Nucleus Laminaris Neurons Research Articles

Dendritic Calcium Channels and Their Activation by Synaptic Signals in Auditory Coincidence Detector Neurons

The avian nucleus laminaris (NL) encodes the azimuthal location of low-frequency sound sources by detecting the coincidence of binaural signals. Accurate coincidence detection requires precise developmental regulation of the lengths of the fine, bitufted dendrites that characterize neurons in NL. Such regulation has been suggested to be driven by local, synaptically mediated, dendritic signals such as Ca(2+). We examined Ca(2+) signaling through patch clamp and ion imaging experiments in slices containing nucleus laminaris from embryonic chicks. Voltage-clamp recordings of neurons located in the NL showed the presence of large Ca(2+) currents of two types, a low voltage-activated, fast inactivating Ni(2+) sensitive channel resembling mammalian T-type channels, and a high voltage-activated, slowly inactivating Cd(2+) sensitive channel. Two-photon Ca(2+) imaging showed that both channel types were concentrated on dendrites, even at their distal tips. Single action potentials triggered synaptically or by somatic current injection immediately elevated Ca(2+) throughout the entire cell. Ca(2+) signals triggered by subthreshold synaptic activity were highly localized. Thus when electrical activity is suprathreshold, Ca(2+) channels ensure that Ca(2+) rises in all dendrites, even those that are synaptically inactive.

Detection of Interaural Time Differences in the Alligator

The auditory systems of birds and mammals use timing information from each ear to detect interaural time difference (ITD). To determine whether the Jeffress-type algorithms that underlie sensitivity to ITD in birds are an evolutionarily stable strategy, we recorded from the auditory nuclei of crocodilians, who are the sister group to the birds. In alligators, precisely timed spikes in the first-order nucleus magnocellularis (NM) encode the timing of sounds, and NM neurons project to neurons in the nucleus laminaris (NL) that detect interaural time differences. In vivo recordings from NL neurons show that the arrival time of phase-locked spikes differs between the ipsilateral and contralateral inputs. When this disparity is nullified by their best ITD, the neurons respond maximally. Thus NL neurons act as coincidence detectors. A biologically detailed model of NL with alligator parameters discriminated ITDs up to 1 kHz. The range of best ITDs represented in NL was much larger than in birds, however, and extended from 0 to 1000 micros contralateral, with a median ITD of 450 micros. Thus, crocodilians and birds employ similar algorithms for ITD detection, although crocodilians have larger heads.

Compartment‐specific regulation of plasma membrane calcium ATPase type 2 in the chick auditory brainstem

Calcium signaling plays a role in synaptic regulation of dendritic structure, usually on the time scale of hours or days. Here we use immunocytochemistry to examine changes in expression of plasma membrane calcium ATPase type 2 (PMCA2), a high-affinity calcium efflux protein, in the chick nucleus laminaris (NL) following manipulations of synaptic inputs. Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Deprivation of the contralateral projection from NM to NL leads to rapid retraction of ventral, but not the dorsal, dendrites of NL neurons. Immunocytochemistry revealed symmetric distribution of PMCA2 in two neuropil regions of normally innervated NL. Electron microscopy confirmed that PMCA2 localizes in both NM terminals and NL dendrites. As early as 30 minutes after transection of the contralateral projection from NM to NL or unilateral cochlea removal, significant decreases in PMCA2 immunoreactivity were seen in the deprived neuropil of NL compared with the other neuropil that continued to receive normal input. The rapid decrease correlated with reductions in the immunoreactivity for microtubule-associated protein 2, which affects cytoskeleton stabilization. These results suggest that PMCA2 is regulated independently in ventral and dorsal NL dendrites and/or their inputs from NM in a way that is correlated with presynaptic activity. This provides a potential mechanism by which deprivation can change calcium transport that, in turn, may be important for rapid, compartment-specific dendritic remodeling.

