Sensory transduction and neuronal transmission as related to ultrastructure and encoding of information in different labyrinthine receptor systems of vertebrates.

Mechano-electric transduction and neuronal transmission were studied in sensory systems ascending from and descending to single receptor cells of the labyrinth organs in submammalian vertebrates. The animals were young crocodiles (Caiman crocodilus), geckos (Gekko gecko, Tarentola mauritanica), and turtles (Pseudemys scripta elegans, Chinemys reevesii). Intracellular receptor potentials from the apical region of the hair cell (or from the ciliary surface) were recorded in the ampullar, macular, and papillar sensory cells. These single-cell responses are, within limits, proportional to stimulus amplitude, frequency, or phase and are bidirectional in that they show depolarization by kinociliopetal stereociliar displacement and hyperpolarization by kinociliofugal displacement. Synaptic potentials (presynaptic from the basal region of the hair cell, postsynaptic from the contacting nerve endings) were recorded in the utricular, saccular, and lagenar neuroepithelia with electron-optic localization of the in situ fixed microelectrode tip. As local excitatory or inhibitory processes, respectively, they follow the stimulus and receptor potential with latency and with nonlinear distortion. Action potentials (spikes), as synchronized by the excitatory synaptic potentials, were recorded from single nerve fibers or bipolar cells, related to ampullar, macular, or papillar receptor units. Unit responses and synaptic potentials were recorded from the first, second, and following centripetal and central neurons of the ascending systems, or from neurons of the descending systems in the brain stem or from centrifugal neurons. Such records were achieved during adequate mechanical or acoustical stimulation of the different receptor systems, with additional electrical stimulation, uni- or bilaterally. Thus, the influence of centripetal-centrifugal bilateral interaction on the receptor functions was measured, as inhibition or disinhibition, respectively. The input-output relations of these sequential stages of information transfer were plotted as histograms of different types, as characteristic curves, power spectra, or by correlation operations, with or without feedback, from the different systemic levels.U