A study of neurone activity in the spiral ganglion of the cat's basal turn

Evaluation of a model of the cochlear neural membrane. I. Physiological measurement of membrane characteristics in response to intrameatal electrical stimulation

To understand the auditory neural response to electrical stimuli similar to those used in a cochlear implant, it will be necessary to understand the neural refraction and summation response kinetics. Evidence exists indicating that the cell soma may alter the auditory neural response kinetics and could be the site of conduction failure for excitation initiated on the peripheral process. There is, however, reason to believe that the excitation site in some healthy, type I neurons and in pathological, type III neurons is the central process of the cell. To characterize the neural response to activation at a controlled central process site, cat auditory neurons were stimulated with an intrameatal electrode, and the summation and refraction response kinetics were measured. This approach was used to: (1) characterize the behavior of the neural response to central process excitation; (2) make comparisons between intrameatal excitation at a known central site and scala tympani excitation at an unknown site; and (3) provide membrane characterization free from the possible alteration of membrane kinetics produced by the cell soma. The membrane kinetics measured using intrameatal stimulation differ from those recorded with scala tympani stimulation indicating that the mechanisms for scala tympani and intrameatal stimulation differ.

Recovery characteristics of auditory nerve fibres in the normal and noise-damaged guinea pig cochlea

Spontaneous activity was analysed in auditory-nerve fibres innervating normal and noise-damaged cochleas. Spike occurrences were conceived as point processes. Joint interval distributions and serial correlation coefficients reveal a weak history effect for succeeding intervals. The point process is regarded as a renewal and the recovery function, being proportional to the hazard function, is determined from the interval probability density function. In 29 out of 60 fibres the latter shows peculiarities which result in a deviation from a monotonically increasing recovery function. For three fibers of low characteristic frequency the interval probability function shows an oscillatory pattern and for 26 fibres this function exhibits an early, sharp peak around 1.1 ms irrespective of characteristic frequency, spontaneous rate, or cochlear damage. The recovery function is not different between fibres with normal and those with abnormally high thresholds and exhibits an exponential recovery with one time constant of average value 1.6 ms. Bursting activity is found in only one fibre from the abnormally high threshold group.

Basilar membrane mechanics in the hook region of cat and guinea-pig cochleae: sharp tuning and nonlinearity in the absence of baseline position shifts

A heterodyne laser interferometer was used to observe the movements of small (approximately 20 microns) stainless-steel beads placed on the basilar membrane in the hook region of cat and guinea-pig cochleae. In several preparations, the displacement patterns observed exhibited sharp nonlinear tuning; in one cat this tuning was comparable to that commonly observed in single auditory-nerve fibers. The most sensitive frequencies of the preparations ranged from 31-40 kHz in the cat, and 28-32 kHz in the guinea-pig. The sharp tuning and nonlinearity of the basilar membrane responses was not apparent in surgically or acoustically traumatized preparations. The response nonlinearities were susceptible to temporary threshold shifts and disappeared within a few minutes post-mortem. Stimulus-related shifts in the baseline position of the basilar membrane were not apparent at low stimulus levels. Such shifts were occasionally observed at higher stimulus levels (e.g., > 90 dB SPL), but never approached the fundamental (oscillatory) component of basilar membrane vibration in magnitude. These findings are discussed in relation to previous observations by other workers.

Basilar membrane tonotopicity in the hook region of the cat cochlea

Middle-ear to basilar membrane (BM) velocity transfer functions are reported for seven locations in the hook region of a single cat cochlea. These transfer functions were recorded at high sound pressure levels in a linearized, or passive cochlea, and resemble those reported previously by Wilson and Evans (1983). They demonstrate longitudinal tonotopicity with a gradient of approximately 3.6 mm/octave. When allowances are made for the nonlinear mechanisms previously demonstrated in active hook region preparations (Cooper and Rhode, 1992), the data are also consistent with the tonotopic map derived from the intracellular dye-filling studies of Liberman (1982).

