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M Mizunami

Publications and source records attributed to M Mizunami.

13 recordsLinked to original sources
PubMed1998

Giant input neurons of the mushroom body: intracellular recording and staining in the cockroach.

The mushroom body (MB) of the insect brain is critical to associative memory formation. Intrinsic neurons within the MB (called Kenyon cells, KCs) receive sensory signals from input neurons in the calyces. The calyces of the cockroach MB receive branches of four giant neurons (calycal giants, CGs) which exhibit gamma-aminobutyric acid (GABA)-like immunoreactivity. Here we examined the CGs by intracellular recording and staining. The CGs have dendritic arborizations in the lateral horn (lateral protocerebral lobe) and the neuropil anterior to the alpha and beta lobes (output neuropils of the MB); their terminal arborizations cover the entire calyces. The CGs exhibit a spontaneous and rhythmic burst of spikes, which are suppressed by olfactory, visual, tactile or air current stimulation. The CGs may facilitate, by disinhibition, the acquisition of sensory signals by the KCs when the insect is aroused by sensory stimuli.

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PubMed1998

Three classes of GABA-like immunoreactive neurons in the mushroom body of the cockroach.

The mushroom body (MB) is a higher center of the insect brain and is critical to some forms of associative memory. Each MB consists of calyces connected to alpha and beta lobes via pedunculus. In the calyces, input neurons make synaptic connections with intrinsic neurons. In the pedunculus and lobes, intrinsic neurons make synaptic connections with output neurons. Here, the distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in the MB of the cockroach Periplaneta americana was investigated, using an antiserum against a GABA-protein conjugate, to elucidate inhibitory pathways of the MB. We report that three classes of extrinsic neurons of the MB exhibit GABA-like immunoreactivity. The first is four large neurons which arborize in a diffuse neuropil surrounding the alpha lobe and project into whole areas of the calyces. Their cell bodies are 30-50 micron in diameter, among the largest in the brain. The second group is 7-9 neurons ascending from the circumesophageal connective and projecting into the calyces, which probably represent inhibitory input neurons. The third group is ca. 40 neurons with dendritic arborizations in the junction between the pedunculus and the lobes, which probably represent inhibitory output neurons.

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PubMed1998

Function-specific distribution patterns of axon terminals of input neurons in the calyces of the mushroom body of the cockroach, Periplaneta americana.

Input neurons (INs) in the calyces of the mushroom bodies (MBs) of the cockroach brain were examined by single- or multiple-staining with cobalt lysine and by Golgi impregnation. Olfactory INs had axon terminals with tuft-like, button-like or spiny-blebbed arbors in specific concentric zones in calycal neuropil. INs which responded to light stimulation had thick brush-like arbors along with axonal branches extending radially along the inner layer of calycal neuropil. Some of multiglomerular INs and two types of protocerebral INs extended blebbed axonal branches to the outer surface layer of calycal neuropil or thick bush-like axonal branches with many varicosities to entire calycal neuropil. The distribution patterns of dendrites and axon terminals of INs in the calyces suggest the existence of functional subdivisions in calycal neuropil.

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PubMed1997

Modular structures in the mushroom body of the cockroach.

The mushroom body (MB) is a higher center of the insect brain and is critical to olfactory and other forms of associative memory. Here, we report that repetitive modular subunits, which we refer to as slabs, are present in the internal matrix of the alpha lobe, a major output neuropil of the MB in the cockroach. The methods employed were osmium-ethyl gallate, Bodian-reduced silver, and Golgi staining procedures. A total of 15 dark and 15 pale slabs, each consisting of specific subsets of intrinsic neurons (Kenyon cells), alternate throughout the length of the alpha lobe. One of the major classes of MB output neurons, which are postsynaptic to Kenyon cells, exhibited segmented dendritic arbors that interact with every other slabs, i.e. only either dark or pale slabs. As each output neuron interacts with each specific set of dark or pale slabs, the slab likely functions as a unit for transmitting MB output signals.

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PubMed1996

Gain control of synaptic transfer from second- to third-order neurons of cockroach ocelli.

