Local nonuniformities in the syncytium of the horizontal cells of the retina in the turtle

Electrical coupling between horizontal cells of the turtle retina was investigated by means of two microelectrodes (current and recording ones) penetrating neighbouring cells at a fixed distance from each other. The morphological coupling was revealed by means of fluorescent dye Lucifer Yellow. The electrical coupling was confirmed between elements of similar type (L1--axonal terminals, or L2--cell bodies, or R/G type cells) and no coupling was found between elements of different types, though L1 and L2 are directly connected through thin axons. In the L1 syncytium the electrical coupling at small (less than or equal to 50 microns) but fixed distances between microelectrodes could differ several times depending on the minimal displacement of microelectrodes. This local nonuniformity of coupling can be explained on the basis of structural nonuniformities in the L1 (axon terminal) network. It is unlikely however that the structural nonuniformities can influence the functional properties of horizontal cell network when the retina is stimulated adequately (by light).     

Asymmetry in the structural organization of the ganglion layer of the retina in the frog

Silver-impregnated retinal preparations were used to study the distribution density and topographic features of small and large ganglionic cells (GC) of Rana ridibunda and Rana temporaria. For both species the increased density of GC (a streak) stretched higher than the naso-temporal axis passing through the optic disk. Beyond the streak the density of small GC was maximal in the central zone of the retina and decreased towards its periphery. For the upper quadrants of the retina the density of small GC was higher than that for the lower ones by 26% on the average. On the contrary, the density of large GC was higher in the lower part of the retina as compared to the upper one, the difference being more pronounced for R. temporaria. The density of large GC was also asymmetric with respect to the dorso-ventral axis being higher in nasal quadrants than in temporal ones by 40-55%. The highest density of large GC was found in the middle zone of the retina. The found structural asymmetry in the retinal output raster may bear an adaptively ecological meaning and may condition the particularities of the formation of the visually guided prey-catching and avoidance reactions.     

Early stages in the formation of the cerebellum in the frog

The formation of the cerebellum was studied during the first 6 months of the tadpole stage of the bullfrog by using standard histological methods and reconstructions from serial horizontal sections. Three major developmental phases were noted in the formation of the cerebellum. (1) During the first 5 weeks of development, the neuroepithelium proliferated and the dorsal mesencephalic plates increased in size. (2) Starting in the sixth week, a patch of neuroepithelium began to differentiate and gave rise to a small population of Purkinje cells. In subsequent weeks, the area of differentiation continued to spread and a Purkinje cell layer became established along the dorsal margin of the cerebellar plate. (3) In the 12th week, the ventrolateral part of the cerebellar plate began to increase in size and generate two populations of small cells. The lateralmost part of the neuroepithelium in this area generated a group of cells that formed an external granular layer that was one cell deep. Cells of this external granular layer migrated inward into the primitive molecular layer, and by the 26th week only a remnant of an external granular layer remained in the cerebellum. The more medially situated part of the neuroepithelium gave rise to another population of small cells that formed a column, which appeared to be continuous with the Purkinje cells, but differed from them in size. It should be noted that full maturation of the cerebellum occurs during metamorphosis, which in this species remains some 2 years away.     

Inhomogeneities in the propagation of the slow wave in the stomach

The propagation of the slow wave in the stomach and its role in inducing sweeping peristaltic contractions toward the pylorus, essential for a proper digestion and emptying, have been studied for many years. Irregularities in the timing or in the pattern of propagation of the slow wave have been known to induce various gastric malfunctions and, recently, several types of gastric dysrhythmias have been described which could lead to gastric contraction abnormalities. In this study, Du et al. have analyzed the disturbances caused by a simple transmural incision in a human stomach, performed to obtain a biopsy of the muscle, on the propagation pattern of the slow wave. In addition, they show that such an incision may by itself also induce new types of gastric dysrhythmias. These results are important in demonstrating that the function of the stomach can easily be disturbed by such procedures. This mini‐review describes several ways in which inhomogeneities in propagation may affect the conduction pattern of the slow wave, including the genesis of several dysrhythmias, and what is currently known about their impact on gastric contraction and digestion.     

Challenges in the Management of the HIV Patient in the Third Decade of AIDS

The epidemic of AIDS has been increasingly recognized as a major health and socioeconomic problem, not only in the United States or Africa, but also the rest of the world. The face of the epidemic has changed. The role that mental health providers play has also significantly grown as the epidemic continues on. Prior to the introduction of Highly Active Anti-Retroviral Therapy (HAART) and other advances in HIV care, the patients faced issues that related to death and dying. These advances brought with them renewed hope and resurrected lives. The patients fought with issues related to living new lives with HIV no longer an imminent death threat. In the third decade of AIDS, the struggles of the post-HAART era continue but bring with it more challenges. Mental health providers need to familiarize themselves with these issues so that they can better help HIV patients cope with this devastating disease.     

