Masakazu Takahata

Local spikeless interaction of motoneuron dendrites in the crayfishProcambarus clarkii girard

Summary1.Spikeless communication between dendrites of crayfish motoneurons was demonstrated by intracellular current injection.2.Subthreshold depolarization of one motoneuron (Add MN) innervating the adductor exopodite, one of the closer muscles of the uropod, increased the spontaneous discharge rate of another motoneuron (Red MN No. 1) innervating another closer muscle, reductor exopodite.3.Hyperpolarization of the Add MN caused a decrease in the spike frequency of the Red MN No. 1.4.The effects are graded and dependent on the membrane potential changes in the current-injected cells.5.Spikeless communication was also observed between opener motoneurons. Such communication was observed only between synergistic motoneurons and not between antagonistic ones.6.Regions in the Add MN in which current injection effectively changed the Red MNs activity were always distant from the spike-initiating region, and confined to the distal dendritic branches.7.The amplitude of e.p.s.p.s recorded from the Red MN No. 1 in response to antidromic spikes of the Add MN was increased by hyperpolarization and decreased by depolarization at the site of recording. The spikeless, graded interaction appears to be mediated by chemical transmission via a monosynaptic connection.8.It is concluded that the crayfish uropod motoneurons function not only as simple output elements but also as complex integrative elements by forming local circuits at restricted dendritic regions.

Functional characteristics of local non-spiking interneurons as the pre-motor elements in crayfish

SummaryControl of the crayfish uropod motoneurons by the unilateral-type local non-spiking interneurons (LNSNs) in the terminal (sixth) abdominal ganglion was studied with intracellular recording and current injection.1.Current injected into LNSNs affected the spike activity of uropod motoneurons in a graded manner. The effects depended on both the duration and the intensity of current pulses.2.Simultaneous intracellular recordings from an LNSN and an opener motoneuron showed that LNSNs could change the motoneuron membrane potential by their own membrane potential change without generating spikes.3.In fifty-nine cases, we penetrated LNSNs which affected uropod motoneurons bidirectionally. Hyperpolarization of 23 LNSNs decreased and that of other 36 LNSNs increased the spontaneous discharge rate of an identified reductor exopodite motoneuron (Red MN) No. 1. Depolarization had the opposite effect in either case.4.It is suggested that at least one chemical synapse should be intercalated between the LNSN and the motoneuron. although whether their connection is monosynaptic or not still remains open to future study.5.Twenty-four penetrated LNSNs co-actively controlled the synergistic set of motoneurons. Other 23 LNSNs also controlled the antagonistic set in addition to the synergistic set of motoneurons in a reciprocal way, and other 21 LNSNs in a co-activating way.6.The possible role of unilateral-type LNSNs in the motor control, especially in the non-rhythmical, episodic movement such as uropod steering, is discussed.

Statocyst control of the uropod movement in response to body rolling in crayfish

Summary1.The steering movement of crayfish uropods in response to body rolling was analysed quantitatively.2.The uropod on the upper side is spread out while the other one on the lower side is closed. Opening of one uropod was always accompanied by closing of the other. Positional changes of both uropods in response to body rolling were measured at 13 body positions. The response curve followed an approximate sine function in an intact animal3.In a single-statocyst animal, increase of receptor activity in the intact statocyst caused opening of the ipsilateral uropod and closing of the contralateral one, while decrease of receptor activity had no effect on the uropod position.4.A response curve obtained by algebraic summaion of two response curves in the animals with right and left statolith removed respectively was consistent with that of an intact animal with both statocysts.5.It was concluded that each statocyst can produce bilateral uropod movement only in a limited range of roll angle since the receptor activity increases only when the statocyst hairs are deflected toward the inside of the sensory crescent. Bilateral uropod movement in an intact animal is produced by simple summation of inputs from the right and left statocysts.6.Unilateral statolith removal caused an asymmetrical configuration of the uropods in the resting (0°) body position. The uropod on the operated side opened and that on the opposite side closed. This asymmetry was normalized day by day and the original symmetrical configuration was finally restored about 7 days after operation.

