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Biomedical subjects

D H Cohen

Publications and source records attributed to D H Cohen.

At least 37 records · Page 2Linked to original sources
PubMed1981

Modification of the discharge of vagal cardiac neurons during learned heart rate change.

Visually conditioned heart rate change in the pigeon has been developed as a vertebrate model system for the cellular neurophysiological analysis of associative learning. In previous studies of the "final common path," it was shown that both the vagal and sympathetic cardiac innervations contribute to this response. The present experiments indicate that, prior to any behavioral training, the visual stimulus elicits a small decrease in the discharge of vagal cardiac neurons. During conditioning, this stimulus evokes a progressively greater decrease in discharge that parallels the acquisition of the conditioned cardioacceleration. In contrast, nonassociative control animals show habituation of the initial decrease in discharge. These data confirm the involvement of the vagal cardiac innervation in conditioned heart rate change, indicate that the vagal innervation acts synergistically with the sympathetic to produce cardioacceleration, and suggest that a short-latency pathway mediates the conditioned response.

Action Potentials↗
PubMed1981

Heart rate conditioning in the pigeon immobilized with alpha-bungarotoxin.

Previous in vitro studies have yielded conflicting results concerning the physiological effects of alpha-bungarotoxin (alpha-BTX) at avian autonomic ganglia. In the present study heart rate change was classically conditioned in pigeons immobilized with alpha-BTX to assess the effects of this agent on autonomic function in the intact animal. When compared with non-immobilized controls, alpha-BTX had no significant effect on basal heart rate or on the rate of acquisition, magnitude or dynamics of conditioned heart rate change. Furthermore, a significant difference in performance between groups receiving conditioning and sensitization paradigms clearly indicated the establishment of conditioned responding. It is concluded that alpha-BTX has little or no effect on the cardiac innervation at doses sufficient to block neuromuscular transmission. The irreversible blockade of neuromuscular transmission by alpha-BTX without the side-effects produced by most other paralytics makes it an ideal choice as an immobilizing agent in acute preparations.

Animals↗
PubMed1981

Enhancement of heart rate responses during conditioning and sensitization following interruption of raphe-spinal projections.

Recent evidence suggests that neurons within the ventromedial medulla give rise to an inhibitory pathway(s) which projects directly upon the sympathetic preganglionic neuropil. The present investigations were initiated to determine the effects of interrupting this pathway on the expression of a learned autonomic response, defensively conditioned cardioacceleration in the pigeon. Experiment I included two groups. Each experimental animal was trained concomitantly with a sham-operated control, and all birds received 100 trials of conditioning. Each conditioning trial consisted of a 6-sec light presentation followed immediately by a 0.5-sec foot shock. The first experimental group included birds with lesions involving the rostral medullary raphe nuclei (RMR), while the second included birds sustaining lesions of the caudal pontine raphe region (CPR). The principal findings were that (a) RMR-lesioned birds exhibited heart rate increases significantly greater than CPR-lesioned and control birds, (b) heart rate changes in CPR-lesioned birds were not significantly different from control values, and (c) RMR and CPR lesions did not affect base line heart rates significantly. Experiment II was undertaken to determine whether the elevated responsivity following RMR lesions represented a performance increment or an actual enhancement of associative learning. RMR-lesioned animals and their paired controls received 60 trials of sensitization training (explicitly unpaired lights and foot shocks) followed by 60 trials of conditioning. The primary result was that, independent of the training paradigm, RMR-lesioned animals, had significantly greater heart rate increases than control animals. In summary, the "lability" of heart rate changes following RMR lesions apparently is a performance effect and does not represent an enhancement of associative learning. Additionally, the data are consistent with the hypothesis that lesions of this midline medullary region disrupt a descending pathway important for the phasic reflex modulation of chronotropic cardiac responses to exteroceptive stimuli.

Animals↗
PubMed1979

Origin, course and terminations of the rubrospinal tract in the pigeon (Columba livia).

The red nucleus and its spinal projections in the pigeon (Columba livia) have been studied using both normal and experimental material. The cytoarchitecture of the nucleus is described on the basis of Nissl-stained sections and reveals an organization generally similar to that of mammals. The large neurons (40-50 mum) tend to be located dorsomedially and ventrolaterally at more caudal nuclear levels, while the small- and medium-sized neurons (15-35 mum) predominate at rostral levels. However, neurons of all sizes are present throughout the nucleus. Following lesions of the nucleus, the course of degenerating axons stained with the Fink-Heimer method has been traced throughout the brainstem and spinal cord. The rubrospinal tract crosses the midline, courses past the ventrocaudal aspect of the contralateral nucleus ruber, and then descends rostro-ventral and lateral to the nucleus tegmenti pontinus. In its caudal continuation the tract lies ventral to the brachium conjunctivum and the entering radix of the trigeminal nerve. It then assumes a ventrolateral position in the caudal brainstem before shifting to a dorsolateral position in the lateral funiculus of the spinal cord. Within the spinal grey the rubrospinal tract terminates in laminae V, VI and to a lesser extent VII. The possibility of a topographical organization of the nucleus was investigated with injections of horseradish peroxidase into brachial, thoracic and lumbar spinal cord. Regardless of the level of injection, labelled neurons of all sizes were present throughout the contralateral nucleus ruber, indicating the absence of an obvious topography.

