Lamina-specific cortical dynamics in human visual and sensorimotor cortices

Distinct anatomical and spectral channels are thought to play specialized roles in the communication within cortical networks. While activity in the alpha and beta frequency range (7 - 40 Hz) is thought to predominantly originate from infragranular cortical layers conveying feedback-related information, activity in the gamma range (>40 Hz) dominates in supragranular layers communicating feedforward signals. We leveraged high precision MEG to test this proposal, directly and non-invasively, in human participants performing visually cued actions. We found that visual alpha mapped onto deep cortical laminae, whereas visual gamma predominantly occurred more superficially. This lamina-specificity was echoed in movement-related sensorimotor beta and gamma activity. These lamina-specific pre- and post- movement changes in sensorimotor beta and gamma activity suggest a more complex functional role than the proposed feedback and feedforward communication in sensory cortex. Distinct frequency channels thus operate in a lamina-specific manner across cortex, but may fulfill distinct functional roles in sensory and motor processes.

Maturation of NaV and KV Channel Topographies in the Auditory Nerve Spike Initiator before and after Developmental Onset of Hearing Function

Auditory nerve excitation and thus hearing depend on spike-generating ion channels and their placement along the axons of auditory nerve fibers (ANFs). The developmental expression patterns and native axonal locations of voltage-gated ion channels in ANFs are unknown. Therefore, we examined the development of heminodes and nodes of Ranvier in the peripheral axons of type I ANFs in the rat cochlea with immunohistochemistry and confocal microscopy. Nodal structures presumably supporting presensory spiking formed between postnatal days 5 (P5) and P7, including Ankyrin-G, NaV1.6, and Caspr. These immature nodal structures lacked low-voltage-activated KV1.1 which was not enriched at juxtaparanodes until approximately P13, concurrent with the developmental onset of acoustic hearing function. Anatomical alignment of ANF spike-initiating heminodes relative to excitatory input from inner hair cell (IHC) ribbon synapses continued until approximately P30. High-voltage-activated KV3.1b and KV2.2 were expressed in mutually exclusive domains: KV3.1b was strictly localized to nodes and heminodes, whereas KV2.2 expression began at the juxtaparanodes and continued centrally along the first internode. At spike-initiating heminodes in the distal osseous spiral lamina, NaV1.1 partly overlapped NaV1.6 and ankyrin-G. ANFs displayed KV7.2 and KV7.3 at heminodes, nodes, internodes, and the unmyelinated synaptic terminal segments beneath IHCs in the organ of Corti. In response to sound, spikes are initiated at the heminode, which is tightly coupled to the IHC ribbon synapse ∼20-40 μm away. These results show that maturation of nodal alignment and ion channel content may underlie postnatal improvements of ANF excitability and discharge synchrony.

SIGNIFICANCE STATEMENT

Acoustic and electrical hearing depends on rapid, reliable, and precise spike generation in auditory nerve fibers. A limitation of current models and therapies is a lack of information on the identities and topographies of underlying ion channels. We report the developmental profile of the auditory nerve spike generator with a focus on NaV1.1, NaV1.6, KV1.1, KV2.2, KV3.1b, KV7.2, and KV7.3 in relation to the scaffold ankyrin-G. Molecular anatomy of the spike generator matures in the weeks after developmental onset of hearing function. Subcellular positioning of voltage-gated ion channels will enable multicompartmental modeling of auditory nerve responses elicited by afferent chemical neurotransmission from hair cells and modulated by efferent neurotransmitters or evoked by extracellular field stimulation from a cochlear implant.

Orthotropic material properties of the gerbil basilar membrane

In this paper, two sets of experimental results to extract the two effective elastic moduli, the effective shear modulus, and the effective Poisson's ratio for the gerbil cochlear partition are analyzed. In order to accomplish this, a geometrically nonlinear composite orthotropic plate model is employed. The model is used to predict both out-of-plane and in-plane motion of the partition under a static finite area distributed load. This loading condition models the small, but finite size, probe tips used in experiments. Both in-plane and out-of-plane motion are needed for comparison with recent experimental results. It is shown that the spatial decay rate (the space constant) for the in-plane deflection is different than for the out-of-plane deflection, which has a significant effect on the derived partition properties. The size of the probe tip is shown to have little influence on the results. Results are presented for two types of boundary conditions. Orthotropy ratios determined from the experimental data are found to vary with longitudinal position and choice of boundary conditions. Orthotropy ratios (the ratio of the two elastic moduli) are in the range of 65 close to the base to 10 in the upper middle turn of the cochlea.

Cochlear development: hair cells don their wigs and get wired

PURPOSE OF REVIEW

Hair cells and spiral ganglion neurons form functional pairings in the cochlea that transduce the mechanical energy of sound into signals that are carried to the brainstem. Mutations of genes affecting the development and maintenance of these two cell populations cause deafness in humans and other animals. This review highlights recent findings regarding the development of hair cell stereocilia and spiral ganglion neurons in the cochlea.

