The responses of peripheral and central mechanosensory lateral line units of weakly electric fish to moving objects

Mechanosensory lateral line afferents of weakly electric fish (Eigenmannia) responded to an object which moved parallel to the long axis of the fish with phases of increased spike activity separated by phases of below spontaneous activity. Responses increased with object speed but finally may show saturation. At increasingly greater distances the responses decayed as a power function of distance. For different object velocities the exponents (mean±SD) describing this response falloff were -0.71±0.4 (20 cm/s object velocity) and-1.9±1.25 (10 cm/s). Opposite directions of object movement may cause an inversion of the main features of the response histograms. In terms of peak spike rate or total number of spikes elicited, however, primary lateral line afferents were not directionally sensitive.Central (midbrain) lateral line units of weakly electric fish (Apteronotus) showed a jittery response if an object moved by. In midbrain mechanosensory lateral line, ampullary, and tuberous units the response to a rostral-tocaudal object movement may be different from that elicited by a caudal-to-rostral object motion. Central units of Apteronotus may receive input from two or more sensory modalities. Units may be lateral line-tuberous or lateral line-ampullary. Multimodal lateral line units were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. The receptive fields of central units demonstrate a weak somatotopic organization of lateral line input: anterior body areas project to rostral midbrain, posterior body areas project to caudal midbrain.Abbreviation EOD electric organ discharge     

Physiology of lateral line mechanoreceptive regions in the elasmobranch brain

The physiology of mechanoreceptive lateral line areas was investigated in the thornback guitarfish, Platyrhinoidis triseriata, from medulla to telecephalon, using averaged evoked potentials (AEPs) and unit responses as windows to brain functions. Responses were analysed with respect to frequency sensitivity, intensity functions, influence of stimulus repetition rate, response latency, receptive field (RF) organization and multimodal interaction. 1. Following a quasi-natural vibrating sphere stimulus, neural responses were recorded in the medullary medial octavolateralis nucleus (MON), the dorsal (DMN) and anterior (AN) nucleus of the mesencephalic nuclear complex, the diencephalic lateral tuberal nucleus (LTN), and a telencephalic area which may correspond to the medial pallium (Figs. 2, 3, 13, 14, 15, 16). 2. Within the test range of 6.5-200 Hz all lateral line areas investigated responded to minute water vibrations. Best frequencies (in terms of displacement) were between 75 and 200 Hz with threshold values for AEPs as low as 0.005 microns peak-to-peak (p-p) water displacement calculated at the skin surface (Fig. 6). 3. AEP-responses to a vibrating sphere stimulus recorded in the MON are tonic or phasic-tonic, i.e., responses are strongest at stimulus onset but last for the whole stimulus duration in form of a frequency following response (Fig. 3). DMN and AN responses are phasic or phasic-tonic. Units recorded in the MON are phase coupled to the stimulus, those recorded in the DMN, AN or LTN are usually not (Figs. 5, 8, 9). Diencephalic LTN and telencephalic lateral line responses (AEPs) often are purely phasic. However, in the diencephalic LTN tonic and/or off-responses can be recorded (Fig. 11). 4. For the frequencies 25, 50, and 100 Hz, the dynamic intensity range of lateral line areas varies from 12.8 to at least 91.6 dB (AEP) respectively 8.9 and 92 dB (few unit and single unit recordings) (Fig. 7). 5. Mesencephalic, diencephalic, and telecephalic RFs, based on the evaluation of AEPs or multiunit activity (MUA), are usually contralateral (AN and LTN) or ipsi- and contralateral (telencephalon) and often complex (Figs. 10, 12, 16). 6. In many cases no obvious interactions between different modalities (vibrating sphere, electric field stimulus, and/or a light flash) were seen. However, some recording sites in the mesencephalic AN and the diencephalic LTN showed bimodal interactions in that an electric field stimulus decreased or increased the amplitude of a lateral line response and vice versa (Fig. 13 B).     

