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K Akert

Publications and source records attributed to K Akert.

At least 19 recordsLinked to original sources
PubMed1989

Fine structure of the tuberous electroreceptor of the high-frequency electric fish, Sternarchus albifrons (gymnotiformes).

Sternarchus emits low voltage biphasic pulses at about 700-900/s. These signals (and changes in them caused by external objects) are detected by the tuberous or phasic electroreceptors. We used electron microscopy to examine extracellular compartments in the current pathway to the receptor cells, which are delineated by cells joined by tight junctions. Highly specialized accessory cells were found to separate the receptor cells from the extracellular space continuous with the exterior. Except for synaptic specializations complements of intramembrane particles of cell membranes were unremarkable and did not correlate with presumed high and low resistivity. We propose an equivalent electrical circuit that is consistent with the morphological and physiological observations.

Animals↗
PubMed1986

Further morphological (freeze-fracture) evidence for an impulse generating function of Mauthner axon collaterals in the tench (Tinca tinca L.) spinal cord.

The freeze-etching technique was applied to the tench spinal cord. Replicas of Mauthner axons revealed that the non-junctional axolemma at the tip of axon collaterals alone exhibits intramembranous E-face particles whose size, distribution and overall density are comparable with those at Ranvier nodes. Since typical Ranvier nodes are lacking along the stem of the heavily myelinated Mauthner giant axon, this finding contributes further evidence to our earlier observations suggesting that the spiny Mauthner axon collaterals represent true nodal equivalents and most likely the long-sought substrate of impulse propagation.

Animals↗
PubMed1985

Cytochemical evidence for an inhomogeneous distribution of sodium channels in the squid giant axon.

In a light microscopic study the ferric ion-ferrocyanide staining of the squid giant axon was investigated. In other neurons this stain is known to indicate excitable membranes with high sodium channel density, such as the node of Ranvier or the initial segment. In the squid the stain reacted in small patches of the axolemmal, suggesting an aggregation of sodium channels. These axolemma patches were always adjacent to the nuclei of special Schwann cells, suggesting a glial-axonal interaction.

Animals↗
PubMed1985

Coupling of gap junctions by induction of impulse conductivity in cultured epithelium of newt embryo (Cynops orientalis).

Impulse generation and propagation was previously shown to occur in skin epithelium of newt (Cynops orientalis) embryos during certain stages of development and to be correlated with morphological changes of gap junctions. These properties are not detected in embryonic epithelia explanted and grown in culture. However, early explants when transplanted to a host embryo develop conductivity, and relatively large gap junctions with loose arrangement of connexons occur as soon as the host embryo reaches the stage when conductivity is at its maximum. In contrast, morphological and physiological characteristics of impulse propagation are lost when the transplanted epithelium is extirpated from the host embryo and returned to in-vitro conditions. Therefore, it appears that impulse propagation is dependent not solely on the differentiation of epithelial cells but upon signals from non-epithelial (possibly mesodermal) tissue as well.

Animals↗
PubMed1979

Increase of large intramembranous particles in the presynaptic active zone after administration of 4-aminopyridine.

The effects of 4-aminopyridine (4-AP, 1 mg/kg i.v.) on the distribution of intramembranous particles in spinal motoneuronal synapses was examined in anesthetized (pentobarbital sodium, 50 mg/kg, i.p.) rats by freeze-etching electron microscopy. The particles were classified according to their size into small (4.2--10.5 nm in diameter) and large (10.5--18.9 nm in diameter). The active zone of the presynaptic membrane was identified on the basis of ultrastructural criteria. The large intramembranous particles in both protoplasmic (PF) and external fracture faces (EF) were aggregated more densely in the active zone as compared with its surroundings both in the control and 4-AP-treated group (P less than 0.001). The density of large particles in the PF rose from 300/sq.micron under the influence of 4-AP (P less than 0.001). The increased frequency of vesicle attachment sites at the presynaptic membrane (from 115/sq.micron in barbiturized controls to 150/sq.micron in 4-AP treated animals) seemed to be positively correlated with the increasing density of large particles. Based on these findings it is suggested that the large intramembranous particles of the presynaptic membrane may participate in the control of calcium fluxes which are essential to the transmitter release by exocytosis.