RECENT FINDINGS

Genes underlying Usher syndrome 1A have shed light on possible molecular participants in the development and structure of the hair cell stereocilia. Analysis of deaf mouse mutants have uncovered genes involved in stereocilia elongation and the orientation of the stereociliary bundles. Studies on the regulation of spiral ganglion neuronal survival and guidance suggest that the timing of expression of specific growth factors along the cochlear spiral is involved in maintaining the divergence of vestibular and cochlear nerve fibers.

SUMMARY

Examining human and mouse genes affecting hearing has not only provided insight into causes of human deafness, but has also opened a window into how stereociliary bundles are constructed and spiral ganglion neurons are preserved and guided during development. Synthesis of information from diverse lines of research pinpoints genes for screening or repair in the genetic medicine of the future and dramatizes the intimate relationship between strict adherence to complex developmental programs and hearing. In addition, future improvements in the efficacy of cochlear implants may depend on the preservation and manipulation of adult spiral ganglion neurons. Developmental mechanisms promise to yield insight into possible interventions to redirect or reconnect spiral ganglion neurons in damaged cochlea.

Reticular lamina vibrations in the apical turn of a living guinea pig cochlea

The reticular lamina of the apical turn of a living guinea pig cochlea was viewed through the intact Reissner's membrane using a slit confocal microscope. Vibrations were measured at selected identified locations with a confocal heterodyne interferometer, in response to tones applied with an acoustic transducer coupled to the ear canal. The position coordinates of each location were recorded. Mechanical tuning curves were measured along a radial track at Hensen's cells, outer hair cells, inner hair cells and at the osseous spiral lamina, over a frequency range of 3 kHz, using five sound pressure levels (100, 90, 80, 70 and 60 dB SPL). The carrier to noise ratio obtained throughout the experiments was high. The response shape at any measuring location was not found to change appreciably with signal level. The response shape also did not change significantly with the radial position on the reticular lamina. However, the response magnitude increased progressively from the inner hair cell to the Hensen's cell. The observed linearity of response at the fundamental frequency is explained by the presence of negative feed back in the apical turn of the cochlea.

[In vivo cochleoscopy through the round window]

The overall aim of the present investigation was to develop a technique for endoscopic investigation of the cochlea. In the experiments reported here, the possible effect of the endoscope-called the "cochleoscope"-on the electrophysiology of the cochlea was investigated by recording the cochlear action potential (CAP) threshold tuning curve from (0.1-34 kHz). The dorsolateral bulla of anesthesized guinea pigs (with ketamine 60 mg/kg and Rompun 12 mg/kg) was opened, after which the cochleoscope was introduced under micromanipulator control through the round window membrane. Three cochleoscopes were used and had diameters of 0.29 mm, 0.7 mm and 0.89 mm, respectively, containing 2000, 3000 and 3000 fibers each. Experiments in 7 animals showed that the cochleoscope did not influence CAP thresholds. Although the present resolution of the endoscopes is limited, the basilar membrane can be clearly distinguished from the osseous spiral lamina. It is anticipated that improved resolution will allow the cochleoscope to be used for diagnostic purposes in cases of sensorineural hearing loss.

Microanatomy of the mouse osseous cochlea: a scanning electron microscopic study

This is the first scanning electron microscopic demonstration of the three-dimensional architecture and detailed surface structures of the entire osseous labyrinth of the cochlea. Mouse cochleae were observed after dissolving the soft tissues with KOH and NaOCl solutions. The precise shapes, surface structures, and orientations of the primary osseous spiral lamina and secondary osseous spiral lamina in the cochlea were observed along their entire course from the hook at the base to the helicotrema at the apex. The primary osseous spiral lamina showed three half turns after the hook; the lengths of the hook and each half turn and the slope angle of the spiral were obtained. The widths of the primary and secondary spiral laminae and the spiral fissure for the basilar membrane between the free edges of the two spiral laminae were measured along the course of the cochlear duct. The surface of the lateral wall under the stria vascularis was also viewed. Scanning electron microscopy can provide more precise microanatomical data than has been previously available for the osseous cochlea, giving a better understanding of hearing mechanisms with regard to the width, support, and movement of the basilar membrane and the functions of various components of the cochlea.

Structural features of the basal lamina in Reissner's membrane of the guinea pig

The structural features of the basal lamina in Reissner's membrane were examined electron microscopically using tannic acid fixation, poly-cationic staining (ruthenium red), and mono-cationic staining (safranin O). Tannic acid fixation revealed 5-10-nm filaments which ran perpendicularly between the lamina densa and the cytoplasmic surface of squamous epithelial cells. Ruthenium red staining showed a lattice-like arrangement of anionic sites on both sides of the lamina densa. Safranin O staining revealed filamentous networks at the same sites. These structural features of the basal lamina in the Reissner's membrane were very similar to those of the glomerular and the alveolar basement membrane. Therefore, the possibility that anionic sites of Reissner's membrane may play the role of a size and charge-selective barrier between the scala media and the scala vestibuli, in much the same way that the glomerular basement membrane plays the same role of barrier, must be taken into account.