The lateral line mechanoreceptive mesencephalic,diencephalic, and telencephalic regions in the thornback ray,Platyrhinoidis triseriata (Elasmobranchii)

Central lateral line pathways were mapped in the thronback ray, Platyrhinoidis triseriata, by analyzing depth profiles of averaged evoked potentials (AEPs), multiunit activity (MUA), and single unit recordings. Neural activity evoked by contra- or ipsilateral posterior lateral line nerve (pLLN) shock is restricted to the tectum mesencephali, the dorsomedial nucleus (DMN) and anterior nucleus (AN) of the mesencephalic nuclear complex, the posterior central thalamic nucleus (PCT), the lateral tuberal nucleus of the hypothalamus, and the deep medial pallium of the telencephalon (Figs. 2, 3, 4, 6, 7). Neural responses (AEPs and MUA) recorded in different lateral line areas differ with respect to shape, dynamic response properties, and/or latencies (Figs. 9, 10 and Table 1). Ipsilaterally recorded mesencephalic and diencephalic AEPs are less pronounced and of longer latency than their contralateral counterpart (Fig. 9 and Table 1). In contrast, AEP recorded in the telencephalon show a weak ipsilateral preference. If stimulated with a low amplitude water wave most DMN, AN, and tectal lateral line units respond in the frequency range 6.5 Hz to 200 Hz. Best frequencies (in terms of least displacement) are 75-150 Hz with a peak-to-peak water displacement of 0.04 micron sufficient to evoke a response in the most sensitive units (Fig. 11A, B, C). DMN and AN lateral line units have small excitatory receptive fields (RFs). Anterior, middle, and posterior body surfaces map onto the rostral, middle, and posterior brain surfaces of the contralateral DMN (Fig. 12). Some units recorded in the PCT are bimodal; they respond to a hydrodynamic flow field--generated with a ruler approaching the fish--only if the light is on and the eye facing the ruler is left uncovered (Fig. 13).     

Effects of running water on brainstem lateral line responses in trout,<Emphasis Type="Italic"> Oncorhynchus mykiss</Emphasis>, to sinusoidal wave stimuli

We investigated how single units in the medial octavolateralis nucleus of the rainbow trout, Oncorhynchus mykiss, respond to a 50-Hz vibrating sphere in still and running water. Four types of units were distinguished. Type MI units (N=16) were flow-sensitive; their ongoing discharge rates either increased or decreased in running water, and as a consequence, responses of these units to the vibrating sphere were masked if the fish was exposed to water flow. Type MII units (N=7) were not flow-sensitive; their ongoing discharge rates were comparable in still and running water, and thus their responses to the vibrating sphere were not masked. Type MIII units (N=7) were also not flow-sensitive; nevertheless, their responses to the vibrating sphere were masked in running water. Type MIV units (N=14) were flow-sensitive, but their responses to the vibrating sphere were not masked. Our data confirm previous findings in the goldfish, Carassius auratus, indicating that the organization of the peripheral lateral line is reflected to a large degree in the medial octavolateralis nucleus. We compare data from goldfish and trout and discuss differences with respect to lateral line morphology, lifestyle and habitat of these species.Abbreviations CN canal neuromast - MON medial octavolateralis nucleus - SN superfical neuromast - a.c. alternating current - d.c. direct current     

Brainstem lateral line responses to sinusoidal wave stimuli in the goldfish, Carassius auratus

Extracellular recordings were made from single lateral line units in the medial octavolateralis nucleus in the brainstem of goldfish, Carassius auratus. Units were defined as receiving lateral line input if they responded to the water motions generated by a stationary, sinusoidally oscillating sphere and/or a moving sphere but not to airborne sound and vibrations. Units which responded to airborne sound or vibrations were assumed to receive input from the inner ear and were not further investigated. Responses of lateral line units were quantified in terms of the number of evoked spikes and the degree of phase-locking to a 50 Hz vibrating sphere presented at various stationary locations along the side of the fish. Receptive fields were characterized based on spike rate, degree of phase-locking and average phase angle as a function of sphere location. Four groups of units were distinguished: 1, units with receptive fields comparable to those of primary afferents; 2, units with receptive fields which consisted of one excitatory and one inhibitory area; 3, units with receptive fields which consisted of more than two excitatory and/or inhibitory areas; 4, units with receptive fields which consisted of a single excitatory or a single inhibitory area. The receptive fields of most units were characterized by adjacent excitatory and inhibitory areas. This organization is reminiscent of excitatory-inhibitory receptive field organizations in other vertebrate sensory systems.     

