Individual Nerve Fibers Research Articles

Automated determination of axonal orientation in the deep white matter of the human brain.

The wide-spread utilization of diffusion-weighted imaging in the clinical neurosciences to assess white-matter (WM) integrity and architecture calls for robust validation strategies applied to the data that are acquired with noninvasive imaging. However, the pathology and detailed fiber architecture of WM tissue can only be observed postmortem. With these considerations in mind, we designed an automated method for the determination of axonal orientation in high-resolution microscope images. The algorithm was tested on tissue that was stained using a silver impregnation technique that was optimized to resolve axonal fibers against very low levels of background. The orientation of individual nerve fibers was detected using spatial filtering and a template-matching algorithm, and the results are displayed as color-coded overlays. Quantitative models of WM fiber architecture at the microscopic level can lead to improved interpretation of low-resolution neuroimaging data and to more accurate mapping of fiber pathways in the human brain.

A point process framework for modeling electrical stimulation of the auditory nerve

Model-based studies of responses of auditory nerve fibers to electrical stimulation can provide insight into the functioning of cochlear implants. Ideally, these studies can identify limitations in sound processing strategies and lead to improved methods for providing sound information to cochlear implant users. To accomplish this, models must accurately describe spiking activity while avoiding excessive complexity that would preclude large-scale simulations of populations of auditory nerve fibers and obscure insight into the mechanisms that influence neural encoding of sound information. In this spirit, we develop a point process model of individual auditory nerve fibers that provides a compact and accurate description of neural responses to electric stimulation. Inspired by the framework of generalized linear models, the proposed model consists of a cascade of linear and nonlinear stages. We show how each of these stages can be associated with biophysical mechanisms and related to models of neuronal dynamics. Moreover, we derive a semianalytical procedure that uniquely determines each parameter in the model on the basis of fundamental statistics from recordings of single fiber responses to electric stimulation, including threshold, relative spread, jitter, and chronaxie. The model also accounts for refractory and summation effects that influence the responses of auditory nerve fibers to high pulse rate stimulation. Throughout, we compare model predictions to published physiological data of response to high and low pulse rate stimulation. We find that the model, although constructed to fit data from single and paired pulse experiments, can accurately predict responses to unmodulated and modulated pulse train stimuli. We close by performing an ideal observer analysis of simulated spike trains in response to sinusoidally amplitude modulated stimuli and find that carrier pulse rate does not affect modulation detection thresholds.

Dynamic longitudinal investigation of individual nerve endings in the skin of anesthetized mice using in vivo two-photon microscopy

Visualization of individual cutaneous nerve endings has previously relied on laborious procedures of tissue excision, fixation, sectioning and staining for light or electron microscopy. We present a method for non-invasive, longitudinal two-photon microscopy of single nerve endings within the skin of anesthetized transgenic mice. Besides excellent signal-to-background ratio and nanometer-scale spatial resolution, this method offers time-lapse "movies" of pathophysiological changes in nerve fine structure over minutes, hours, days or weeks. Structure of keratinocytes and dermal matrix is visualized simultaneously with nerve endings, providing clear landmarks for longitudinal analysis. We further demonstrate feasibility of dissecting individual nerve fibers with infra-red laser and monitoring their degradation and regeneration. In summary, our excision-free optical biopsy technique is ideal for longitudinal microscopic analysis of animal skin and skin innervations in vivo and can be applied widely in preclinical models of chronic pain, allergies, skin cancers and a variety of dermatological disorders.

Temporal response properties of the auditory nerve: Data from human cochlear-implant recipients