Functional polarization of statocyst receptors in the crayfishProcambarus clarkii Girard

SummaryDirectionality of response in the statocyst receptors of the crayfish (Procambarus clarkii Girard) was investigated by artificially deflecting individual statocyst hairs in various directions. 1.The hairs attached to the receptors are aligned in a crescent on the statocyst floor. Hair deflection toward the center of the crescent elicited maximal excitatory responses in the majority of the receptors. Hair deflection in the opposite direction elicited maximal suppressive responses.2.It was concluded that most of the statocyst receptors are functionally polarized toward the center of the crescent. The plane of functional polarization was consistent with that of morphological polarization.3.The receptors could be classified into two types; tonic (42%) and phasic (50%), according to the time course of the excitatory responses. The remainder (8%) could not be classified as either of these, showing intermediate responses.4.Some (ca. 40%) of the phasic-type receptors responded transiently regardless of the direction of hair deflection. It is suggested that the role of those receptors without any plane of functional polarization, is different from that of the other receptors in producing equilibrium responses.

Physiological and morphological characterization of anaxonic non-spiking interneurons in the crayfish motor control system

Spike activation of the motoneurons innervating uropod muscles in crayfish is controlled by anaxonic interneurons located within the terminal (the 6th abdominal) ganglion. These neurons do not generate spikes either spontaneously at the resting potential level or in response to current injection of either polarity. Yet the change in the membrane potential of these non-spiking interneurons caused an increase or decrease in the discharge frequency of motoneuron spikes, depending upon the direction of the membrane potential change. These non-spiking interneurons within the terminal ganglion presumably integrate various descending command signals and select the adequate information to be gated to the motoneurons.

Cholinergic transmission at mechanosensory afferents in the crayfish terminal abdominal ganglion

Electrical stimulation of mechanosensory afferents innervating hairs on the surface of the exopodite in crayfish Procambarus clarkii (Girard) elicited reciprocal activation of the antagonistic set of uropod motor neurones. The closer motor neurones were excited while the opener motor neurones were inhibited. This reciprocal pattern of activity in the uropod motor neurones was also produced by bath application of acetylcholine (ACh) and the cholinergic agonist, carbamylcholine (carbachol). The closing pattern of activity in the uropod motor neurones produced by sensory stimulation was completely eliminated by bath application of the ACh blocker, d-tubocurarine, though the spontaneous activity of the motor neurones was not affected significantly. Bath application of the acetylcholinesterase inhibitor, neostigmine, increased the amplitude and extended the time course of excitatory postsynaptic potentials (EPSPs) of ascending interneurones elicited by sensory stimulation. These results strongly suggest that synaptic transmission from mechanosensory afferents innervating hairs on the surface of the tailfan is cholinergic.Bath application of the cholinergic antagonists, dtubocurarine (vertebrate nicotinic antagonist) and atropine (muscarinic antagonist) reversibly reduced the amplitude of EPSPs in many identified ascending and spiking local interneurones during sensory stimulation. Bath application of the cholinergic agonists, nicotine (nicotinic agonist) and oxotremorine (muscarinic agonist) also reduced EPSP amplitude. Nicotine caused a rapid depolarization of membrane potential with, in some cases, spikes in the interneurones. In the presence of nicotine, interneurones showed almost no response to the sensory stimulation, probably owing to desensitization of postsynaptic receptors. On the other hand, no remarkable changes in membrane potential of interneurones were observed after oxotremorine application. These results suggest that ACh released from the mechanosensory afferents depolarizes interneurones by acting on receptors similar to vertebrate nicotinic receptors.

