Library

Notes: [Auszug] Amputated arms continue to make reflex responses to tactile stimulation1*3-5. The accept reflex can almost always be elicited by placing small objects against the suckers but the reject reflex rarely occurs1. Even so, it is assumed that both reflexes are organized by the arm nerve cord1'6. In ...

Notes: Summary 1. Five interneurones that receive auditory input are described in the Suboesophageal ganglion [SOG] of the locustLocusta migratoria [Fig. 1]. The neurones have been characterised physiologically and anatomically by intracellular recording and staining with Lucifer Yellow. 2. The newly described neurones all have their somata in the SOG (Fig. 1). They are classified into three groups: (a) ascending neurones with axons in the circumoesophageal connectives (SA1, 2, 3); (b) a local neurone with no axon (SL1); and (c) a descending neurone with its axon in the cervical connective (SD1). 3. Transverse sections of the labial and maxillary neuromeres show that the arborisations of the SOG auditory neurones occupy two main areas, medio-dorsal and medio-ventral (Figs. 2, 3). Two auditory interneurones that ascend from the thorax to the brain, the G and B neurones, have segmentally arranged axon collaterals in the SOG (Fig. 1) which project into the same areas of neuropil. 4. In the brain, SA1 terminates in the same dorsal areas of the lower lateral lobe of the protocerebrum as the thoracic ascending auditory interneurones (Fig. 4). SA2 and SA3 terminate more medially where local and descending interneurones arborise. 5. The SOG neurones show sigmoid intensity-response curves (Fig. 7) and broad-band frequency-response curves with maxima between 10 and 20 kHz (Fig. 6), similar to those of the thoracic ascenders at comparable intensities. The SOG neurones have spiking thresholds for auditory stimuli somewhat higher than those of thoracic ascending neurones. 6. All neurones except SD1 spike in response to tones. Discharges are phasic or phasic/tonic in response to short (20 ms) tones, but can become tonic when stimulated with longer tones and depending on the extent of habituation. The SD1 spiked in response to sound signals only with concomitant depolarisation by current injection (Fig. 8B). The oscillating nature of the responses to tones in SD1 (Fig. 8 Biv) and SA2 (Figs. 10B, 11) suggest a recurrent pathway, perhaps involving axon collaterals. 7. Auditory information is conducted from the thorax to the SOG as spikes, not as passively conducted potentials (Fig. 5). Spike latencies in the SOG neurones are long (37–85 ms) compared with those of thoracic ascenders recorded in the SOG (Table 1). In SL1, however, the delay between the spike in a thoracic ascender and the first stimulusrelated EPSP is much shorter (ca. 1 ms) making it a likely candidate to receive information directly from through-fibres. 8. In SA2, SA3, SL1 and SD1, light stimuli also produce EPSPs (Fig. 10) which are not influenced by EPSPs to sound because responses to light and to sound decrement independently. The visual input to ascending SOG neurones indicates there are information loops between the brain and SOG. 9. The responses of SA2, SA3, SL1 and SD1, but not SA1, decrement to varying degrees on repetitive stimulation. The SA3 neurone, which has axons in both circumoesophageal connectives (Fig. 1), displays side-dependent habituation. 10. The neurones described here are components of a previously unknown auditory processing centre in the locust CNS. Their anatomical and physiological properties suggest that the role of the SOG in the auditory pathway needs reassessing, and the concept of the locust auditory system as a linear hierarchy may no longer be valid.

Notes: Summary The establishment of the characteristic adult flight and its motor pattern has been followed behaviourally and electrophysiologically in locusts of exactly known ages. In the last two larval instars there is repetitive firing in the flight muscles but the alternation of antagonists typical of adult flight is not present (Pig. 3). Alternation can first be seen late in the last larval instar and the full adult pattern is recognizable in most animals by day 3 of adult life. Competent flight behaviour is established by day 4 or 5. In this period the coupling between elevator and depressor neurones improves and the pattern stabilizes (Figs. 4 and 6) but the time course of the whole process varies considerable between individuals. After this the only major change is an increase in wingbeat frequency from about 15–20 Hz at fledging to 25–35 Hz in the second or third week (Fig. 5). Fixing the wings immovably at fledging, so eliminating normal sensory feedback and practice, does not prevent the co-ordinated pattern from developing (Fig. 8). Muscle firing frequency with the wings fixed remains in the range 13–20 Hz throughout life, which may represent the natural frequency of the intrinsic oscillator (Fig. 10). Input from all the sense organs of the wings has a direct effect on the motor pattern in young animals (Fig. 9) and it is suggested that the increase in wingbeat frequency is due to changes in the phasic sensory input from the wings as the muscles grow and the cuticle thickens.

Notes: Summary 1. Electrical stimulation of the hindwing tegula inSchistocerca gregaria can elicit synchronous firing in tegula afferents. Recordings from the metathoracic wing sensory nerve, 1C1, showed that both the exact position of the stimulus wires in the tegula and the stimulus voltage determined which of two groups of tegula afferents were predominantly activated (Fig. 2). 2. Recordings were made from metathoracic nerve 1 and flight motoneuron (MN) cell bodies during tegula stimulation. It was found that the faster conducting fibre group from the tegula elicited an EPSP in a wing depressor motoneuron (127) and an IPSP in a wing elevator motoneuron (113). The slightly slower conducting fibre group produced the opposite effects, i.e., an IPSP in the depressor (127) and an EPSP in the elevator motoneuron (113). Latency measurements and responses to high frequency stimulation indicated that the excitatory connections are monosynaptic whereas the inhibitory connections are probably mediated by nonspiking interneurons (Figs. 4–7). 3. Stimulation of the hindwing tegula produced PSP's of the same sign in a contralateral motoneuron as in its ipsilateral homologue. The contralaterally evoked EPSP's were mediated by a polysynaptic pathway. These EPSP's often potentiated with repetitive stimulation whereas the ipsilateral responses did not (Fig. 9). 4. Stimulating both hindwing tegulae caused interactions at the motoneurons. Stimulation of the contralateral hindwing tegula potentiated the response of MN 113 to an ipsilateral stimulus given up to 30–40 ms later. Ipsilateral stimulation did not potentiate the contralateral response (Fig. 10). This interaction between the two inputs probably occurs at the tegula-to-motoneuron synapse. 5. Simultaneous stimulation of the fore- and hindwing tegulae at flight frequency could evoke a larger EPSP in MN 113 than when the hindwing alone was stimulated (Fig. 11). 6. These interactions between tegulae result in a considerable amplification of their individual inputs to motoneurons during flight.

Notes: Summary Aggregates of synaptic vesicles, stained black by the zinc iodide-osmium procedure, can be visualised with the light microscope in 1 μm plastic sections. This allows the main branches of a neurone to be reconstructed relatively rapidly and the associated vesicle aggregates to be plotted. By resectioning, the identity of the vesicle aggregates has been confirmed with the electron microscope. Two flight motor neurones in the mesothoracic ganglion of the locust have been examined. One is identified as a dorsal longitudinal muscle motor neurone (muscle 112) and the other is probably a subalar neurone (muscle 99). Both have a large density of vesicle aggregates on the neuropilar segment, the widest part of the main neuronal axis, but few on the neurite within 250 μm of the cell body. The larger branches arising from the neuropilar segment tend to have a lower density of aggregates than fine branches, which suggests that synapses to the branches may occur mainly on the distal twigs. These results are an important preliminary step in determining the integrative functions of such neurones and have immediate implications in the interpretation of microelectrode recordings.