Postsynaptic inhibition can explain the concentration of short inter-spike-intervals in avian auditory nerve fibres

Spontaneous and sound-evoked single-unit activity was recorded from afferent neurones in the cochlear ganglion of the anaesthetized pigeon. The histogram of successive intervals of spontaneous activity of 51% of neurones exhibited more short intervals than expected from a Poisson point-process description of spike times; for another 43% of neurones the point-process was Poisson. A model of spike generation was developed to account for the concentration of short spike-intervals. The proposed model contains inhibitory postsynaptic potentials at the afferent dendrite, in addition to the excitatory postsynaptic potentials. Not only does the model reproduce the first-order interval statistics of neural activity, but it provides a mechanism for improving phase-locking to the fundamental frequency of a sinusoid, and also offers an explanation for the presence of reciprocal synapses in the human cochlea.

Ultrastructural studies of the peripheral extensions (dendrites) of type I ganglion cells in the cat

An ultrastructural study of peripheral extensions (dendrites) of type I ganglion cells in seven healthy adult cats and one cat that underwent bilateral cochlear injection of neomycin was undertaken. Morphologic evidence revealed that the peripheral process (dendrite) consistently has a smaller diameter than the central process. As the dendrite reaches the cell body, there is a constricted segment with a length that ranges from 10 to 30 microns, and a diameter of 0.5 microns. This region is covered by a continuous myelin sheath that does not thin. The central process (axon) does not have a corresponding constriction, and consists of a myelin covered, smoothly tapered segment that extends to the first axonal node of Ranvier. In the deafened cat, some cell bodies of the surviving ganglion cells appeared to have a residual portion of dendrite. These morphologic characteristics might have physiologic implications both in the mechanisms of normal hearing and in cochlear implantation.

Physiological properties of the electrically stimulated auditory nerve. II. Single fiber recordings

Single fiber recordings from the electrically stimulated auditory nerve yield post-stimulus time (PST) histograms demonstrating several response patterns. With pulsatile stimulation of the cochlea, the PST histogram for most fibers at threshold consists of a long-latency (500-800 microseconds), broad response peak with significant latency variability. At increased stimulus intensities, the response pattern changes to a short-latency (300-500 microseconds), high-synchrony peak. In preparations where stimulation is applied directly to the axons of the auditory nerve, the response pattern consists solely of a short-latency, high-synchrony peak. It is postulated that threshold excitation of normal auditory neurons occurs on the dendritic processes. At higher stimulus intensities, the site of excitation appears to shift to the axonal region of the cells. Two additional response patterns to electrical stimulation which are attributed to synaptic excitation of the auditory neurons via the hair cells are described.

Physiological properties of the electrically stimulated auditory nerve. I. Compound action potential recordings

The electrically evoked compound action potential (CAP) of the auditory nerve exhibits two peaks, termed N0, at 350 microseconds latency, and N1, at 550 microseconds latency. At low stimulus intensities the CAP consists solely of the long latency N1 peak. As the stimulus strength is increased the higher threshold N0 appears. At high stimulus intensities N1 disappears and only the N0 component of the CAP remains. It is postulated that N1 represents action potentials propagated from the dendritic processes of the auditory neurons and that N0 represents action potentials initiated on the axons of these cells. The N1 peak exhibits anomalous refractory behavior which can be identified in the electrically evoked auditory brainstem response (EABR). That behavior may be useful diagnostically in assessing the extent of dendrite degeneration in cochlear implant candidates and users.

Effect of divalent cations on spontaneous and evoked activity of single mammalian auditory neurones

Recordings were obtained from single primary auditory neurones in the guinea pig cochlea during perfusion of the scala tympani with solutions containing elevated concentrations of Mg2+, Mn2+ and Co2+, or with lowered concentrations of Ca2+. Such perfusions caused a reversible depression of spontaneous firing rates. This is consistent with the notion that spontaneous firing is the result of background release of excitatory transmitter from cochlear hair cells, in the absence of acoustic stimulation. The above ion modifications also produced varying changes in single neurone response curves to acoustic stimuli. In one half of the 14 neurons studied these changes were also compatible with a classical blockage of synaptic transmission. The other half however, showed little or no change in sensitivity at low frequencies while large threshold elevations occurred at high frequencies.