Synaptic transmission from second- to third-order neurons of cockroach ocelli occurs in an exponentially rising part of the overall sigmoidal characteristic curve relating pre- and postsynaptic voltage. Because of the nonlinear nature of the synapse, linear responses of second-order neurons to changes in ligh intensity are half-wave rectified, i.e., the response to a decrement in light is amplified whereas that to an increment in light is compressed. Here I report that the gain of synaptic transmission from second- to third-order neurons changes by ambient light levels and by wind stimulation applied to the cerci. Transfer characteristics of the synapse were studied by simultaneous intracellular recordings of second- and third-order neurons. Potential changes were evoked in second-order neurons by a sinusoidally modulated light with various mean luminances. With a decrease in the mean luminance (a) the mean membrane potential of second-order neurons was depolarized, (b) the synapse between the second- and third-order neurons operated in a steeper range of the exponential characteristic curve, where the gain to transmit modulatory signals was higher, and (c) the gain of third-order neurons to detect a decrement in light increased. Second-order neurons were depolarized when a wind or tactile stimulus was applied to various parts of the body including the cerci. During a wind-evoked depolarization, the synapse operated in a steeper range of the characteristic curve, which resulted in an increased gain of third-order neurons to detect light decrements. I conclude that the nonlinear nature of the synapse between the second- and third-order neurons provides an opportunity for an adjustment of gain to transmit signals of intensity change. The possibility that a similar gain control occurs in other visual systems and underlies a more advanced visual function, i.e., detection of motion, is discussed.

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PubMed1995

Morphology of higher-order ocellar interneurons in the cockroach brain.

Most species of insects have two or three ocelli, in addition to a pair of compound eyes. In the cockroach ocellus, a large number of photoreceptors converge upon four second-order neurons, which exit the ocellus and project into the ocellar tract neuropil of the brain, where they form synapses with at least 15 third-order neurons. Third-order neurons project into a variety of neuropils in the brain, including the posterior slope, a premotor center from which descending neurons originate. I examined the morphology and ocellar response of neurons in the posterior slope of the cockroach, using intracellular recording and stainings. Most ocellar neurons of the posterior slope exhibited transient depolarizations at the cessation of ocellar illumination, which seem to reflect sign-conserving synaptic input from third-order neurons. Ocellar neurons of the posterior slope project into various areas of the brain, including 1) the central complex, a higher center implicated in higher locomotory control, 2) the pedunculus of the mushroom body, an associative center, 3) the lamina (the first neuropil of the culus of the mushroom body, an associative center, 3) the lamina (the first neuropil of the optic lobe), 4) the antennal lobe (olfactory center), 5) the tritocerebrum (mechanosensory center), and 6) the subesophageal and thoracic motor centers. These results suggest that the posterior slope is a high-order ocellar center from which ocellar signals are transmitted to various target neuropils of the brain, as well as a premotor center to form descending motor commands.

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PubMed1995

Neural organization of ocellar pathways in the cockroach brain.

A large number of photoreceptors of insect ocelli converge onto a smaller number of second-order neurons. Second-order neurons exit the ocellus and project into the ocellar tract neuropil of the brain. Here, the anatomy and physiology of ocellar interneurons of the ocellar tract neuropil of the cockroach are described. The total number and gross morphologies of ocellar tract neurons were examined by extracellular cobalt impregnations into the neuropil. Morphology and physiology of individual neurons were examined using intracellular recording and stainings. Each ocellar tract neuropil contains at least 25 interneurons comprising: 1) four second-order neurons, 2) 15 third-order neurons that receive synapses from second-order neurons at the ocellar tract and project into a number of target neuropil areas of the brain, 3) two possible efferent neurons, 4) three third-order or efferent neurons, and 5) one neuron still to be characterized. The projection areas of ocellar third-order neurons include 1) visual, olfactory, and mechanosensory centers; 2) the mushroom body (a higher associative center); 3) the posterior slope, a premotor center from which descending brain neurons originate; and 4) the thoracic motor systems. By comparing the present results to those reported from other insects, I conclude that the cockroach ocellar system has two distinctive features. 1) The ratio of convergence at synapses between photoreceptors and second-order neurons is higher than those reported for other insects so far studied. 2) Ocellar signals are transmitted to various target neuropils by third-order neurons, whereas ocellar systems of all other insects possess pathways in which ocellar signals are transmitted to target neuropils by second-order neurons. The functional significance of these features of the cockroach ocellar system is discussed.

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PubMed1995

Functional diversity of neural organization in insect ocellar systems.

Most insects have simple eyes (ocelli), in addition to compound eyes. Although the ocelli can detect only changes in light intensity averaged over their large visual field, they do play various roles in the behavior of insects, as they have advantages over compound eyes in terms of photic sensitivity and the speed of signal transmission. I summarize here our present knowledge on the neural organization of ocellar systems of a number of insects, and propose that they can be classified into three types: (1) the "cockroach type" ocellar system where signals from the photoreceptors first converge onto a small number of second-order neurons, and then diverge to a large number of third-order neurons which project into a number of target neuropils of the brain; (2) the "bee type" ocellar system where photoreceptor signals are passed to various target neuropils by a large number of second-order neurons; and (3) the "locust type" ocellar system where ocellar signals are transmitted to various target neuropils by both second- and third-order neurons. I propose that: (1) the "cockroach type" ocellar system can be characterized as a sensitive type where higher sensitivity is attained because of the higher ratio of convergence of photoreceptor signals onto second-order neurons; (2) the "bee type" ocellar system is a fast type where a high speed of signal transmission is attained at the possible cost of sensitivity; and (3) the "locust type" ocellar system is an intermediate type where both speed and sensitivity are emphasized. A possible phylogenetic relationship between the three types of ocellar systems is discussed.