The primary goal of this study was to characterize the variability in auditory-nerve temporal response patterns obtained with the electrically evoked compound action potential (ECAP) within and across a relatively large group of cochlear-implant recipients. ECAPs were recorded in response to each of 21 pulses in a pulse train for five rates (900, 1200, 1800, 2400, and 3500pps) and three cochlear regions (basal, middle, and apical). An alternating amplitude pattern was typically observed across the pulse train for slower rates, reflecting refractory properties of individual nerve fibers. For faster rates, the alternation ceased and overall amplitudes were substantially lower relative to the first pulse in the train, reflecting cross-fiber desynchronization. The following specific parameters were examined: (1) the rate at which the alternating pattern ceased (termed stochastic rate), (2) the alternation depth and the rate at which the maximum alternation occurred, and (3) the average normalized ECAP amplitude across the pulse train (measure of overall adaptation/desynchronization). Data from 29 ears showed that stochastic rates for the group spanned the entire range of rates tested. The majority of subjects (79%) had different stochastic rates across the three cochlear regions. The stochastic rate occurred most frequently at 2400pps for basal and middle electrodes, and at 3500pps for apical electrodes. Stimulus level was significantly correlated with stochastic rate, where higher levels yielded faster stochastic rates. The maximum alternation depth averaged 19% of the amplitude for the first pulse. Maximum alternation occurred most often at 1800pps for basal and apical electrodes, and at 1200pps for middle electrodes. These differences suggest some independence between alternation depth and stochastic rate. Finally, the overall amount of adaptation or desynchronization ranged from 63% (for 900pps) to 23% (for 3500pps) of the amplitude for the first pulse. Differences in temporal response properties across the cochlea within subjects may have implications for developing new speech-processing strategies that employ varied rates across the array.

Visualising vagal afferent neurons and their terminals whilst silencing TRPV1

Visualising vagal afferent neurons and their terminals whilst silencing TRPV1

When cold becomes hot

When cold becomes hot

BYPASS COAPTATION FOR CERVICAL ROOT AVULSION

Previously, we reported bypass coaptation of the C3 and C4 anterior rami to the upper trunk of the brachial plexus for restoration of the muscles denervated as a result of C5 and C6 nerve root avulsion. This procedure is thought to be superior to the transfer of individual peripheral nerve fibers to the brachial plexus branches. Therefore, the benefits of the bypass coaptation procedures in the treatment of various root avulsions are presented. Twenty-six patients were selected as suitable candidates for bypass coaptation procedures. They were divided into 3 groups: 1) Erb-Duchenne palsy due to C5 and C6 root avulsion, 2) Klumpke palsy due to C8 and T1 root avulsion, and 3) the flail arm (or flail upper limb) due to C5 through T1 root avulsion. The surgical techniques are described in detail. The coaptation procedures for the first group resulted in excellent recovery of all the denervated muscles. The patients in the second group showed reinnervation of the finger muscles and finger sensory distributions in infants within the first year after surgery. The flail arm group regained satisfactory proximal muscle function but only mild distal muscle function. One exception was a child who showed significant recovery in proximal and distal motor and sensory function. We recommend the bypass coaptation as a useful procedure for the following categories: Erb-Duchenne palsy due to C5 and C6 root avulsion in all ages, Klumpke palsy due to the C8 and T1 avulsion, and the flail arm due to C5 through T1 avulsion in young children. However, bypass procedures for the flail limb in adults require additional innovative methods to facilitate the growth rate of regenerating nerves.

Volumetric retinal imaging with ultrahigh-resolution spectral-domain optical coherence tomography and adaptive optics using two broadband light sources

Ultrabroadband sources, such as multiplexed superluminescent diodes (SLDs) and femtosecond lasers, have been successfully employed in adaptive optics optical coherence tomography (AO-OCT) systems for ultrahigh resolution retinal imaging. The large cost differential of these sources, however, motivates the need for a performance comparison. Here, we compare the performance of a Femtolasers Integral Ti:Sapphire laser and a Superlum BroadLighter T840, using the same AO-OCT system and the same subject. In addition, we investigate the capability of our instrument equipped with the Integral to capture volume images of the fovea and adjacent regions on a second subject using the AO to control focus in the retina and custom and freeware image registration software to reduce eye motion artifacts. Monochromatic ocular aberrations were corrected with a woofer-tweeter AO system. Coherence lengths of the Integral and BroadLighter were measured in vivo at 3.2 microm and 3.3 microm, respectively. The difference in dynamic range was 5 dB, close to the expected variability of the experiment. Individual cone photoreceptors, retinal capillaries and nerve fiber bundles were distinguished in all three dimensions with both sources. The acquired retinal volumes are provided for viewing in OSA ISP, allowing the reader to data mine at the microscope level.