Action of the octavolateralis efferent system upon the lateral line of free-swimming toadfish,Opsanus tau

Summary The activation and action of the octavolateralis efferent system was studied by chronic recordings of discharge patterns from putative efferent and single primary afferent neurons in alert, free-swimming toadfish. Efferent axons isolated in the anterior lateral line nerve showed phasic discharges following touch stimuli applied to the head or trunk and demonstrated sustained discharges to visual stimuli. Resting discharge patterns of primary afferents were categorized into irregular, burster, regular, and silent classes. Afferent discharges were often modulated by low frequency (< 1 Hz) water movement around the head generated during respiratory movements. When fish with recording electrodes implanted in the lateral line nerve were visually stimulated, modulated peak discharges and average (DC) firing rates were inhibited in irregular-type units only. Inhibition of irregular-type afferent neurons also followed visual presentation of natural prey and persisted long after prey stimuli were removed from view. The inhibitory action upon lateralis afferents when activated by biologically significant visual stimuli leads to the hypothesis that the octavolateralis efferent system functions in the peripheral processing of information carried by the lateral line in natural settings.Abbreviations DC average - IO infraorbital - IPSPs inhibitory postynaptic potentials - MXC maxillary canal - OMC operculomandibular canal - SOC supraorbital canal     

Directionality of antennal sweeps elicited by water jet stimulation of the tailfan in the crayfish Procambarus clarkii

Summary Directionality and intensity dependence of antennal sweeps elicited by water jet stimulation of the tailfan in tethered, reversibly blinded adult and juvenile crayfish (Procambarus clarkii) were analyzed.Resting crayfish keep their antennae at about 50° symmetrically to the longitudinal body axis (Figs. 2 bottom, and 3).In adults, tailfan stimulation elicits synchronous backward sweeps of both antennae, which increase for more caudal stimulus directions (Figs. 2–4 and 5A). Directions differing by 30°–60° are significantly distinguished (Fig. 4). The mean sweep of the ipsilateral antenna significantly overrides that of the contralateral antenna for rostrolateral stimulation at 40–200 mm/s stimulus velocity and lateral to caudolateral stimulation at 40 mm/s and thus lateralization of the stimulus is revealed (Figs. 2 top, 4 and 5A). Mean antennal sweeps at a given stimulus direction and distance increase with increasing stimulus velocity (40–250 mm/s, Fig. 5A).In juveniles, the directional dependence of antennal sweeps is reduced compared to that of adults, while a similar intensity dependence is found (Fig. 5B).The pronounced directionality of the antennal response in adult crayfish vanishes and response latencies increase after reversibly covering the tailfan with a small bag or the telson with waterproof paste (Figs. 6 and 7). Thus, tailfan and especially telson mechanoreceptors play an important role in the localization of water movements elicited by predators or prey behind the crayfish.     

Responses of brainstem lateral line units to different stimulus source locations and vibration directions

We recorded responses of lateral line units in the medial octavolateralis nucleus in the brainstem of goldfish, Carassius auratus, to a 50 Hz vibrating sphere and studied how responses were affected by placing the sphere at various locations alongside the fish and by different directions of vibration. In most units (88%), stimulation with the sphere from one or more spatial locations caused an increase and/or decrease in discharge rate. In few units (10%), discharge rate was increased by stimulation from one location and decreased by stimulation from an adjacent location in space. In a minority of the units (2%), changing sphere location did not affect discharge rates but caused a change in phase coupling. Units sensitive to a distinct sphere vibration direction were not found. The data also show that the responses of most brainstem units differ from those of primary afferent nerve fibers. Whereas primary afferents represent the pressure gradient pattern generated by the sphere and thus encode location and vibration direction of a vibrating sphere, most brainstem units do not. This information may be represented in the brainstem by a population code or in higher centers of the ascending lateral line pathway.     

