Response properties of wind-sensitive giant interneurons in the fourth-instar nymphs of the cricket, Gryllus bimaculatus

The response properties of four wind-sensitive giant interneurons (GIs) 8-1, 9-1, 9-2 and 9-3 in the fourth-instar nymphs of the cricket Gryllus bimaculatus were investigated to clarify the differences and/or similarities of the escape eliciting neural system between nymphs and adults. Air current was presented to the animal from 12 different directions in the horizontal plane, and the intensity-response curves for each GI were obtained at each stimulus direction. The intensity-response curves showed that the response magnitudes of GI 8-1 in the fourth-instar crickets increased with stimulus velocity up to 300 mm/s regardless of the stimulus direction. The response magnitudes of GI 9-1 in the nymphs reached a plateau at a stimulus velocity of 30 mm/s in most stimulus directions. The response magnitudes of GIs 9-2 and 9-3 increased with stimulus velocity up to 300 mm/s regardless of the stimulus direction. The directional sensitivity curves plotted on the basis of threshold velocities revealed that the preferential directions of the GIs were the ipsilateral-side in GI 8-1, the ipsilateral-front and contralateral-rear in GI 9-1, the ipsilateral-rear in GI 9-2 and the ipsilateral-front in GI 9-3, designated with respect to the side of the ventral nerve cord containing the axons. Although the GIs in nymphs occasionally showed higher threshold velocities and larger response magnitudes, the directional sensitivities, i.e., the preferred directions, of the GIs were basically the same with those of adults.     

Cercal sensory system and giant interneurons in Gryllodes sigillatus

The external morphologies of two cricket species, Gryllodes sigillatus and Gryllus bimaculatus, were investigated. Despite its small body length, G. sigillatus possessed longer cerci and longer cercal filiform hairs than G. bimaculatus. The estimated number of filiform hairs on a cercus was also larger in G. sigillatus than in G. bimaculatus. Wind-sensitive interneurons receiving sensory inputs from cercal filiform hairs and running in the ventral nerve cord (VNC) were investigated in G. sigillatus both morphologically and physiologically. By intracellular staining, these interneurons were proved to be morphologically homologous with previously identified giant interneurons (GIs 8-1, 9-1, 9-2, 9-3, 10-2, and 10-3) in G. bimaculatus and Acheta domesticus. In G. sigillatus, the intensity-response relationship (I-R curve) for each GI was investigated using a unidirectional air current stimulus. The stimulus was applied from 12 different directions, and an I-R curve was obtained for each stimulus direction. Each GI showed a characteristic I-R curve depending on stimulus direction. The directionality curve expressed in terms of threshold velocity showed that each GI had a distinctive directional characteristic. The functional properties of GIs in G. sigillatus, such as I-R curve, threshold velocity, and directional characteristics, were compared with those of homologous GIs in G. bimaculatus in Discussion.     

Functional changes of cricket giant interneurons caused by chronic unilateral cercal ablation during postembryonic development

One of a pair of cerci was ablated in the first-, fourth- and last-instar nymphs of the cricket, Gryllus bimaculatus. The insects were then reared until the final molt, after which the intensity-response (I-R) relationships for four giant interneurons (GIs) 8-1, 9-1, 9-2 and 9-3 with regard to a controlled air current stimulus were measured. In order to examine the functional changes during postembryonic development and the differences in the physiological plasticity of GIs between nymphs and adults, the obtained I-R curves for each GI were compared with those measured in intact and unilaterally cercus-ablated adult crickets. Each GI showed a distinctive change in response magnitudes after the long-term unilateral cercal ablation. In most cases, the I-R curves for each GI in the crickets ablated from nymphal periods were different from those in the adult crickets mentioned above. Moreover, the pattern of change in response magnitude was different from GI to GI. In contrast to these observations, it was reported that some important characteristics of the wind-evoked escape behavior such as relative occurrence and escape direction in unilaterally cercus-ablated crickets investigated after a long-term rearing were almost identical with those in intact crickets. Therefore, the results obtained in the present study suggest that functional changes occur not only in GIs but also in many other neural elements in the escape-eliciting system in order to maintain the features of wind-evoked escape behavior.     

