Manning J. Correia

An electrophysiological comparison of solitary type I and type II vestibular hair cells

We have studied the electrophysiological properties of enzymatically dissociated adult pigeon semicircular canal type I (chalice shaped) and type II (cylindrical shaped) hair cells using whole cell patch clamp techniques. Under current clamp conditions, type I hair cells often exhibit more hyperpolarized resting potentials than type II hair cells, and type I hair cells also have higher input conductances (measured with negative current steps) than type II hair cells. Under voltage clamp conditions, type I hair cells showed large, persistent outward currents over the range of about -70 to -50 mV, whereas, type II hair cells showed little or no current over this potential range. The persistent outward current of type I hair cells was not inactivated at a holding potential of -30mV, a potential that inactivated the rapidly inactivating, IA, and delayed rectifier, IK, currents of type II hair cells. This current probably contributes to both the resting potential and input conductance of type I hair cells.

MODIFICATION OF VESTIBULAR RESPONSES AS A FUNCTION OF RATE OF ROTATION ABOUT AN EARTH-HORIZONTAL AXIS,

Eight men completed an experiment in which they were rotated about an Earth-horizontal axis at velocities of 10 and 30 rpm. Both nystagmus and subjective estimates of body position in space were modified by the higher rate of rotation. Subjects who gave essentially veridical estimates of body position at 10 rpm became disoriented at 30 rpm and gave responses closely resembling those of subjects with labyrinthine dysfunction. Subjects who produced sustained unidirectional horizontal nystagmus during constant velocity rotation at 10 rpm produced a reversing horizontal nystagmus during comparable intervals of rotation at 30 rpm. Nystagmus slow phase velocity for both 10 and 30 rpm exhibited a cyclic modulation which was related to orientation relative to gravity. As in previous studies, sickness was produced by rotation about a horizontal axis, and a relationship between mental task and incidence of sickness was again noted.

Responses of semicircular canal and otolith afferents to small angle static head tilts in the Gerbil

The discharge activity of first-order vestibular neurons was recorded in anesthetized or decerebrated gerbils from the post-ganglionic fibers of the vestibular nerve. Semicircular canal afferents were distinguished from otolith afferents on the basis of their responses to linear and angular head acceleration. In decerebrated preparations, canal afferents exhibited significantly faster discharge activity (average = 87.8 impulses/s) than that of canal afferents in anesthetized preparations (average = 66.2 impulses/s), when the head was held to position the lateral semicircular canals coplanar with the earth horizontal plane (standard position). The effects of changes in linear forces on vestibular afferent activity were determined by statically tilting the head +/- 10 degrees about either the fore-aft and/or left-right head axes. A change in activity, from that recorded in the standard position, of 10% or greater was considered significant. Using this criterion, significant changes in the tilt response in anesthetized animals were observed in both anterior (23 of 48 neurons, 48%) and lateral (22 of 31, 71%) canal afferents as well as otolith (18 of 25, 72%) afferents. In decerebrated preparations for tilts around the pitch (left-right) axis, comparable effects were measured in (19 of 36, 53%) anterior and (17 of 30, 57%) lateral canal afferents. Neurons with irregular firing activity were more likely than regularly firing canal afferents to change their average discharge rate during static tilt. No significant differences in response magnitude to +/- 10 degrees head tilt were found between canal and otolith afferents in anesthetized animals. Mechanisms to account for the responses to linear acceleration of canal afferents are discussed.

The delayed rectifier, IKI, is the major conductance in type I vestibular hair cells across vestibular end organs

Hair cells were dissociated from the semicircular canal, utricle, lagena and saccule of white king pigeons. Type I hair cells were identified morphologically based on the ratios of neck width to cuticular plate width (NPR < 0.72) as well as neck width to cell body width (NBR < 0.64). The perforated patch variant of the whole-cell recording technique was used to measure electrical properties from type I hair cells. In voltage-clamp, the membrane properties of all identified type I cells were dominated by a predominantly outward potassium current, previously characterized in semicircular canal as IKI. Zero-current potential, activation, deactivation, slope conductance, pharmacologic and steady-state properties of the complex currents were not statistically different between type I hair cells of different vestibular end organs. The voltage dependence causes a significant proportion of this conductance to be active about the cell′s zero-current potential. The first report of the whole-cell activation kinetics of the conductance is presented, showing a voltage dependence that could be best fit by an equation for a single exponential. Results presented here are the first data from pigeon dissociated type I hair cells from utricle, saccule and lagena suggesting that the basolateral conductances of a morphologically identified population of type I hair cells are conserved between functionally different vestibular end organs; the major conductance being a delayed rectifier characterized previously in semicircular canal hair cells as IKI.