Multimodal interneurones in the polyclad flatworm,Alloeoplana californica

Summary Two morphological types of interneurones were found in the brainof Alloeoplana californica (Figs. 1, 2). Both respond to water vibration and to light offset (Fig. 3). These responses are blocked by Mg⁺⁺ or Cd⁺⁺ (Fig. 4), and habituate to repetitive stimuli (Figs. 6, 10). Even when the light response is habituated, light offset will dishabituate the vibration response (Figs. 7, 10); no other regime tested produced dishabituation of either response. These neurones receive higher-order sensory input, and make subthreshold excitatory synapses on motor pathways; intracellular tetraethylammonium lengthens the time course of the spikes (Fig. 5), and each such spike elicits a contraction in the anterior margin of the animal. We believe that they form part of the neuronal circuitry underlying arousal.Abbreviation TEA tetraethylammonium     

Directionality of phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens

Summary A dorsal approach to the eighth nerve and free-field stimulation were used to investigate the effect of sound direction and intensity on phase locking in auditory nerve fibers of the leopard frog Rana pipiens pipiens.Tuning curves of 75 auditory neurons were analyzed (Fig. 2). Amphibian papillar neurons, but not basilar papillar neurons, exhibit significant phase locking to short tone bursts at the characteristic frequency (CF), the degree of phase locking (vector strength) decreasing with the neuron's CF (Figs. 3, 4 and 10E). Vector strength increases with sound pressure level to saturate about 20 dB above threshold, while the preferred firing phase is only slightly affected (Figs. 5 and 6).In contrast, sound direction hardly affects vector strength (Figs. 7, 8, 9A and 10A and C), but has a strong influence on the preferred firing phase (Figs. 7, 8, 9B and C, 10B and D): With respect to anterior tone presentation there are phase lags for ipsilateral and phase leads for posterior and contralateral presentation. Phase differences between both ears show a sinusoidal or cardioid/ovoidal directional characteristic; maximum differences are found with antero-lateral tone presentation (Fig. 11). The directionality of phase locking decreases with the neuron's CF (Fig. 10F) and only slightly changes with sound pressure level (Fig. 12). Thus, phase locking of amphibian papilla neurons can potentially provide intensity-independent information for sound localization.Abbreviations SPL sound pressure level - FTC frequency threshold curve - CF characteristic frequency - TF test frequency - VS vector strength - AP amphibian papilla - BP basilar papilla     

Intracellular activity in cricket neurons during generation of song patterns

Summary During production of song patterns by the semi-isolated CNS of Gryllus campestris, intracellullar recordings were made in fibers of the mesothoracic ganglion, including synaptic areas of identified wing opener and closer motor neurons. The normal calling song pattern and some transitional songs toward courtship and toward aggression were generated by the CNS in the absence of any phasic sensory timing (Figs. 1, 4). Intracellular activity of the opener motor neurons was characterized by an absence of events in the interchirp interval, an EPSP underlying each burst, and an IPSP following the burst if the closer motor neurons were to be activated (Fig. 1). Intracellular activity of the closer motor neurons was characterized by an absence of events in the interchirp interval, an IPSP immediately following the onset of the opener motor neuron burst, and an EPSP after the IPSP (Figs. 2, 3). Units were found which fired in a burst during the period when both the opener and closer motor neurons were inhibited (Fig. 5). Complementary sets of units were found which displayed an oscillation of activity at the chirp rhythm but not at the pulse rhythm (Fig. 6). Gaps in the calling song were observed whose characteristics indicated that motor neuron activity was neither required for, nor effective in, resetting the chirp timing oscillator (Fig. 8). A possible model for the song generating mechanism is outlined.     