Animals↗
PubMed1979

Raphe inhibition of sympathetic preganglionic neurons.

Anatomical and electrophysiological studies suggest that the medullary raphe gives rise to a monosynaptic, inhibitory projection on sympathetic preganglionic neurons. Physiological and behavioral data indicate that this sympathoinhibitory pathway participates in the central control of cardiovascular function.

Animals↗
PubMed1978

Electrophysiological and electron microscopic analysis of the vagus nerve of the pigeon, with particular reference to the cardiac innervation.

The compound action potential components and their associated fiber contingents were investigated in the pigeon vagus nerve with a view toward identifying the vagal cardioinhibitory fibers. In the cervical vagus, the compound action potential evoked by electrical stimulation included four major components that conducted at 17.0-30.0 (A-wave), 8.0-14.5 (B2-wave) and 0.8-2.0 (C-wave) m/sec. Cardiac slowing was not elicited until activation of the Bl-wave, and the bradycardic response was maximal when this component was maximized. Electron microscopic analysis of the cervical vagus revealed myelinated fibers 1.1-6.8 micron in diameter and unmyelinated fibers 0.3-1.4 micron in diameter. A contingent of myelinated fibers approximately 2-4 micron in diameter apparently generated the Bl-wave, while the prominent unmyelinated fiber contingent (37%) accounted for the C-wave. Analysis of various vagal branches indicated that approximatley 20% of the cervical vagal fibers exit the main trunk between cervical and mid-thoracic levels, but few of these are the larger myelinated fibers greater than 2 micron in diameter. The upper abdominal vagus consists largely of unmyelinated and small myelinated fibers, and consequently the vast majority of larger myelinated fibers found in the cervical vagus exit between mid-thoracic and upper abdominal levels, presumably in the cardiac branches. Direct examination of the cardiac branches confirmed this. Thus, it is concluded that the Bl-wave of the compound action potential is uniquely associated with cardiac slowing, that this component is generated by myelinated fibers ranging from 2 to 4 micron in diameter, and that almost all such fibers are destined for the cardiac branches of the vagus.

Action Potentials↗
PubMed1978

Field potential and single unit analyses of the avian dorsal motor nucleus of the vagus and citeria for identifying vagal cardiac cells of origin.

Field potentials evoked by mid-cervical vagal stimulation were systematically mapped in the dorsal motor and solitary nuclei of the pigeon. Since responses varied predictably with microelectrode position, they could be used for localization in the dorsal medulla. By varying stimulus intensity and monitoring the vagal compound action potential, contributions of the different compound action potential waves were then established. Activation of the Bl-wave, which includes cardioinhibitory fiber activity, has its most prominent effect in the intermediate rostrocaudal zone of the dorsal motor nucleus in the region of subnuxleus b. This is where the cells of origin of the vagal cardiac fibers have previously been anatomically localized. Single unit experiments then established that (a) vagal motoneurons with axonal conduction velocities in the cardioinhibitory fiber range (8.0-14.5 m/sec) are primarily localized to the intermediate rostrocaudal zone of the dorsal motor nucleus in the region of subnucleus b, and (b) motoneurons in this zone that conduct at 8.0-14.5 m/sec distribute their axons in the cardiac branches. Furthermore, no error is introduced by identifying such neurons with mid-cervical rather than midthoracic vagal stimulation. Thus, the following criteria establish a neuron as giving rise to a vagal cardioinhibitory fiber: (a) localizing it to the intermediate rostrocaudal zone of the dorsal motor nucleus on the basis of the field potential evoked by mid-cervical vagal stimulation; (b) antidromically activating it with mid-cervical vagal stimulation; and (c) demonstrating that its axon conducts at 8.8-14.5 m/sec.

Animals↗
PubMed1977

Avian sympathetic cardiac fibers and their cells of origin: anatomical and electrophysiological characteristics.

