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P Valli

Publications and source records attributed to P Valli.

At least 73 records · Page 4Linked to original sources
PubMed1980

[Effects of some bivalent and trivalent ions and of EGTA on the process of activation of the ampullar receptors].

In order to define the role of Ca++ in labyrinthine receptors the endolymph in isolated canal preparations was replaced with modified solutions containing Ca-substitutes(Sr++, Ba++), Ca-antagonists(La+++, Cd++, Mg++) and Ca-chelating agents(EGTA). The response of the receptors was evaluated by recording the slow and propagated potentials from the ampullar nerve. All the tested solutions proved able to depress or cancel the mechanically evoked responses of ampullar receptors. The presence of Ca++ in the endolymph is therefore essential to grant the mechanosensitivity of the hair cells. The receptor blockade produced by Ca-withdrawal develops much earlier under continuous mechanical stimulation. This suggests that Ca++ might be bound in some structures and released during cupula deflections.

Animals↗
PubMed1980

Activation of the efferent system in the isolated frog labyrinth: effects on the afferent EPSPs and spike discharge recorded from single fibres of the posterior nerve.

Intra-axonal recordings were obtained from single afferent fibres of the posterior nerve in the isolated labyrinth of the frog (Rana esculenta). EPSPs and spike discharge were recorded both at rest and during rotatory stimulation of the canal. Electrical stimulation of either the distal end of the cut posterior nerve or of the central stumps of the anterior-horizontal nerves elicited a frequency-dependent inhibitory effect on the afferent discharge arising from the posterior canal. Denervation experiments revealed that inhibition is mediated by efferent fibres exhibiting a high degree of branching in the proximal part of the eighth nerve. The inhibitory effect was selectively cancelled by (1)D-tubocurarine 10(-6) M; (2) atropine 5 x 10(-5) M; (3) acetylcholine or carbachol 10(-4) M; (4) eserine 10(-5) M. Inhibition is thus most likely to be sustained by the release of acetylcholine from the efferent nerve terminals. Experiments in which the ionic composition of the external medium was modified suggest that the transmitter acts mainly by opening the chloride ion channels of the hair cell membrane. In some units the same stimulation pattern evoked a consistent increase in both EPSP and spike discharge, instead of inhibition. Such facilitation was unaffected by drugs for ionic modifications which block the efferent synapse, but disappeared after denervation. Inhibition and facilitation, therefore, act as two control mechanisms which are able to modify substantially, at the first stage of processing, the sensory information which is sent to the vestibular second order neurons.

Acetylcholine↗
PubMed1977

Post-synaptic potentials recorded from afferent nerve fibres of the posterior semicircular canal in the frog.

Glass microelectrode recordings were made from single fibres of the posterior ampullary nerve in the isolated labyrinth of the frog (Rana esculenta). Potentials were recorded both at rest and during rotatory stimulation of the canal. At rest, the tracings revealed an intense background of small, largely summated potentials (0.5-10 mV amplitude, 3-6 msec duration), which underlay the discharge of spikes in all the impaled units. The frequency of the subthreshold events was related to the frequency of the propagated spikes, the latter ranging from 0 to 40/sec. Stimulation modulated the frequency of both spikes and subthreshold potentials, whose summation during excitation led to a positive shift of the fibre membrane potential. The small potentials proved to be dependent on Ca2+ and Mg2+ levels in the bath. Antidromic-stimulation of the posterior ampullary nerve indicated that the observed events do not represent an artifact due to extracellular field interference related to spike activity in the neighbouring fibres. Tetrodotoxin (10(-7)-10(-6) g/ml) applied externally to the preparation or previously perfused through the frog vessels abolishes the propagated spikes but left unaffected the small potentials which, even under drug treatment, were normally modulated by the stimulus. The subthreshold potentials thus appear to be EPSPs generated at the cyto-neural junction between the hair cells and the endings of the ampullary nerve fibres.

Action Potentials↗
PubMedDate not supplied

The origin of slow potentials in semicircular canals of the frog.

Slow potentials generated in the sensory organ of the ampulla were recorded in isolated semicircular canals of the frog by means of fluid electrodes. These potentials, which may be picked up from the intrampullar fluid and those from the ampullar nerve appear to be generated at different stages of the process taking place in crista ampullaris. Slow intra-ampullar potentials apparently reflect receptor potentials of hair cells. They are preserved after degeneration of the nerve fibre endings and are relatively insensitive to DNP poisoning; their amplitude is maximum at high K+ concentrations. Slow nerve potentials appear to be due to electronic spreading of post-synaptic excitatory potentials generated at cytoneural junctions. They disappear after degeneration of the nerve fibres, in low Ca++ and high Mg++ solutions and are extremely sensitive to DNP poisoning. An analysis of the time-course of the slow ampullar and nerve potentials referred to the discharge of impulses in afferent fibres was performed with a view to interpreting the transduction mechanism of semicircular canals.