Primary afferent fibers conduct impulses in both directions under physiological stimulus conditions

Summary In electric fish of the family Mormyridae some primary afferent fibers conduct impulses not only from electroreceptors to the brain but also from the brain to the receptors. The efferent impulses may be elicited by electrical stimulation which is within the physiological range, i.e., by stimulation which is similar in amplitude and duration to the stimulation that is caused by the fish's own electric organ discharge. Afferent and efferent impulses in the same afferent fiber were identified by: simultaneously recording from a fiber at two different points, at the receptor and at the nerve trunk (Figs. 2C-H; 3B-D); by cutting the afferent fiber between the brain and the recording site as well as between the recording site and the periphery; and by intra-axonal recording from the afferent fiber near its entry into the brain (Fig. 4). The efferent impulses result from the central integration of a corollary discharge of the electric organ motor command with excitatory and inhibitory input from several different receptors near the one from which afferent impulses originate (Fig. 4). The centrally originating impulse may be capable of modifying the effect of signals originating in the periphery.Abbreviations ELLL electrosensory lateral line lobe - EOCD electric organ corollary discharge - EOD electric organ discharge - epsp excitatory postsynaptic potential - NPLL posterior lateral line nerve     

Model studies on the mechanical significance of grouping in compound spider slit sensilla (Chelicerata,Araneida)

Summary The mechanical implications of various types of slit arrangements found among the strain-sensitive slit sensilla in the arachnid exoskeleton (Fig. 3) were studied by measuring the deformation of model slits, cut into plastic discs, under static load applied in the plane of the disc and from varying directions (Figs. 1, 2).1. Close parallel, lyriform arrangements. Compression of slits (adequate stimulus) reaches much higher values than dilatation. It is highest with the load direction at right angle to the slit axes. Also, in the majority of slits the range of load angles resulting in compression is considerably larger than that leading to dilatation. Length distribution and lateral shift of slits in the models have a pronounced effect on slit deformability (Figs. 4-5): (a) In the oblique bar arrangement with slits of equal length and regular lateral shift (Fig. 4A) deformation of all slits is very similar at all load directions. In all slits compression results from a range of load angles larger than 120°. (b) In arrangements with a regular increase in slit length and a triangular outline shape deformability differs greatly among the slits at all load angles (Fig. 4B). (c) The slit configuration with a heartshaped outline (Fig. 4C) is peculiar for the large spread of load angles at which the compression of the different slits is highest. — These properties recommend different arrangements for the solution of different strain measuring problems, with for instance, the particular need of a wide angular working range (arrangement a), of a large spectrum of absolute sensitivities (b), or of the analysis of load direction (c).2. Angle and distance between slits. Due to the mechanical directionality inherent in an elongated slit the divergence of slit axes within a group of slits is likely to indicate the importance of the analysis of strain direction (Fig. 6). The mechanical interaction between closely neighbouring slits decreases with their distance from each other. In a parallel arrangement of equally long slits it disappears if the distance is about 1.5 times the slit length (Fig. 7).3. Aiming towards a mechanical model which would explain the complex deformation found in a lyriform organ, we consider the outline of the organ as a hole traversed by beams of material. Slit deformation can be calculated from the elastic lines of the beams which separate the slits and information drawn from photoelastic experiments (Figs. 8-11).     

Warm and cold receptors of two sensilla on the foreleg tarsi of the tropical bont tickAmblyomma variegatum

Summary A pair of antagonistic thermal receptors has been identified in each of two long, tapering, poreless setae located distally on the foreleg tarsi of the tropicalbont tick,Amblyomma variegatum (Fig. 1). One, the cold receptor, responds to a rapiddrop in temperature (T) with a sudden rise in impulse frequency (F). The other, a warm receptor, responds to a rapidrise inT with a sudden rise inF (Figs. 2, 4). These two units are unusual for sharing their seta with two other units which are mechanosensitive. The four are distinguishable on the basis of spike amplitude and form (Fig. 3). Hence the thermal sensitivity displayed is hardly attributable to the pair of cells with tubular bodies but rather to the two extending up into the seta (for structure, see Hess and Vlimant 1982, 1983 a).As based on the first 100 ms of the response, differential sensitivity to rapidT change is –16.1± 10.4 (imp/s)/°C for cold units, 17.6 ± 9.5 (imp/s)/°C for warm (Table 1). As progressively larger segments of the spike train are employed to determineF, differential sensitivity of the warm unit drops off much more quickly than that of the cold (Table 2, Figs. 5, 6). In the cold unit resolving power (the difference in rapid temperature change discriminable with 90% probability by a pair of responses of a single unit at average sensitivity) continues to increase as the segment of the spike train determiningF is lengthened (from 0.58 °C for 100 ms segments to 0.41 °C for 1,100 ms segments). Resolving power of the warm unit, on the other hand, tends to decrease as longer segments are employed (from 0.52 °C for the first 100 ms to 0.80 °C for the first 1,100 ms). These relationships provoke the question of whether the spike trains may be evaluated in the CNS in different fashions.Abbreviations b slope of characteristic curve - F impulse frequency in impulses per second (imp/s) - n number of individuals examined - Pw partial pressure of water vapor in Torr - r correlation coefficient - s SD of responses from characteristic curve - SD standard deviation - T temperature in °C - T difference inT Refers to difference between initial and end temperature in abruptT changes     