Analysis of the habituation-like changes in transmission in the temporodentate pathway of the rat

Habituation-like decrements in extracellular measures of synaptic activation (population EPSP) and cell discharge (population spike) were analyzed in the dentate gyrus of the rat following repetitive low-frequency stimulation of the medial and lateral entorhinal cortex. Stimulation of either subdivision of the entorhinal projection system resulted in comparable habituation-like response decrements with similar stimulation regimens. However, habituating stimulation of one subdivision did not result in decreased responsiveness to stimulation of the other. Repetitive low-frequency stimulation or even a single pulse delivered to either subdivision did, however, result in a potentiation of granule cell discharge in response to stimulation of the other subdivision (a form of heterosynaptic potentiation). This heterosynaptic potentiation of granule cell discharge was not accompanied by any increase in the extracellular EPSP. Comparisons of the relationship between the population EPSP and population spike before and during habituating stimulation revealed changes in cell discharge in response to the habituating stimulus which could not be accounted for by changes in synaptic activation alone. The results suggest that repetitive activation of the temporodentate pathway alters granule cell output as a result of two processes, a habituation-like decrement in synaptic activation, and a potentiation of granule cell discharge as a consequence of prior activation.     

Gradients of histogenesis in the ependymal lining of the third ventricle in the rabbit

Tritiated thymidine autoradiography was used to analyze the site, time of origin, and developmental gradients of the specialized lining of the ependymal surface of the third ventricle. Cells destined to form the ependyma are generated between days 15 and 22 of embryogenesis (gestation: 30 +/- 2 days), the majority of the cells undergoing final division on the 18th day of gestation. Ependymal cells originate in an orderly fashion according to 3 gradients. Two gradients of opposite direction (ventrodorsal and dorsoventral) are found in the parasaggital plane. Both gradients start at the level of the hypothalamic sulcus, progressively departing from this anatomical landmark as histogenesis progresses. A third gradient occurs in the caudorostral axis, such that cells located in caudal regions originate earlier than those located in rostral sectors. Thus, an orderly relationship exists between the time of origin of ependymal cells and their final location within the lining of the ventricular wall. These findings indicate, once again, the topographic nature of the gradients of histogenesis. The histogenic gradients displayed by the ependymal lining of the third ventricle appear strongly related to those exhibited by other diencephalic derivatives. The latter suggests that common factors govern the developmental sequence of all diencephalic derivatives as a function of their relative topographic location, independently of their functional role in the adult.     

Role of the subthalamo-nigral input in the control of amygdala-kindled seizures in the rat

The substantia nigra pars reticulata (SNpr) is a critical site for the control of epileptic seizures. Potentiation of the inhibitory GABAergic input from the striatum to the SNpr suppresses primary or secondary generalized seizures in the rat. The purpose of this study was to examine the possible involvement of the excitatory glutamatergic input from the subthalamic nucleus to the SNpr in the control of both the electroencephalographic and the motor components of amygdala-kindled seizures in the rat. Microinjections of either an N-methyl- -aspartate (NMDA) antagonist in the substantia nigra or a GABA_A agonist in the subthalamic nucleus, significantly reduced motor seizures but did not modified the afterdischarges. These results provide evidence for the involvement of the subthalamo-nigral projection in the modulation and the propagation of the motor components of amygdala-kindled seizures.     

Patterning in the regeneration of electroreceptors in the fin of Kryptopterus

The influence of the target tissue on afferent nerve regeneration was studied in the adult glass catfish, Kryptopterus. In this fish, electroreceptors in the anal fin are distributed in a characteristic pattern in the proximal part of the fin and are absent in the distal portion of the fin. We tested whether axons were more likely to induce electroreceptors in certain regions of fin epidermis than in others. We rotated fin transplants so that the location of the degenerating electroreceptors was altered with respect to the regenerating axons in the host tissue dorsal to the fin. The effects of these rotations were observed in the living animal with differential interference contrast optics over a period of 10 weeks. When transplants were reversed rostrocaudally, new electroreceptors formed in the caudal half of the interradial zone, where degenerating electroreceptors were at the time of transplantation. When transplants were rotated so that the dorsoventral and rostrocaudal axes were reversed, some new receptors formed in the old target site regions that were located in the caudal interradial zones (in the distal half of the graft with respect to the host). Regenerating axons reached these regions of the transplant by taking unusual routes around the electroreceptor-free regions of fin. Very few electroreceptors formed in the distal/caudal or proximal/caudal interradial quadrants of grafts where the original orientation of the tissue was maintained. We suggest that old target sites have a neurotropic influence on the regenerating afferent axons and discuss the possibility that the distal fin epidermis is not as permissive to electroreceptor formation as proximal fin epidermis.     

