (EPSPs) with large unitary components just prior to giant tailflips, excitation of the FFs during nongiant tailflips is due to summation of many small EPSPs, and these build up for about 60 ms prior to the tailflip; we call the period of excitation prior to FF firing the preflexion phase and the period during which FFs fire, the flexion phase of the tailflip. Even FFs that will not fire during a given tailflip become depolarized during preflexion and flexion periods. Throughout the preflexion and flexion periods there is activity in dorsal nerve cord axons (DCAs) that lie below the giants. Many DCAs are interneurons that excite FFs at short latency. Some DCAs fire uniquely during the preflexion phase, while some fire only during the flexion phase. Which DCAs fire is highly variable, and in some cases firing of particular DCAs can be correlated with particular forms of tailflips. Two identified DCAs, 12 and 13, that fire during the flexion phase were studied. These interneurons originate and receive their synaptic input in the second and third abdominal ganglia, respectively, and project to the last ganglion exciting FFs caudal to their ganglion of origin en route. Their pattern of synaptic input prior to and during nongiant tailflips is indistinguishable from that of FFs. Their input to FFs is weak, but when they fire they tend to promote intersegmental synchrony of FFs in the segments they feed. It appears likely to us that nongiant tailflips are synthesized from a small library of component tailflip movements that can be combined to produce a variety of complete tailflips and that the component movements are produced by a limited group of premotor interneurons, of which 12 and 13 are members.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1983

Formation of the receptive fields of leech mechanosensory neurons during embryonic development.

We have charted the embryonic development of the epidermal receptive fields of the PV and PD mechanosensory neurons present in each segmental ganglion of the ventral nerve cord of the glossiphoniid leech Haementeria ghilianii. The receptive field of either of these neurons is established by several peripheral axons, each of which grows into its own skin territory, so that the receptive field is subdivided into a quiltwork of contiguous, mutually exclusive subfields of various sizes. The largest subfield develops first from the primary peripheral axon of the PD or PV neuron. The PD and PV primary axons grow directly to their respective territories and begin to innervate them at approximately the same time (see also Kuwada, J.Y., and A.P. Kramer (1983) J. Neurosci. 3: 2098-2111). The smaller secondary and minor subfields develop later from the secondary and intersegmental peripheral axons, respectively. These axons, too, grow directly to their appropriate skin territories. The arborizations of the peripheral axons expand until the adult receptive field pattern is established late in embryogenesis; they do not appear to initially overgrow and later trim down to the normal boundaries of the adult receptive field. The receptive field of the PV neuron develops from a stereotyped skeletal branching pattern of the peripheral axons that is elaborated in a regular way to form a grid-like pattern that matches the arrangement of muscle fibers in the body wall. Thus, this branching pattern may be the result of axon growth along prespecified pathways, perhaps delineated by the muscles. Throughout embryonic development, as in the adult leech, axon branches of neighboring homologous P neurons overlap considerably, but separate axon branches of the same neuron are virtually non-overlapping. These observations suggest that developing branches of a neuron are excluded from territory occupied by other branches of the same cell, in contrast to branches belonging to different neurons which can occupy the same territory.

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PubMed1983

Embryonic development of the leech nervous system: primary axon outgrowth of identified neurons.

This paper describes the embryonic development of the leech nervous system and focuses on the differentiation of two identified pressure sensory (P) neurons, the PD and PV neurons. In the adult leech the P neurons have distinctive cell body locations in the central nervous system (CNS), different peripheral axon branching patterns, and different receptive field territories in the skin. The embryonic P neurons also have distinct and reproducible locations in the CNS and have been studied with recording and dye-filled microelectrodes from the time the first growth cones are projected from their somata. The peripheral axons of the P neurons are among the earliest peripheral axons to develop and may play an important role in the formation of peripheral nerves. The first or primary peripheral axons of the P neurons grow directly to their separate target territories. The specificity of the P neurons for their targets is probably not due to temporal differences in the outgrowth of their primary axons. Instead, the PD neuron seems to exhibit a preference early in embryogenesis for the target of its primary axon despite an apparent opportunity to occupy the target of the PV primary axon. It is hypothesized that the primary peripheral axons of the P neurons are among the first axons projected from the CNS and follow environmental cues to reach and innervate their target territories.

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PubMed1982

Serotonin analog selectively ablates indentified neurons in the leech embryo.

Exposure of embryonic leeches to 5,7-dihydroxytryptamine a cytotoxic analog of the monoamine neurotransmitter serotonin, results in the selective ablation of serotonin-containing neurons in the ventral nerve cord. Other neurons appear to be unaffected by this treatment, including those that contain another monoamine neurotransmitter, dopamine. Embryos with ablations continue to develop into juvenile leeches, but as juveniles they are unable to make normal swimming movements. However, normal swimming movements can be instated in such leeches by injecting them with serotonin.