Electromyographic characterization of walking behavior initiated spontaneously in crayfish

Crayfish initiate walking behavior not only reflexively in response to external stimuli but also spontaneously in the absence of any specific stimulus. In order to analyze the initiation mechanism underlying these different types of walking, we made simultaneous electromyographic (EMG) recordings from thoracic legs when animals initiated walking, either reflexively or spontaneously, and video recorded their movements synchronously with the EMG recording. Two different stimuli, mechanical and chemical, were used to reflexively induce walking. A non-rhythmic, sustained activation of leg muscles was found to precede the behavioral initiation of either type of walking. The duration of this non-rhythmic muscle activation was significantly longer in the spontaneously initiated walking than in the mechanical stimulus-evoked walking, although no difference was observed between the spontaneous and chemical stimulus-evoked walking. EMG recordings from all eight legs revealed that their non-rhythmic muscle activation occurred almost simultaneously prior to initiation of rhythmical stepping movements. When an animal was suspended without a leg substratum, the timing of muscle activation was more variable among the legs than in the free condition on the substratum. When the circumesophageal commissures were both severed to eliminate signals descending from the brain to the thoracic ganglia, the bilaterally coordinated rhythmic burst activity was not observed in the walking legs. These findings suggest that the spontaneous initiation of walking behavior requires sensory feedback signals from leg proprioceptors, subserved by a different descending activation mechanism from that for stimulus-driven initiation of walking.     

Rearing under different conditions results in different functional recoveries of giant interneurons in unilaterally cercus-ablated crickets, Gryllus bimaculatus

The effects of rearing conditions on the functional recovery of wind-sensitive giant interneurons (GIs) after unilateral cercal ablation were investigated in the cricket, Gryllus bimaculatus. Crickets were reared in a glass vials to prohibit free walking for 14 days after unilateral cercal ablation ("14-day vial" crickets). Other crickets were reared in an apparatus called a "walking inducer" (WI) to increase the walking distance during the same 14-day period ("14-day WI" crickets). In these crickets, the response properties of GIs 8-1, 9-1, 9-2, and 9-3 to air currents from various directions were investigated. From the intensity-response curves obtained, directionality curves expressed in terms of threshold velocity and response magnitude were made independently. To understand changes in the functional recovery of GIs more thoroughly, the directional characteristics of GIs in crickets 1 day after unilateral cercal ablation ("1-day free" crickets) were also compared. Between the 1-day free and 14-day vial crickets, all the GIs showed differences in both threshold velocity and response magnitude for some stimulus directions. Between the 14-day vial and 14-day WI crickets, differences in the threshold velocities of GIs 9-1, 9-2, and 9-3, and in the response magnitudes of GIs 8-1, 9-1, and 9-3 were detected. Because the rearing condition after unilateral cercal ablation largely affects the compensatory recovery in some parameters of wind-evoked escape behavior, such as relative occurrence and escape direction, we discuss the functional differences in GIs revealed here in relation to the roles of GIs in the neural system that controls escape behavior.     

Dynamic analysis of cockroach giant interneuron activity evoked by forced displacement of cercal thread-hair sensilla

This investigation involved extracellular recordings of cockroach abdominal giant interneuron (GI) action potentials evoked by cercal “threadlike” hair sensilla (THS) stimulation with a galvanometric device, by controlled displacements of about seven THS. Small and large GIs, distinguished by their amplitudes, were studied simultaneously. Only the small GIs were spontaneously active. Responses to sine, pulse, and ramp stimulation of sensilla produced phasic responses in both GI types. Some GIs were directionally sensitive and had shorter response latencies in the direction of best sensitivity while others were omnidirectional. Contralateral stimulation decreased responses to homolateral stimuli. In experiments using paired pulses (less than 50-ms intervals) there is a period of hyperexcitability, in large GIs, in which the response to the second stimulus is greater. Repeated stimulation caused an exponential decline in the response which was steeper in all GIs at higher stimulating frequencies and had a faster time course in large GIs. Because of this last property GIs function as low-pass filters limiting the flow of information, with large GIs having a lower frequency “cutoff” than smaller GIs.     

