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G M Yasargil

Publications and source records attributed to G M Yasargil.

13 recordsLinked to original sources
PubMed1990

Topography and ultrastructure of commissural interneurons that may establish reciprocal inhibitory connections of the Mauthner axons in the spinal cord of the tench, Tinca tinca L.

This study was made to identify the inhibitory interneurons belonging to the spinal circuitry activated by the Mauthner axons in the tench (Tinca tinca L.). The histological investigations were focused on a segmental pair of commissural interneurons that were reconstructed in toto from their distinguishing topographical and ultrastructural features. These features are: (a) the adendritic soma located 100-150 microns dorsal to the central canal; (b) the first node of Ranvier which is precommissural and connected to the ipsilateral Mauthner axon via gap junction; (c) the second node of Ranvier, from which two first-order branches arise postcommissurally each supplying roughly the rostral and caudal half of the contralateral spinal cord segment; (d) their second-order branches, which arise at intervals that correspond closely to those of the Mauthner axon collaterals; (e) the postsynaptic targets of the second-order branches, which are exclusively all the motoneurons and interneurons innervated by the contralateral Mauthner axon; (f) the axon terminals of these branches, which contain F-type vesicles, form Gray type-2 synapses, and abut either on the initial segment or on the first node of Ranvier of the target neurons. Thus, it appears that this segmental interneuron has all the appropriate features that could provide the structural basis for the reciprocal fast-acting inhibitory coupling underlying the startle reflex elicited by the Mauthner neurons in response to auditory stimuli.

Animals↗
PubMed1988

Axonal domains within shared touch domes in the rat: a comparison of their fate during conditions favoring collateral sprouting and following axonal regeneration.

Low-threshold mechanosensory nerves in the adult rat differ both from their counterparts in lower vertebrates and from high-threshold nociceptive nerves in mammals in that they appear not to undergo collateral sprouting into adjacent denervated skin, although they will clearly regenerate into it after they are damaged. We have now studied the growth capabilities of the low-threshold nerves supplying touch domes, the visible mechanosensory structures scattered throughout the hairy skin. Touch domes in the rat are often multiply innervated. A serendipitous observation on such domes allowed us to investigate the possibility that a functional collateral sprouting of their nerves can indeed occur, but only to a spatially very restricted extent, e.g., within the confines of a partially denervated dome. We used a "prodder" with a tip diameter of 16 micron to examine the mechanosensory profile across single domes that were preselected as being supplied by only two axons, one running in each of two adjacent dorsal cutaneous nerves (DCNs). Simultaneous recordings were made of the afferent discharges evoked in these nerves when the prodder was applied at about 17 or more locations on a selected dome; the spatial resolution was better than 55 micron. We found that within such a shared dome, one axon can supply a discrete territory (its "domain"), which may or may not overlap with the corresponding domain of the other axon. In a preliminary electron microscopic study, we found no evidence for a sharing of single Merkel cells, which are the specialized sensory cells in touch domes, even in the regions of a shared dome where two domains overlapped; each innervated Merkel cell appeared to be contacted by a single nerve ending, implying that in a shared dome each axon probably supplies an exclusive subpopulation of the Merkel cells. We tested for functional collateral sprouting by eliminating one nerve to a shared dome, and at a selected time thereafter mapping the domain of the remaining axon to see whether it had enlarged. The result was the same whether the two domains initially had a region of overlap or not; no expansion of the surviving domain occurred over postoperative periods up to 4 months (an expansion of the domain by 55 micron would have been detected). Thus functional collateral sprouting had failed to occur.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Morphology of the Mauthner axon inhibitory system in tench (Tinca tinca L.) spinal cord.

Using ultrastructural features as a natural tracer we identified a commissural interneuron in the tench (Tinca tinca L.) spinal cord with the following characteristics: (1) the unipolar cell soma is located 100-150 micron dorsal from the central canal, the frequency of its occurrence being one cell on each side per spinal cord segment; (2) the first node of Ranvier of its axon is connected to a presynaptic branch of the ipsilateral Mauthner axon via gap junctions; (3) its postsynaptic targets are contralateral moto- and interneurons; (4) its terminal boutons contain f-type vesicles and form Gray-type-2 synapses abut on the initial segment or node of Ranvier of these target neurons distal to the input site of monosynaptic excitatory links from the contralateral Mauthner axon. Thus, it appears that this segmental interneuron may provide the structural basis for the well-known mutual crossed inhibition within the spinal circuit of Mauthner axons.

Animals↗
PubMed1987

Vasospastic phenomena on the luminal replica of rat brain vessels.

Strikingly localized ring-like constrictions (other than sphincters, cushions and offshoot furrows) have appeared on the casted vessels of some of our preparations. Morphology of the phenomena reveals diverse degree of active contraction of the vessel wall, ranging from corrugation of the luminal surface to near severance of the acrylic cast. Distribution of the vasospastic phenomena reveals as particularly affected the arteries in the diameter range between 25 and 75 micron, that belong either to intra-arterial anastomoses between the branches of each of the three major cerebral arteries or to their terminal junctions in the border zones (inter-arterial anastomoses). Among the possible causes for the occurrence of the observed vasospasms, we indicate the susceptibility to raised intraluminal pressure during injection, producing contraction of the smooth muscle cell. Reactivity appears heightened in the anastomotic districts of the circulation. The evidence of so-called "plastic strips" clinging to constricted sections of affected vessels prompts re-examination of their proposed origin as "plastic wrapping". Rather, they appear to be remnants of dynamic elements of the vascular wall (smooth muscle cells) that resisted corrosion.

