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FIGURE 4.3 (a) represents dorsal view of the central ganglion (whole mount) showing central projections of LFR 1 (left) and MFR (right) receptor neurons. (b) represents threshold curves of LFR 1 (filled circles) and MFR (open circles) neurons. (c) represents responses of the middle leg LFR 1 neuron to 50 Hz, 100 msec duration stimulus of varying intensities. (d) represents responses of the middle leg MFR neuron to 100 and 400 Hz stimuli of 100 msec duration (shown as black bars) and varying intensities.

bushcrickets (Kalmring et al., 1966) and in Type 5 auditory fibres in crickets (Eibl and Huber, 1979), whereas the fibres most likely to originate in the subgenual organ of crickets show completely different arborisation (Eibl and Huber, 1979; Esch et al., 1980). Recent investigations (Zorovic et al., 2004) indicate that MFR receptor neurons are presynaptically inhibited as described for crickets (Poulet and Hedwig, 2003), locusts (Burrows, 1996) and stick insects (Stein and Sauer, 1999). Wolf and Burrows (1995) proposed that central neurons contribute to presynaptic inhibition of proprio-receptive sensory neurons.

Joint Chordotonal Organs

Chordotonal organs were described in N. viridula close to leg joints in the femur (the femoral chordotonal organ) (Figure 4.1a), distal part of the tibia (the tibial distal chordotonal organ) (Figure 4.1c) and in the last tarsal joint (the tarso-pretarsal chordotonal organ) (Figure 4.1d) (Michel et al., 1983). The femoral chordotonal organ, situated in the lateral distal third of the femur at its anterior ridge (Figure 4.1a), is composed of 12 scolopidia, each with two sensory cells. Their axons run in a sensory nerve, branch together with axons of both subgenual organ sensory cells and join the main leg nerve in the posterior part of the femur, close to the joint with the trochanter. The organ is divided into a proximal part with eight scolopidia, and into a distal part with four scolopidia identified by Debaisieux (1938) as the condensed and dispersed scoloparium, respectively. The proximal part of the organ is fixed to the wall of the femur, and its proximal three scolopidia are separated from the rest; their cap cells form a ligament distally attached to the musculus levator tibia. The other five scolopidia are positioned distally and their cap cells form a separate ligament, distally fixed directly at the apodeme. In this ligament, the authors described four scolopidia of the dispersed femoral chordotonal organ; the cap cells run within the ligament and attach at the apodeme.

The tibial distal chordotonal organ lies in the blood channel proximally (400 ^im) to the tibio-tarsal joint (Figure 4.1c) and has just two scolopidia, one with one and the other with two sensory cells, respectively. The round-shaped ligament of cap cells of both scolopidia extends distally and is attached to the joint membrane between the tibia and the first tarsal joint. The sensory cells lie at the dorsal side of n. tibialis anterior, with dendrites separated from the nerve and scolopals with cilia freely stretched in the blood channel. Axons of sensory cells run with other nerve fibres of the n. tibialis anterior.

The tarso-pretarsal chordotonal organ consists of two separate scoloparia (Figure 4.1d). The proximal one contains one scolopidium with two and one scolopidium with one sensory cell. The proximal scoloparium is distally fixed by a ligament at the tendon of the unguitractor moving the pretarsus. The distal scoloparium consists of three scolopidia, which are arranged into two separate parts, each with a separate ligament. The cap cell of the one-celled scolopidium is distally attached to the inner wall of the posterior claw. The second part of the tarso-pretarsal organ distal scoloparium contains two scolopidia, one with two and the other with one sensory cell. Their cap cells are attached to the inner wall of the anterior claw. Axons of the distal scoloparium form a thin nerve, which joins with others in the n. tibialis anterior.

The joint chordotonal organs and campaniform sensilla detect low frequency substrate vibrations (Figure 4.2). The low frequency receptor neurons (LFR) in N. viridula respond in the frequency range below 120 Hz in a phase-locked manner with best velocity sensitivity (threshold between 3 and 6 X 10"5 m/sec) between 50 and 70 Hz. Their threshold curves run in parallel with the line of equal displacement value (around 10"7 m) (Cokl, 1983). According to the phase of response, three types of LFR were identified electrophysiologically.

Two LFR1 neurons were marked with Lucifer yellow during intracellular recording from the central ganglion (Figure 4.3) (Zorovic et al., 2004). Their central arborisation and projection areas are restricted to the ipsilateral half of the ganglion. The fibres enter the central ganglion in the posterior third of the leg nerve. The main branch arches anteriorly until it reaches the ganglion midline. The side branches diverge from the main axon mostly on the anterior side and decrease in size towards the midline. This branching pattern is somewhat similar to those of the fCO fibres in locusts (Burrows, 1996) although the phase-locked response characteristics suggest that fibres may originate in campaniform sensilla (Kiihne, 1982).

Antennal Mechanoreceptors

Observations of mating behaviour (Ota and Cokl, 1991) indicate that the antennal mechanoreceptors of N. viridula may contribute to the detection of vibratory signals. Antennal campaniform sensilla and Johnston's organ with the central chordotonal organ were described by Jeram and Pabst (1996) and Jeram and Cokl (1996). On each antenna 12 campaniform sensilla were identified; six of them are grouped at the base of the first antennal segment. The Johnston's organ is located in the distal part of the pedicel in nymphs and in the distal part of the second pedicellite in adults. It is composed of 45 amphinematic scolopidia distributed around the periphery of the distal part of the third antennal segment (distal pedicellite). Scolopidia are attached separately in invaginations of cuticle between the pedicel and flagellum. Each scolopidium has three sensory cells and three enveloping cells (scolopale, attachment and accessory cell). Two sensory cells have short sensory cilia, attached distally to the extracellular tube, which has direct connection to the joint cuticle.

The third sensory cell with a longer sensory cilium attaches distally to the cuticle. Axons of 17 scolopidia join one antennal nerve, and 28 scolopidia of the opposite side join fibres of the other antennal nerve. The central organ described in the pedicel of N. viridula consists of seven mononematic scolopidia located proximally and centrally in regard to amphinematic scolopidia. Four of these are grouped into two scoloparia (each with two scolopidia). The four grouped and three separate scolopidia attach to the same place as those of the Johnston's organ. The axons of four scolopidia join one antennal nerve and those of the other three run together with fibres of the second antennal nerve.

Responses of receptor neurons to vibrations of the proximal flagellar segment revealed highest sensitivity around 50 Hz with threshold velocity sensitivity of 2 X 10"3 m/sec (Jeram, 1993, 1996; Jeram and Cokl, 1996). The phase-locked response pattern is characteristic for all of them. Back-fill staining of the N. viridula antennal nerve (Jeram, 1996) revealed mechano-sensory fibres with branches passing the antennal lobus and finally terminating at the ipsilateral side in the suboesophageal or prothoracic ganglion. A few of them project down to the abdominal region of the central ganglion. Such long axons were demonstrated in Drosophila to originate in campaniform sensilla of the pedicel (Strausfeld and Bacon, 1983).

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