Postembryonic changes in the response properties of wind-sensitive giant interneurons in cricket

The intensity-response (I-R) relations for four wind-sensitive giant interneurons (GIs 8-1, 9-1, 9-2 and 9-3) in the fourth-, sixth- and last-instar nymphs of the cricket, Gryllus bimaculatus, were investigated using a unidirectional air current stimulus in order to explore the functional changes of GIs during postembryonic development. Contrary to our expectations, the response properties of GIs in nymphs were largely different from those in adults. The response magnitude of GI 8-1 in an intact cricket decreased during development, i.e. the GI in younger insects showed a larger response magnitude. Although the response magnitudes of GIs 9-1 and 9-2 were almost identical during the nymphal period, a significant decrease was observed after the imaginal ecdysis. During the nymphal period, the response magnitude of GI 9-3 increased according to the developmental stage. However, it decreased significantly after the imaginal ecdysis. We also investigated the response magnitudes of the GIs in nymphs after unilateral cercal ablation. From the results of ablation experiments, the changes in excitatory and/or inhibitory connections between filiform hairs and each GI during postembryonic development were revealed.     

Synaptic specificity in the first instar cockroach: patterns of monosynaptic input from filiform hair afferents to giant interneurons

Summary Direct evidence for monosynaptic connections between filiform hair sensory axons and giant interneurons (GIs) in the first instar cockroach, Periplaneta americana, was obtained using intracellular recording and HRP injection followed by electron microscopy. GIs 1–6 all receive monosynaptic input from at least one filiform afferent axon. GI1, GI2 and GI5 receive input only from the medial (M) axon, while GI3, GI4 and GI6 receive input from both M and lateral (L) axons. The dendrites of GI3 and GI6 which are contralateral to the cell bodies receive input from both axons whereas the smaller ipsilateral dendritic fields have synapses only from the L axon. GI5 has M axon input only onto its contralateral dendrites. In 50% of preparations GI7 receives weak input from the ipsilateral L axon. There is no obvious relationship between the morphology of the giant interneurons and the pattern of input they receive from the filiform afferents.Abbreviations GI giant interneuron - HRP horseradish peroxidase - L lateral axon - M medial axon     

Prioritization of emotional signals by the human auditory system: evidence from a perceptual hysteresis protocol

It is expected that natural selection has endowed our auditory apparatus with the ability to adaptively prioritize information that is crucial for survival and reproduction, such as vocal emotional signals emitted by our conspecifics, even in a noisy and dynamic natural environment (signals progressively emerge or fade away in noise as conspecifics move toward or away from us). Here, we tested the hypothesis that emotional signals are detected more easily (i.e., at lower signal-to-noise levels) and retained for a longer time (i.e., persisting in your sensory system at greater distance from the physical source) than signals bearing no emotional content, using a perceptual hysteresis protocol. Trials consisted of emotional signals (i.e., laughter and screams) or neutral signals (spectrally-rotated versions of the emotional stimuli) progressively emerging from white noise (ascending sequences) or progressively fading away in white noise (descending sequences). We demonstrated that vocal emotional signals were significantly detected at lower signal-to-noise levels than emotionally neutral signals in both ascending and descending sequences, suggesting that the human auditory system prioritizes signals bearing adaptive value.     

Flight-correlated activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria

The study investigates activity changes in neurons of the lateral accessory lobes in the brain of the locust Schistocerca gregaria during wind-elicited tethered flight. Neurons with ascending projections from the ventral nerve cord to the lateral accessory lobes showed flight-associated excitations which were modulated in the flight motor rhythm. Descending neurons with ramifications in the lateral accessory lobes were tonically excited corresponding to flight duration. The onset of wind-elicited responses in the descending neurons preceded the onset of flight motor activity by 22–60 milliseconds. Neurons connecting the lateral accessory lobes with the central body, the anterior optic tubercles, or other brain areas showed a variety of responses including activity changes during flight initiation and flight termination. Activity in many of these neurons was less tightly coupled to the flight situation and often returned to background levels before flight was terminated. Most of the recorded neurons responded, in addition, to stationary visual stimuli. The results suggest that the lateral accessory lobes in the locust brain are integrative links between the central body, visual pathways, and the ventral nerve cord. The possible involvement of these brain areas in flight control is discussed.     

Dissemination and loss of a biofilm‐related genomic island in marine Pseudoalteromonas mediated by integrative and conjugative elements

Acquisition of genomic islands (GIs) plays a central role in the diversification and adaptation of bacteria. Some GIs can be mobilized in trans by integrative and conjugative elements (ICEs) or conjugative plasmids if the GIs carry specific transfer‐related sequences. However, the transfer mechanism of GIs lacking such elements remains largely unexplored. Here, we investigated the transmissibility of a GI found in a coral‐associated marine bacterium. This GI does not carry genes with transfer functions, but it carries four genes required for robust biofilm formation. Notably, this GI is inserted in the integration site for SXT/R391 ICEs. We demonstrated that acquisition of an SXT/R391 ICE results in either a tandem GI/ICE arrangement or the complete displacement of the GI. The GI displacement by the ICE greatly reduces biofilm formation. In contrast, the tandem integration of the ICE with the GI in cis allows the GI to hijack the transfer machinery of the ICE to excise, transfer and re‐integrate into a new host. Collectively, our findings reveal that the integration of an ICE into a GI integration site enables rapid genome dynamics and a new mechanism by which SXT/R391 ICEs can augment genome plasticity.     