Multi-electrode recordings of delay lines in nucleus laminaris of the barn owl

Event Abstract Back to Event Multi-electrode recordings of delay lines in nucleus laminaris of the barn owl Nico Lautemann1*, Paula Kuokkanen2, Richard Kempter3 and Hermann Wagner1, 4 1 Rheinisch-Westfaelische Technische Hochschule, Department for Zoology and Animal Physiology, Georgia 2 Humboldt University, Institute for Theoretical Biology, Germany 3 Bernstein Center for Computational Neuroscience, Germany 4 University of Maryland , Faculty of Electrical Engineering and Computing, United States Barn owls (Tyto alba) are nocturnal hunters that are able to catch their prey in complete darkness by using auditory cues. The cue used to localize the azimuthal position of a sound source is the interaural time difference (ITD). ITD is the difference of the arrival time of a sound at the two ears. The time pathway computing the ITD starts in the cochlear nucleus magnocellularis (NM). The axons of NM neurons project bilaterally to nucleus laminaris (NL), making NL the first binaural stage in the time pathway. The NL neurons are narrowly tuned to sound frequency and act as coincidence detectors. Simultaneous inputs from the right and left side cause the neurons to be maximally active. Their firing frequency changes periodically in dependence on an imposed phase shift between the left and right inputs. Nucleus laminaris contains both, a tonotopic map and a map of ITD. The projections from the ipsi- and contralateral NM are supposed to form delay lines. The ipsilateral axon collaterals contact and penetrate NL from dorsal, while the contralateral axon collaterals run on the ventral side and transverse NL from ventral to dorsal. In the barn owl the map of ITD results from the synapses of the axon collaterals with NL neurons at different dorso-ventral depths. In this way a time-code, present in the NM collaterals, is converted into a place-code in NL neurons. The key elements and features of such a sound-localization circuit have been proposed by Jeffress in 1948 [1]. Since then a large amount of evidence has been accumulated, supporting the hypothesis that this model is realized in birds. However, the existence of delay lines in the barn owl has not yet been directly shown. To do so, we used acute coronal slice preparations of the NM-NL circuit of the barn owl brainstem and recorded the extracellular multi-unit activity in the network at many different positions with planar multi-electrode arrays (MEA) while electrically stimulating the NM fibers (Fig. 1A). We demonstrate the propagation of the response along and within NL, directly showing the existence of delay lines (Fig. 1B). The delays inside and outside of NL were quantified by determining propagation velocities, showing different propagation velocities of the fibers in- and outside NL. Since the network is still developing in the first weeks, we used animals of different ages (P2-P11) to study the maturation of the delay lines, taking place in the first days post hatch.

Adaptation of firing rate and spike-timing precision in the avian cochlear nucleus.

Adaptation is commonly defined as a decrease in response to a constant stimulus. In the auditory system such adaptation is seen at multiple levels. However, the first-order central neurons of the interaural time difference detection circuit encode information in the timing of spikes rather than the overall firing rate. We investigated adaptation during in vitro whole-cell recordings from chick nucleus magnocellularis neurons. Injection of noisy, depolarizing current caused an increase in firing rate and a decrease in spike time precision that developed over approximately 20 s. This adaptation depends on sustained depolarization, is independent of firing, and is eliminated by alpha-dendrotoxin (0.1 microM), implicating slow inactivation of low-threshold voltage-activated K+ channels as its mechanism. This process may alter both firing rate and spike-timing precision of phase-locked inputs to coincidence detector neurons in nucleus laminaris and thereby adjust the precision of sound localization.

Sound-Intensity-Dependent Compensation for the Small Interaural Time Difference Cue for Sound Source Localization

Interaural time difference (ITD) is a major cue for sound source localization. However, animals with small heads experience small ITDs, making ITD detection difficult, particularly for low-frequency sound. Here, we describe a sound-intensity-dependent mechanism for compensating for the small ITD cues in the coincidence detector neurons in the nucleus laminaris (NL) of the chicken aged from 3 to 29 d after hatching. The hypothesized compensation mechanisms were confirmed by simulation. In vivo single-unit recordings revealed an improved contrast of ITD tuning in low-best-frequency (<1 kHz) NL neurons by suppressing the firing activity at the worst ITD, whereas the firing rate was increased with increasing sound intensity at the best ITD. In contrast, level-dependent suppression was so weak in the middle- and high-best-frequency (> or =1 kHz) NL neurons that loud sounds led to increases in firing rate at both the best and the worst ITDs. The suppression of firing activity at the worst ITD in the low-best-frequency neurons required the activation of the superior olivary nucleus (SON) and was eliminated by electrolytic lesions of the SON. The frequency-dependent suppression reflected the dense projection from the SON to the low-frequency region of NL. Thus, the small ITD cues available in low-frequency sounds were partly compensated for by a sound-intensity-dependent inhibition from the SON.