Single unit activity in the peripheral lateral line system of the cichlid fishSarotherodon niloticus L.

Summary The activities of single afferent fibers were recorded in the trunk lateral line nerve of the cichlid fishSarotherodon niloticus L. Using both electrophysiological recordings and neuroanatomical tracing techniques, the number, arrangement, and innervation of superficial (SNs) and canal (CNs) neuromasts were determined. Both, SNs and CNs, are innervated by several afferent fibers of different diameters and efferent fibers. The CNs and SNs are neuronally separated: afferent fibers which innervate both CNs and SNs were not found. Whereas the single CN is innervated by a separate set of afferent fibers, fibers innervating the SNs within rows often branched to reach all or several SNs. The SNs within a row were thus considered to form a functional unit. With the exception of SNs on the tail fin, functional units of neuromasts were in general topographically restricted to single scales.The majority of lateral line units had resting activity. On the basis of the time interval distribution of the resting activity, 4 types of units were classified: these were labelled irregular (type I), regular (type II), bimodal (type III) and silent (type IV). Type I was the most common type of resting activity (obtained in 47.8% of the recorded units). Units with this resting activity type were identified as afferents innervating either SNs or CNs. Units with resting activity of type II represented mostly afferents of CNs if their mean activity was high (around 40 imp/s). If the mean activity of this type was below 20 imp/s the units were unresponsive to local water movements and at least some were identified as efferent fibers. Resting activity of type III was found only in units originating from CNs. Only 4% of the units were silent (type IV). These units were often identified as injured neuromasts. Units originating from CNs show higher mean resting activity than those from SNs. For both SN and CN units, the mean discharge rate of the resting activity correlated with the sensitivity to stimulation for sinusoidal water movements.During stimulation of the neuromasts by sinusoidal water movements of small amplitude and different frequencies, the response characteristics of SN and CN units were determined by linear frequency analysis under steady state conditions. Most units responded linearly to small stimulus amplitudes. In this amplitude range the units' resting activity was modulated according to the stimulus frequency. Small stimulus amplitudes proportionally changed the amount of modulation but did not alter the phase of the response. CN and SN units that responded linearly produce differing frequency responses. Whereas CNs were most sensitive at frequencies of up to 200 Hz (center frequencies between 100 and 200 Hz), the center frequencies of SNs were distributed between 10 and 70 Hz with a maximum number at about 30 Hz. Bode plots for many CN and SN units indicated that the neuromasts were sensitive to the acceleration component of the water movement.The functional significance of the differences between the two types of lateral line neuromasts (SNs and CNs) were discussed.Abbreviations SN superficial neuromast - CN canal neuromast     

Ontogenesis of tonotopy in inferior colliculus of a hipposiderid bat reveals postnatal shift in frequency-place code