Multiple feedback loops in the flying cockroach: excitation of the dorsal and inhibition of the ventral giant interneurons

1. In a tethered cockroach (Periplaneta americana) whose wings have been cut back to stumps, it is possible to elicit brief sequences of flight-like activity by puffing wind on the animal's body. 2. During such brief sequences, rhythmic bursts of action potentials coming from the thorax at the wingbeat frequency, descend the abdominal nerve cord to the last abdominal ganglion (A6). This descending rhythm is often accompanied by an ascending rhythm (Fig. 2). 3. Intracellular recording during flight-like activity from identified ascending giant interneurons, and from some unidentified descending axons in the abdominal nerve cord, shows that: (a) ventral giant interneurons (vGIs) remain silent (Fig. 3); (b) dorsal giant interneurons (dGIs) are activated at the onset of the flight-like activity and remain active sporadically throughout the flight sequence (Fig.4); (c) some descending axons in the abdominal nerve cord show rhythmic activity phase-locked to the flight rhythm (Fig. 5). 4. Also during such brief sequences, the cercal nerves, running from the cerci (paired, posterior, wind sensitive appendages) to the last abdominal ganglion, show rhythmic activity at the wingbeat frequency (Fig. 6). This includes activity of some motor axons controlling vibratory cercal movements and of some sensory axons. 5. More prolonged flight sequences were elicited in cockroaches whose wings were not cut and which flew in front of a wind tunnel (Fig. 1B). 6. In these more prolonged flight sequences, the number of ascending spikes per burst was greater than that seen in the wingless preparation (Fig. 8; compare to Fig. 2). Recordings from both ventral and dorsal GIs show that: in spite of the ongoing wind from both the tunnel and the beating wings, which is far above threshold for the vGIs in a resting cockroach, the vGIs are entirely silent during flight. Moreover, the vGIs response to strong wind puffs that normally evoke maximal GI responses is reduced by a mean of 86% during flight (Fig. 9). The dGIs are active in a strong rhythm (Figs. 11 and 12). 7. Three sources appear to contribute to the ascending dGI rhythm (1) the axons carrying the rhythmic descending bursts; (2) the rhythmic sensory activity resulting from the cercal vibration; and (3) the sensory activity resulting from rhythmic wind gusts produced by the wingbeat and detected by the cerci. The contribution of each source has been tested alone while removing the other two (Figs. 13 and 14). Such experiments suggest that all 3 feedback loops are involved in rhythmically exciting the dGIs (Fig. 15).     

Directional sensitivity of wind-sensitive giant interneurons in the cave cricket Troglophilus neglectus.

Unlike the situation in most cockroach and cricket species studied so far, the wind-sensitive cerci of the cave cricket Troglophilus neglectus Krauss (Rhaphidophoridae, Orthoptera) are not oriented parallel to the body axis but perpendicular to it. The effects of this difference on the morphology, and directional sensitivity of cercal giant interneurons (GIs), were investigated. In order to test the hypothesis that the 90 degrees change in cercal orientation causes a corresponding shift in directional sensitivity of GIs, their responses in both the horizontal and vertical planes were tested. One ventral and four dorsal GIs (corresponding to GIs 9-1a and 9-2a, 9-3a, 10-2a, 10-3a of gryllid crickets) were identified. The ventral GI 9-1a of Troglophilus differed somewhat from its cricket homologue in its dendritic arborisation and its directional sensitivity in the horizontal plane. The morphology and horizontal directionality of the dorsal GIs closely resembled that of their counterparts in gryllids. In the vertical plane, the directionality of all GIs tested was similar. They were all excited mainly by wind puffs from the axon-ipsilateral quadrant. The results suggest that directional sensitivity to air currents in the horizontal plane is maintained despite the altered orientation of the cerci. This is presumably due to compensatory modifications in the directional pReferences of the filiform hairs.     

