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Biomedical subjects

K Akert

Publications and source records attributed to K Akert.

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

Projection of precentral, premotor and prefrontal cortex to the basilar pontine grey and to nucleus reticularis tegmenti pontis in the monkey (Macaca fascicularis).

In the present autoradiographic study the projections from frontal lobe cortex to basilar pontine grey (GP) and to N. reticularis tegmenti pontis (RTP) have been traced in 24 monkeys. GP receives marked ipsilateral and scant contralateral projections from primary motor as well as from premotor cortex (including supplementary motor area and frontal eye field). This projection terminates in the form of thin lamellae which are folded along the longitudinal axis and which receive the input in a partly separate and partly overlapping somatotopical order. These lamellae provide the enlarged interface which seems necessary for the interaction and the processing of information converging from frontal cortex of the two hemispheres and dealing with posture and movement of functionally related parts of limbs and body. Comparatively weak and widely spread ipsilateral projections are noted from prefrontal area 9. This may indicate a functionally significant difference of prefrontal cortex with respect to the "dynamic operation of cerebro-cerebellar circuitry" (Eccles). The fronto-cortical projections to RPT and to the caudal pole of GP are also described.

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PubMed1980

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish, Sternarchus albifrons.

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adrendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 micrometers in diameter) and large neurons (approx. 30 micrometers in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called "relay cells" of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to "pacemaker cells." The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the elctromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and SV-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.

Animals↗
PubMed1980

Use of the B-IIb tetanus toxin derived fragment as a specific neuropharmacological transport agent.

It has been previously demonstrated that the non-toxic B-IIb tetanus toxin-derived fragment is transported retrogradely within neurons of the rat peripheral nervous system. In the present work we have shown that when a foreign polypeptide, in this case the Ibc tetanus toxin fragment, is coupled to B-IIb by a disulfide bond, it is transported retrogradely from the axonal endings within muscle to the motoneuronal perikarya. In contrast, Ibc fragment alone was found not to be transported. From these results we draw the conclusion that fragments like B-IIb may serve as specific carriers for chemical and chemotherapeutic agents into the central nervous system.

Animals↗
PubMed1980

Bacilliform inclusions in cells of the proximal pars distalis in the pituitary of four species of Tilapia (Teleostei).

Thin sections and replicas of freeze-etched pituitaries from six species of the teleostean family of Cichlidae were studied by electron microscopy. Four species belonging to the genus Tilapia exhibit rod-like structures, i.e., "bacilliform inclusions" (BI), about 1-2 micrometer in length in cells of the proximal pars distalis. The BI are found either isolated or fused in small groups. They are enclosed in an envelope similar to that of secretory granules. Both the BI and the secretory granules give a positive PAS-reaction.

Animals↗
PubMed1980

The influence of high pressure freezing on mammalian nerve tissue.

Vitrification of biological specimens in liquid nitrogen can be achieved under high pressure (2,100 bars). This procedure obviates the use of aldehyde fixation and cryoprotection (glycerol). The present work demonstrates its applicability to the freeze-etching of mammalian brain tissue. Freeze-fracture repicas from rat cerebellar cortex and subfornical organ prepared by this method are compared to conventionally processed material using aldehyde fixation, glycerination and freezing with Freon. The formation of large ice crystals is prevented in tissue blocks up to 0.5 mm thick; deep etching is markedly enhanced. Cytoplasmic microstructures such as mitochondrial cristae, microtubules and microfilaments, are readily observable against a finely granulated cytosol matrix. An additional advantage is the combined application with freeze-substitution.

Animals↗
PubMed1980

Relationships of precentral premotor and prefrontal cortex to the mediodorsal and intralaminar nuclei of the monkey thalamus.

The projections of precentral, premotor and prefrontal cortical areas to the mediodorsal and intralaminar thalamic nuclei have been reexamined by means of the anterograde labelling technique in 21 macaque monkeys. Three principles which have been already mentioned in previous reports could be confirmed within limits: (i) The dichotomy of agranular and granular frontal cortices related to lateral and medial thalamic nuclear division respectively, (ii) the reciprocity of fronto-thalamic connections and (iii) the thalamic matrix consisting of longitudinally arranged cell columns (Kievit and Kuypers 1977) from which the cortical connections originate and where they end. One main exception to these principles involves the intralaminar and paralaminar nuclear continuum which seems to disrupt the nearly parallel system of thalamically relayed information channels by forming complex patterns of converging and diverging connectivities with cortical, subcortical and possibly intrathalamic regions. Another exception concerns the partial bilaterality of fronto-thalamic projections which is not reciprocated as far as we know by the thalamo-cortical counterparts. Major emphasis is laid upon the fact that agranular frontal cortex, i.e., precentral and premotor (including supplementary motor) areas not only project to the ventrolateral nuclear complex and to intralaminar nuclei but in addition to MD (paralaminar zone) whose efferent connections are known to be directed mainly towards the frontal eyefield and the caudate nucleus and to a lesser degree to the agranular frontal cortex.

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.

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