Immunohistochemical characterization of the innervation of human colonic mesenteric and submucosal blood vessels

The aim was to characterize quantitatively the classes of nerves innervating human mesenteric and submucosal vessels. Specimens of uninvolved normal human mesentery and colon were obtained with prior informed consent from patients undergoing elective surgery for bowel carcinoma. Mesenteric and submucosal vessels were processed for double-labelling immunohistochemical localization of tyrosine hydroxylase (TH), neuropeptide Y (NPY), calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), somatostatin (SOM), vesicular acetylcholine transporter (VAChT) and enkephelin (ENK), each compared to the pan-neuronal marker protein gene product 9.5. Branching patterns of individual nerve fibres were investigated using in vitro anterograde tracing. Sympathetic neurons containing TH and NPY were the largest population, accounting for more than 85% on all vessels. Extrinsic sensory axons, containing SP but not CGRP comprised a second major population on mesenteric vessels: these axons generally lacked TH, NPY and VAChT. On submucosal, but not mesenteric vessels, an additional population of SOM-immunoreactive fibres was present: these axons did not co-localize with TH. Major similarities and differences with enteric vessel innervation in laboratory animals were identified. Sympathetic neurons comprise the largest input. Extrinsic sensory neurons in humans largely lack CGRP but contain SP. Submucosal vessels receive an additional source of innervation not present in mesenteric vessels, which contain SOM, but are rarely cholinergic. These results have significant implications for understanding the control of blood flow to the human gut.

Speech in noise and the medial olivo‐cochlear efferent system

A central question in auditory scene analysis is how we are able to follow speech against a background of interfering noise. The question is particularly acute for artificial speech recognition algorithms and the hearing impaired. The medial efferent system has been suggested as one contributor to our ability to hear speech in noise. We have recently added efferent suppression to our model of the auditory periphery and evaluated it against physiological observations at the level of the basilar membrane and auditory nerve. We have also replicated a study using compound action potentials where a tone in a noise background became more salient when the efferent system was artificially activated. Visual representations of the computed auditory nerve response to speech in noise show considerable improvement when the efferent system is activated. Tests using the auditory model as a front end to a connected‐word recognition algorithm also showed improved performance in the presence of noise when efferent effects were included. The benefits of efferent suppression include reduced compression and extension of the dynamic range of individual auditory nerve fibers.

Deafness alters auditory nerve fibre responses to cochlear implant stimulation

Here we characterized the relationship between duration of sensorineural hearing loss and the response of the auditory nerve to electrical stimulus rate. Electrophysiological recordings were made from undeafened guinea pigs and those ototoxically deafened for either 5 weeks or 6 months. Auditory neuron survival decreased significantly with the duration of deafness. Extracellular recordings were made from auditory nerve fibres responding to biphasic, charge-balanced current pulses delivered at rates of 20 and 200 pulses/s via a monopolar scala tympani stimulating electrode. The response to 20 pulses/s electrical stimulation of the deafened cochlea exhibited a decrease in spike latency, unaltered temporal jitter and unaltered dynamic range (of nerve firing rate against stimulus current), and a reduction in threshold after 6 months of deafness. The response to a 200-pulse/s stimulus was similar except that the dynamic range was greater than with 20 pulses/s and was also greater in deafened animals than in undeafened animals. Deafness and pulse rate are related; in deaf animals spike recovery appears to be complete between successive stimulus pulses at a low rate (20 pulses/s), but incomplete between pulses at a moderate pulse rate (200 pulses/s). These results suggest that changes in the function of individual auditory nerve fibres after deafness may affect clinical responses during high-rate stimulation such as that used in contemporary speech processing strategies, but not during lower rate stimulation such as that used to record evoked potentials.

Quantifying retinal nerve fiber layer thickness in whole-mounted retina

In order to relate optical measurement of the retinal nerve fiber layer (RNFL) to the underlying structure, one must have accurate values for RNFL thickness at the locations measured optically. The purpose of this study was to develop a method for measuring RNFL thickness at any location on retinal tissue previously studied by other optical imaging. The method developed used confocal laser scanning microscopy (cLSM) to provide both en face and cross-sectional images of a whole-mounted retina. Isolated rat retina was fixed with 3% glutaraldehyde. Nerve fiber bundles were identified by using phalloidin to label F-actin and ganglion cell bodies were identified by DAPI fluorescent counterstain of nuclei. The flat-mounted retina was examined by cLSM. 2-D images were collected through the retina to a depth at least covering the ganglion cell layer. The images were stacked to reconstruct cross-sectional images of the measured retina. Thickness of nerve fiber bundles was measured on these synthesized cross sections and compared with the measurement from conventional histologic sections. The en face image displayed individual nerve fiber bundles and ganglion cells between bundles as different colors. Blood vessels, which also bound phalloidin, were easily distinguished from nerve fiber bundles. The en face image displayed the same pattern of nerve fiber bundles as seen in imaging measurements and simplified the identification of corresponding areas in the two modalities. The cross-sectional images provided thickness measurements of the RNFL over the entire field-of-view, not just at the points represented by the conventional histologic section, resulting in a large increase in available data.