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↗
PubMed1986

The capillary bed in the choroid plexus of the lateral ventricles: a study of luminal casts.

Micro-angioarchitecture of the choroid plexus of the lateral ventricles is investigated in microcorrosion casts of animal and human preparations studied with the scanning electron microscope. The capillary bed in the diverse regions of the tissue belongs to one of three patterns: (1)-a network of capillary meshes that envelop the larger arteries and veins predominates in the central segment. (2)-in the villous regions a "leaf-like" organization of sinusoids is found together with (3)-fronds of "glomerular" formations. "Glomeruli" are formed when arterial afferents and venous efferents converge in a quasi hilar structure before branching in arterio-venous loops. Nodular thickenings are observed on glomerular capillaries. The preparations studied (rat, dog, human) are remarkably similar and differ mostly in degree of occurrence of common architectural patterns. Arterio-venous communications are found at the hilus of human glomerular formations.

Adult↗
PubMed1979

Ultrastructure of the Mauthner axon collateral and its synapses in the goldfish spinal cord.

The ultrastructure of the synapses formed by the Mauthner axon collateral (MAC) was examined in the goldfish spinal cord. All such collaterals form axo-dendritic chemical synapses with dendrites of motor and interneurons. Besides these chemical synapses, contacts have been found between some MAC and axons of interneurons, which can be characterized as gap junctions with presumed electrical transmission. All cellular processes contacted by the MAC contain 'dense bands' of material in their cytoplasm. These 'dense bands' are absent from other parts of the fish C.N.S. These observations and the peculiar synaptic pattern are discussed and correlated with electrophysiological results.

Animals↗
PubMed1973

The membrane effects, and sensitivity to strychnine, of neural inhibition of the Mauthner cell, and its inhibition by glycine and GABA.

1. Anionic conductance changes in Mauthner neurones of goldfish were measured during synaptically evoked inhibition and inhibition caused by iontophoretic application of the putative inhibitory transmitters glycine and gamma-aminobutyric acid (GABA).2. The effects of either amino acid were indistinguishable from those of the neural inhibitory transmitter(s). The membrane permeability during the neural or drug response was increased to Br(-), Cl(-), I(-), SCN(-), NO(3) (-), ClO(3) (-), and formate (HCOO(-)), but not to HCO(3) (-), BrO(3) (-), IO(3) (-), SO(4) (-), HPO(4) (-), H(2)PO(4) (-), acetate and citrate.3. Strychnine was injected intramuscularly, iontophoretically, or applied topically to the exposed brain in order to compare quantitatively its ability to prevent inhibition evoked by synaptic activation and by pharmacological means. Inhibitions were measured by the increase in membrane conductance.4. Strychnine, at concentrations just adequate to block completely the late collateral inhibition (LCI) and crossed VIII nerve inhibition, had little effect on the pharmacological inhibition caused by glycine, and sometimes there was no detectable effect at all. In one experiment even a local iontophoretic application of strychnine in a sufficient dose to diffuse over the cell and block the LCI almost completely, merely halved the effect of a small dose of glycine applied to the same localized region of the membrane.5. Higher concentrations of strychnine than those necessary to block synaptically evoked inhibition would reduce the effect of glycine but not that of GABA. The evidence indicated that any apparent effect of strychnine upon GABA could be explained by displacement of the GABA-containing iontophoretic pipette.6. The glycine-blocking action of iontophoretic pulses of strychnine was of relatively very slow onset and long duration compared to the effects of pulses of glycine and GABA.7. These findings can be interpreted as either (1) strychnine has a presynaptic action, preventing the release of inhibitory neurotransmitter, in addition to its less potent post-synaptic one in blocking pharmacological inhibition, or (2) strychnine acts entirely post-synaptically, but the physiological transmitter action differs from that of glycine and GABA in being considerably more sensitive to strychnine antagonism. In either case, the use of strychnine as evidence for the claim that glycine is an inhibitory neurotransmitter at the Mauthner cell is questionable.

Aminobutyrates↗
PubMedDate not supplied

Relationships between MR-imaged total amount of tissue removed, resection scores of specific mediobasal limbic subcompartments and clinical outcome following selective amygdalohippocampectomy.

Of 204 patients who underwent a selective amygdalohippocampectomy at our hospital, we studied 30 in whom special pre- and postoperative MR imaging was carried out and who were followed up for at least 1 year postoperatively. We measured the total size of the resection and the extent to which the following specific mediobasal temporal lobe structures had been removed: amygdala, hippocampus, pes hippocampi, dentate gyrus, parahippocampal gyrus, uncus and subiculum. Postoperative seizure control was correlated with the overall size of the resection as well as with the resection scores of the above mentioned limbic subcompartments. The mean size of the removed tissue was 7.2 cm3 (range: 2.1-17.7). The mean resection scores of the limbic subcompartments (in percentages) are: amygdala 92%, hippocampus 46%, pes hippocampi 92%, dentate gyrus 45%, parahippocampal gyrus 32%, uncus 92%, subiculum 40%. Although a small resection did not exclude a good outcome, the general tendency was that a better outcome was obtained from a larger resection. With regard to the resection scores of the limbic subcompartments, a positive correlation emerged between good postoperative outcome and the radicality of the removal of the parahippocampal gyrus (and the subiculum, which has been evaluated separately). These findings support our previously formulated amplifier hypothesis for the parahippocampal gyrus.

Adult↗