Animals↗
PubMedDate not supplied

The importance of potassium in the function of frog semicircular canals.

The slow potentials and afferent discharge of impulses in frog semicircular canals have been studied at different endolymphatic and perilymphatic K+ concentrations. Results indicate that the presence of K ions in the bathing fluids is essential for maintaining the receptor function in crista ampullaris, although very low concentrations of this ion in the perilymph are sufficient to preserve the receptor responsiveness to mechanical stimuli. The hypothesis is put forward that K+ may be pumped from the exterior of the canal towards the intracupular structures, where it accumulates. A K-rich endolymphatic environment does, however, appear to be necessary to ensure the resting activity of ampullar receptors and their ability to be "disfacilitated" during inhibitory cupula deflections.

Action Potentials↗
PubMedDate not supplied

Ionic composition of the endolymph and sensory transduction in labyrinthine organs.

The role of various ions in stimuli conversion has been investigated in isolated frog semicircular canals, by replacing the endolymph with solutions deprived in turn of Na+, K+, Cl- and Ca++. Mechanical and electrical stimuli were employed and slow ampullar and nerve potentials were recorded, together with the afferent discharge of impulses in the eighth nerve. The results support evidence that K+ is the carrier of the receptor current, as postulated in Davis' hypothesis, while Ca++ is essential in the processes which allow the current to be modulated during cupula deflections and therefore endon hair cells with mechanical sensitivity.

Acoustic Stimulation↗
PubMedDate not supplied

Ionic mechanisms sustaining activity in ampullar receptors of the frog.

Ionic mechanisms sustaining sensory transduction in crista ampullaris sensory cells have been investigated chiefly by replacing the endolymph with solutions with K+ and Ca++ chelators added. The effects of the modified solutions were evaluated by extracellular and intracellular recordings of both and the presynaptic and postsynaptic activity of ampullar receptors. The results strongly suggest that the receptor current in labyrinthine cells is carried exclusively by K+. Moreover evidence are reported indicating that the transducer membrane in ampullar receptors is provided with Ca++ sensitive potassium channels whose opening is depending on free Ca++ released from cupular structures during excitatory deflections.

Animals↗
PubMedDate not supplied

Local mechanisms in vestibular receptor control. Effects of curare on the EPSPs and spike discharge recorded from single afferent fibres of the posterior canal nerve of the frog.

In the frog, the afferent discharge of EPSPs and spikes has been recorded intracellularly from single fibres of the posterior ampullar nerve before and after administration of D-tubocurarine (5 X 10(-6) M) in the following experimental conditions: (a) in animals in which the labyrinth was merely exposed (intact frogs), (b) in isolated acute labyrinth preparations, (c) in isolated chronic labyrinth preparations after degeneration of the efferent fibres. In both (a) and (c), curare induced either facilitation or inhibition of the resting discharge in a large number of units. By contrast, after degeneration of the efferent fibres, the effects of the drug were almost completely suppressed. These results indicate the existence of peripheral mechanisms involving the efferent nerve terminals which can control the release of afferent transmitter even in the absence of any central influence.

Animals↗
PubMedDate not supplied

[On the mechanism of the formation of traumatic lesions of the larynx: observations on three cases (author's transl)].

The authors, having drawn attention to the rarity of traumatic lesions of the larynx, illustrate three cases mentioning in particular the disproportion between the external and internal lesions, (as has already been pointed out by other authors), and the mechanism of the formation of such lesions. This, in particular, on the strenght of interpretations of slight cutaneous lesions.

Accidents, Traffic↗
PubMedDate not supplied

[Anatomo pathological and medicolegal data on 4 cases of unwitnessed death of infants].

The Author gives his contribution to the study of "untestified" death of children still unweaned and describes four cases. Except for one case (porpora encefalica) the macroscopic appearance is insignificant. On the other hand, the histologic findings demonstrate presence of several foci of infiltration of the heart, lung, liver and brain. The Author suggests the diagnosis of viral disease even-thoug more accurate bacterial and viral exames are indispensable for a precise definition of the cases reported.

Age Factors↗