5,7-Dihydroxytryptamine↗

PubMed1982

Segmental giant: evidence for a driver neuron interposed between command and motor neurons in the crayfish escape system.

1. The giant command neurons for tailflip escape behavior in crayfish have been thought to excite the nongiant fast flexor (tailflip producing) motor neurons (FFs) via monosynaptic connections. We show here that excitation of FFs instead occurs via a bilateral pair of segmental giant neurons (SGs) interposed between the command axons and FFs in each segment. 2. Anatomically, the SGs appear to make numerous contacts with ipsilateral command axons and FFs and fewer contacts contralaterally. In contrast, the command axons have only sparse direct connections to the FFs. An SG has an axon in the ipsilateral first ganglionic root and may be a modified swimmeret motor neuron. 3. Each SG is depolarized well beyond threshold by the firing of an ipsilateral command axon and is depolarized to near threshold by the firing of a contralateral command axon. The synapses between command axons and SGs are electrical and probably rectifying. 4. Each FF is excited to a level near firing threshold by the SG ipsilateral to its axon and is excited weakly by the contralateral SG. The synapses between SGs and FFs are electrical and nonrectifying. 5. Variations in excitatory postsynaptic potentials (EPSPs) recorded in FFs during prolonged, high-frequency firing of the command axons can be accounted for by refractoriness of SG spikes, as opposed to refractoriness of dendritic branch spikes as had previously been delivered. 6. These findings illustrate the limitations of conventional tests for monosynapticity. 7. The functional significance of having driver neurons interposed between command neurons and motor neurons is discussed.

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PubMed1981

Different command neurons select different outputs from a shared premotor interneuron of crayfish tail-flip circuitry.

In the crayfish a bilateral pair of interneurons (the 13's) are involved in the generation of two types of tail-flip escape responses, one mediated by giant neurons and the other by nongiant circuitry. The 13's make a variety of output connections with the motoneurons and with other interneurons involved in tail flipping. The motoneuronal outputs include strong synapses on telson flexor motoneurons, whose activity during tail flips mediated by lateral giant fibers would be maladaptive. The lateral giants always drive the 13's, but also drive inhibitory neurons that prevent the undesirable outputs of the 13's while permitting their adaptive outputs to be expressed. It is often adaptive for tail flips initiated by nongiant circuitry to utilize the telson flexor muscles that 13 strongly excites. During such tail flips 13 is often fired, and this firing is important in driving the telson flexors.

Action Potentials↗

PubMed1981

Interneurons between giant axons and motoneurons in crayfish escape circuitry.

1. Crayfish giant fibers are generally believed to generate tailflip movements by means of direct connections to two classes of phasic flexor muscle motoneurons, the motor giants (MoGs) and the nongiant fast flexor motoneurons (FFs). It is shown here that the giants also stimulate a network of interneurons that make connections with the FFs. 2. This network includes an intraganglionic neuron, the segmental giant (SG), in each abdominal hemisegment and a number of intersegmental neurons, two of which (I2 and I3) were studied in detail. 3. The SGs are driven reliably by the giant fibers and they in turn drive the FFs of their hemisegment about as effectively as do the giant fibers themselves; it is possible that the giant fibers excite the FFs mainly by way of the SGs. The SGs also have an efferent first root axon whose peripheral targets we have been unable to determine. 4. I2 and I3 originate in the second and third abdominal ganglia, respectively, and descend to the last ganglion. In their ganglia of origin they are reliably driven by the giant fibers and by the SGs. In addition, I2 weakly excites I3 and both receive weak, apparently direct, excitatory input from FFs as well as less direct excitatory and inhibitory input from unidentified afferent sources. Both weakly excite most FFs in ganglia behind the one in which they originate. This excitation adds to that produced directly by giant fibers and SGs and, we believe, is sometimes decisive in causing FF firing. Their firing also causes inhibition involved in suppressing effects of reafference, as do the giant fibers themselves. 5. I3 strongly excites the motoneurons of certain tail fan muscles (the ventral and posterior telson flexors). However, the contraction of these muscles would be maladaptive during some giant fiber-mediated tailflips. Accordingly, when the giant fibers, which always recruit I3, fire, they cause an inhibition of the motoneurons that nullifies the excitatory input from I3. At a formal level this means that the giants, viewed as command neurons, not only drive but also alter or modulate the subordinate motor pattern-generating network that they control. 6. Tailflips that are less stereotyped than those mediated by giant fibers are known to occur without participation of the giants. It is suggested that the presence of complex circuitry mediating between giant fibers and FFs may be related to the use of portions of this circuitry as well as the FFs themselves in production of nongiant tailflips.

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