Aminopyridines↗
PubMed1979

Freeze-etch study of the teleostean pituitary.

The structure of membranes in the pituitary of several teleostean species was studied with the freeze-etching technique. Nuclear pore density is higher in hormone-secreting than in stellate cells, suggesting a higher rate of nucleo-cytoplasmic interactions in the secretory cells. The perinuclear endoplasmic reticulum (ER) in the MAS-secreting cells has a large number of fenestrae, whereas in the ER whorls of the prolactin-secreting cells the fenestration occurs only rarely. Three different types of membrane specializations are described, presumably related to exo-endocytotic events at the level of the plasmalemma.

Animals↗
PubMed1979

Ultrastructural effects of 4-aminopyridine on the presynaptic membrane in the rat spinal cord.

The effects of 4-aminopyridine (4-AP, 1 mg/kg, i.v.) on the ultrastructure of synapses in the ventral horn of the spinal cord were examined in rats anesthetized with 50 and 100 mg/kg, i.p., pentobarbital sodium and alpha-chloralose respectively. The presynaptic membrane of the 4-AP-treated animals, in both S- and F-type synapses, was wrinkled and gave frequently rise to omega-shaped profiles. However, it was not prominently lifted in comparison with the controls. In both extracellular (EF) and protoplasmic fracture faces (PF) of freeze-etched replicas, the frequency of the presynaptic membrane modulations (PMM) per unit area rose significantly under the influence of 4-AP. The ratio of open (crater-like) to total PMM in EF increased almost twice under the experimental condition. No differential 4-AP effects were seen between pentobarbital- and chloralose-treated animals. The morphological evidence is consistent with electrophysiological data showing that 4-AP facilitates the transmitter release at synaptic endings.

Aminopyrine↗
PubMed1979

Projections of precentral and premotor cortex to the red nucleus and other midbrain areas in Macaca fascicularis.

Midbrain projections from the precentral and premotor cortex were studied autoradiographyically in seven macaques. Earlier findings of Kuypers and Lawrence (1967) based on silver impregnated axonal degeneration were largely confirmed and partly extended. Laminar projections into the parvocellular red nucleus were found bilaterally from the somatotopic representation areas of the precentral cortex and three main divisions of premotor cortex (ventral, dorsal, and medial). The heaviest contralateral projection was recorded from the medial hemispheric aspects of area 6 (partly identical with the supplementary motor cortex). Homolateral projections were found from the limb areas of the precentral motor cortex to the magnocellular portion of the red nucleus and direct evidence for the overlap between corticorubral projections and corresponding areas of origin of the rubrospinal tract was obtained. Other projections concerned N. Darkschewitsch, N. Edinger-Westphal, suprarubral and dorsal mesencephalic reticular formation and adjacent lateral periaqueductal grey, substantia nigra, pretectal region, superior colliculus and N. tegmenti pedunculo-pontinus.

Animals↗
PubMed1979

Projection of precentral motor cortex upon nucleus medialis dorsalis thalami in the monkey.

The efferent connections of the lateral aspects of the precentral, premotor and granular frontal cortex were reexamined in 12 monkeys (Macaca fascicularis) by means of anterograde labelling techniques using radioactive proline and leucine. A distinct projection of the precentral gyrus upon the paralamellar portion of the nucleus medialis dorsalis (MD) was observed. The somatotopic arrangement within this projection is less well defined than within the ventrolateral thalamic complex. The facial division of area 4 has not only a marked ipsilateral but in contrast to the body representation also a distinct if much weaker contralateral projection. No label was found in MD after injections in the postcentral gyrus. Thus, MD represents a thalamic link between agranular (motor) and granular (association) cortex of the frontal lobe.

Animals↗
PubMed1979

Three types of membrane modulations during transmitter release in rat spinal cord synapses.

Protoplasmic fracture faces of vesicle attachment sites (VSA) in the presynaptic active zone of rat spinal motoneurons were classified morphologically on the basis of particle-to-membrane relationships. Intramembranous particles are absent in type 1, located around the edges of type 2 and covering the center of type 3. Application of 4-aminopyridine (4-AP), which is known to enhance transmitter release, affected strikingly the total number of VAS (P less than 0.001) but not the percentage of the three types. It is postulated that the particle-free VAS (types 1 and 2) sharing 80--85% of VAS within the active zone, are related to exocytosis. The predominance of type 3 outside the active zone and its similarity to particle-loaded indentations possibly corresponding to coated vesicle formation, suggested that type 3 may represent endocytosis.