Further exploration into the adaptive design of the arthropod “microbrain”: I. Sensory and memory-processing systems

Abstract Arthropods have small but sophisticated brains that have enabled them to adapt their behavior to a diverse range of environments. In this review, we first discuss some of general characteristics of the arthropod “microbrain” in comparison with the mammalian “megalobrain”. Then we discuss about recent progress in the study of sensory and memory-processing systems of the arthropod “microbrain”. Results of recent studies have shown that (1) insects have excellent capability for elemental and context-dependent forms of olfactory learning, (2) mushroom bodies, higher olfactory and associative centers of arthropods, have much more elaborated internal structures than previously thought, (3) many genes involved in the formation of basic brain structures are common among arthropods and vertebrates, suggesting that common ancestors of arthropods and vertebrates already had organized head ganglia, and (4) the basic organization of sensori-motor pathways of the insect brain has features common to that of the mammalian brain. These findings provide a starting point for the study of brain mechanisms of elaborated behaviors of arthropods, many of which remain unexplored.

Exploration into the Adaptive Design of the Arthropod “Microbrain”

Abstract Arthropods have small but sophisticated brains which have enabled them to adapt their behavior to a diverse range of environments. The enormous evolutionary success of arthropods in terms of species richness and diversity depends on the sophistication of their brains. Advances in neurobiology have clarified some of the sensory and motor mechanisms of the arthropod brain, but the basic rules of computation underlying the central functions of the arthropod brain remain unknown. Consequently, it is not known how the basic design of the arthropod brain differs from, or is analogous to, that of other animals, especially mammals. In this report, we argue that characteristic features of the arthropod “microbrain” can be ascribed not only to the limited number of its constituting neurons but also to the optimization to life with a small body.

Readiness Discharge for Spontaneous Initiation of Walking in Crayfish

Animals initiate behavior not only reflexively but also spontaneously in the absence of external stimuli. In vertebrates, electrophysiological data on the neuronal activity associated with the self-initiated voluntary behavior have accumulated extensively. In invertebrates, however, little is known about the neuronal basis of the spontaneous initiation of behavior. We investigated the spike activity of brain neurons at the time of spontaneous initiation of walking in the crayfish Procambarus clarkii and found neuronal signals indicative of readiness or preparatory activities in the vertebrate brain that precede the onset of voluntary actions. Those readiness discharge neurons became active >1 s before the initiation of walking regardless of stepping direction. They remained inactive at the onset of mechanical stimulus-evoked walking in which other descending units were recruited. These results suggest that the parallel descending mechanisms from the brain separately subserve the spontaneous and stimulus-evoked walking. Electrical stimulation of these different classes of neurons caused different types of walking. In addition, we found other descending units that represented different aspects of walking, including those units that showed a sustained activity increase throughout the walking bout depending on its stepping direction, as well as one veto unit for canceling out the output effect of the readiness discharge and three termination units for stopping the walking behavior. These findings suggest that the descending activities are modularized in parallel for spontaneous initiation, continuation, and termination of walking, constituting a sequentially hierarchical control.

Afferent response patterns of the crayfish statocyst with ferrite grain statolith to magnetic field stimulation

Summary1.Magnetic field stimulation to simulate the rotation of the crayfish statocyst has been performed to determine the response characteristics of the statocyst hair cells. Ferrite grains were introduced about one month before the experiment as a replacement for the natural statoliths to effect magnetic pulling.2.Several types of response discharge could be recognized. Excitatory response types included tonicon, tonic-on-off, phasic-on, and phasic-on-off types. Inhibitory types can be divided into tonic-inhibition with off-excitation and phasic-inhibition with offinhibition.3.Statocyst afferent neurons could be classified into several groups depending on the direction of the preferred sensitivity within the simulated rotation plane tested. Among 21 units examined, 9 units showed a single preferred direction with excitatory response and were thus unidirectional units; 3 units were bidirectional, and 5 units were multidirectional. There were 2 bidirectional inhibitory units and 2 multidirectional inhibitory units.4.Analysis of the response pattern suggests that the central nervous system in the crayfish should be capable of averaging responses of more than one unit among the statocyst afferent neurons to detect the coincidence of discharge frequency change in order to obtain the positional information.