Bilateral communication between vestibular labyrinths in pigeons

Extracellular action potentials from single horizontal semicircular canal primary afferent fibers were recorded in paralysed decerebrate pigeons during pulse mechanical stimulation of the contralateral horizontal semicircular canal. Clear responses to the contralateral membranous duct displacement stimuli were observed in 51% of the tested 158 horizontal semicircular canal afferents. Generally, three different types of responses were obtained in the primary afferent fibers including excitation, inhibition, and a few complex type neural activity profiles. Inhibitory responses were of larger amplitude and had longer time constants than did excitatory responses. The few complex type responses observed were characterized by an initial excitatory discharge followed by a longer duration decrease in the fibers firing rate. The sensitivity to stimulation and type of response obtained for each afferent was significantly correlated with the fibers coefficient of variation value. Regular firing afferents were less sensitive and exhibited primarily excitatory responses (71%) to contralateral canal stimulation. Irregular firing afferents were more sensitive and exhibited mostly inhibitory responses (84%). The present results demonstrate that a communication network for information exchange between the bilateral labyrinths exists in pigeons. The observed responses in primary afferent fibers to contralateral horizontal semicircular canal stimulation are proposed to be mediated by the vestibular efferent system, which could provide an anatomical pathway for information exchange from vestibular receptors on opposite sides of the head.

Models of membrane resonance in pigeon semicircular canal type II hair cells

Pigeon vestibular semicircular canal type II hair cells often exhibit voltage oscillations following current steps that depolarize the cell membrane from its resting potential. Currents active around the resting membrane potential and most likely responsible for the observed resonant behavior are the Ca++-insensitive, inactivating potassium conductance IA (A-current) and delayed rectifier potassium conductance IK. Several equivalent circuits are considered as representative of the hair cell membrane behavior, sufficient to explain and quantitatively fit the observed voltage oscillations. In addition to the membrane capacitance and frequency-independent parallel conductance, a third parallel element whose admittance function is of second order is necessary to describe and accurately predict all of the experimentally obtained current and voltage responses. Even though most voltage oscillations could be fitted by an equivalent circuit in which the second order admittance term is overdamped (i.e., represents a type of current with two time constants, one of activation and the other of inactivation), the sharpest quality resonance obtained with small current steps (around 20 pA) from the resting potential could be satisfactorily fit only by an underdamped term.

Neuromathematical Concepts of Point Process Theory

The mathematical foundations for describing a discrete series of events such as a spike train are presented by way of a point process theoretic approach. Various significance tests and empirical descriptors are discussed. These may be used in fitting a model to spike train data. The application of these techniques to the single unit, spontaneous activity from peripheral semicircular canal units in the pigeon is demonstrated as an example.

Ionic currents in regenerating avian vestibular hair cells.

By applying the conventional whole‐cell patch‐clamp technique in combination with the slice procedure, we have investigated the properties of avian semicircular canal hair cells in situ. Passive and active electrical properties of hair cells from control animals have been compared with those of regenerating hair cells following streptomycin treatment (that killed almost all hair cells). Regenerating type II hair cells showed patterns of responses qualitatively similar to those of normal hair cells. However, parameters reflecting the total number of ionic channels and the surface area of type II hair cells changed during recovery—suggesting that new hair cells came from smaller precursors which (with time) reacquired the same electrophysiological properties as normal hair cells. Finally, we have investigated the ionic properties of a small sample of type I hair cells. Ionic currents of regenerating type I hair cells did not show, at least in the temporal window considered (up to 10 weeks from the end of the streptomycin treatment), the typical ionic currents of normal type I hair cells, but expressed instead ionic currents resembling those of type II hair cells. The possibility that regenerating type I hair cells can transdifferentiate from type II hair cells is therefore suggested.

Organization of ascending auditory pathways in the pigeon (Columbia livia) as determined by autoradiographic methods

A mixture of tritiated proline and fucose was injected into the labyrinthine endolymphatic space of 5 white king pigeons (Columba livia). Using standard autoradiographic techniques, we observed transsynaptic labeling in ascending auditory pathways to the level of the mesencephalon. Auditory system structures, ipsilateral to the injection site, which labeled heavily were the cochlear nerve, the magnocellular and angular nuclei, and the superior olive. Those ipsilateral structures which were slightly labeled were the lateral lemniscus and the dorsal part of the lateral mesencephalic nucleus. Contralateral structures which labeled were the superior olive, lateral lemniscus, and dorsal part of the lateral mesencephalic nucleus. The results of this study suggest that ascending auditory pathways (to the level of the mesencephalon) in the pigeon are more similar to those described for mammals in general than previously thought.

A method for controlled mechanical stimulation of single semicircular canals

In the present technical report, we describe a method of mechanical stimulation for single semicircular canals that is reproducible, sensitive, and discrete. The mechanical stimulator is capable of delivering reliable controlled stimuli of different waveforms over a wide frequency range that produce endolymph movements and consequent cupula deflections without motion being imparted to the animal. The results of electrophysiological experiments where the responses from pigeon (Columba livia) single horizontal semicircular canal afferent fibers produced by mechanical stimulation across a broad frequency bandwidth are reported. Comparisons between afferent fiber responses elicited by natural yaw rotation and mechanical stimulation were conducted, with the results indicating that the two stimulation methods produced responses from the same afferent unit that could be equated in magnitude. In addition, results are described from several control experiments that were conducted in order to determine the efficacy of the mechanical stimulation technique.