Cochlear axon calibres are adjusted to characteristic frequencies

Dendritic Degeneration of Human Auditory Nerve Fibers and Its Impact on the Spiking Pattern Under Regular Conditions and During Cochlear Implant Stimulation

Due to limitations of human in vivo studies, detailed computational models enable understanding the neural signaling in the degenerated auditory system and cochlear implants (CIs). Four human cochleae were used to quantify hearing levels depending on dendritic changes in diameter and myelination thickness from type I of the auditory nerve fibers (ANFs). Type I neurons transmit the auditory information as spiking pattern from the inner hair cells (IHCs) to the cochlear nucleus. The impact of dendrite diameter and degree of myelination on neural signal transmission was simulated for (1) synaptic excitation via IHCs and (2) stimulation from CI electrodes. An accurate three-dimensional human cochlear geometry, along with 30 auditory pathways, mimicked the CI environment. The excitation properties of electrical potential distribution induced by two CI were analyzed. Main findings: (1) The unimodal distribution of control dendrite diameters becomes multimodal for hearing loss cases; a group of thin dendrites with diameters between 0.3 and 1 μm with a peak at 0.5 μm appeared. (2) Postsynaptic currents from IHCs excite such thin dendrites easier and earlier than under control conditions. However, this advantage is lost as their conduction velocity decreases proportionally with the diameter and causes increased spike latency and jitter in soma and axon. Firing probability reduces through the soma passage due to the low intracellular current flow in thin dendrites during spiking. (3) Compared with dendrite diameter, variations in myelin thickness have a small impact on spiking performance. (4) Contrary to synaptic excitation, CIs cause several spike initiation sites in dendrite, soma region, and axon; moreover, fiber excitability reduces with fiber diameter. In a few cases, where weak stimuli elicit spikes of a target neuron (TN) in the axon, dendrite diameter reduction has no effect. However, in many cases, a spike in a TN is first initiated in the dendrite, and consequently, dendrite degeneration demands an increase in threshold currents. (5) Threshold currents of a TN and co-stimulation of degenerated ANFs in other frequency regions depend on the electrode position, including its distance to the outer wall, the cochlear turn, and the three-dimensional pathway of the TN.

A role of oligodendrocytes in information processing

Myelinating oligodendrocytes enable fast propagation of action potentials along the ensheathed axons. In addition, oligodendrocytes play diverse non-canonical roles including axonal metabolic support and activity-dependent myelination. An open question remains whether myelination also contributes to information processing in addition to speeding up conduction velocity. Here, we analyze the role of myelin in auditory information processing using paradigms that are also good predictors of speech understanding in humans. We compare mice with different degrees of dysmyelination using acute multiunit recordings in the auditory cortex, in combination with behavioral readouts. We find complex alterations of neuronal responses that reflect fatigue and temporal acuity deficits. We observe partially discriminable but similar deficits in well myelinated mice in which glial cells cannot fully support axons metabolically. We suggest a model in which myelination contributes to sustained stimulus perception in temporally complex paradigms, with a role of metabolically active oligodendrocytes in cortical information processing.

The Significance of Internode Length for Saltatory Conduction: Looking Back at the Age of 90

The development of peripheral nerve fibers involves interdependence between the timing of Schwann cell recruitment during myelination and elongation of the nerve. This adjusts the number and the length of internodes to the length of the fiber. Saltatory conduction in longer nerves involves longer saltations; this makes internode length the factor that determines conduction velocity, thereby adjusting impulse transmission in circuits of different lengths. Myelination increases conduction velocity by means of saltatory conduction but what determines the saltatory conduction is not so much the indispensable insulating adjunct of myelin as the length of the internodes that separate the excitable membrane segments. We have previously studied the development of the length and proportion of internodes in some detail. If the anatomical data are combined, the data fall in place for a revised understanding of conduction velocity and the system that adapts the conduction properties of peripheral nerves to fiber lengths and to body size.

Why do axons differ in caliber?

CNS axons differ in diameter (d) by nearly 100-fold (∼0.1-10 μm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate.

The use of long-duration current pulses to assess nerve survival

This study investigated the usefulness of long-duration current pulses in assessing the status of the auditory nerve in ears with various degrees of retrograde neural degeneration. Guinea pigs were deafened with aminoglycosides prior to acute implantation of the cochlea and collection of electrically evoked auditory brainstem responses (EABRs). Analysis of wave I evoked with long-duration current pulses suggests that this evoked response is sensitive to degeneration of the peripheral processes of the auditory nerve. Correlations with spiral ganglion cell density show that EABR measures obtained with long-duration pulses are comparable to those previously established for estimating nerve survival. Further analysis indicates that this measure may provide unique information about the degenerative state of the nerve. Threshold EABR measures using long-duration pulses are evidently more place-specific than other measures. Also, results suggest that long-duration pulses may be sensitive to two phases of the degenerative process: degradation of the peripheral processes and subsequent degeneration of neural processes central to the spiral ganglion.