Variability Reduction in Interaural Time Difference Tuning in the Barn Owl

The interaural time difference (ITD) is the primary auditory cue used by the barn owl for localization in the horizontal direction. ITD is initially computed by circuits consisting of axonal delay lines from one of the cochlear nuclei and coincidence detector neurons in the nucleus laminaris (NL). NL projects directly to the anterior part of the dorsal lateral lemniscal nucleus (LLDa), and this area projects to the core of the central nucleus of the inferior colliculus (ICcc) in the midbrain. To show the selectivity of an NL neuron for ITD requires averaging of responses over several stimulus presentations for each ITD. In contrast, ICcc neurons detect their preferred ITD in a single burst of stimulus. We recorded extracellularly the responses of LLDa neurons to ITD in anesthetized barn owls and show that this ability is already present in LLDa, raising the possibility that ICcc inherits its noise reduction property from LLDa.

Early development of intrinsic and synaptic properties of chicken nucleus laminaris neurons

Onset of auditory brainstem responses in chickens takes place at about embryonic day 11/12 (E11/12). We investigated early development of neuronal properties of chicken nucleus laminaris neurons, the third-order auditory neurons critically involved in sound localization. Whole-cell patch recordings were performed in brainstem slices obtained at E10, E11, E12, E14, E16, and E18. At E18 neurons acquired an adult-like firing pattern in response to prolonged depolarizing current injections, with a single spike at the onset of the current injection followed by a plateau of membrane potential. At earlier ages, however, multiple spikes and/or subthreshold membrane potential oscillations were generated. We observed a >threefold reduction in input resistance from E10 to E18, and progressive changes in excitability properties, such as elevated threshold currents for spike generation, increased spike rising and falling rates, accompanied by reduced spike width and enhanced ability to follow high frequency inputs. Consistent with development of firing properties, the amplitude of voltage-gated potassium channel (Kv) currents increased by ∼threefold from E10 to E18, with a dramatic increase (∼ninefold) in the low threshold component. Excitatory postsynaptic potentials (EPSPs) were first recorded at E10, prior to and independent of the cochlear afferent inputs from the auditory nerve to the cochlear nucleus. EPSPs became markedly briefer in duration during the period studied. We conclude that the basic features of the key neuronal properties of NL neurons are well constructed during early development from E10 to E18.

Mapping function onto neuronal morphology.

Neurons have a wide range of dendritic morphologies the functions of which are largely unknown. We used an optimization procedure to find neuronal morphological structures for two computational tasks: first, neuronal morphologies were selected for linearly summing excitatory synaptic potentials (EPSPs); second, structures were selected that distinguished the temporal order of EPSPs. The solutions resembled the morphology of real neurons. In particular the neurons optimized for linear summation electrotonically separated their synapses, as found in avian nucleus laminaris neurons, and neurons optimized for spike-order detection had primary dendrites of significantly different diameter, as found in the basal and apical dendrites of cortical pyramidal neurons. This similarity makes an experimentally testable prediction of our theoretical approach, which is that pyramidal neurons can act as spike-order detectors for basal and apical inputs. The automated mapping between neuronal function and structure introduced here could allow a large catalog of computational functions to be built indexed by morphological structure.