Summary The postnatal development of midbrain tonotopy was investigated in the inferior colliculus (IC) of the south Indian CF-FM batHipposideros speoris. The developmental progress of the three-dimensional frequency representation was determined by systematic stereotaxic recordings of multiunit clusters from the 1st up to the 7th postnatal week. Additional developmental measures included the tuning characteristics of single units (Figs. 3f; 4f; 5f), the analysis of the vocalised pulse repertoire (Figs. 3e, 4e, 5e), and morphometric reconstructions of the brains of all experimental animals (Fig. 1).The maturation of auditory processing could be divided into two distinct, possibly overlapping developmental periods: First, up to the 5th week, the orderly tonotopy in the IC developed, beginning with the low frequency representation and progressively adding the high frequency representation. With regard to the topology of isofrequency sheets within the IC, maturation progresses from dorsolateral to ventromedial (Figs. 3c, 4c). At the end of this phase the entire IC becomes specialised for narrowly tuned and sensitive frequency processing. This includes the establishment of the auditory fovea, i.e. the extensive spatial representation of a narrow band of behaviorally relevant frequencies in the ventromedial part of the IC. In the 5th postnatal week the auditory fovea is concerned with frequencies from 100–118 kHz (Fig. 4c, d). During subsequent development, the frequency tuning of the auditory fovea increases by 20–25 kHz and finally attains the adult range of ca. 125–140 kHz. During this process, neither the bandwidth of the auditory fovea (15–20 kHz) nor the absolute sensitivity of its units (ca. 50 dB SPL) were changed. Further maturation occurred at the single unit level : the sharpness of frequency tuning increased from the 5th to the 7th postnatal weeks (Q-₁₀-dB-values up to 30–60), and upper thresholds emerged (Figs. 4f, 5f).Although in the adult the frequency of the auditory fovea matches that of the vocalised pulses, none of the juvenile bats tested from the 5th to the 7th weeks showed such a frequency match between vocalisation and audition (Figs. 4e, 5e).The results show that postnatal maturation of audition in hipposiderid bats cannot be described by a model based on a single developmental parameter.Abbreviations BF best frequency - CF constant frequency - Cer cerebellum - CN cochlear nucleus - CO auditory cortex - CUF cuneiform nucleus - DAB days after birth - FAL forearm length - FM frequency modulation - IC inferior colliculus - NLL nucleus of the lateral lemniscus - PAG periaqueductal gray - SC superior colliculus     

The role of retinula cell types in fixation behaviour of walkingDrosophila melanogaster

Summary Fixation behaviour of free walking wild typeDrosophila and various retinal mutants was tested in a circular arena. Optomotor response was also measured as a test of the function of R1-6.ora andsev,ora do not fixate a narrow stripe (10° or 20°, Fig. 1) but are able to orient towards broad stripes (110° or 180°, Fig. 1). The behaviour ofsev is not different from wild type. Fixation behaviour ofw rdgB is similar toora (Figs. 5, 6). The mutantora has a maximum optomotor response at low contrast frequencies (Fig. 2), but the threshold for this response is at least one log unit higher than in wild type orsev (Fig. 8). The light intensity threshold at 550 nm of fixation to a broad stripe (110°) is 1–2 log units higher inora than in wildtype, and 4 log units higher insev,ora and the structural brain mutantVam (Fig. 7).The conclusions are that retinula cells R1-6 mediate fixation to a narrow stripe at high and low ambient light intensities, and to a broad stripe at low ambient light levels. R8, possibly in conjunction with R1-6, contributes to orientation towards broad stripes at high light intensities. This hypothesis is supported by evidence that blue-adapted white-eyed flies are able to orient towards a broad stripe at high blue light intensities (Figs. 9 and 12). Blue adaptation totally eliminates the optomotor response (Figs. 10, 11) and so the optomotor response observed inora at low contrast frequencies (Figs. 2 and 8) is most likely due to the small remnants of the rhabdomeres of R1-6 that remain.Abbreviations PDA prolonged depolarising afterpotential - ERG electroretinogram     

Stimulus filtering and electroreception: Tuberous electroreceptors in three species of Gymnotoid fish

