C. Casella

Activation of the efferent system in the isolated frog labyrinth: Effects on the afferent EPSPs and spike discharge recorded from single fibers 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.

Electrical properties and synaptic connections of the sympathetic neurons in the rat and guinea-pig superior cervical ganglion

SummaryFeatures of action potentials and synaptic potentials as obtained by intracellular recordings from the sympathetic ganglion cells in the superior cervical ganglion of both the rat and the guinea-pig are described. Time-constant, imput resistance and capacitance of the ganglion cell membrane were also measured. Rheobasic currents and chronaxies are evaluated from strength-latency recordings.Single stimulating shocks applied to pre-ganglionic fibres produce repeated activation in the sympathetic ganglion cells and neuron response appear to be dispersed over a considerable length of time. Simultaneous recordings of extracellular post-ganglionic discharge and intracellular response from single ganglion units showed that pre-ganglionic fibres with different conduction velocities converge on the neurons. This convergence can fully justify the temporal dispersion of the ganglion cell responses.

The effect of glutamate on the frog semicircular canal.

L-glutamate (Glu) has at least two sites of action in the frog semicircular canal: the hair cell (presynaptic) and the primary afferent nerve fibres (postsynaptic). Glus action on the hair cell results in an increased release of the natural transmitter which is responsible for a substantial increase in the frequency of firing in primary afferents. The presynaptic action of Glu is antagonized by D-alpha-aminoadipate (D alpha AA). Glu produces a long-lasting depolarization on the afferent nerve fibres which does not by itself elicit any afferent discharge of impulses when the release of the natural transmitter is prevented. Glu-induced nerve depolarization is only partially antagonized by D alpha AA. The difficulty of reconciling some of the observations made of the effects of Glu in semicircular canals with its presumed role as an afferent transmitter in this organ is discussed, but this role is not definitely rejected.

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.

Synaptically mediated potentials elicited by the stimulation of post-ganglionic trunks in the guinea-pig superior cervical ganglion.

SummaryStimulation of a post-ganglionic nerve trunk (internal carotid nerve) leads to synaptic activation of the majority of the sympathetic neurons in the superior cervical ganglion of the guinea-pig. Curarization of the ganglion and the section of the cervical sympathetic trunk eliminate any sign of synaptic activation in the ganglion cells produced by stimulating the internal carotid nerve. Stimulation of other post-ganglionic nerve trunks (superior cardiac nerve) gives rise only to pure antidromic responses in the cells. A compound action potential of considerable amplitude, which is completely cancelled by curarization of the ganglion, may be led off from the superior cardiac nerve after stimulating the internal carotid nerve; no propagated potential in the opposite direction was recorded at all. The conclusion is drawn that pre-ganglionic fibres projecting into the internal carotid nerve give rise to an elaborate system of collateral endings impinging onto most of the ganglion neurons and are responsible for synaptic activation of the ganglion cells observed after stimulating post-ganglionic nerves. The characteristics of action potentials evoked antidromically in the sympathetic neurons are also described.

Peripheral organization of the vestibular efferent system in the frog: an electrophysiological study

The distribution and the properties of efferent fibers in vestibular nerve were studied in the isolated frog labyrinth. Electrical stimulation of the central stump of any vestibular nerve branchlet elicited compound action potentials in all the other eighth nerve branchlets, indicating the existence of neural links between the various vestibular organs. The same experimental paradigm, when repeated in frogs with chronic section of the eighth nerve roots, demonstrated that these pathways are efferent collaterals extending to all vestibular organs. There are more collaterals linking the 3 semicircular canals than the otolith organs and the otoliths with the canal organs. Efferent connections in the eighth nerve were preserved in full after ablation of the ipsilateral hemi-cerebellum, suggesting that the efferent pathways probably originate in the brainstem. Intracellular recordings from single afferent fibers of both canal and otolith organs revealed that efferent fiber activation could elicit either inhibition or facilitation of the receptor discharge. It was concluded that the frog efferent vestibular system is endowed with non-selective control channels which allow single neurons to influence the receptor activity of different labyrinthine organs.

Interactions between taste receptors in the frog tongue.

SummaryReceptors sensitive to stimulation with dilute CaCl2 solutions and located in different fungiform papillae of the frog tongue are cross-connected via branches of the glossopharyngeal nerve fibers to form “chemical sensory units” comprising on average about 3 papillae; often one or more papillae were found to be common to different units.The receptor response observed when single papillae were individually stimulated increased, when the strength of the stimulus was raised, at a much lower rate than that obtained by stimulating the whole tongue surface. This proves that the antidromic impulses travelling in the cross-connections linking different papillae result in an evident depression of the receptor response to CaCl2.The possible functional significance of mutual interaction between the receptors in the frog tongue is discussed.

Functional organization of the peripheral efferent vestibular system in the frog.

The functional organization of the efferent vestibular system (EVS) was studied in the isolated frog labyrinth. To ascertain whether, besides the efferent branching fibres that innervate several end-organs, the EVS is also endowed with efferent non-branching axons which might control a given population of sensory units in each end-organ, the 8th nerve and one of its branchlets were electrically stimulated while recordings of the spontaneous activity arising from the different sensors were made by impaling single afferent axons in all the 8th nerve branchlets. The results demonstrated that the vast majority of the sensory units whose activity was modified by stimulating the whole 8th nerve was also affected by stimulating an 8th nerve branchlet. These findings therefore rule out the possibility that the EVS is endowed with projective fibres and strengthen the view that the EVS is a highly divergent system with collaterals arising from single parent axons that innervate several end-organs. These experiments have also shown that the percentage of sensory units which are actually controlled by the EVS varies amongst the different labyrinthine organs. It is maximal in the sacculus (ca. 90%), somewhat lower in canal organs (ca. 80%) and the utriculus (ca. 70%) and considerably lower in the lagena (ca. 50%). This EVS arrangement therefore might allow information arising from some organs to be modified more extensively than that from others.

Adaptive distortions in the generator potential of semicircular canal sensory afferents

The generator potential in sensory afferents of frog crista ampullaris was extracellularly recorded from the cut end of the posterior ampullary nerve by means of suction electrodes. A servocontrolled turntable allowed suitable rotatary stimulations. The analysis of the recorded generator potential revealed a different time course from that predicted on the basis of the pendulum model. Adaptation and undershoots in the responses to velocity ramps, steps and sinusoids, were mainly responsible for the deviations, which became very evident only when fairly high acceleration rates were applied. Both adaptation and undershoots were produced presumably by the activation of an electrogenic pump, probably located in nerv terminals contacting the hair cells. In fact, the time course of the generator potential became much more consistent with the predictions from the pendulum model under treatments capable of hindering the ion pump activity.

Stretch receptors stimulation in frog's lungs

SummaryThe response of the stretch receptors of the frogs lung to inflation together with pulmonary volume and pressure have been quantitatively evaluated by counting the action potentials led off from the vagus nerve in isolated lung-vagus preparations. It is seen that sustained and repeated inflations deeply change the mechanical properties of the lung and the receptors response to inflation. The response of the receptors, however, appears to be constantly and linearly related with the “tension” of the pulmonary wall considered as a stretched elastic lamina. It is concluded that tension of the pulmonary wall is the natural stimulus for the receptors of the frogs lung. Adaptation of the receptors to the stimulus appeared to be negligible over the inflation period.