The diameters of guinea pig auditory nerve fibres: distribution and correlation with spontaneous rate

In the mammalian auditory nerve physiological recordings revealed that the spontaneous discharge rate of single auditory fibres correlates with the diversity of input-output functions which may be important for intensity discrimination (e.g., Sachs and Abbas, 1974, Liberman, 1978; Winter et al., 1990). In this study we determined if the spontaneous discharge rate of auditory nerve fibres in the guinea pig is correlated with an anatomical feature, namely the diameter of the respective fibres. The diameter of myelinated (Type I) guinea pig auditory nerve fibres was measured after staining with different techniques. Measurements were made on semithin sections using a video image analysis system. The diameters of fibres stained with toluidine blue from the portion of the auditory nerve containing fibres from the basal turn of the cochlea were found to have a normal distribution. Fibres were also labelled with horseradish peroxidase by bulk injection into the spiral ganglion. It was found that the presence of horseradish peroxidase within the fibres reduced the measured diameter in comparison to adjacent unlabelled fibres. A number of fibres were physiologically characterized with respect to spontaneous discharge rate and subsequently intracellularly labelled with horseradish peroxidase. Fibre diameter of a selected sample of intracellularly fibres was measured over a distance of 800 microns within the internal auditory meatus. At the positions nearest to the spiral ganglion fibres possessing low spontaneous rates were found to have smaller diameters than high spontaneous rate fibres. No difference in fibre diameter was found for the positions near the cochlear nucleus.

Morphometric analysis of the cochlear nerve in man

Fiber diameters were analyzed in the meatal segment of the cochlear nerve from 7 temporal bones obtained from 7 patients. Two patients had normal hearing for their age. Two had sustained noise exposure and one had presbyacusis of predominantly neural type. The cochleae displayed characteristic degeneration patterns. The other two manifested hearing loss of unspecified type. The fiber diameters ranged from 0.5 to 11 microns. The diameter distribution was unimodal in all seven nerves. The means of the diameters ranged from 4.2 to 5.5 microns. They were significantly different between patients with age-related normal hearing on the one hand and patients with noise induced hearing loss and neural presbyacusis on the other. The findings are discussed in relation to changes in nerve conduction speed and hearing loss; a possible correlation between the fiber diameter distribution and the tonotopical arrangement of the cochlea is suggested.

Reduced conduction velocity of retinal Y-cell axons following early partial removal of their synaptic targets

Following unilateral visual cortex ablation in neonatal kittens, the extraretinal conduction velocities of retinal Y-cell axons projecting to the lesioned hemisphere were measured and compared to the velocities of Y-cell axons projecting to the intact hemisphere. The extraretinal velocities of Y-cells projecting to the lesioned hemisphere were 16-28% lower than the velocities of Y-cells projecting to the intact hemisphere. This effect was seen in axons originating in both the ipsilateral and contralateral retinae. These data are consistent with the hypothesis that one factor regulating axon size is the amount of synaptic territory which is available to the cell. When the velocities of the unmyelinated intraretinal portions of Y-axons were examined in experimental animals, no significant differences could be detected between axons projecting to the lesioned hemisphere and those projecting to the intact hemisphere.

Analysis of the human auditory nerve

In human temporal bones of patients with normal hearing or sensory neural deafness, the cochlear neurons were quantitatively and qualitatively evaluated at the level of the osseous spiral lamina, the spiral ganglion and the cochlear nerve. We found from 32,000 to 31,000 myelinated nerve fibres in the cochlear nerve of normal hearing individuals and any lower number in cases of sensory neural deafness. There was in general a good correspondence between the counted numbers of the myelinated nerve fibres in the osseous spiral lamina, the spiral ganglion cells and the myelinated nerve fibres in the cochlear nerve in the inner acoustic meatus. The diameter of the peripheral axons of the type I neurons are about half the diameter of the central axons. The average diameter of the central axons is 2.5 millimicrons with a narrow distribution in children, but an increasingly larger range of fiber calibers with increasing age (0.5 to 7 microns in the 40 to 50 year age group adults).

Changes of the ratio between myelin thickness and axon diameter in human developing sural, femoral, ulnar, facial, and trochlear nerves

Previous studies on sural nerves were extended to human femoral, ulnar, facial and trochlear nerves. As asynchronous development of axon diameter and myelin sheath thickness was noted in all nerves studied. Whereas axons reach their maximal diameter by or before 5 years of age, maximal myelin sheath thickness is not attained before 16-17 years of age, i.e., more than 10 years later. The slope of the regression lines for the ratio between axon diameter and myelin thickness is significantly steeper in older than in younger individuals; it also differs if small and large fibers with more or less than 50 myelin lamellae are evaluated separately. The number of Schmidt-Lanterman incisures during later stages of development is related to myelin thickness, but the length of the spiral of the myelin lamella, thought to unrolled, in relation to its width, i.e., internodal length, varies considerably during development. The changes of the relationship between axons and myelin sheath thickness during normal human development have to be taken into account if hypomyelination is considered as a significant pathological phenomenon in peripheral neuropathies, especially in children. The implications of the present findings concerning conduction velocity of peripheral nerve fibers and other electrophysiologic parameters are discussed.

Cochlear nerve topography and fiber spectrum in the pigmented mouse

The topographical and cytological features of the pigmented mouse (CBA/CBA) cochlear nerve were analyzed. The cochlear nerve is very short and is approximately 0.2-0.3 mm in its length. The entire cochlear nerve and a portion of the ventral cochlear nucleus are located within the internal acoustic meatus, and are closely surrounded by bone. Approximately 10,000 nerve fibers are present, of which only about 3% are unmyelinated. The distribution of the axon diameters in the myelinated nerves is close to unimodal.