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PubMed1990

Synaptic rectification model equivalent to the correlation-type movement detector.

Neural models which are equivalent to the correlation-type movement detector are described. The models involve contrast-coding channels which comprise bandpass linear filters followed by synaptic retifiers. Linear, one-directional lateral interactions are assumed among the contrast-coding channels. Synaptic rectifiers convert linear spatial interaction into a multiplication-like (quadratic) interaction, which is the core of the correlation-type movement detector. One of the neural models (E-I model) well approximates the correlation model in both time-averaged and dynamic (instantaneous) responses. Possible applicability of the model to movement detection by insects is discussed.

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PubMed1990

Nonlinear signal transmission between second- and third-order neurons of cockroach ocelli.

Transfer characteristics of the synapse made from second- to third-order neurons of cockroach ocelli were studied using simultaneous microelectrode penetrations and the application of tetrodotoxin. Potential changes were evoked in second-order neurons by either an extrinsic current or a sinusoidally modulated light. The synapse had a low-pass filter characteristic with a cutoff frequency of 25-30 Hz, which passed most presynaptic signals. The synapse operated at an exponentially rising part of the overall sigmoidal input/output curve relating pre- and postsynaptic voltages. Although the response of the second-order neuron to sinusoidal light was essentially linear, the response of the third-order neuron contained an accelerating nonlinearity: the response amplitude was a positively accelerated function of the stimulus contrast, reflecting nonlinear synaptic transmission. The response of the third-order neuron exhibited a half-wave rectification: the depolarizing response to light decrement was much larger than the hyperpolarizing response to light increment. Nonlinear synaptic transmission also enhanced the transient response to step-like intensity changes. I conclude that (a) the major function of synaptic transmission between second- and third-order neurons of cockroach ocelli is to convert linear presynaptic signals into nonlinear ones and that (b) signal transmission at the synapse between second- and third-order neurons of cockroach ocelli fundamentally differs from that at the synapse between photoreceptors and second-order neurons of visual systems so far studied, where the synapse operates in the midregion of the characteristic curve and the transmission is essentially linear.

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PubMed1988

Dynamic relationship between the slow potential and spikes in cockroach ocellar neurons.

The relationship between the slow potential and spikes of second-order ocellar neurons of the cockroach, Periplaneta americana, was studied. The stimulus was a sinusoidally modulated light with various mean illuminances. A solitary spike was generated at the depolarizing phase of the modulation response. Analysis of the relationship between the amplitude/frequency of voltage modulation and the rate of spike generation showed that (a) the spike initiation process was bandpass at approximately 0.5-5 Hz, (b) the process contained a dynamic linearity and a static nonlinearity, and (c) the spike threshold at optimal frequencies (0.5-5 Hz) remained unchanged over a mean illuminance range of 3.6 log units, whereas (d) the spike threshold at frequencies of less than 0.5 Hz was lower at a dimmer mean illuminance. The voltage noise in the response was larger and the mean membrane potential level was more positive at a dimmer mean illuminance. Steady or noise current injection during sinusoidal light stimulation showed that (a) the decrease in the spike threshold at a dimmer mean illuminance was due to the increase in the noise variance: the noise had facilitatory effects on the spike initiation; and (b) the change in the mean potential level had little effect on the spike threshold. We conclude that fundamental signal modifications occur during the spike initiation in the cockroach ocellar neuron, a finding that differs from the spike initiation process in other visual systems, including Limulus eye and vertebrate retina, in which it is presumed that little signal modification occurs at the analog-to-digital conversion process.

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PubMed1986

Dynamics of cockroach ocellar neurons.

The incremental responses from the second-order neurons of the ocellus of the cockroach, Periplaneta americana, have been measured. The stimulus was a white-noise-modulated light with various mean illuminances. The kernels, obtained by cross-correlating the white-noise input against the resulting response, provided a measure of incremental sensitivity as well as of response dynamics. We found that the incremental sensitivity of the second-order neurons was an exact Weber-Fechner function; white-noise-evoked responses from second-order neurons were linear; the dynamics of second-order neurons remain unchanged over a mean illuminance range of 4 log units; the small nonlinearity in the response of the second-order neuron was a simple amplitude compression; and the correlation between the white-noise input and spike discharges of the second-order neurons produced a first-order kernel similar to that of the cell's slow potential. We conclude that signal processing in the cockroach ocellus is simple but different from that in other visual systems, including vertebrate retinas and insect compound eyes, in which the system's dynamics depend on the mean illuminance.

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