Neural computations in the owl’s sound localization

Neural models predict mathematical operations taking place in systems of neurons. The plausibility of such operations is a matter of debate. The owl’s auditory system offers a good substrate to approach this question. To determine what and where acoustic objects are, the brain must compare and combine noisy information coming from each ear. Comparison, combination, and noise reduction can be represented by equations. A common tool used to extract signals from variable sets of stimuli is averaging. Similarly, cross‐correlation and multiplication are mathematical correlates of comparison and combination selectivity, respectively. In the owl’s auditory pathway, we can find neurons that effectively perform these types of processing. The nucleus Laminaris neurons perform a running cross‐correlation that indicates the degree of similarity between right‐ and left‐side inputs. Midbrain neurons become selective to combinations of spatial cues by effectively multiplying inputs from independent processing pathways. ...

Passive Soma Facilitates Submillisecond Coincidence Detection in the Owl's Auditory System

Neurons of the avian nucleus laminaris (NL) compute the interaural time difference (ITD) by detecting coincident arrivals of binaural signals with submillisecond accuracy. The cellular mechanisms for this temporal precision have long been studied theoretically and experimentally. The myelinated axon initial segment in the owl's NL neuron and small somatic spikes observed in auditory coincidence detector neurons of various animals suggest that spikes in the NL neuron are generated at the first node of Ranvier and that the soma passively receives back-propagating spikes. To investigate the significance of the "passive soma" structure, we constructed a two-compartment NL neuron model, consisting of a cell body and a first node, and systematically changed the excitability of each compartment. Here, we show that a neuron with a less active soma achieves higher ITD sensitivity and higher noise tolerance with lower energy costs. We also investigate the biophysical mechanism of the computational advantage of the "passive soma" structure by performing sub- and suprathreshold analyses. Setting a spike initiation site with high sodium conductance, not in the large soma but in the small node, serves to amplify high-frequency input signals and to reduce the impact and the energy cost of spike generation. Our results indicate that the owl's NL neuron uses a "passive soma" design for computational and metabolic reasons.

Formation of the avian nucleus magnocellularis from the auditory anlage

In the avian auditory system, the neural network for computing the localization of sound in space begins with bilateral innervation of nucleus laminaris (NL) by nucleus magnocellularis (NM) neurons. We used antibodies against the neural specific markers Hu C/D, neurofilament, and SV2 together with retrograde fluorescent dextran labeling from the contralateral hindbrain to identify NM neurons within the anlage and follow their development. NM neurons could be identified by retrograde labeling as early as embryonic day (E) 6. While the auditory anlage organized itself into NM and NL in a rostral-to-caudal fashion between E6 and E8, labeled NM neurons were visible throughout the extent of the anlage at E6. By observing the pattern of neuronal rearrangements together with the pattern of contralaterally projecting NM fibers, we could identify NL in the ventral anlage. Ipsilateral NM fibers contacted the developing NL at E8, well after NM collaterals had projected contralaterally. Furthermore, the formation of ipsilateral connections between NM and NL neurons appeared to coincide with the arrival of VIIIth nerve fibers in NM. By E10, immunoreactivity for SV2 was heavily concentrated in the dorsal and ventral neuropils of NL. Thus, extensive pathfinding and morphological rearrangement of central auditory nuclei occurs well before the arrival of cochlear afferents. Our results suggest that NM neurons may play a central role in formation of tonotopic connections in the auditory system.

Noise Reduction of Coincidence Detector Output by the Inferior Colliculus of the Barn Owl

A recurring theme in theoretical work is that integration over populations of similarly tuned neurons can reduce neural noise. However, there are relatively few demonstrations of an explicit noise reduction mechanism in a neural network. Here we demonstrate that the brainstem of the barn owl includes a stage of processing apparently devoted to increasing the signal-to-noise ratio in the encoding of the interaural time difference (ITD), one of two primary binaural cues used to compute the position of a sound source in space. In the barn owl, the ITD is processed in a dedicated neural pathway that terminates at the core of the inferior colliculus (ICcc). The actual locus of the computation of the ITD is before ICcc in the nucleus laminaris (NL), and ICcc receives no inputs carrying information that did not originate in NL. Unlike in NL, the rate-ITD functions of ICcc neurons require as little as a single stimulus presentation per ITD to show coherent ITD tuning. ICcc neurons also displayed a greater dynamic range with a maximal difference in ITD response rates approximately double that seen in NL. These results indicate that ICcc neurons perform a computation functionally analogous to averaging across a population of similarly tuned NL neurons.