Structural and Functional Characterization of a Caenorhabditis elegans Genetic Interaction Network within Pathways

A genetic interaction (GI) is defined when the mutation of one gene modifies the phenotypic expression associated with the mutation of a second gene. Genome-wide efforts to map GIs in yeast revealed structural and functional properties of a GI network. This provided insights into the mechanisms underlying the robustness of yeast to genetic and environmental insults, and also into the link existing between genotype and phenotype. While a significant conservation of GIs and GI network structure has been reported between distant yeast species, such a conservation is not clear between unicellular and multicellular organisms. Structural and functional characterization of a GI network in these latter organisms is consequently of high interest. In this study, we present an in-depth characterization of ~1.5K GIs in the nematode Caenorhabditis elegans. We identify and characterize six distinct classes of GIs by examining a wide-range of structural and functional properties of genes and network, including co-expression, phenotypical manifestations, relationship with protein-protein interaction dense subnetworks (PDS) and pathways, molecular and biological functions, gene essentiality and pleiotropy. Our study shows that GI classes link genes within pathways and display distinctive properties, specifically towards PDS. It suggests a model in which pathways are composed of PDS-centric and PDS-independent GIs coordinating molecular machines through two specific classes of GIs involving pleiotropic and non-pleiotropic connectors. Our study provides the first in-depth characterization of a GI network within pathways of a multicellular organism. It also suggests a model to understand better how GIs control system robustness and evolution.     

Use of bilateral information to determine the walking direction during orientation to a pheromone source in the silkmoth Bombyx mori

Odor source localization is an important animal behavior. Male moths locate mates by tracking sex pheromone emitted by conspecific females. During this type of behavior, males exhibit a combination of upwind surge and zigzagging flight. Similarly, the male walking moth Bombyx mori responds to transient pheromone exposure with a surge in movement, followed by sustained zigzagging walking. The initial surge direction is known to be influenced by the pheromone input pattern. Here, we identified the sensory input patterns that determine the initial walking direction of males. We first quantified the stimulus by measuring electroantennogram values, which were used as a reference for subsequent tests. We used a brief stimulus pulse to examine the relationship between sensory stimulus patterns and the turning direction of initial surge. We found that the difference in input timing and intensity between left and right antennae affected the walking direction, indicating that B. mori integrate bilateral pheromone information during orientation behavior. When we tested pheromone stimulation for longer periods, turning behavior was suppressed, which was induced by stimulus cessation. This study contributes toward understanding efficient strategies for odor-source localization that is utilized by walking insects.     

Visually elicited turning behavior in Rana pipiens: comparative organization and neural control of escape and prey capture

High-speed videography was used to describe the initial turning movement of visually triggered escape in frogs and to compare it with the initial turn of frog prey capture behavior. These two types of turning had some general similarities, e.g. turn duration and peak velocity were positively correlated with turn angle. However, there were kinematic differences: for turns of a given angular amplitude, escape turns consistently demonstrated shorter duration and higher peak velocity than prey capture turns. There also were differences predictably matched to stimulus angles; escape turn angles were more variably related to stimulus angles. Both turning movements are believed to depend upon the optic tectum. However, given the observed differences in kinematics and spatial organization, we used lesion experiments to determine if distinct tectal efferent pathways subserve turning under each circumstance. Large unilateral lesions of the brainstem simultaneously disrupted both types of turning. However, smaller laterally placed lesions disrupted escape turning without disrupting prey capture turns. The kinematic differences in combination with the lesion results support the idea that the post-tectal circuitry for visually elicited turning movements is based upon separate descending pathways that control turning toward prey and turning away from threat.Abbreviations CG central gray - OT optic tectum - SEM standard error of the mean     

Cholinergic neurotransmission from mechanosensory afferents to giant interneurons in the terminal abdominal ganglion of the cricket Gryllus bimaculatus

Crickets respond to air currents with quick avoidance behavior. The terminal abdominal ganglion (TAG) has a neuronal circuit for a wind-detection system to elicit this behavior. We investigated neuronal transmission from cercal sensory afferent neurons to ascending giant interneurons (GIs). Pharmacological treatment with 500 muM acetylcholine (ACh) increased neuronal activities of ascending interneurons with cell bodies located in the TAG. The effects of ACh antagonists on the activities of identified GIs were examined. The muscarinic ACh antagonist atropine at 3-mM concentration had no obvious effect on the activities of GIs 10-3, 10-2, or 9-3. On the other hand, a 3-mM concentration of the nicotinic ACh antagonist mecamylamine decreased spike firing of these interneurons. Immunohistochemistry using a polyclonal anti-conjugated acetylcholine antibody revealed the distribution of cholinergic neurons in the TAG. The cercal sensory afferent neurons running through the cercal nerve root showed cholinergic immunoreactivity, and the cholinergic immunoreactive region in the neuropil overlapped with the terminal arborizations of the cercal sensory afferent neurons. Cell bodies in the median region of the TAG also showed cholinergic immunoreactivity. This indicates that not only sensory afferent neurons but also other neurons that have cell bodies in the TAG could use ACh as a neurotransmitter.     