Electroreceptive neurons in the posterior branch of the anterior lateral line nerve of three species of electric fish (Gymnotoidei):Sternopygus macrums, Eigenmannia virescens, andApteronotus albifrons, show speciesspecific differences in the filtering of electrical stimuli. All of the tuberous electroreceptor fibers of an individual are tuned to the same frequency: that of the electric organ discharge (EOD) of the species, more specifically, to that of the individual. The fibers inSternopygus are tuned to 50–150 Hz; those inEigenmannia to 250–500 Hz, and those inApteronotus to 800–1,200 Hz (Figs. 3, 5, 8). Two classes of organs inSternopygus andEigenmannia, P and T units, respond to sinusoidal stimuli at the unit's best frequency (BF) with a phase-locked partially-adapting (P), or tonic (sustained) (T) discharge. T-units are more sharply tuned and are more sensitive than P-units. Only one class of organs,P or partially adapting units, have been found inApteronotus and phase-locking is less evident than it is in other species.Nerve section proximal to the recording site does not alter the tuning curves inSternopygus (Fig. 18), but local warming and cooling of the cutaneous receptor site in bothSternopygus andEigenmannia shifts the tuning curve to higher and lower frequencies, respectively (Fig. 17).     

Filter characteristics of cercal afferents in the cockroach

Summary The response dynamics of cercal afferents in the cockroach, Periplaneta americana, were determined by means of a cross-correlation technique using a Gaussian white noise modulation of wind as a stimulus. The white noise stimulus could evoke sustained firing activity in most of the afferents examined (Fig. 1). The spike discharges were unitized and then cross-correlated with the stimulus to compute 1st- and 2nd-order Weiner kernels. The Ist-order kernels from a total of 28 afferents were biphasic and closely matched the time differential of a pulse (Figs. 1, 3 and 4). The amplitude and waveform of the kernels depended on the stimulus angle in such a way that the kernels were the mirror image of those on the polar opposite side (Figs. 2 and 3). The 2nd-order kernels were also differential. They had 2 diagonal peaks and 2 off-diagonal valleys in a 2-dimensional plot with 2 time axes (Figs. 1, 5 and 6). This 4-eye configuration was basically invariant irrespective of the stimulus angle, although the kernels varied in amplitude when the stimulus angle was changed. The time between the peak and a following trough of the 1st-order kernel was constant and had a mean of 4.6±0.1 ms, whereas the time between 2 diagonal peaks of the 2nd-order kernels was 4.7±0.1 ms (Figs. 4 and 6), suggesting that wind receptors (filiform sensilla) on cerci act as a band-pass filter with a peak frequency of about 106 Hz. The peak time, however, varies from 2.3 to 6.9 ms in both kernels, which may reflect the spatial distribution of the corresponding hairs on the cercus. The summation of the 1st- (linear) and 2nd-order (nonlinear) models precisely predicted the timing of the spike firing (Fig. 8). Thus, these 2 lower-order kernels can totally characterize the response dynamics of the wind receptors. The nonlinear response explains the directional sensitivity of the sensory neurons, while the differentiating 1st-order kernel explains the velocity sensitivity of the neurons. The nonlinearity is a signal compression in which one of the diagonal peaks of the 2nd-order kernel always offsets the downward phase of the 1st-order kernel (Fig. 7) and obviously represents a half-wave rectification property of the wind receptors that are excited by hair movement in only one direction and inhibited by hair movement in the polar opposite direction.     

Differing afferent connections of spiking and nonspiking wind-sensitive local interneurons in the terminal abdominal ganglion of the cricket Gryllus bimaculatus

Fifteen local spiking interneurons (LSIs) and twentyone local non-spiking interneurons (LNIs) were identified in the terminal abdominal ganglion (TAG) of the cricket Gryllus bimaculatus on the basis of intracellular recording and staining (Figs. 1, 5, 6). Although the majority of LNIs showed sharp directionalities (Fig. 7) the LSIs did not (Fig. 3). The directionality of LNIs varied with the recording sites within a single cell (Fig. 8). Electrical stimulations of the cereal sensory nerve suggested that the LNIs are connected monosynaptically with the sensory afferents of both the cerci, and that LSIs may possess a variety of bilateral combinations of polysynaptic connections with the sensory afferents. We found that the spiking and the non-spiking local interneurons in the cereal sensory system differ not only in their membrane properties, but also in their afferent connections, and concluded that their differing connectivity to the sensory afferents will associate them with different roles in signal processing.Abbreviations TAG terminal abdominal ganglion - LSI local spiking interneuron - LNI local non-spiking interneurons - CNS central nervous system - PSP post synaptic potential - GI giant interneuron