The Level and Integrity of Synaptic Input Regulates Dendrite Structure

How localized synaptic input regulates dendritic branch structure is not well understood. For these experiments, we used single-cell electroporation, live cell imaging, in vitro deafferentation, pharmacology, and electrophysiological stimulation to study how local alterations in synaptic input affect dendritic branch structure in nucleus laminaris (NL). We found that interrupting or modulating synaptic input to distinct sets of NL dendrites can regulate their structure on a very short timescale. Specifically, eliminating synaptic input by deafferenting only one set of the bitufted NL dendrites caused a selective reduction in the total dendritic branch length of the deafferented dendrites but relatively few changes in the normally innervated dendrites on the same cell. An analysis of individual dendritic branch changes demonstrated that both control and deafferented NL dendrites exhibit branch extension and retraction. However, the presence of intact synaptic inputs balanced these changes, maintaining the total dendritic branch length of control dendrites. When glutamate receptor signaling was blocked (DNQX and AP-5), NL neurons exhibited significant dendrite retraction, demonstrating that NL dendrite maintenance depends in part on presynaptic glutamatergic input. Electrophysiological experiments further confirmed that modulating the level of synaptic input regulates NL dendrite structure. Differential stimulation of the two sets of dendrites resulted in a selective reduction in the total dendritic branch length of the unstimulated dendrites and a selective increase in the total dendritic branch length of the stimulated dorsal dendrites. These results suggest that balanced activation of the two sets of NL dendrites is required to maintain the relative amount of dendritic surface area allotted to each input.

Interaural timing difference circuits in the auditory brainstem of the emu (Dromaius novaehollandiae)

In the auditory system, precise encoding of temporal information is critical for sound localization, a task with direct behavioral relevance. Interaural timing differences (ITDs) are computed using axonal delay lines and cellular coincidence detectors in nucleus laminaris (NL). We present morphological and physiological data on the timing circuits in the emu, Dromaius novaehollandiae, and compare these results with those from the barn owl (Tyto alba) and the domestic chick (Gallus gallus). Emu NL was composed of a compact monolayer of bitufted neurons whose two thick primary dendrites were oriented dorsoventrally. They showed a gradient in dendritic length along the presumed tonotopic axis. The NL and nucleus magnocellularis (NM) neurons were strongly immunoreactive for parvalbumin, a calcium-binding protein. Antibodies against synaptic vesicle protein 2 and glutamic acid decarboxlyase revealed that excitatory synapses terminated heavily on the dendritic tufts, while inhibitory terminals were distributed more uniformly. Physiological recordings from brainstem slices demonstrated contralateral delay lines from NM to NL. During whole-cell patch-clamp recordings, NM and NL neurons fired single spikes and were doubly rectifying. NL and NM neurons had input resistances of 30.0 +/- 19.9 Momega and 49.0 +/- 25.6 Momega, respectively, and membrane time constants of 12.8 +/- 3.8 ms and 3.9 +/- 0.2 ms. These results provide further support for the Jeffress model for sound localization in birds. The emu timing circuits showed the ancestral (plesiomorphic) pattern in their anatomy and physiology, while differences in dendritic structure compared to chick and owl may indicate specialization for encoding ITDs at low best frequencies.

The analysis of interaural time differences in the chick brain stem

The brain stem auditory system of the chick has proven to be a useful model system for analyzing how the brain encodes temporal information. This paper reviews some of the work on a circuit in the brain stem that compares the timing of information coming from the two ears to determine the location of a sound source. The contralateral projection from the cochlear nucleus, nucleus magnocellularis (NM), to nucleus laminaris (NL) forms a delay line as it proceeds from medial to lateral across NL. NL neurons function like coincidence detectors in that they respond maximally when input from the two ears arrive simultaneously. This arrangement may allow NL to code sound space by the relative level of activity across the nucleus. The head anatomy of the chick allows for enhancement of the functional interaural time differences. Comparing the functional interaural time differences to the length of the neural delay line suggests that each NL can encode approximately one hemifield of sound space. Finally it is suggested that inhibitory input into the NM–NL circuit may provide a means to dynamically adjust the gain of the circuit to allow accurate coding of sound location despite changes in overall sound intensity.