An intact peripheral nerve preparation for monitoring the activity of single, periosteal afferent nerve fibres

A preparation is described in which it is possible to selectively activate and monitor the activity of the individual periosteal afferent nerve fibres arising from the humerus bone of the cat. The nerve is a fine branch of the median nerve that accompanies the small artery and vein that enter the nutrient foramen of the humerus. By freeing this fine nerve from nearby tissue over a length of ∼1–2 cm and placing it over a silver hook recording electrode, it becomes possible to identify and monitor electrophysiologically, the impulse activity of individual periosteal afferent fibres activated by focal mechanical stimulation of the periosteum. With this preparation it will be possible to examine the central actions and security of transmission at central synaptic targets for single, small-diameter afferent fibres arising from bone.

An experimental study into the acousto-mechanical effects of invading the cochlea

The active and nonlinear mechanical processing of sound that takes place in the mammalian cochlea is fundamental to our sense of hearing. We have investigated the effects of opening the cochlea in order to make experimental observations of this processing. Using an optically transparent window that permits laser interferometric access to the apical turn of the guinea-pig cochlea, we show that the acousto-mechanical transfer functions of the sealed (i.e. near intact) cochlea are considerably simpler than those of the unsealed cochlea. Comparison of our results with those of others suggests that most previous investigations of apical cochlear mechanics have been made under unsealed conditions, and are therefore likely to have misrepresented the filtering of low-frequency sounds in the cochlea. The mechanical filtering that is apparent in the apical turns of sealed cochleae also differs from the filtering seen in individual auditory nerve fibres with similar characteristic frequencies. As previous studies have shown the neural and mechanical tuning of the basal cochlea to be almost identical, we conclude that the strategies used to process low frequency sounds in the apical turns of the cochlea might differ fundamentally from those used to process high frequency sounds in the basal turns.

Substance P immunoreactive nerve fibres in the domestic chick ankle joint before and after acute urate arthritis

Substance P immunoreactive (SP-IR) nerve fibres were identified using fluorescence immunohistochemistry and the length of the labelled individual nerve fibres was measured using confocal microscopy of whole mounts of the lateral wall of the ankle joint in 2-week-old domestic chicks. In the normal ankle there was an extensive network of SP-IR fibres in the synovial and subsynovial tissue of the joint capsule. Fours hours after injection of sodium urate into the joint space the joint was inflamed and there was a significant reduction in the labelled nerve fibres. These results demonstrate experimentally that painful gouty arthritis produces a neurogenic depletion of substance P from peripheral nerve fibres in the synovial and subsynovial tissue.

Peripheral nerve fascicles: anatomy and clinical relevance.

Within a peripheral nerve, the individual nerve fibers are grouped together in fascicles. Whether there is somatotopic organization within these fascicles has long been of interest, the subject of many investigations, and somewhat controversial. Evidence from diverse sources now points to important somatotopic clustering of nerve fibers within most of the length of the nerve. Information is lacking regarding proximal segments, particularly the plexus and spinal nerve root levels. As a result of this somatotopic arrangement, partial focal nerve lesions can produce restricted clinical deficits that defy the classic rules of localization. Examples of such restricted nerve lesions are provided in this review. Recognition of fascicle somatotopy is also important in the surgical approach to disorders of peripheral nerves.

Localization of NO-Synthase in Gonads of the Freshwater Bivalve Mollusc Nodularia vladivostokensis

Ultrastructural localization of NO-synthase in the gonad of the freshwater bivalve mollusc Nodularia vladivostokensis was studied. Individual NO-containing nerve fibers are revealed in the gonad wall. In the interacinar connective tissue, a high level of the enzyme activity is found in neurons and neural fibers. Reaction product in neurons is located in the nucleus, granular endoplasmic reticulum, on plasma membrane, and in the area of neuromuscular synapses. NO-containing granules are found in various nerve processes, but only cholinergic ones are identified.