Organization in the descending tracts of the dorsolateral funiculus in the cat

Organization of the fibers in the descending tracts of the dorsolateral funiculus of the cervical spinal cord was investigated in cats. The spinal cord was penetrated with microelectrodes at 400 mum intervals in the medio-lateral direction at the c5/c6 and c6/c7 segmental borders. Silicon substrate microelectrodes with a linear arrangement of activated iridium contacts were used. The stimulus consisted of a 20 ms train of charge balanced biphasic current pulses at 330 Hz. The evoked activities from selected forelimb muscles were acquired into computer. Only the data points with an activation threshold of less than 35 muA were considered in the analysis. Muscle contractions were mostly in the form of short twitches. In both spinal segments, an area of high threshold was found in the middle of the dorsolateral funiculus. Majority of the muscles studied had a dorsal or ventral concentration of activation points. The distal muscles were mostly activated in the ventro-lateral aspect of the funiculus, while the elbow muscle maps spread to both dorsal and ventral sides. These results show a functional organization in both cervical segments studied, with overlapping regions between the areas dedicated for each forelimb muscle.     

Pursuit of the origin of the large myelinated fibers of the anterolateral funiculus in the spinal cord in humans in relation to the pathomechanism in amyotrophic lateral sclerosis

To determine the origin of the large myelinated fibers in the anterolateral funiculus (ALF) in the spinal cord of humans, myelinated fibers in the ALF of the mid-cervical spinal cord were examined quantitatively. Five groups of subjects were examined, consisting of control subjects, patients with cerebral lesions and showing complete degeneration of the unilateral/bilateral pyramis of the medulla oblongata, those with lesions of the pontine tegmentum, those with lesions of the lower cervical spinal cord, and those with thoracic/lumbar lesions. The results indicate that the large myelinated fibers in the ALF of the mid-cervical spinal cord of humans originate from the tegmentum of the brain stem and the lower cervical spinal cord, and not from the cerebrum, or the thoracic or lumbar spinal cord. Thus, they are descending fibers from the brain stem tegmentum and ascending fibers from the lower cervical cord, and not corticospinal tracts or long-ascending fibers from the thoracic or lumbar spinal cord. The origin of the large myelinated fibers in the ALF of the spinal cord in humans, the number of which was severely decreased in patients with amyotrophic lateral sclerosis, is considered to be the long-descending neurons in the brain stem tegmentum and the propriospinal neurons in the spinal cord. Received: 23 December 1998 / Revised, accepted: 29 March 1999     

Changes in the calcium action potentials of the neurons of the spinal ganglia in the rat in response to divalent cation substitution in the extracellular medium

Rat dorsal root ganglion neurons were investigated by intracellular microelectrodes using current clamp technique. After elimination of sodium ions from the extracellular medium and addition of TEA pure "calcium" action potentials were recorded. Progressive and selective disturbance of such potentials was observed during continuous intracellular recording indicating a decrease in the calcium conductance while sodium and potassium conductances remained uneffected. Such a disturbance is supposed to be due to excessive influx of calcium, strontium or barium ions into the cell. The calcium component of the action potential was also dependent on the stimulation frequency, such dependence being different for strontium or barium solutions. Possible participation of current-dependent inactivation in the origin of this phenomenon is discussed.     

Participation of calcium-dependent potassium conductance in the processes of membrane hyperpolarization of the pyramidal neurons in the sensorimotor cortex in the cat

In acute experiments on immobilized cats intracellular injection of Ca+ decreased of IPSP and postburst hyperpolarization amplitudes in pyramidal neurons of the sensorimotor cortex. Intracellular injection of ethylene glycol tetraacetic acid had almost the same effect. This substance also reduced the late part of spike afterhyperpolarization, while the early part remained practically unchanged. It is concluded that Ca2+-dependent K+-conductance might play an important role in the genesis of IPSP, postburst and spike afterhyperpolarization in the membrane of pyramidal neurons of the cat sensorimotor cortex.