Tonotopic Specialization of Auditory Coincidence Detection in Nucleus Laminaris of the Chick

The interaural time difference (ITD) is a cue for localizing a sound source along the horizontal plane and is first determined in the nucleus laminaris (NL) in birds. Neurons in NL are tonotopically organized, such that ITDs are processed separately at each characteristic frequency (CF). Here, we investigated the excitability and coincidence detection of neurons along the tonotopic axis in NL, using a chick brainstem slice preparation. Systematic changes with CF were observed in morphological and electrophysiological properties of NL neurons. These properties included the length of dendrites, the input capacitance, the conductance of hyperpolarization-activated current, and the EPSC time course. In contrast to these gradients, the conductance of low-threshold K+ current and the expression of Kv1.2 channel protein were maximal in the central (middle-CF) region of NL. As a result, the middle-CF neuron had the smallest input resistance and membrane time constant, and consequently the fastest EPSP, and exhibited the most accurate coincidence detection. The specialization of middle-CF neurons as coincidence detectors may account for the high resolution of sound-source localization in the middle-frequency range observed in avians.

A systems and neurobiological model of nucleus angularis and nucleus laminaris in the barn owl

The representation of binaural sound localization cues was investigated in a model of the barn owl’s auditory nerve, nucleus angularis (NA), nucleus magnocellularis (NM), nucleus laminaris (NL), and superior olivary nucleus (SON) using spiking leaky integrate and fire neurons. A phenomenological model of the barn owl’s auditory-nerve spike response was created to reproduce responses to tonal stimuli and exhibit two-tone rate suppression effects. The representation of stimulus level by NA was investigated by decoding the envelope of the input signal from populations of NA model neurons where the envelope was defined as in a previous computational model of sound localization in the barn owl [B.J. Fischer and C.H. Anderson, Adv. NIPS 16 (2004)]. Simulations demonstrated that inclusion of NA neurons with type IV responses increased the accuracy of the representation of the stimulus envelope. The effect of inhibition on the output of the population of NL model neurons was investigated and compared to a cross-correlation model. The output of the model SON provides inhibitory feedback to ipsilateral NA, NM, and NL model neurons. Simulations demonstrated that inclusion of inhibition of NL neurons by SON output increased peak–trough differences in ITD tuning curves.

EphA4 signaling promotes axon segregation in the developing auditory system

Precision of synaptic connections within neural circuits is essential for the accurate processing of sensory information. Specificity is exemplified at cellular and subcellular levels in the chick auditory brainstem, where nucleus magnocellularis (NM) neurons project bilaterally to nucleus laminaris (NL). Dorsal dendrites of NL neurons receive input from ipsilateral, but not contralateral, branches of NM axons whereas ventral dendrites are innervated by contralateral NM axons. This organization is analogous to that of the mammalian medial superior olive (MSO) and represents an important component of the circuitry underlying sound localization. However, the molecular mechanisms that establish segregated inputs to individual regions of NL neurons have not been identified. During synapse formation in NL, the EphA4 receptor is expressed in dorsal, but not ventral NL, neuropil, suggesting a potential role in targeting synapses to appropriate termination zones. Here, we directly tested this role by ectopically expressing EphA4 and disrupting EphA4 signaling using in ovo electroporation. We found that both misexpression of EphA4 and disruption of EphA4 signaling resulted in an increase in the number of NM axons that grow aberrantly across NL cell bodies into inappropriate regions of NL neuropil. EphA4 signaling is thus essential for targeting axons to distinct subsets of dendrites. Moreover, loss of EphA4 function resulted in morphological abnormalities of NL suggestive of errors in cell migration. These results suggest that EphA4 has multiple roles in the formation of auditory brainstem nuclei and their projections.

Sensitivity to interaural time difference in neurons of nucleus laminaris in the chicken in vivo

Sensitivity to interaural time difference in neurons of nucleus laminaris in the chicken in vivo