The sympathetic cardiac innervation of the pigeon was investigated to describe certain anatomical and physiological properties of the cardiac nerve fibers and their postganglionic cells of origin. The compound action potential of the right cardiac nerve has two major components, one conducting at 2.0-5.6 m/sec with no chronotropic effect on the heart and the other conducting at 0.4-1.0 m/sec with a cardioacceleratory effect. Postganglionic neurons responding antidromically to cardiac nerve stimulation were then studied in ganglion 14 which contains most cells of origin of the cardiac fibers. These neurons have refractory periods of approximately 4 msec, following frequencies of less than 4 HZ, and axons conducting at 0.4-2.0 m/sec; this conduction velocity range corresponds to the slower compound action potential component. Electron microscopy of the cardiac nerve revealed unmyelinated fibers ranging in diameter from 0.2 to 1.2 micrometer and a population of myelinated fibers 1.0-3.6 micrometer in diameter. The unmyelinated fibers account for the slower compound action potential component and are largely postganglionic cardioaccelerator axons. The myelinated fibers account for the faster compound action potential component which has no chronotropic effect and is not reflected in postganglionic antidromic latencies; it is suggested that these myelinated fibers are cardiac sympathetic afferents. This study thus establishes electrophysiological criteria for identifying cardiac postganglionic neurons and describes the anatomical basis of these criteria.

Animals↗
PubMed1977

Anatomical and physiological characterization of avian sympathetic cardiac efferents.

In the preceding paper6 describing the right sympathetic cardiac nerve of the pigeon we reported a fast compound action potential component with no chronotropic effect, and single unit recordings from sympathetic postganglionic neurons demonstrated that this action potential component was not reflected in the antidromic postganglionic latencies. Electron microscopy then indicated the presence of numerous myelinated fibers in the nerve, and together these findings suggested the existence of a substantial number of sympathetic cardiac afferents in the right cardiac nerve. The present paper confirms this by demonstrating with retrograde transport of horseradish peroxidase that some fibers of the right sympathetic cardiac nerve have their cells of origin in dorsal root ganglia. This was also shown to be the case for the left sympathetic cardiac nerve. Selective activation of the myelinated fiber contingent was then shown to elicit a short latency decrease in arterial blood pressure that could be further augmented by the activation of the smaller unmyelinated fibers. This reflex depressor response to activation of sympathetic cardiac afferents survived bilateral vagotomy but was blocked by atropine. It is therefore concluded that both the left and right sympathetic cardiac nerves of the pigeon contain afferent fibers, both myelinated and unmyelinated, and that the reflex effect of activating these fibers is a sympathetically mediated vasodilation that is atropine-sensitive.

Afferent Pathways↗
PubMed1976

Involvement of the avian hypothalamus in defensively conditioned heart rate change.

Hypothalamic involvement in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 128 pigeons by evaluating conditioning performance following lesions of various hypothalamic areas. Extensive destruction of the posterior hypothalamus severely impaired development of conditioned heart rate change. Anterior hypothalamic lesions also produced serious deficits, though not as severe as following posterior hypothalamic damage. Partial posterior hypothalamic, unilateral and tuberal lesions produced only minor to moderate deficits. The critical locus for profound impairment of conditioned response development appeared to be the medial hypothalamus, and it is suggested that more specifically it is the terminal field of the archistriatal projection upon the medial hypothalamus. It is concluded that the medial hypothalamus is essential for the development of defensively conditioned heart rate change, and based on previous findings it is suggested that the critical descending pathway for expression of this conditioned response involves the archistriatal projection upon the medial hypothalamus and subsequently a polysynaptic pathway through the ventral brainstem.

Animals↗
PubMed1975

A cytoarchitectonic analysis of the spinal cord of the pigeon (Columba livia).

The spinal gray of the pigeon is described cytoarchitectonically to establish a foundation for anatomical and physiological studies of the pigeon spinal cord. The material includes segments from the high cervical cord through the lumbosacral enlargement, and nine cellular layers are described. In addition to this laminar organization, various distinct cell groups such as the dorsal magnocellular column, column of Terni, marginal cells and lobes of Lachi are described. Layers I-IV occupy the head of the dorsal horn, are apparent at all spinal levels examined, and represent the clearest case of laminar organization of the spinal gray of the pigeon. Layer V occupies the full extent of the neck of the dorsal horn at all segmental levels investigated. Also, the dorsal magnocellular column is situated in the central region of this layer from the rostral pole of the cervical enlargement through the lumbosacral enlargement, and arguments are advanced that this cell column is homologous to the column of Clarke. In the intermediate zone a Layer IV is defined, but it is apparent only at the enlargements. Layers VII-IX constitute the ventral horn, Layer IX being the motoneuronal cell groups. With the exception of the motoneuronal groups, the boundaries of the ventral horn layers are considerably less distinct than those of the dorsal horn, and no attempt is made to distinguish Layers VII and VIII at lumbosacral levels. At the enlargements there is a prominent lateral motoneuronal cell group consisting of large cells. It is generally concluded that the cytoarchitectonic organization of the spinal gray of the pigeon bears a rather close resemblance to that described for various mammalian species, particularly with respect to the dorsal horn.

