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Hiroshi Kuba

Publications and source records attributed to Hiroshi Kuba.

10 recordsLinked to original sources
PubMed2006

Axonal site of spike initiation enhances auditory coincidence detection.

Neurons initiate spikes in the axon initial segment or at the first node in the axon. However, it is not yet understood how the site of spike initiation affects neuronal activity and function. In nucleus laminaris of birds, neurons behave as coincidence detectors for sound source localization and encode interaural time differences (ITDs) separately at each characteristic frequency (CF). Here we show, in nucleus laminaris of the chick, that the site of spike initiation in the axon is arranged at a distance from the soma, so as to achieve the highest ITD sensitivity at each CF. Na+ channels were not found in the soma of high-CF (2.5-3.3 kHz) and middle-CF (1.0-2.5 kHz) neurons but were clustered within a short segment of the axon separated by 20-50 microm from the soma; in low-CF (0.4-1.0 kHz) neurons they were clustered in a longer stretch of the axon closer to the soma. Thus, neurons initiate spikes at a more remote site as the CF of neurons increases. Consequently, the somatic amplitudes of both orthodromic and antidromic spikes were small in high-CF and middle-CF neurons and were large in low-CF neurons. Computer simulation showed that the geometry of the initiation site was optimized to reduce the threshold of spike generation and to increase the ITD sensitivity at each CF. Especially in high-CF neurons, a distant localization of the spike initiation site improved the ITD sensitivity because of electrical isolation of the initiation site from the soma and dendrites, and because of reduction of Na+-channel inactivation by attenuating the temporal summation of synaptic potentials through the low-pass filtering along the axon.

Action Potentials↗
PubMed2005

Hyperpolarization-activated cyclic nucleotide-gated cation channels regulate auditory coincidence detection in nucleus laminaris of the chick.

Coincidence detection of bilateral acoustic signals in nucleus laminaris (NL) is the first step in azimuthal sound source localization in birds. Here, we demonstrate graded expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels along the tonotopic axis of NL and its role in the regulation of coincidence detection. Expression of HCN1 and HCN2, but not HCN3 or HCN4, was detected in NL. Based on measurement of both subtype mRNA and protein, HCN1 varied along the tonotopic axis and was minimal in high-characteristic frequency (CF) neurons. In contrast, HCN2 was evenly distributed. The resting conductance was larger and the steady-state activation curve of Ih was more positive in neurons of middle to low CF than those of high CF, consistent with the predominance of HCN1 channels in these neurons. Application of 8-Br-cAMP or noradrenaline generated a depolarizing shift of the Ih voltage activation curve. This shift was larger in neurons of high CF than in those of middle CF. The shift in the activation voltage of Ih depolarized the resting membrane, accelerated the EPSP time course, and significantly improved the coincidence detection in neurons of high CF, suggesting that Ih may improve the localization of sound sources.

Animals↗
PubMed2005

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.

Action Potentials↗
PubMed2004

Electrophysiological correlates of associative visual agnosia lesioned in the ventral pathway.

Visual agnosia has been well studied by anatomical, neuropsychological and neuroimaging studies. However, functional changes in the brain have been rarely assessed by electrophysiological methods. We carried out electrophysiological examinations on a 23-year-old man with associative visual agnosia, prosopagnosia and cerebral achromatopsia to evaluate the higher brain dysfunctions of visual recognition. Electrophysiological methods consisted of achromatic, chromatic and category-specific visual evoked potentials (CS-VEPs), and event-related potentials (ERPs) with color and motion discrimination tasks. Brain magnetic resonance imaging revealed large white matter lesions in the bilateral temporo-occipital lobes involving the lingual and fusiform gyri (V4) and inferior longitudinal fasciculi due to multiple sclerosis. Examinations including CS-VEPs demonstrated dysfunctions of face and object perception while sparing semantic word perception after primary visual cortex (V1) in the ventral pathway. ERPs showed abnormal color perception in the ventral pathway with normal motion perception in the dorsal pathway. These electrophysiological findings were consistent with lesions in the ventral pathway that were detected by clinical and neuroimaging findings. Therefore, CS-VEPs and ERPs with color and motion discrimination tasks are useful methods for assessing the functional changes of visual recognition such as visual agnosia.

Adult↗
PubMed2003

Evaluation of the limiting acuity of coincidence detection in nucleus laminaris of the chicken.

The localization of sounds requires the detection of very brief inter-aural time differences (ITDs). In birds, ITDs are first encoded in neurons of the nucleus laminaris (NL) through the precise coincidence of binaural synaptic inputs. We examined the effects of temperature on acuity of coincidence detection in chick NL, by utilizing whole-cell and cell-attached recording techniques in brain slices while applying electrical stimuli bilaterally to axonal projections from the nucleus magnocellularis to NL. The precision of coincidence detection was measured as a time window, corresponding to the time interval that gave the half-maximum spiking probability. Acuity improved with the elevation of recording temperature, and at 40 degrees C, the avian body temperature, the time window was 0.38 ms. Although all synaptic events were briefer at higher temperature, the duration of EPSPs were equivalent to or faster than that of EPSCs at 40 degrees C. Activation of low-threshold K+ currents by a slight membrane depolarization during an EPSP was responsible for this EPSP acceleration. EPSPs were prolonged following inhibition of low-threshold K+ currents by dendrotoxin (40 nM) or hyperpolarization-activated cation currents by Cs+ (3 mM). The EPSP time course had a strong positive correlation with the sharpness of coincidence detection. The limiting value of the time window (0.16 ms), calculated from the estimated EPSP time course, was narrow enough to explain the acuity of ITD detection at NL in vivo.