Aminopyridines↗
PubMed1979

[The morphologic diversity and complexity of synapses and microcircuits].

After mentioning the dendrite arborizations and the dendritic spines which represent an important site of synaptic interactions and neuronal plasticity, the main synaptic types, chemical and electrical, excitatory and inhibitory are mentioned and their morphological characteristics and dynamic properties are discussed. In the second part, the synaptic connections between several nerve cells, and the module principle of cerebellar and cerebral cortex are described. Significant progress is expected from the application of the 14C-deoxyglucose technique which is used as a histochemical marker of neuronal activity.

Animals↗
PubMed1978

Comparison of active zones in spinal cord of anesthetized and unanesthetized rats: a high voltage electron microscope study.

High voltage electronmicroscopy was applied to the study of presynaptic nerve terminals and specifically the arrangement and staining properties of synaptic vesicles in and around the active zone. The zinc iodideosmium tetroxide technique was used for the impregnation of neuropil derived from the spinal cord of anesthetized (40 mg/kg pentobarbital sodium) and unanesthetized rats. Under optimal conditions of section thickness and tilt angles stereoscopic views of a paracrystalline lattice arrangement of synaptic vesicles could be ascertained. The ZIO affinity was significantly higher in synaptic vesicles of the anesthetized as compared with the unanesthetized preparations.

Animals↗
PubMed1978

Projections of the precentral motor cortex and other cortical areas of the frontal lobe to the subthalamic nucleus in the monkey.

Corticofugal projections were examined by means of the autoradiographic tracer method in 21 macaca fascicularis. The labeled material was injected into the main body representation areas of the precentral motor cortex and into various regions of Brodmann's areas 6, 8 and 9 of the frontal lobe. The ipsilateral subthalamic nucleus receives a moderately strong and somato-topically organized projection from Woolsey's precentral motor cortex. This projection is mainly restricted to the lateral moiety. The remaining nucleus is occupied by less intensive projections from premotor and prefrontal areas. It is concluded that the subthalamic nucleus is a convergence site of pallidal and corticomotor and frontal projections. Cortical afferents may exert an influence on the pallido-subthalamic-pallidal inhibitory feedback loop.

Animals↗
PubMed1978

Junctional complexes and variations in gap junctions between spinal cord ependymal cells of a teleost. Sternarchus albifrons (Gymnotoidei).

Junctions between ependymal cells lining the central canal in the spinal cord of the weakly electric teleost, Sternarchus albifrons, were investigated by means of thin-section and freeze-fracture electron microscopy. Junctional complexes representing the 'terminal bar' consist of a zonula occludens, followed by intermediate junctions and desmosomes. This pattern is common in many types of epithelia but uncommon in ependyma. Three types of gap junctions were found below the level of the terminal bar and intermixed with desmosomes: (1) gap junctions at least partially enclosed by a single strand of apparent tight junction, (2) ordinary gap junctions, partially surrounded by a particle-free halo and (3) segmented gap junctions, in which linear arrays several particles wide are separated by fairly regular linear particle-free regions. Some of the observations may have implications for the mechanism of formation of gap junctions.

Animals↗
PubMed1978

Intramembranous particles at the nodes of Ranvier of the cat spinal cord: a morphometric study.

Size and distribution of intramembranous particles at nodes of Ranvier of the cat spinal cord were investigated by the freeze-etching technique and compared with those at the internodal axon. The particles are larger (up to 20 nm) at the nodal than at the internodal segment (up to 13 nm), and these large particles are more densely packed in the nodal (400 per sq micrometer) than in the internodal E (external) face (4 per sq micrometer). The nodal E face reveals a much denser overall population (1200--1300 per sq micrometer) of particles than the internodal E face (100--200 per sq micrometer), while at the P (protoplasmic) face the particle density is similar in nodal and internodal segments (1200--1600 per sq micrometer). It is suggested that the large nodal particles may be related to the mechanism of nerve excitation.

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