Animals↗
PubMed1975

Spinal terminal fields of dorsal root fibers in the pigeon (Columba livia).

This paper describes the spinal pattern of brachial and lumbosacral dorsal root terminations in the pigeon. These are presented within the framework of the cytoarchitectonic analysis of the preceding report (Leonard and Cohen, '75). At the level of dorsal root section and immediately adjacent sections dense terminal fields were evident throughout the ipsilateral dorsal horn (Layers I-V), including the substantia gelatinosa. However, degeneration in the substantia gelatinosa was prominent only with short survival times. There were mediolateral differences in the density of degenerating material in the dorsal horn, as well as variations between the patterns at the cervical and lumbosacral enlargements. The general pattern of degeneration in the dorsal horn extended approximately three segments rostral and two segments caudal to the level of root section, becoming progressively less dense with distance from the level of rhizotomy. An exception to this was degeneration in the dorsal magnocellular (Clarke's) column where, following section of either brachial or lumbosacral roots, degeneration could be traced into the thoracic spinal cord. However, these ascending and descending projections upon column at thoracic levels overlap minimally if at all. Degeneration was sparse in the intermediate zone and ventral horn (Layers VI-IX). It tended to concentrate centrally in Layer VI at the enlargements, and much of the degeneration ventral to this was interpreted as fibers en passage to the motoneuronal cell groups, Layer IX. Terminal fields in Layer IX were evident at both cervical and lumbosacral enlargements. They were largely restricted to the lateral motoneuronal cell group and were more prominent at cervical levels where degeneration was commonly observed around cell bodies and their proximal dendrites. As with the cytoarchitectonic organization of the spinal gray in the pigeon, the terminal fields of brachial and lumbosacral dorsal root fibers appear to have a pattern similar to that reported in mammalian literature.

Animals↗
PubMed1975

Congenital left posterolateral diaphragmatic hernia with previously normal chest x-ray.

Three patients who presented with left congenital posterolateral diaphragmatic hernia at the ages of six months, two years and six years and who had a normal chest x-ray earlier in life are reported. In two children the late onset of symptoms and previously normal radiographic appearance might be explained by the spleen acting as a plug in the diaphragmatic defect. A normal chest x-ray in early infancy does not exclude the diagnosis of congenital posterolateral diaphragmatic hernia.

Child↗
PubMed1975

Retinal afferents to the pigeon optic tectum: discharge characteristics in response to whole field illumination.

Responses of pigeon retinotectal afferents to changes in whole field illumination are quantitatively analyzed for 313 units recorded in the stratum opticum. One hundred per cent of these units were responsive in unanesthetized birds and 92.9 percent in urethane anesthetized preparations. Of the responsive units, 92.4 percent (anesthetized) and 100 percent (unanesthetized) gave on-responses whose discharge characteristics showed an orderly relationship to stimulus intensity. Following the on-response, discharge generally ceased during sustained illumination. At stimulus termination off-responses frequently occurred, their probability being a function of stimulus intensity and duration. This probability, as well as specific response characteristics, could be predicted to a large extent from on-response characteristics. Anesthesia and level of background illumination predictably affected these response characteristics quantitatively but did not alter them qualitatively. It is hypothesized that with respect to whole field illumination the pigeon's retinotectal afferents constitute a homogeneous population continuously distributed according to threshold for intensity change.

Animals↗
PubMed1975

Involvement of the avian amygdalar homologue (archistriatum posterior and mediale) in defensively conditioned heart rate change.

Archistriatal involvement in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 198 pigeons by evaluating conditioning performance following lesions of different archistriatal subdivisions or interruption of their subtelencephalic projections. Lesions of the anterior-intermedium archistriatum or interruption of its descending projection (tractus occipitomesencephalicus) had no effect on conditioned response development. In contrast, destruction of the posterior-mediale region or interruption of its descending projection (tractus occipitomesencephalicus, pars hypothalami) produced profound conditioning deficits. The findings are entirely consistent with a recent anatomical analysis (Zeier and Karten, '71) suggesting that only the posterior-mediale archistriatum constitutes the avian amygdalar homologue. It is concluded that the integrity of the posterior-mediale region and its projection to the hypothalamus via the tractus occipitomesencephalicus, pars hypothalami are essential for establishing visually conditioned heart rate change in a defensive conditioning paradigm, and these findings are discussed in the context of structures previously implicated in mediating this conditioning autonomic response.

Animals↗