According to Davis, the transduction mechanism in labyrinthine receptors depends on a biological battery, located somewhere in the sense organ, which drives a steady current across the hair cells. When the hairs are bent, current flow could be modulated across the cells by varying ohmic resistance in their apical membranes, thereby controlling transmitter release at cytoneural junctions and the resulting discharge of impulses in afferent nerve fibres. On the basis of this hypothesis, we thought it would be of interest to test the effects of current on the sensory epithelium of crista ampullaris. Isolated preparations of posterior semicircular canals were dissected from frogs (Rana esculenta). Appropriate mechanical and electrical stimuli were applied to obtain receptor excitation or inhibition of comparable intensities. The responses were evaluated by counting the number of action potentials in ampullary nerve branches during the first 5 s following the onset of stimulation. The underlying level of resting discharge was subtracted. Anodic DC currents (5-10 μV inside the ampulla positive) were found to produce an immediate rise in receptor discharge frequency. The effects of cathodic DC currents of comparable intensities were the opposite in all respects, with the discharge rate falling steeply and inhibition ensuing. The marked sensitivity of ampullary receptors to electric currents stresses their similarity with fish electroreceptors thus supporting phylogenetical and morphological evidence. Mechanical and electrical effects summed each other algebraically. In particular, it was possible, over a wide range of intensities, to suppress the current excitation by inhibition following suitable mechanical stimulation. As a consequence of adaptation, excitatory and inhibitory effects vanished within 25-30 s of the beginning of stimulation. In a medium having lowered Ca++ (0.1 mM) and enhanced Mg++ (10 mM) concentrations, the responses to both types of stimulation were cancelled, as was the resting discharge. Both low Ca++ or high Mg++ concentrations in the bathing fluid produced a parallel decline in responses to mechanical and electrical stimuli. Conversely, the resting discharge was depressed in high Mg++ but enhanced in low Ca++ Ringer, which is known to produce autorythmicity in nerve membranes. The parallel impairment of receptor responses in the presence of different Ca++ and Mg++ concentrations appeared to be solely related to a reduction in transmitter release at the cyto-neural junctions of crista ampullaris, irrespective of the excitability in post-synaptic endings. The main effect of currents is therefore merely presynaptic, as it is also suggested by the fact that inhibitory mechanical stimuli of suitable intensities can prevent any excitation brought about by currents. The present observations apparently support Davis' theory, since a moderate current flow through the sensory cells proved able to modulate transmitter release at their base poles. The severe depression of receptor resting discharge in low Ca++ and high Mg++ media suggests that this is due not to spontaneous transmitter release, but is actually sustained by a steady current driven across the synaptic membrane of the hair cells by a physiological battery. There is however a puzzling discrepancy between the effects of the physiological current sustaining endless resting activity and those of any superimposed stimulating current, which on the contrary displays marked adaptation. It may tentatively be supposed that the currents driven by the physiological battery and those acting as a stimulus may follow different pathways through the sensory cells, possibly affecting membranes having different adaptation properties.
Electrical polarization of the sensory epithelium in semicircular canals: effect of calcium and mafnesium on discharge frequency of ampullar receptors in response to mechanical and and electrical stimulation
ROSSI, Marialisa
1975
Abstract
According to Davis, the transduction mechanism in labyrinthine receptors depends on a biological battery, located somewhere in the sense organ, which drives a steady current across the hair cells. When the hairs are bent, current flow could be modulated across the cells by varying ohmic resistance in their apical membranes, thereby controlling transmitter release at cytoneural junctions and the resulting discharge of impulses in afferent nerve fibres. On the basis of this hypothesis, we thought it would be of interest to test the effects of current on the sensory epithelium of crista ampullaris. Isolated preparations of posterior semicircular canals were dissected from frogs (Rana esculenta). Appropriate mechanical and electrical stimuli were applied to obtain receptor excitation or inhibition of comparable intensities. The responses were evaluated by counting the number of action potentials in ampullary nerve branches during the first 5 s following the onset of stimulation. The underlying level of resting discharge was subtracted. Anodic DC currents (5-10 μV inside the ampulla positive) were found to produce an immediate rise in receptor discharge frequency. The effects of cathodic DC currents of comparable intensities were the opposite in all respects, with the discharge rate falling steeply and inhibition ensuing. The marked sensitivity of ampullary receptors to electric currents stresses their similarity with fish electroreceptors thus supporting phylogenetical and morphological evidence. Mechanical and electrical effects summed each other algebraically. In particular, it was possible, over a wide range of intensities, to suppress the current excitation by inhibition following suitable mechanical stimulation. As a consequence of adaptation, excitatory and inhibitory effects vanished within 25-30 s of the beginning of stimulation. In a medium having lowered Ca++ (0.1 mM) and enhanced Mg++ (10 mM) concentrations, the responses to both types of stimulation were cancelled, as was the resting discharge. Both low Ca++ or high Mg++ concentrations in the bathing fluid produced a parallel decline in responses to mechanical and electrical stimuli. Conversely, the resting discharge was depressed in high Mg++ but enhanced in low Ca++ Ringer, which is known to produce autorythmicity in nerve membranes. The parallel impairment of receptor responses in the presence of different Ca++ and Mg++ concentrations appeared to be solely related to a reduction in transmitter release at the cyto-neural junctions of crista ampullaris, irrespective of the excitability in post-synaptic endings. The main effect of currents is therefore merely presynaptic, as it is also suggested by the fact that inhibitory mechanical stimuli of suitable intensities can prevent any excitation brought about by currents. The present observations apparently support Davis' theory, since a moderate current flow through the sensory cells proved able to modulate transmitter release at their base poles. The severe depression of receptor resting discharge in low Ca++ and high Mg++ media suggests that this is due not to spontaneous transmitter release, but is actually sustained by a steady current driven across the synaptic membrane of the hair cells by a physiological battery. There is however a puzzling discrepancy between the effects of the physiological current sustaining endless resting activity and those of any superimposed stimulating current, which on the contrary displays marked adaptation. It may tentatively be supposed that the currents driven by the physiological battery and those acting as a stimulus may follow different pathways through the sensory cells, possibly affecting membranes having different adaptation properties.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