Projections from the raphe nuclei to the suprachiasmatic nucleus of the rat

The presence of serotonergic afferents in the hypothalamic suprachiasmatic nucleus (SCN) is well documented and several functional roles of serotonin (5-HT) in circadian function are well established. However, there is some controversy about the precise location of the serotonergic neurones from where this input arises. Discrete injection of the tracer Cholera toxin, subunit B, (ChB) was centred in the rat SCN, and a few retrograde labelled neurones were distributed in the dorsal and median raphe nuclei (MnR) and in the rostral part of the raphe magnus (RMg), but no neurones were found in the raphe pallidus or raphe obscurus. In addition, a group of neurones was consistently found in the medial part of the pontine supra lemniscal nucleus but not including the serotonergic B 9 region. A combination of retrograde tracing with Fluoro-Gold together with 5-HT-immunolabelling, showed few double-labelled neurones in the dorsal, MnR and B 9. However, the majority of projecting neurones were not co-storing 5-HT immunoreactivity. Phaseolus vulgaris-leucoagglutinin (PHA-L) injections in the dorsal raphe resulted in faint labelling, whereas the MnR gave rise to several labelled fibres in the SCN. Individual delicate PHA-L nerve fibres were found in all compartments of the SCN both in terms of rostrocaudal, ventromedial and dorsomedial extent, without any sign of a topographical organisation of the MnR input to the SCN. PHA-L injections into RMg gave rise to labelling of a few processes within the SCN. In conclusion, the main serotonergic input to the rat SCN originates from a few neurones in the MnR.

Pathological sympathoexcitation: how is it achieved?

Congestive heart failure (CHF) and obstructive sleep apnoea syndrome (OSAS) are both associated with an intense sympathoexcitation, including an increased muscle sympathetic nerve activity (MSNA). We have studied the firing characteristics of single vasoconstrictor fibres to the muscle vascular bed in CHF and OSAS patients, at rest and during transient sympathoexcitatory stimuli, to elucidate the mechanisms by which vasoconstrictor output is augmented in these conditions. The main alternatives for augmenting sympathetic output are an increased firing frequency of individual nerve fibres and an increased recruitment of nerve fibres. Starting with the frequency alternative, the inherent bursting character of MSNA provides two possibilities to increase the firing of individual fibres: (1) by increasing the proportion of neural bursts in which the fibre is active (increased firing probability) and (2) by increasing the number of spikes a fibre generates per burst (increased multiple within-burst firing). At rest and in cardiac sinus rhythm, an increased firing probability is seen in both CHF and OSAS patients, whereas increased multiple within-burst firing is found in OSAS but not in CHF. In response to transient sympathoexcitatory stimuli (such as pre-mature heart beats), both patient groups show marked shifts towards multiple within-burst firing. Thus, both mechanisms for augmenting discharge frequency are operating in these two pathological conditions, but the firing characteristics at rest differ significantly. During recording sessions in sympathoexcited patients, we have encountered vasoconstrictor fibres that are active almost exclusively during periods of transient sympathoexcitation, while being virtually silent at rest. This suggests that recruitment of previously inactive vasoconstrictor fibres, the second main alternative for increasing vasoconstrictor output, contributes to transient sympathoexcitatory responses in these patients. Although it seems reasonable to assume that recruitment may also contribute to the resting level of MSNA in CHF and OSAS, this issue is difficult to resolve in microneurographic studies. In conclusion, pathological sympathoexcitation appears to depend on both recruitment and increased firing frequency. A shift towards multiple within-burst firing, at rest or in response to transient stimuli, may constitute a risk factor per se as it entails neural volleys with high instantaneous firing frequencies and consequently higher release of neurotransmitters.

Immunohistochemical Localization of μ-Opioid Receptors in Human Dental Pulp

Studies in both animal and clinical models suggest that opioids exert their analgesic effects not only through activation of receptors in the CNS but also through interaction with peripheral opioid receptors. This study evaluated the presence and distribution of mu-opioid receptors in human dental pulp. Human third molars indicated for extraction were removed, fixed in 4% paraformaldehyde and 0.2% picric acid, and decalcified in 10% EDTA and 7.5% polyvinylpyrrolidone. The teeth were cut using a cryostat, and the avidin-biotin peroxidase immunohistochemistry technique was used. Immunostaining for mu-opioid receptors was detected along the nerve bundle of the radicular as well as coronal dental pulp. Positive immunostaining was also observed in the individual nerve fibers in the coronal region. This demonstration of opiate receptors on pulpal nerve fibers suggests a peripheral site in the dental pulp where endogenous or exogenous opioids can interact with mu-opioid receptors.