Animals↗
PubMed2002

Development of membrane conductance improves coincidence detection in the nucleus laminaris of the chicken.

Coincidence detection at the nucleus laminaris (NL) of a chicken was improved between embryos (embryonic days (E) 16 and 17) and chicks (post-hatch days (P) 2-7) in slice preparations. Electrical stimuli were applied bilaterally to the projection fibres to the NL at various intervals. The response window corresponding to the temporal separation of electrical stimuli that resulted in half-maximal firing probability was adopted as the measure of coincidence detection, and was narrower in chicks (1.4 ms) than in embryos (3.9 ms). Between these two ages, the membrane time constant of NL neurons was reduced from 18.4 to 3.2 ms and the membrane conductance was increased 5-fold, while no difference was measured in the input capacitance. Evoked EPSCs decayed slightly faster in chicks, while the size and the time course of miniature EPSCs were unchanged. Action potentials had lower thresholds and larger after-hyperpolarization in chicks than in embryos. Dendrotoxin-I depolarized cells and increased their input resistance significantly at both ages, eliminated the after-hyperpolarization, and delayed the decay phase of action potentials, indicative of the expression of low-threshold K(+) channels. Cs(+) hyperpolarized the cells, increased the input resistance and eliminated sags during hyperpolarization at both ages, while the hyperpolarization sag was affected by neither Ba(2+) nor TEA. These data indicate the expression of hyperpolarization-activated cation channels. Between these two ages, the maximum conductance of low-threshold K(+) channels increased 4-fold to about 16 nS, and hyperpolarization-activated channels increased 6-fold to about 10 nS. Improvement of coincidence detection correlated with the acceleration of the EPSP time course as a result of the increase of these conductances.

Action Potentials↗
PubMed2002

Occurrence of subdural hematoma and resolution of gait disturbance in a patient treated with shunting for normal pressure hydrocephalus.

A 66-year-old man with gait disturbance was diagnosed with normal pressure hydrocephalus (NPH) and treated with ventriculoperitoneal shunting using a programmable valve. The valve ultimately set at a pressure of 40 mm H(2)O after higher settings no longer relieved symptoms. However, this pressure setting was excessively low and was associated with occurrence of bilateral subdural hematomas. Paradoxically, this event was associated with stable improvement of gait. Our patient's gait disturbance was unassociated with muscle weakness, spasticity, cerebellar ataxia, or Romberg's sign, and, therefore, was consistent with a frontal gait disorder. Cerebral cortical blood flow as measured after shunting by single photon emission computed tomography (SPECT) was slightly increased from the value before shunting, possibly because of intracranial hypotension related to the valve setting. Lasting improvement of gait in our case may be a result of increased blood flow in the supplementary motor area (SMA).

Aged↗
PubMed2002

Synaptic depression improves coincidence detection in the nucleus laminaris in brainstem slices of the chick embryo.

Neurons in the nucleus laminaris detect the coincidence of binaural signals, and are the first neurons to calculate the interaural time difference for the sound source localization in birds. In this paper, we have studied contributions of synaptic depression to the coincidence detection in the nucleus laminaris in a slice preparation of the chick embryo (E16-18), using the whole-cell patch recording technique. Under voltage clamp, EPSCs decreased progressively in their amplitude during the course of tetanic stimuli. This synaptic depression was primarily ascribed to the reduction of transmitter release from the presynaptic terminal, because the depression was decreased by reducing transmitter release with 2.5 microm Cd2+ but was not affected by reducing desensitization of postsynaptic AMPA receptors with 20 microm cyclothiazide. Under current clamp, trains of 10 stimuli of 100 Hz were applied bilaterally with changing the time intervals systematically between both sides. Response window, defined as the time interval corresponding to the half-maximum firing probability, was narrowed during the course of the stimulus train, and this occurred in parallel with a decrease in the EPSP amplitude. In addition, the reduction of the EPSP amplitude due to 2.5 microm Cd2+ or 2 microm CNQX improved the accuracy of coincidence detection. These results indicate that the synaptic depression may improve the coincidence detection in the chick laminaris neurons.

Action Potentials↗
PubMed2002

Gait disturbance in patients with low pressure hydrocephalus.

Using criteria of the classification recently described by Nutt et al., we examined gait disorder in five patients with normal pressure hydrocephalus (NPH). Their cerebrospinal fluid (CSF) pressures were in the normal range, and trials of CSF removal produced temporary improvement of symptoms. Surgical procedures to relieve hydrocephalus improved gait disorders in all patients. No patient showed spasticity, sensory ataxia, cerebellar ataxia, extrapyramidal signs, or limb apraxia. All walked slowly with a wide base and a short stride. The arm swing normally associated with walking was preserved. In standing, patients were unsteady and fell easily when pushed. Four patients showed hesitation in initiating walking and in turning. These clinical features fit Nutt's criteria for frontal gait disorder and frontal disequilibrium. Unlike findings in Parkinson's disease, where similar gait disorders may occur, other extrapyramidal signs, Myerson's sign, and upper limb dysfunction were absent in NPH, and arm swing while walking was preserved. We suspect that ventricular dilatation disturbs neuronal connections between the supplementary motor area and the globus pallidus in NPH patients.

Aged↗