Two types of information processing in cercal systems of insects: directional sensitivity of giant interneurons

Summary Cercal systems of seven insect species (cricketMelanogryllus desertus, mole cricketGryllotalpa gryllotalpa, katydidsPholidoptera pustulipes andTettigonia viridissima, cockroachesPeriplaneta americana andBlatta orientalis, and locustLocusta migratoria) were examined for direction-sensitive giant interneurons (GIs) that are excited by cercal receptors but have directional preferences independent of cercus position. Such GIs are known for the cricketsAcheta domesticus andGryllus bimaculatus. Directional sensitivity diagrams (DSDs) of GIs were obtained by recording and analysing the electrical responses of abdominal connectives to sound stimuli from various directions. For each animal DSDs were plotted in the form of polar graphs for two or three positions of the stimulated cercus so that the effect of cercus position on the orientation of the DSD could be evaluated.All insects studied had GIs whose DSDs for fixed cercus positions were similar in appearance to the DSDs described for GIs of the cricketsAcheta domesticus andGryllus bimaculatus. Most of these DSDs were shaped like a figure 8 (when airflow is used as the stimulus instead of sound, each DSD has only one lobe). However, not all GIs demonstrated a constant directional preference. GIs with constant directionality were found only inMelanogryllus desertus, Pholidoptera pustulipes, Tettigonia viridissima andLocusta migratoria. In these insects DSDs from one GI plotted for different cercus positions had approximately the same orientation (Figs. 4–7). In contrast, GIs inGryllotalpa gryllotalpa, Periplaneta americana andBlatta orientalis had DSDs whose orientation changed in accordance with a change in position of the stimulated cercus (Figs. 8–10).Thus, direction-sensitive GIs investigated here can be divided into two types: (1) GIs with constant directionality (whose DSDs are fixed to the body, and (2) GIs with variable directionality (whose DSDs are fixed to the cerci). To date, in each species only GIs of the same type have been encountered. This may be an indication that cercal systems can be divided into two categories according to how they process information. However, since we have not tested all GIs in each species, we cannot rule out the possibility that a species might have both types of GIs.Abbreviations DSD directional sensitivity diagram - GI giant interneuron - TAG terminal abdominal ganglion     

Positional information determines the anatomy and synaptic specificity of cockroach filiform hair afferents using independent mechanisms

Summary Mutant first instar cockroaches (Periplaneta americana) with supernumerary filiform hair sensilla on their cerci were used to study the effects of cell body position on axonal morphology and synaptic connections. The wild-type cercus has two hairs, one lateral (L) and the other medial (M), each with an underlying sensory neuron. Silver-intensified cobalt fills show that the supernumerary lateral neuron (SIN) in the mutant has the same shape of arborization as L, and electrophysiological recording shows that it forms synaptic connections with the same subset of giant interneurons (GIs) as L in the terminal ganglion: GI3 and GI6. The supernumerary medial neuron (SuM) has the same axonal morphology as M and synapses with the same GIs as does M: ipsilateral GIs 1 and 2 and contralateral GIs 1, 2, 3, 5 and 6. In 0.1% of approximately 8000 animals screened, a supernumerary hair arose on the cereal midline (C hair). The C neuron sends its axon to the CNS in the same branch of the cereal nerve as the L and SIN, and has a similar arborization. However, the C neuron forms synapses with the same GIs as do M and SuM. Electron microscopy of horseradish peroxidase-injected neurons was used to confirm that the C afferent forms a monosynaptic connection to GI2. It was concluded that the position of the sensory neuron cell body does control its axonal morphology and synaptic connectivity, but that these characteristics are produced by independent mechanisms.Abbreviations GI giant interneuron - L lateral - M medial - SI Space Invader - SuM supernumerary medial - C cereal midline